Introduction
The naturally occurring stable isotopes of hydrogen and oxygen in the water
molecule have been highly instructive as tracers in hydrology and
eco-hydrology. This is mainly based on naturally occurring variations in the
relative abundance of two rare, heavy isotopes (i.e., 2H or D and
18O), arising from phase changes and mixing as water passes through the
hydrologic cycle (Dansgaard, 1964; Gat, 1996). The hydrogen and oxygen
isotopic composition of precipitation varies both spatially and temporally,
due to fluctuations (i) at the site of evaporation, e.g., in meteorological
conditions such as relative humidity (RH), wind and sea surface temperature;
and (ii) at the site of precipitation, e.g., in the degree of rainout of
particular air masses (Craig, 1961; Dansgaard, 1964; Gat, 1996;
Araguás-Araguás et al., 2000; Gibson et al., 2008). The stable isotopes of
hydrogen and oxygen in precipitation show a distinct empirical relationship,
described by the Global Meteoric Water Line (GMWL: δ2H = 8.1*δ18O + 10.3 ‰; Rozanski et al.,
1993). The δ18O–δ2H relationship in precipitation
at any single location is however better described by a Local Meteoric Water
Line (LMWL), which can have a different slope and intercept depending on the
conditions in which the local water source was formed, and LMWLs can be used
to compare different (sub)surface water bodies with local precipitation
(Rozanski et al., 1993; Breitenbach et al., 2010).
Variation of δ2H or δ18O in xylem water of plants
has been used extensively to determine water sources used by plants and
their functional rooting depth (Zimmermann et al., 1967; Brooks et al.,
2010). Evaporation directly from the soil causes an isotopic enrichment of
soil water available for plant roots (Sachse et al., 2012, and references
therein). Water uptake through the roots and transport in plants is
generally considered to occur without fractionation (White et al., 1985;
Dawson and Ehleringer, 1991, 1993; Zhao et al., 2016), so that the isotopic
composition of xylem water represents the composition of the plant water
source (Dawson and Ehleringer, 1991; Evaristo et al., 2015). Fractionation
during root water uptake has been found for plants living under xeric
conditions and in mangroves (Ellsworth and Williams, 2007), and recent
research stated that fractionation likely occurs during the water
redistribution after uptake (Zhao et al., 2016). In contrast, the isotopic
composition of leaf water differs markedly from that of xylem water. This is
because evaporation discriminates towards lighter isotopologues. As a
result, the remaining leaf water after transpiration becomes more enriched
in heavy isotopes. The degree of enrichment from xylem to leaf water is a
function of temperature, RH and the isotopic composition of water vapor
surrounding the plant (Kahmen et al., 2008; Sachse et al., 2012).
As δ2H values of leaf wax n-alkanes
obtained from lake-surface sediments and soils showed tight correlations
with δ2H values of precipitation (Sachse et al.,
2004; Hou et al., 2008; Polissar and Freeman, 2010; Garcin et al., 2012),
the δ2H signature of n-alkanes, long-chain
hydrocarbons with 25–35 carbon atoms derived from fossil plant leaf waxes
incorporated in lake sediments, is increasingly being used as a paleoclimate
proxy (Eglinton and Hamilton, 1967; Mayer and Schwark, 1999; von Grafenstein
et al., 1999; Thompson et al., 2003; Tierney et al., 2008; Costa et al.,
2014). Consequently, a better understanding of the hydrogen
fractionation is also needed, which occurs during its incorporation from
precipitation via leaf water into plant leaf waxes. The present study is
developed in the context of such an application of hydrogen isotope
geochemistry for paleoclimate research, focusing on the sediment record of
Lake Chala (Verschuren et al., 2009; Barker et al., 2011; Tierney et al.,
2011).
The study area is located in equatorial East Africa. In this semiarid
tropical region, the biannual passage of the Intertropical Convergence Zone
(ITCZ) induces a strongly bimodal pattern of seasonal rainfall (Nicholson,
2000). Plants in this region are exposed to a prolonged dry season between
June and September, during which little precipitation falls (< 20 mm month-1). Adaptations to survive this period of water shortage include
stem succulence, leathery leaves and deep roots providing access to deep and
permanent water sources (Elffers et al., 1964; Corbin et al., 2005). Meinzer
et al. (1999) suggested that, at least in pristine dryland ecosystems,
competition for water among species may actually be limited due to
pronounced spatial and temporal partitioning of water resources resulting
from maximized species diversity. Differential water resource utilization
has been shown across different (Ehleringer et al., 1991; Jackson et al.,
1999; Goldstein et al., 2008) and within similar growth forms (Field and
Dawson, 1998; Meinzer et al., 1999; Stratton et al., 2000). It furthermore
appears that the relationship between the root biomass in a particular soil
layer and the degree of contribution of that soil layer to plant water
uptake is not always straightforward (Jackson et al., 1995; Stahl et al.,
2013). Therefore, analysis of the dual stable isotope composition of xylem
water could be a valuable tool to elucidate the water sources effectively
used by plants (Dawson and Ehleringer, 1991; Liu et al., 2010).
In this study, we measured the δ2H and δ18O content of precipitation, lake water, groundwater, and of xylem
and leaf water in three individuals of each of the 14 different species from
three distinct habitats around Lake Chala. Sampling was carried out during
successive wet and dry seasons of 1 complete year. Our primary research
questions were as follows: (1) are seasonal differences in the isotopic composition of
precipitation reflected in xylem water? (2) do plants show habitat-specific
variability and temporal shifts in their water source use? (3) what is
the influence of plant family, growth form, phenology, season and habitat on
the deuterium enrichment from xylem to leaf water? The work that we present
here reports isotope data from a data-scarce region and is intended to
provide a basis for the interpretation of leaf wax n-alkane δ2H values applied as (paleo)hydrological proxies.
Lake Chala, situated in equatorial East Africa, on a
continent-scale vegetation map (left) and with the different sampled habitats
(right). Sampling sites in savannah, at the lakeshore and on the crater rim
are indicated by red dots. Adapted from Wikimedia Commons (2010).
Materials and methods
Study site
Lake Chala is a 4.2 km2, ∼ 92 m deep crater
lake with near-vertical inner crater walls (Moernaut et al., 2010), situated
on the southeastern slope of Mt Kilimanjaro (3∘19′ S,
37∘42′ E) at 880 m above sea level (m a.s.l.) in equatorial East
Africa (Fig. 1). The biannual passage of the Intertropical Convergence Zone (ITCZ)
induces a bimodal rainfall pattern, with southeastern (SE) monsoon
winds bringing “long rains” normally from March to mid-May and northeastern (NE)
monsoon winds bringing “short rains” from late October to December
(Nicholson, 2000; Fig. 2). The local climate is tropical semiarid, with
lowest mean monthly night- and daytime temperatures in July–August (ca. 18
and 28 ∘C, respectively) and highest values in February–March (ca. 21 and 33 ∘C, respectively; Fig. 1a) for Voi, 80 km east of Lake
Chala (Buckles et al., 2014). Given the total annual rainfall of ca. 565 mm
and an estimated annual lake-surface evaporation of ca. 1735 mm (Payne,
1970), the water budget of Lake Chala must be balanced by substantial
groundwater input, the main source of which is precipitation falling onto montane
forests of Mt Kilimanjaro's east-facing slope at 1800–2800 m a.s.l. (Hemp,
2006a).
The vegetation of the crater basin containing Lake Chala consists of
different forest and woodland types (Hemp, 2006b). On the upper part of the
inner slopes a dry forest occurs, with succulents such as Euphorbia quinquecostata (Euphorbiaceae) and deciduous species such as
Commiphora baluensis (Burseraceae) and Haplocoelum foliolosum (Sapindaceae) dominating the tree layer. Near the lakeshore an
evergreen forest with Sorindeia madagascariensis (Anacardiaceae),
Ficus sycomorus (Moraceae) and Trichilia emetica
(Meliaceae) grows. In contrast, the outer crater slopes are covered with dry
savannah woodlands, with a lower and more open canopy. The stunted, fruit-tree-like appearance of the woody species, mainly Combretaceae, Burseraceae
and Anacardiaceae, inspired the first botanists to describe this vegetation
formation as “Obstgartensteppe” (“fruit tree garden steppe”; Volkens, 1897).
Whereas all these vegetation types grow on rocky slopes with very shallow
soils, the soils of the flat foothills are deeper. Here, most of the former
natural savannah vegetation is converted into agricultural fields or meadows.
The savannah woodlands still existing in this area are dominated by acacias
(A. nilotica, A. senegal; Mimosaceae).
Sampling
Two duplicate rain gauges were installed in the savannah just outside Chala
crater (at 3∘19′ S, 37∘42′ E and 842 m a.s.l.; Fig. 1) to
sample precipitation on a monthly basis between September 2013 and August 2014. The
collectors consisted of a 5 L plastic container with a plastic funnel of ca. 15 cm diameter, in which a plastic fiber mesh net was placed to prevent dirt
from entering the bottle. A layer of mineral oil (thickness ca. 1.5 cm) was
poured in the jars to avoid evaporation and exchange with air moisture,
which could alter isotopic composition of collected water (Friedman et al.,
1992). As only six precipitation samples reached the laboratory, precipitation
was additionally collected in the savannah about 20 km to the west of the
Chala crater (at 3∘23′ S, 37∘27′ E and 820 m a.s.l.) by
an evaporation-free collector (Groning et al., 2012) of the brand PALMEX
(Croatia) on a monthly basis from November 2014 until November 2015. Lake
water was sampled 30 cm below the water surface in the middle of the lake on
a monthly basis from January 2013 until October 2014. A groundwater sample
was obtained from the Miwaleni Spring (3∘25′ S, 37∘27′ E)
in July 2015. We assumed that the isotopic composition of groundwater is
seasonally stable, as the groundwater recharge occurs on a decadal scale in
the region (Zuber, 1983).
Studied plant species and family with their respective growth form,
leaf phenology and habitat.
Species
Family
Growth form
Leaf phenology
Habitat
Acacia gerrardii
Leguminosae
Tree
Evergreen
Savannah
Boswellia neglecta
Burseraceae
Tree
Deciduous
Crater rim
Ficus sycomorus
Moraceae
Tree
Deciduous
Lakeshore
Lepisanthes senegalensis
Sapindaceae
Tree
Evergreen
Lakeshore
Sideroxylon sp.
Sapotaceae
Tree
Evergreen
Lakeshore
Commiphora africana
Burseraceae
Shrub
Deciduous
Crater rim
Euphorbia tirucalli
Euphorbiaceae
Shrub
Evergreen
Crater rim
Grewia tephrodermis
Tiliaceae
Shrub
Deciduous
Savannah, crater rim
Maerua sp.
Capparaceae
Shrub
Evergreen
Crater rim
Thylachium africanum
Capparaceae
Shrub
Evergreen
Lakeshore, crater rim
Vepris uguenensis
Rutaceae
Shrub
Evergreen
Savannah, crater rim
Ximenia americana
Oleaceae
Shrub
Evergreen
Savannah
Enteropogon macrostachyus*
Poaceae
Grass
Perennial
Savannah
Themeda triandra*
Poaceae
Grass
Perennial
Savannah, crater rim
* The whole plant was sampled, which consisted mainly of green leaves and
thus represented leaf water.
Plant material was collected during the main dry season in September 2013
and July 2014, during the NE monsoon season in December 2013, during the
short dry season in February 2014, and during the SE monsoon season in April
2014. However, because the 2014 long rainy season atypically started already
in February, our plant sampling for the short dry season was already
influenced by fresh SE monsoon rainfall. Fourteen plant species with varying
growth forms (grass, shrub or tree) and leaf phenology (deciduous or
evergreen, the latter including succulent) were collected in three distinct
habitats (savannah, crater rim, lakeshore), representative of the region,
around Lake Chala (Fig. 1, Table 1). Shrubs were defined as woody plants
with multiple stems, while trees had one erect perennial stem. Lakeshore
vegetation was sampled at the northeast side of the lake, savannah was
sampled outside the crater ca. 500 m to the northwest, and crater-rim
vegetation at the top of the crater's western rim (1100 m a.s.l.). The
locally most abundant plant species within each habitat were chosen with the
aid of an experienced local guide, although the choice of species was
sometimes restricted by practical limitations such as difficulties in
reaching certain locations. For each habitat, three individuals of each
species were sampled.
Two different sampling techniques were used to collect xylem water from
plants, the choice of which depended on the plant type. When the plant had a
trunk with a diameter of more than 10 cm, a core drill sample (300 mm,
diameter 4.30 mm, hardwood head; Pressler, Recklinghausen, Germany) was
extracted, from which the outer layer (epidermis, cortex, bark fibres and
phloem) was removed to prevent contamination with phloem sap. In the case of
smaller trees and shrubs, a piece of twig was sampled, the outer layer was
scraped off using a knife, and it was enclosed in sealed vials. For leaf
water analysis, leaves were taken from each plant and placed in vials. If
twigs were sampled, leaves on those twigs were sampled. In the case of core
sampling, leaves were sampled randomly at different heights and at the four
cardinal points and merged to one bulk sample per replicate to provide
samples that were representative of the entire plant. Entire leaves were
collected in order to ensure the integration of the signal from the entire leaf,
given likely isotopic gradients along the length of the leaf (Helliker and
Ehleringer, 2000; Sessions, 2006). Leaves were sampled between 10:00 and
15:00 UTC + 3 to eliminate additional variability induced by previously reported
large diurnal variations in the isotopic composition of leaves (Cernusak et
al., 2002; Li et al., 2006; Kahmen et al., 2008). From grasses, only leaves
were sampled. The whole plant was sampled which consisted mainly of green
leaves and thus represented leaf water. Stem, twig and leaf samples were
stored frozen until the water was quantitatively extracted via cryogenic
vacuum distillation (West et al., 2006). Following Araguás-Araguás
et al. (1995), isotopic data were retained for interpretation only if the
extraction efficiency, determined by further drying of the sample at
105 ∘C for at least 48 h, exceeded 98 %. A variety of water
extraction methods for the analysis of the stable isotope water exist.
Cryogenic vacuum extraction of soil pore water remains a challenge but is
an effective method for plant water extractions (Orlowski et al., 2016).
Analysis
The δ2H and δ18O values of water
samples were determined using cavity ringdown spectrometry (WS-CRDS,
L2120-i, Picarro, USA), coupled with a vaporizing module (A0211
high-precision vaporizer) and a micro-combustion module, which eliminates
the interference of organic compounds (Martín-Goméz et al., 2015). Each
sample was measured 10 times, of which the first 5 injections were
eliminated in order to overcome memory effects. The measurement uncertainty
(±1σ) of our CRDS was 0.1 and 0.4 ‰ for δ18O and δ2H,
respectively. δ isotopic composition is expressed in terms of
2H / 1H and 18O / 16O ratios, represented by δ
values: δsample= (Rsample/Rstandard-1) with
Rsample and Rstandard being the isotopic ratio (2H / 1H or
18O / 16O) measured in the sample and the standard, respectively
(Gat, 2005). The reference standard used is Vienna Standard Mean Ocean Water
(VSMOW), which, by definition, has δ2H and δ18O
concentrations of 0 ‰.
The enrichment factor εl/x characterizes the hydrogen-isotopic
fractionation between xylem and leaf water and is defined as
(Eq. 1)
εl/x=(δ2Hleaf+1)/(δ2Hxylem+1)-1.
Enrichment factors and δ values are typically reported in per mil
(‰) (Cohen et al., 2007). Both the δ2H and δ18O values of water samples were
measured, but particular focus was given to the δ2H
values as this research is intended to provide a basis for the
interpretation of leaf wax n-alkane δ2H
values as (paleo)hydrological proxies.
The average isotopic signature of the source of xylem water was determined
from the intersection of xylem water samples (aligned along a local
evaporation line, LEL) with the LMWL (Eqs. 2 and 3):
δ18OLMWL-int=δ2H-slopeLEL*δ18O-interceptLMWLslopeLMWL-slopeLELδ2HLMWL-int=δ18OLMWL-int*slopeLMWL+interceptLMWL.
The isotopic signatures of xylem water were further characterized with a
parameter describing the relative degree of evaporation. We developed the
evaporation distance, defined as ED and calculated as the distance from the
LMWL along an evaporation line, scaled to the δ2H axis (Eq. 4).
The higher this ED value, the further away from the LMWL and the more
evaporated the water will be, i.e., the concentration of heavy
isotopes will be higher.
Evaporationdistance(ED)=δ2H-δ2HLMWL-int2+slopeLMWL*δ18O-δ18OLMWL-int2
Analyses of variance (ANOVA) were used for comparisons of δ2H
and δ18O isotopic signatures among plant species, growth forms,
leaf phenologies, seasons and habitats. Tukey post hoc comparisons were used
to further examine differences. All statistical analyses were performed
using R (version 3.2.3.). Slopes and intercepts of LMWL and LEL were
estimated with linear regressions. A discussion of the different slopes and
intercepts is not within the scope of this paper, but they were used to calculate
δ2HLMWL-int.
(a) At Lake Chala, the monthly average temperature varied slightly
between 23.9 ∘C in June–July and 27.1 ∘C in April. The
minimum and maximum temperatures for both the study area and nearby Voi
(Kenya) are shown. (b) Monthly rainfall distribution from September 2013 to
August 2014, with the isotopic composition of precipitation and lake water
(δ2Hprec and δ2Hlake). The total amount
of rainfall during the study period (692 mm) was slightly above reported
values for the long-term mean annual precipitation in the Chala region.
Note that the 2014 long rainy season atypically already started in February
and ceased in June.
Xylem and leaf water δ2H and δ18O values of all plant species, seasons and plant habitats in the
vicinity of Lake Chala, and lake, precipitation and groundwater against the
LMWL (δ2H = 7.12*δ18O + 10.69 ‰,
black line). The box plots show the mean (bold line), minimum, first
quartile, median, third quartile and maximum for the isotopic composition of
leaf water, xylem water, precipitation and lake water. LMWL: local meteoric
water line; GMWL: global meteoric water line (δ2H = 8.1*δ18O + 10.3 ‰; Rozanski et al.,
1993).
The average isotopic signature of the source of xylem water
(δ2HLMWL-int), determined from the intersection of xylem
water samples with the LMWL, among habitats and seasons. Nov.: November; n:
amount of samples; R: rainfall amount (mm).
The evaporation distance (ED) of xylem samples, describing
the relative degree of evaporation by calculating the distance from the LMWL
along an evaporation line, among habitats and seasons. n: amount of
samples.
Results
Isotopic composition of precipitation, lake water and
groundwater
During our main sampling period from September 2013 until August 2014, local
rainfall was highest in December (peak NE monsoon rains) and April (peak SE
monsoon rains) with 135 and 122 mm, respectively, and lowest in January
(short dry season) and August (long dry season) with two instances of 1 mm of rain
(Fig. 2b). During the 12-month monitoring period, the total amount of local
precipitation was 692 mm. The monthly average temperature varied between
23.9 ∘C in June–July and 27.1 ∘C in April, resulting
in an overall annual mean of 25.5 ± 1.2 ∘C (mean ± 1σ standard deviation). The monthly minimum (nighttime) temperature
followed a similar pattern (19.6 ± 1.0 ∘C), while the
monthly maximum (daytime) temperature showed greater variability (37.5 ± 2.1 ∘C),
with an atypical minimum in February (34.8 ∘C) and a maximum in April (41.2 ∘C).
The isotopic composition of precipitation is most enriched during the dry
month of July, with values of 36.6 for δ2Hprec and 4.2 ‰ for δ18Oprec, and most depleted
during rainy November, with values of -47.9 and -7.2 ‰ (Fig. 2), respectively. Comparing the two rainy seasons
revealed considerable differences (p= 0.08), with more enriched rain during
the SE monsoon (δ2Hprec of 16.0 ± 2.5 ‰
and δ18Oprec of 0.8 ± 0.7 ‰; n= 3) compared to the NE monsoon (δ2Hprec of -26.5 ± 21.5 ‰ and δ18Oprec of -4.9 ± 2.3 ‰; n= 2). In
order to draw an LMWL, precipitation samples (n= 12) covering the period
November 2014–November 2015 were added (from a savannah site 20 km west of
Lake Chala, cf. above). Based on this dataset (n= 12), the yearly
volume-weighted average values are -6.5 ‰ for δ2Hprec and -2.5 ‰ for δ18Oprec. Compared to the global meteoric water
line (δ2H = 8.1*δ18O + 10.3 ‰; Rozanski et al., 1993) and the LMWL of central Kenya
(δ2H = 8.3*δ18O + 11.0 ‰; Soderberg et al., 2013), the LMWL of the study region (δ2H = 7.1*δ18O + 10.7 ‰; n= 18) has a
slightly lower slope and intermediate intercept (Fig. 3).
The isotopic measurements on lake water during 22 consecutive months (from
January 2013 until October 2014) yielded mean δ2Hlake and
δ18Olake values of 17.4 ± 0.7 ‰ and 2.9 ± 0.2 ‰, respectively (Figs. 2 and 3), with
very little variation through the year but highest values in the warm months
of February–March and a modest minimum around August. The groundwater
isotopic composition equaled -20.2 ‰ for δ2H and -4.6 ‰ for δ18O in July 2015
(Fig. 3).
Xylem water
The δ2H of xylem water (δ2Hxylem) in a total of 154 analyzed samples (no grasses) ranged between
-87 and 25 ‰ (Fig. 3), with an overall mean value of -18 ± 17 ‰. δ2Hxylem
varied between plants at the lakeshore (-2 ± 10 ‰; n= 48) and isotopically more depleted plants in
the savannah (-25 ± 12 ‰; p < 0.01; n= 34) and on the crater rim
(-26 ± 15 ‰; p < 0.001; n= 72). The δ2Hxylem of
trees (three species; n= 38) at the lakeshore (1 ± 8 ‰)
was significantly higher than that of the single
shrub species sampled in this habitat (-13 ± 5 ‰; p < 0.001; n= 10). In the savannah and on the crater rim, no
difference (p > 0.05) could be observed between the δ2Hxylem of trees and shrubs. Across all sampled
plants, leaf phenology (deciduous or evergreen) did not significantly
influence δ2Hxylem value (p > 0.05).
Only two of the sampled species showed seasonality in δ2Hxylem, but in a dissimilar pattern. The tree species
Sideroxylon sp. had lower δ2Hxylem
values during the long dry season (0 ± 5 ‰; p < 0.05) than during the short (10 ± 1 ‰)
and long rainy seasons (6 ± 8 ‰). The tree
species Ficus sycomorus showed lower values (p < 0.01)
during the short rainy season (-5 ± 2 ‰) than
during the long dry season (4 ± 3 ‰) and long
rainy season (13 ± 4 ‰).
The hydrogen isotopic signatures of xylem samples follow an evaporation line
(LEL). To determine the mean isotopic composition of the water source from
which xylem water originated, LELs were calculated for each of the three
different plant habitats and used to estimate (Eqs. 2 and 3) the intersection
points of xylem water with the LMWL (δ2HLMWL-int). LELs
with a slope of about 3 fitted best with our data from savannah and the
crater rim, while a slope of about 5 fitted best at the lakeshore. The LELs
with slope of 3 correspond well with the modeled evaporation lines for soil
water in our study area (Gibson et al. 2008). A slope of 5 corresponds more
with Gibson et al.'s (2008) modeled evaporation lines for surface water, which can
be explained by the lakeshore trees and shrubs mostly using lake water. The
δ2HLMWL-int values ranged between -79 and -13 ‰,
with an overall mean value of -41 ± 13 ‰. No statistical differences (p > 0.05)
could be observed among δ2HLMWL-int values analyzed by
habitat, species, growth form or leaf phenology (Fig. S1 in the Supplement). Plants at the
lakeshore showed only a weak temporal trend in δ2HLMWL-int
(mean -45 ± 12 ‰; p > 0.05), whereas
plants in both the savannah (-42 ± 9 ‰; p < 0.01)
and on the crater rim (-38 ± 15 ‰; p < 0.001)
showed significant seasonal variability in δ2HLMWL-int (Fig. 4).
The ED (Eq. 4) is a parameter describing the relative
degree of evaporation of a xylem water sample. It is derived by calculating
the distance of xylem data points from the LMWL along the LEL. The higher
this ED value, the further away from the LMWL and the more evaporated the
water will be. A great range in ED values was observed, varying between 1
and 94 ‰ across all samples (Fig. 5). Plants at the
lakeshore produced systematically higher ED values (49 ± 13 ‰) than those
in savannah (23 ± 14 ‰; p < 0.001) or on the crater rim
(17 ± 9 ‰; p < 0.001). Growth form also influenced ED
(p < 0.05), with lower values for shrubs than for trees both at the
lakeshore (respectively, 42 ± 16 and 50 ± 12 ‰) and on the crater rim (respectively, 16 ± 9
and 22 ± 8 ‰; Fig. S2). No
difference (p > 0.05) in ED value was found between evergreen and
deciduous plants. The evaporation distance showed a clear temporal effect
during the study period at the lakeshore (p < 0.01) and on the
crater rim (p < 0.01), but in the savannah the trend was not
significant (p > 0.05; Fig. 5). Among the seven non-grass plant
species sampled at the crater rim, a significant difference (p < 0.01) was observed between the low ED value for Vepris uguenensis
(9 ± 7 ‰) and the high ED value for
Euphorbia tirucalli (25 ± 7 ‰).
Leaf water
The number of samples investigated for leaf water δ2H (δ2Hleaf) totaled 186, including the two
species of grasses. Across this complete dataset, δ2Hleaf ranged from -83 to 37 ‰, with an overall mean
value of 5 ± 20 ‰ (Fig. 3). The δ2Hleaf of grasses (mean -2 ± 14 ‰; n= 18)
were less enriched than those of trees (10 ± 16 ‰; p < 0.01; n= 63), while those
of shrubs were intermediate (3 ± 22 ‰; p= 105). Leaf phenology had a significant effect (p < 0.001) on the
δ2Hleaf of shrubs with values of 16 ± 7 and
-4 ± 24 ‰ for deciduous
(n= 38) and evergreen (n= 67) shrubs, respectively. The two evergreen
shrubs of the Capparaceae family, in particular, showed strongly depleted
δ2Hleaf values: -11 ± 19 ‰
for Thylachium africanum and -57 ± 14 ‰
for Maerua sp. Even if these outliers are
removed from the group of evergreen shrubs, the difference in δ2Hleaf between deciduous and evergreen remains
significant (p < 0.05). No such difference (p > 0.05) was
observed between the δ2Hleaf of deciduous (n= 23) and evergreen (n= 40) trees. At the lakeshore, trees showed
higher δ2Hleaf values (9 ± 20 ‰; n= 38) than shrubs
(-6 ± 20 ‰; p < 0.05; n= 12). Shrubs in the savannah
had higher δ2Hleaf values (13 ± 13 ‰; n= 31)
than those on the crater rim (0 ± 25 ‰; p < 0.05; n= 62) and at the lakeshore
(-6 ± 20 ‰; p < 0.05; n= 12), while
δ2Hleaf values of shrubs at the lakeshore and
on the crater rim did not differ significantly (p > 0.05). Only
the shrub Vepris uguenensis (in both habitats) and the trees
Sideroxylon sp. and Lepisanthes senegalensis (only
lakeshore habitat) showed an effect of seasonality on δ2Hleaf. V. uguenensis showed depleted δ2Hleaf values during the dry season (-11 ± 16 ‰) vs.
enriched values during both rainy seasons (7 ±11 ‰), whereas Sideroxylon sp. and
L. senegalensis showed depleted δ2Hleaf during the long rainy season
(-10 ± 20 and -6 ± 11 ‰, respectively)
and enriched δ2Hleaf during the short rainy
season (27 ± 3 and 33 ± 3 ‰, respectively).
δ2H enrichment factor
εl/x, characterizing the hydrogen-isotopic
fractionation between xylem and leaf water, among habitat and leaf
phenology. Deciduous plants gave higher εl/x than
evergreens. Note that Grewia tephrodermis,
Vepris uguenensis and Thylachium africanum showed similar εl/x independent of sample habitat. n: amount of samples.
Factor of deuterium enrichment from xylem to leaf water
(εl/x)
The enrichment factor εl/x of deuterium fractionation between xylem
and leaf water could be determined on a total of 133 pairs of δ2Hxylem and δ2Hleaf
values. This yielded an average εl/x for δ2H of 24 ± 28 ‰ across all habitats, plant species (trees and
shrubs only) and seasons (Fig. 6); the εl/x for δ18O is
not reported here but showed the same trends. The enrichment factor showed a
significant difference between plants at the lakeshore (7 ± 23 ‰; p < 0.001; n= 42) and the savannah (36 ± 19 ‰; p < 0.001; n= 29) and
crater-rim plants (30 ± 30 ‰; n= 62). Growth
form had no significant effect (p > 0.05) on εl/x, with
values of 27 ± 29 ‰ for shrubs (n= 80) and 19 ± 25 ‰ for trees (n= 53). Significant
differences (p < 0.001) were found between species according to
their leaf phenology with εl/x values of
37 ± 25 ‰ for deciduous plants (n= 50) and 16 ± 27 ‰
for evergreens (n= 83). An effect of seasonality
was limited and could only be observed in Sideroxylon sp. (p < 0.05) and Lepisanthes senegalensis (p < 0.01),
reflecting the trends in δ2Hleaf.
Discussion
Water sources: isotopic composition of precipitation,
groundwater and lake water
Equatorial East Africa has a pronounced bimodal seasonality in rainfall,
characterized by (long) SE monsoon rains from March until May and (short)
NE monsoon rains from late October until December separated by a long dry
season (Nicholson, 2000). During November–December of 2013, when Indian
Ocean moisture was advected by NE monsoon winds, δ2Hprec
and δ18Oprec were more depleted than during February
through May 2014, when Indian Ocean moisture was advected by SE monsoon
winds (Fig. 2). This result stresses the importance of the air mass
trajectory in controlling seasonal patterns of rainwater isotopic
signatures. For the location of Lake Chala, moist air advected by the NE
monsoon has traveled a longer distance overland compared to moist air
advected by the SE monsoon. However, within the short rainy season, we
recorded a considerable difference between the very strongly depleted
δ2Hprec value for November precipitation (-48 ‰), representing the first rains after the dry season,
and the only modestly depleted δ2Hprec value for December (-5 ‰). This indicates that not only the general air mass
trajectory but also other phenomena such as different degrees of rainout
contributing to the formation of precipitation or temperature and relative
humidity control of the initial vapor (Dansgaard, 1964) play an important
role in determining the isotopic composition of monthly precipitation in any
particular year. The total amount of rainfall during the
study period (692 mm) was slightly above reported values for the long-term
mean annual precipitation in the Chala region, which vary between 583 mm
for Taveta 1989–2005 and 532 mm for Chala 2000–2007 (Fig. 2) and
∼ 650 mm for the Chala region (Hemp, 2006b). According to the
Kenya Food Security Steering Group (2014), the 2013 rainy season started in
mid-November instead of late October and was thus delayed by 2–3 weeks.
Additionally, it already ceased in mid-December, 1 week earlier than normal.
Rainfall amounts in both November and December were well above average. In
addition, the 2014 long rainy season started earlier and ceased later than
normal, with monthly rainfall amounts for February and June more than double
those of an average year (Fig. 2). On the other hand, the month of January
2014 was exceptionally dry, and the main dry season from July to September
was drier than usual (Fig. 2). The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Hess, 2004),
developed by NOAA, confirmed that there is a distinctly different trajectory
for precipitation in November and December (northeast) and April, May and
July (southeast). To compute air parcel trajectories, the model required
data from the NOAA meteorological database, and trajectories were modeled
310 h backwards in time starting from the end of the respective month.
The δ2Hprec and δ18Oprec in the dry
month of July were clearly more enriched than the corresponding values for
both wet periods (Fig. 2). This result illustrates the “amount effect”
(Dansgaard, 1964), which states that tropical regions characterized by
limited temperature variation but strong seasonality in rainfall show a
stronger depletion of the heavy isotopes of water at higher precipitation
rates.
The isotopic composition of the groundwater sampled at Miwaleni Spring in
July 2015 (-4.6 ‰ for δ18O and -20.2 ‰ for δ2H) is consistent with data of
McKenzie et al. (2011) for several groundwater wells around Mt Kilimanjaro
measured in 2006.
Monthly isotopic measurements of lake-surface water revealed very little
seasonal variation in δ18Olake and δ2Hlake (Fig. 2 and 3). Our mean δ2Hlake and
δ18Olake values (17.4 and 2.9 ‰, respectively) during 2013–2014 are very similar to
those measured by McKenzie et al. (2010) in January 2006 (19.5 and 2.3 ‰), indicating that the
isotopic signature of Lake Chala surface water is also stable on an
inter-annual to decadal timescale. We did observe a small seasonal trend in
both δ18Olake and δ2Hlake with lowest
(least enriched) values around August. This is counterintuitive, as
evaporative enrichment is expected to be more pronounced during the dry
season. Although detailed assessment of this seasonal trend is outside the
scope of the present study, we offer two possible explanations. First,
seasonal deep circulation of the lake's water column during the cool and
windy main dry season (June–September; Wolff et al., 2014) may mix the
evaporating (and thus isotopically enriched) surface water with isotopically
more depleted deeper water, resulting in slight depletion of the surface
water. Alternatively, dry-season evaporation may be compensated for by enhanced
inflow of subsurface water carrying the isotopic signature of the
precipitation which fell on the forested slopes of Mt Kilimanjaro during
the previous rainy season (Barker et al., 2011). Whatever the cause of the
modest seasonal trend in lake-water δ2Hlake, plants using
significant amounts of lake water can be expected to show a similarly small
seasonality in the isotopic signature of xylem water, δ2Hxylem, when compared to, e.g., the seasonality of
plants using temporarily more variable surface water.
Xylem water: spatial and seasonal partitioning of plants' water
sources
The average isotopic composition of plants' source water is reflected in the
intersection points of individual xylem samples' LELs with the LMWL. Plants
that rely mostly on water from isotopically depleted NE monsoon rains will
exhibit relatively low δ2HLMWL-int values, while the
opposite is true for plants relying on water from the isotopically less
depleted SE monsoon rains. The distance of individual δ2Hxylem values from the LMWL along their LEL is
proportional to the relative degree of evaporation before uptake by the
plant. The higher this parameter ED, the greater the relative importance of
topsoil water (which is prone to evaporation) compared to deeper soil water.
In a study region experiencing a Mediterranean climate with wet winters and
dry summers, Brooks et al. (2010) found increasingly depleted δ18O and δ2H values with increasing soil depth and argued
that the first and isotopically depleted autumn rains recharged the deep and
withered soil, whereas water in shallow soil contains water from later, more
enriched precipitation. We did not measure the isotopic composition of water
along a soil profile, but in this study the first rains after the main
dry season are also isotopically the most depleted. Access of plants to
groundwater will similarly result in isotopically depleted xylem water
because this water is mainly derived from precipitation on the forests of
Mt Kilimanjaro at 1800–2800 m a.s.l. (Payne, 1970; Hemp, 2006a). The
distribution of precipitation on Mt Kilimanjaro changes with elevation, with
mean annual precipitation (MAP) increasing with altitude along the southern
slope to reach a maximum of ∼ 2700 mm yr-1 at 2200 m a.s.l. and decreasing rapidly further uphill (Hemp, 2001). Maximum
groundwater recharge was found at an altitude of ∼ 2000 m, where precipitation is depleted isotopically compared to Lake Chala due to
the altitude effect (Dansgaard, 1964).
Whether plants in the Lake Chala area are deciduous or evergreen produced
no systematic trends in δ2HLMWL-int and evaporation
distance (Figs. S1–S2) at any of the three principal plant habitats. We
expected that evergreen plants would be adapted to tap water from deep
sources (i.e., isotopically depleted water), allowing them to survive the
long dry season, as observed, e.g., by Jackson et al. (1995) in a tropical
moist lowland forest in Panama. However, in line with our results in a
tropical dry lowland, Stratton et al. (2000) also failed to find a clear
difference in δ2Hxylem between deciduous and evergreen
plants.
At the lakeshore and on the crater rim, the evaporation distance of trees
was higher than that of shrubs (Fig. S2), indicating that trees use more
topsoil water enriched in heavy isotopes by evaporation. In contrast, in the
savannah no such difference between the two growth forms was found. It is
generally accepted that the deeper root systems of trees compared to shrubs
allow them to access deeper soil water or groundwater (Dawson, 1996).
However, Meinzer et al. (1999) found that smaller trees use deeper
sources of water than larger trees, and attributed this to three possible
factors. Firstly, large trees require large amounts of nutrients to maintain
their extensive crown leaf area, and the nutrient content of topsoil water
is much greater than that of water taken from greater depth. Secondly, and
likely related to the first factor, large trees have a relatively more
extensive horizontal root system, in order to compensate for the reduced
water content of the topsoil. Finally, the larger stem water storage
capacity of large trees reduces peak daytime demands for soil water uptake
and delays the onset of diurnal leaf water deficits. Goldsmith et al. (2011)
agreed that soil nutrient availability can be a strong growth-limiting
factor and therefore a driver of root distributions but observed that plant
species occurring in either the understory or canopy of mature and secondary
forests used a similarly shallow water source. In our study, the evaporation
distances of plants (both trees and shrubs) at the lakeshore were
strikingly higher than that of plants growing in the savannah and on the
crater rim (Fig. 5). This was because lake-shore plants use a substantial
fraction of lake water, which has relatively enriched δ2Hlake values. The higher δ2Hxylem and evaporation distance of lake-shore trees compared to
lake-shore shrubs indicate that the former rely more heavily on lake water,
while the latter tap more soil water (i.e., a smaller fraction of lake
water).
Plants in all three local habitats showed similar, very negative δ2HLMWL-int values, indicating that they relied mostly on depleted
NE monsoon rains falling between October and December (Fig. 2). In all three habitats the δ2HLMWL-int of plants sampled in December
approached the δ2Hprec of November rain (-48 ‰; Fig. 4). Probably, this is because these rains
represent the onset of the short rainy season following a distinct 4-month
long dry season. They are thus expected to recharge soils to a relatively
large degree. This is again in accordance with the “two water worlds” (TWW)
hypothesis of Brooks et al. (2010). This hypothesis challenges the
assumption of water being completely mixed in soils and states that the
mobile water compartment eventually enters the streams through translatory
flow, while the static water compartment consists of initial precipitation
that is trapped in soil micropores and remains trapped until the water is
used through transpiration by plants in the following dry months (Brooks et
al., 2010). Plants at the lakeshore predictably showed only a weak
(statistically insignificant) seasonal trend in δ2HLMWL-int (Fig. 4). From observations it was obvious that the
shore was the wettest of all three local habitats, and plants stayed
verdantly green in the dry season as there was plenty of water available.
Plants on the crater rim and in the savannah showed lowest δ2HLMWL-int values during December, which then increased during
the following months to reach their highest recorded values during the dry
season in July. This indicates that the plants' water pool was replenished
stepwise by the isotopically more enriched precipitation that followed from
December onwards. At the top of the crater rim there was greater variability
in monthly mean δ2HLMWL-int, and the compound seasonal
trend was more pronounced than in the savannah. Probably, this is because
the rim is the driest location, with shallow soils on bedrock, while the
savannah site at the foot of the crater has deeper soils. Shallow soils have
a smaller pool of micropores that can trap water and thus the static water
compartment will be more quickly exhausted and replenished with new,
isotopically different water. Altogether, the isotopic composition of xylem
water seems to confirm the two water worlds hypothesis, in which a
soil-bound water pool is used by the plants, while another, highly mobile
pool of precipitation water contributes to streams and groundwater (Brooks
et al., 2010; Goldsmith et al., 2011; Evaristo et al., 2015).
From all plants on the crater rim, Euphorbia tirucalli had the
highest evaporation distance, indicating that this evergreen shrub is
adapted to use the most shallow water sources in this habitat. This allows
it to exploit light precipitation events (“showers”) more effectively
(Caldwell et al., 1998) and to coexist with other species that draw water
from deeper sources. This physiology may be due to the unique combination of
crassulacean acid metabolism (CAM) in the succulent stem of this species and C3 metabolism
in its non-succulent leaves (Van Damme, 1999; Hastilestari et al., 2013).
E. tirucalli has a high drought tolerance as its leaves wither and
die (i.e., become deciduous) under extremely dry conditions, while the stem
continues its CAM photosynthesis (Hastilestari et al., 2013). However, the
succulent stem made it difficult to separate phloem and xylem, so that the
collected water may be a combination of both and its isotopic signature
enriched because of transpiration.
Parameters affecting xylem-to-leaf deuterium enrichment
Averaging all sampled species over all seasons, the δ2H enrichment factor εl/x from xylem to leaf water in
plants of the Lake Chala area is 24 ± 28 ‰.
Simulating global patterns of leaf water δ2H
enrichment, Kahmen et al. (2013) predicted δ2H enrichment to be strong in arid biomes (40–100 ‰),
intermediate in temperate biomes (10–30 ‰) and weaker
in humid tropical biomes (0–20 ‰). Our values for the
semiarid tropical area of Lake Chala lie in the temperate model. The broad
range of δ2Hleaf values measured in this study
(from -83 to 37 ‰) is not
surprising as several environmental variables, including ambient
temperature, relative humidity, wind speed and the δ2H of atmospheric water vapor influence the leaf water deuterium enrichment
(Sachse et al., 2012, and references therein). The drier the atmosphere, the
windier the conditions and the warmer the air, the larger the rates of
transpiration and thus the rates of water uptake (Craig and Gordon, 1965).
In our study region, the monthly average temperature varied only slightly
(Fig. 2a) and will have a minor effect on the variability of δ2Hleaf and εl/x between months. To eliminate
additional variability induced by previously reported large diurnal δ variations in δ2Hleaf (Cernusak et al., 2002;
Li et al., 2006; Kahmen et al., 2008), our samples were taken as much as
possible around the same time of day.
In our study area, leaf phenology appears to exert an important influence on
δ2Hleaf, and this is also reflected in
εl/x (Fig. 6). The species-specific variation in εl/x is likely
explained by differences in plant physiology and biochemistry. The highest
δ2Hleaf and εl/x values were displayed by
deciduous species, at least on the crater rim and in the savannah. Evergreen
plants keeping their foliage during the dry season must be protected against
drought stress by a high degree of succulence or sclerophylly (thickened or
hardened leaves) to reduce moisture loss (Chabot and Hicks, 1982). Burghardt
and Riederer (2003) observed that the peak cuticular transpiration rates of
evergreens are approximately 1 order of magnitude lower than those of
deciduous species. Thus, the adaptive traits of evergreens which reduce
water loss and lower transpiration rates result in lower xylem-to-leaf
deuterium enrichment. Differences in εl/x between habitats are not
surprising, as the plants at the lakeshore are protected by the crater rim
and less transpiration is expected compared to plants in the savannah and on
the crater rim.
Extremely low εl/x values of, respectively, -24 ± 30 and 8 ± 15 ‰ were recorded
for Maerua sp. and Thylachium africanum, two evergreen
Capparaceae growing on the crater rim (Fig. 6). This is indicative of very
limited evapotranspiration. Slightly depleted δ2Hleaf values relative to δ2Hxylem (i.e.,
small, negative εl/x values) have previously been reported for some
trees in Rhizophora mangroves (Ladd and Sachs, 2015). Ladd and Sachs (2015) ascribed this to the high ambient relative humidity, resulting in a small
vapor pressure gradient across the leaf surface (see also Helliker and
Ehleringer, 2000; Farquhar et al., 2007). The atypical εl/x values of
the two Capparaceae in this study are possibly associated with their
xerophytic traits, in particular the waxy appearance of
the leaves in many species of this family (Elffers et al., 1964). These waxy
and leathery leaves are useful adaptations to survive a long dry season, as
plants lose water not only via their stomata but also across the cuticle
(Schönherr, 1982). Oliveira et al. (2003) observed that the waxes of
Capparis yco, a species belonging to the Capparaceae, are very
efficient against water loss due to the predominance of n-alkanes
in their composition. Thus, these waxes on the leaf surfaces of the two
Capparaceae species could reduce the plants' transpiration and thus possibly
explain their small εl/x values. Furthermore, taking into account
highly diverse leaf morphology large variations in δ2Hleaf and εl/x between plant species were expected (Smith and
Freeman, 2006; Kahmen et al., 2008).
The εl/x values of Lake Chala area plants did not show systematic
variation according to growth form (that is, trees vs. shrubs). Judging
from Fig. 6, the plants' habitat did significantly affect εl/x.
However, Grewia tephrodermis, Vepris uguenensis and
Thylachium africanum show similar εl/x values irrespective of
the habitat in which they were sampled (Fig. 6). This suggests that the
overall difference in εl/x values according to habitat is due to
differences in the plant assemblage occurring in each habitat rather than
habitat-specific factors. The temporal variability in εl/x was limited
with only two of the sampled species (Sideroxylon sp. and
Lepisanthes senegalensis) showing significant differences across
seasons. Both species displayed lowest εl/x during the long rainy
season (SE monsoon) and highest εl/x during both the short rainy season
(NE monsoon) and the dry season. Several studies observed that stomatal
conductance in savannah plants declines during the dry season due to
increased vapor pressure deficits and declining soil water availability
(Duff et al., 1997; Prior et al., 1997). Surprisingly, O'Grady et al. (1999)
detected higher transpiration rates in open-canopy eucalyptus forests in
Australia during the dry season than during the wet season, mainly because
of higher evaporative demand. Meinzer et al. (1993), on the other hand,
found similar mean transpiration rates in a lowland tropical forest tree
during the wet and dry seasons despite variation in the leaf-to-air vapor
pressure difference. Our data on the majority of species sampled around Lake
Chala are consistent with the observations of Meinzer et al. (1993) in that
they do not show a significant difference in εl/x among seasons. In
summary, our results point to the fact that on the local scale of a single
study area with several distinct plant habitats, the plant species
assemblage and associated prevailing leaf phenology are the primary factors
influencing xylem-to-leaf water δ2H enrichment,
while growth form and seasonality have negligible effects.
Along a major hydroclimate gradient influencing the composition of plant
assemblages on the (sub-) continental scale, the δ2H of plant
leaf wax n-alkanes (δ2Hwax) varies
with the mean δ2H value of local precipitation (Sachse et al.,
2004; Huang et al., 2004; Hou et al., 2008; Polissar and Freeman, 2010;
Garcin et al., 2012; Tipple and Pagani, 2013); it is this relationship which
underpins the use of leaf wax δ2H signatures in hydroclimate
reconstruction. Precipitation forms the plant's water source and is supposed
to be the primary control of δ2Hwax (Sachse et
al., 2012). However, the relative importance of the potential water sources
(precipitation, xylem water, leaf water) for lipid synthesis in plant leaves
is unknown. Several authors (Sachse et al., 2004; Feakins and Sessions,
2010; Polissar and Freeman, 2010; Kahmen et al., 2013) stated that the leaf
water deuterium enrichment also shapes δ2Hwax.
Another constraint for a robust interpretation is the limited understanding
of the temporal integration of environmental conditions in δ2Hwax. Finally, the net or apparent fractionation between precipitation and leaf wax n-alkanes, which integrate these
uncertainties, is used for paleoclimate reconstructions. Despite
its enormous potential, hydroclimate interpretations remain problematic
because of uncertainties in the effects of past variation in water source δ2H, xylem-to-leaf δ2H enrichment and the biosynthetic isotopic depletion which occurs during n-alkane synthesis (Sessions et al., 1999; Liu and Yang, 2008; Smith and
Freeman, 2006; Sachse et al., 2012). This study investigated the first two
of these sources of uncertainty. The third source of uncertainty requires
investigations into whether the effects of growth form, phenology, habitat
and seasonality that are (not) reflected in εl/x are preserved in the
leaf wax n-alkanes.
Conclusions
In this study, we measured δ18O and δ2H
of precipitation, lake water, groundwater, and plant xylem and leaf water
across different plant species, seasons and habitats with varying distances
to Lake Chala in equatorial East Africa. We found that the trajectory of
the air masses delivering rain to the area considerably influences the
seasonal signature of water isotopes in precipitation but that not all of
its variability can be explained in this way. Lake-surface water showed
stable δ18Olake and δ2Hlake with,
counterintuitively, seasonally lowest isotopic values during the dry season.
No statistical differences were observed between the source water of
evergreen and deciduous plants in the three principal habitats around Lake
Chala, as inferred from the intersection point (δ2HLMWL-int) of the plants' LELs with the LMWL. We found that the
large seasonal variability in δ2H of precipitation
was not reflected in the isotopic composition of xylem water. In all three
habitats, the plants' principal source water was NE monsoon precipitation
falling during the short rainy season (in this year, mostly
November-December), likely because these first rains following the long dry
season recharged the dry soil. The plants' available water pool was
replenished only stepwise by more enriched precipitation from the SE monsoon
falling during the long rainy season (in this year, February–May).
Consequently, only a minor temporal shift in the isotopic composition of
xylem water was observed. These results are in agreement with the two water
worlds hypothesis, where plants rely on a static water pool, while a mobile
water pool recharges groundwater and is exported to streams as runoff. The ED indicates that spatial variability in water
resource use exists in the study region. ED values of trees were higher than
shrubs in both the lakeshore and crater-rim habitat. At the crater rim,
this indicates that trees use more topsoil water, presumably because the
trees' root distribution is driven by their high nutrient needs to sustain a
large canopy. At the lakeshore, this indicates that trees take up a larger
fraction of lake water compared to shrubs. In contrast to water source, leaf
phenology (deciduous vs. evergreen) plays a key role in determining the
xylem-to-leaf water deuterium enrichment in this semiarid tropical
environment. Deciduous species gave highest εl/x values, probably
because evergreens are better protected against loss of moisture.
Our observations have important implications for the interpretation of
δ2H of plant leaf wax n-alkanes from paleohydrological
records in tropical East Africa, as hydroclimate interpretations remain
problematic because of uncertainties in the effects of past variation in water source
δ2H and leaf water deuterium enrichment. Future
studies should establish whether the interspecies variability in xylem–leaf
enrichment (24 ± 28 ‰) has the potential to bias
paleoclimate reconstructions, given the floristic diversity and likelihood
of changes in species assemblage with climate shifts.