Introduction
Oil palm (Elaeis guineensis Jacq.) has become the most rapidly expanding crop in tropical
countries over the past few decades, particularly in South East Asia (FAO,
2014). Asides from losses of biodiversity and associated ecosystem
functioning (e.g., Barnes et al., 2014), potentially negative consequences of
the expansion of oil palm cultivation on components of the hydrological
cycle have been reported (e.g., Banabas et al., 2008). Only a few studies have
dealt with the water use characteristics of oil palms so far (Comte et al.,
2012). Available evapotranspiration estimates derived from
micrometeorological or catchment-based approaches range from 1.3 to 6.5 mm day-1 for different tropical locations and climatic conditions
(e.g.,
Radersma and Ridder, 1996; Henson and Harun, 2005). However, various
components of the water cycle under oil palm remain to be studied for a
convincing hydrological assessment of the hydrological consequences of oil
palm expansion, e.g., regarding the partitioning of the central water flux of
evapotranspiration into transpirational and evaporative fluxes.
Landscapes dominated by oil palms are not necessarily homogeneous in their
water use characteristics. Oil palms are usually planted in mono-specific
and even-aged stands; commonly, stands are cleared and replanted at an age
of approx. 25 years due to difficulties in harvesting operations,
potentially declining yields and the opportunity to plant higher yielding
varieties of oil palm. This creates a mosaic of stands of varying ages, and
hence with possibly different hydrological characteristics.
Substantial differences in transpiration rates of dicot tree stands have
been shown for stands of varying age in several studies (e.g., Jayasuriya et
al., 1993; Roberts et al., 2001; Vertessy et al., 2001; Delzon and Loustau,
2005); commonly, water use increases rapidly after stand establishment,
reaching a peak after some decades (which is associated with high stand
productivity and high stand densities) before declining more or less
consistently with increasing age. This has, e.g., been demonstrated for
Eucalyptus regnans F. Muell. (Cornich and Vertessy, 2001),
Eucalyptus sieberi L. Johnson (Roberts et al., 2001) and
Pinus pinaster Aiton (Delzon and Loustau, 2005) for stands between 10- and 160-years old.
Declines in transpiration rates in older stands were mainly explained by
decreasing leaf and sapwood area with increasing stand age (Roberts et al.,
2001; Vertessy et al., 2001; Delzon and Loustau, 2005). This may not be the
case in palms, as at least at the individual level, for two Amazonian palm
species (Iriartea deltoidea Ruiz and Pav. and Mauritia flexuosa
L.) linear increases of water use with increasing
height, and hence age, have been demonstrated (Renninger et al., 2009, 2010).
Water use patterns over a gradient of plantation age to our knowledge have
not yet been studied for oil palms. Water use could increase or decline with
increasing stand age or could remain relatively stable from a certain age.
Reasons for declining water use at a certain age include decreasing
functionality of trunk xylem tissue with increasing age due to the absence
of secondary growth in monocot species (Zimmermann, 1973), a variety of
other hydraulic limitations (see review of dicot tree studies in Ryan et
al., 2006) and increased hydraulic resistance due to increased pathway
length with increasing trunk height (Yoder et al., 1994). However, for
Mexican fan palms (Washingtonia robusta Linden ex André H Wendl.), no evidence of increasing
hydraulic limitations with increasing palm height was found (Renninger et
al., 2009). Reasons for potentially increasing water use in older
plantations, e.g., include linearly increasing oil palm trunk height with
increasing palm age (Henson and Dolmat, 2003). As trunk height and thus
volume increase, internal water storages probably also increase, possibly
enabling larger (i.e., older) oil palms to transpire at higher rates
(Goldstein et al., 1998; Madurapperuma et al., 2009). Additionally,
increased stand canopy height is expected to result in an enhanced turbulent
energy exchange with the atmosphere, i.e., a closer coupling of transpiration
to environmental drivers, which can facilitate higher transpiration rates
under optimal environmental conditions (Hollinger et al., 1994; Vanclay,
2009). The mentioned reasons for possibly increasing and decreasing water
use with increasing plantations age, respectively, could also partly
outbalance each other, or could be outbalanced by external factors (e.g.,
management related), potentially leading to relatively constant oil palm
transpiration with increasing plantation age.
To investigate the water use characteristics of oil palm stands of varying
age, we derived leaf-, palm- and stand-scale transpiration estimates from
sap flux density measurements with thermal dissipation probes (TDP; Granier,
1985) in 15 different stands (2–25-years old) in the lowlands of Jambi,
Sumatra, Indonesia. We used the oil palm specific calibration equation and
field measurement scheme recently proposed by Niu et al. (2015).
Additionally, in two of these stands (2- and 12-years old) we used the eddy
covariance technique (Baldocchi, 2003) to derive independent estimates of
evapotranspiration rates. For comparative purposes, the measurements were
conducted under similar environmental conditions and partly simultaneously.
Our objectives were (1) to derive transpiration rates of oil palms in stands
of varying age, (2) to estimate the contribution of palm transpiration to
total evapotranspiration, and (3) to analyze the influence of
micro-meteorological drivers on oil palm water use. The study provides some
first insights into the eco-hydrological characteristics of oil palms at
varying spatial (i.e., from leaf to stand) and temporal (i.e., from hourly to
daily) scales as well as first estimates of oil palm stand transpiration
rates and their contribution to total evapotranspiration. It assesses some
of the potential hydrological consequences of oil palm expansion on main
components of the water cycle at the stand level.
Methods
Study sites
The field study was conducted in Jambi, Sumatra, Indonesia (Fig. 1). Between
1991 and 2011, average annual temperature in the region was 26.7 ± 0.2 ∘C
(1991–2011 mean ± SD), with little intra-annual
variation. Annual precipitation was 2235 ± 385 mm, a dry season with
less than 120 mm monthly precipitation usually occurred between June and
September. However, the magnitude of dry season rainfall patterns varied
highly between years (data from Airport Sultan Thaha in Jambi). Soil types
in the research region are mainly sandy and clay Acrisols (Allen et al.,
2015). We had research plots in a total of 15 different oil palm stands
(Table 1), 13 of which were small holder plantations and two of which were
properties of big companies. The stands were spread over two landscapes in
the Jambi province (i.e., the Harapan and Bukit Duabelas regions, Fig. 1),
were all at similar altitude (60 m ± 15 m a.s.l.) and belonged to the
larger experimental set-up of the CRC990 (www.uni-goettingen.de/crc990,
Drescher et al., 2015). Stand age
ranged from 2 to 25 years. Management intensity and frequency (i.e.,
fertilizer and herbicide application, manual and chemical weeding of ground
vegetation and clearing of trunk epiphytes) varied considerably among the
examined oil palm stands, but both were generally higher in larger
plantations, particularly in PTPN6.
Locations of the studied oil palm stands in Jambi
province, Sumatra, Indonesia.
Stand locations, characteristics and study periods.
Plot code
Location/Village name
Age (yr)
Study region
Latitude (S)
Longitude (E)
Altitude (m)
Stand type
Study period
Average radiation/VPD
Average radiation/VPD
Additional comments
(Jambi province, Indonesia)
(H = Harapan,
(S = small holding,
of three selected days
of the full measurement
B = Bukit Duabelas)
C = company)
(MJ m-2 day-1/kPa)
period (MJ m-2 day-1 kPa-1)
PA
Pompa Air
2
H
01∘50′7.62′′
103∘17′44.22′′
75
S
15 Oct 2013 to 14 Jan 2014
21.6/1.4
16.6/1.1
Parallel eddy covariance measurements;
same days used for analysis
HAR_yg
Bungku
3
H
01∘55′38.5′′
103∘15′40.4′′
63
S
28 Sep 2013 to 24 Oct 2013
21.8/1.6
17.6/1.2
BD_yg
Pematang Kabau
5
B
01∘58′50.0′′
102∘36′18.4′′
55
S
9 Jul 2013 to 3 Aug 2013
17.4/1.5
12.3/1.2
BO5
Lubuk Kepayang
9
B
02∘06′48.9′′
102∘47′44.5′′
65
S
1 to 22 Sep 2013
20.4/1.6
15.4/1.1
HO4
Pompa Air
10
B
01∘47′12.7′′
103∘16′14.0′′
48
S
18 Jul 2013 to 5 Aug 2013
19.9/1.4
16.0/1.0
BO4
Dusun Baru
11
B
02∘03′01.5′′
102∘45′12.1′′
34
S
6 to 26 Aug 2013
22.9/1.8
17.6/1.4
BO3
Lubuk Kepayang
12
B
02∘04′15.2′′
102∘47′30.6′′
71
S
3 Jul 2013 to 30 Sep 2013
21.8/1.8
16.1/1.2
PTPN6
PT. Perkebunan Nusantara 6
12
H
01∘41′34.8′′
103∘23′27.6′′
70
C
19 Jul 2014 to 20 Dec 2014
19.7/1.4
16.7/1.1
Parallel eddy covariance measurements;
same days used for analysis
BO2
Lubuk Kepayang
13
B
02∘04′32.0′′
102∘47′30.7′′
84
S
10 Jun 2013 to 4 Jul 2013
24.9/2.1
20.5/1.7
HO2
Bungku
14
H
01∘53′00.7′′
103∘16′03.6′′
55
S
25 Sep 2013 to 19 Nov 2013
21.3/1.5
17.0/1.2
HO1
Bungku
16
H
01∘54′35.6′′
103∘15′58.3′′
81
S
9 to 30 Aug 2013
22.31.9
18.5/1.5
HO3
Pompa Air
17
H
01∘51′28.4′′
103∘18′27.4′′
64
S
7 Dec 2013 to 19 Jan 2014
16.7/1.0
13.0/0.8
PTHI
PT.Humusindo
18
H
01∘57′43.2′′
103∘15′50.3′′
59
C
15 Nov 2013 to 4 Dec 2013
17.5/1.1
17.4/1.1
BD_old
Pematang Kabau
22
B
01∘57′22.4′′
102∘33′39.9′′
73
S
14 to 30 Jul 2013
15.1/1.4
11.8/1.2
HAR_old
Bungku
25
H
01∘56′41.5′′
103∘16′41.9′′
43
S
30 Sep 2013 to 1 Nov 2013
21.1/1.6
17.1/1.2
Sap flux measurements and transpiration
Following a methodological approach for sap flux measurements on oil palms
(Niu et al., 2015), we installed thermal dissipation probe (TDP, Granier,
1985; Uniwerkstätten Universität Kassel, Germany; see Niu et al.,
2015 for technical specifications) sensors in the leaf petioles of 16
leaves, four each on four different palms, for each of the 15 examined
stands. Insulative materials and aluminum foil shielded the sensors to
minimize temperature gradients and reflect radiation. Durable plastic foil
was added for protection from rain. The sensors were connected to AM16/32
multiplexers connected to a CR1000 data logger (both Campbell Scientific
Inc., Logan, USA). The signals from the sensors were recorded every 30 s
and averaged and stored every 10 min. The mV-data from the logger was
converted to sap flux density (g cm-2 h-1) with the
empirically derived calibration equation by Granier (1985), but with a set
of equation parameters a and b that was specifically derived for TDP
measurements on oil palm leaf petioles (Niu et al., 2015).
Individual leaf water use rates were calculated by multiplying respective
sap flux densities (e.g., hourly averages, day sums) by the water conductive
areas of the leaves; the water use values of all individual leaves
measured simultaneously (min. 13 leaves) were averaged (kg day-1). To
scale up to average palm water use (kg day-1), average leaf water use
rates were multiplied by the average number of leaves per palm. Multiplying
the average palm water use by the number of palms per unit of land
(m2) yielded stand transpiration rates (T; mm day-1).
The sap flux measurements were conducted between April 2013 and December
2014, for a minimum of 3 weeks per study plot (Table 1). Three of the plots
(BO3, PA, and PTPN6) ran over several months, partly in parallel to other
plots. Most measurements, however, were conducted successively and thus
partly took place under varying weather conditions. Thus, to minimize
day-to-day variability introduced by varying weather for the analysis of
effects of stand age on water use at different spatial scales, we used the
average of three comparably sunny and dry days from the measurement period
of each stand. Exploratory analyses had shown that unexplained variability
was lower on sunny days than, e.g., on cloudy or intermediate days or when
using the averages of the full respective measurement periods. We chose days
with a daily integrated radiation of more than 17 MJ m-2 day-1 and
an average daytime VPD of more than 1.1 kPa; respective averages (mean ± SD)
of all days included in the analysis were 20.3 ± 2.6 MJ m-2 day-1
and 1.6 ± 0.3 kPa (also see Table 1).
Stand structural characteristics
For all sample leaves, the leaf petiole baseline length was measured between
upper and lower probe of each TDP sensor installed in the field; this
allowed calculating the water conductive area of each leaf (Niu et al.,
2015). For each sample palm, trunk height to the youngest leaf (m) and
diameter at breast height (cm) were measured (see Kotowska et al., 2015 for
detailed methodology) and the number of leaves per palm was counted. Over
time, new leaves emerged and old ones were pruned by the farmers; we assumed
the number of leaves per palm to be constant over our measurement period. On
the stand level, we counted the number of palms per hectare.
Eddy covariance measurements and evapotranspiration
The eddy covariance technique (Baldocchi, 2003) was used to measure
evapotranspiration (ET, mm day-1) in two of the 15 oil palm stands,
the 2-year old (PA) and the 12-year old (PTPN6) stand (Table 1). Towers of 7
and 22 m in height, respectively, were equipped with a sonic anemometer
(Metek uSonic-3 Scientific, Elmshorn, Germany) to measure the three
components of the wind vector, and an open path carbon dioxide and water
analyzer (Li-7500A, Licor Inc., Lincoln, USA) to derive evapotranspiration
rates (Meijide et al., 2015). Fluxes were calculated with the
software EddyPro (Licor Inc), planar-fit coordinate rotated, corrected for
air density fluctuation and quality controlled. Thirty-minute flux data were
flagged for quality applying the steady state and integral turbulence
characteristic tests (Mauder and Foken, 2006). Data were also filtered
according to friction velocity to avoid the possible underestimation of
fluxes in stable atmospheric conditions. Due to the amount of data gaps
created by lack of power and instrument failure, in the 2-year old
plantation we calculated the energy balance closure for the selected three
sunny days included in the analysis (see Table 1), for which it was 82 %.
In the 12-year old stand, the energy balance closure for the respective full
measurement period (May 2014 to February 2015) was 84 %. We selected days
when most of the 30-min measurements during the day where available.
When a single 30-min value was missing, the value was filled by
linear interpolation between the previous and the next 30 min value.
Measurements were conducted between July 2013 and February 2014 in the
2-year old and from May 2014 to February 2015 in the 12-year old stand. For
the analysis, we used the average of the same three sunny days that were
selected for the sap flux analysis in the respective plots (see Table 1).
Daytime (6 a.m. to 7 p.m.) evapotranspiration rates were used for the analyses and
comparison to transpiration rates in order to avoid possible measurement
errors as a consequence of low turbulent conditions during nighttime hours.
To estimate the contribution of stand transpiration to total
evapotranspiration, we confronted sap flux derived transpiration rates with
eddy covariance derived evapotranspiration rates. As described in Niu et
al. (2015), our methodological approach for estimating transpiration is
associated with sample size-related measurement errors of about 14 %. The
eddy covariance measurements were carried out in carefully chosen and
well-suited locations and focused on daytime observations only, when
estimation uncertainties are commonly low (< 30 %, Richardson et
al., 2006). The observed differences between evapotranspiration and
transpiration estimates presented in this study are thus likely largely due
to natural rather than methodological reasons.
Environmental drivers of oil palm water use
A total of three micrometeorological stations were set up in proximity to
the oil palm stands in both landscapes; for the analysis of the water use
characteristics of the respective stands, we used the micrometeorological
data from the closest available station, at a maximum distance of approx. 15
km and at similar altitude (60 m ± 15 m a.s.l.). The stations were
placed in open terrains. Air temperature and relative humidity were measured
at a height of 2 m with a Thermohygrometer (type 1.1025.55.000, Thies Clima,
Göttingen, Germany) to calculate vapor pressure deficit (VPD, kPa). A
short wave radiation sensor (CMP3 Pyranometer, spectral range 300–2800 nm,
Kipp & Zonen, Delf, the Netherlands) was installed at a height of 3 m,
the latter to measure global radiation (Rg, MJ m-2 day-1, from
here on referred to as “radiation”). Measurements were taken every 15 s
and averaged and stored on a DL16 Pro data logger (Thies Clima) every 10 min.
The eddy covariance towers (see eddy covariance measurements and
evapotranspiration) were also equipped with micrometeorological sensors.
Measurements were taken above the canopy, at respective heights of 6.7 and
22 m. Air temperature and humidity (Thermohygrometer, type 1.1025.55.000,
Thies Clima), short wave radiation (BF5, Delta-T, Cambridge, United Kingdom)
and net radiation (CNR4 Net radiometer, Kipp & Zonen) were measured every
15 s and averaged and stored on a DL16 Pro data logger (Thies Clima) every
10 min.
Soil moisture was recorded in the center of eight of the 15 study plots and
at the micrometeorological stations and eddy covariance towers. Soil
moisture sensors (Trime-Pico 32, IMKO, Ettlingen, Germany) were placed 0.3 m
under the soil surface and connected to a data logger (LogTrans16-GPRS, UIT,
Dresden, Germany). Data were recorded every hour, for 16 months from June
2013 on. Exploratory analyses showed no significant effects of soil moisture
on water use rates (linear regression, P > 0.1). Soil moisture
fluctuated only little at the respective locations and during the respective
measurement periods and even on a yearly scale, e.g., between 32 ± 2
and 38 ± 2 % between June 2013 and June 2014 (minimum and
maximum daily values, mean ± SE between the three micrometeorological
stations). Soil moisture did, e.g., also not fall below 36 % during the
measurement period in the long-term monitoring (BO3) stand. It was
non-limiting for plant water use. As it showed no significant relationship
with water use rates, we omitted soil moisture from further analyses of
influences of fluctuations in environmental variables on oil palm water use.
Likewise, further recorded micrometeorological variables (e.g., air pressure,
wind speed) had no significant relationship with water use rates in our
study (linear regression, P > 0.1) and were thus also omitted.
We instead focused on the micrometeorological drivers VPD and global
radiation; among an array of micrometeorological variables (e.g., also
including temperature, humidity, net radiation) exploratory analysis had
shown that they were best suited to explain fluctuations in water use rates.
This has also been demonstrated in other studies on plant water use (e.g.,
Dierick and Hölscher, 2009; Köhler et al., 2009, 2013)
For the diurnal analysis, we averaged the values of three comparably sunny
days and normalized VPD and radiation by setting the highest observed hourly
rates to one. All statistical analyses and graphing were performed with R
version 3.1.1 (R Core Development team, 2014) and Origin 8.5 (Origin Lab,
Northampton, MA, USA).
Results
Stand characteristics
The number of palms per unit of land linearly decreased with increasing
stand age (R2= 0.29, P= 0.04; Fig. 2a). The number of
leaves per palm remained constant and varied little (32–40 leaves per
palm) over stand age (Fig. 2b). The trunk height of oil palms (Fig. 2c)
increased linearly with increasing age (R2= 0.91, P < 0.01),
from about 2 m at an age of 6 to about 9 m at an age of
25 years. The average baseline length of leaf petioles at the location of
sensor installation increased linearly with stand age (R2= 0.65,
P < 0.01). As the number of leaves was constant in mature
stands, the increasing baseline lengths of leaf petioles resulted in a
significant linear increase of the water conductive area per palm with
increasing stand age (R2= 0.53, P < 0.01). In
consequence, the stand-level water conductive area also linearly increased
with stand age (R2= 0.26, P= 0.05; Fig. 2d).
The change of stand density (a), average number of leaves
per palm (b), average trunk height (c), and stand water conductive area (d) over age in the 15 studied oil palm stands.
Transpiration and evapotranspiration
Maximum sap flux densities on three sunny days as measured in the leaf
petioles of oil palms were variable but did not show a significant trend
over age among the examined stands (Fig. 3a). Converted to leaf water use, a
clear non-linear trend over stand age became apparent (R2adj = 0.61,
P < 0.01 for the Hill function, see Morgan et al., 1975,
fit shown in Appendix Fig. 1b, not shown in Fig. 3b): Leaf water use
increased 5-fold from a 2-year old stand to a plot age of about 10 years; it
then remained relatively constant with further increasing age. At the palm
level (Fig. 3c), water use rates closely resemble the relationship of leaf
water use and stand age. At the stand level, oil palm transpiration was very
low (0.2 mm day-1) in the 2-year old stand and increased almost 8-fold
until a stand age of 5 years. It then remained relatively constant with
increasing age at around 1.3 mm day-1 (Fig. 3d). However, three
medium-aged stands (PTPN6, BO5, and HO2) that showed increased sap flux
densities and leaf and palm water use rates also had higher stand
transpiration rates, between 2.0 and 2.5 mm day-1. Potentially, this
could be related to differences in radiation on the respective three sunny
days that were chosen for the analysis. However, there was no significant
relationship between average water use rates on the respective three sunny
days in the 15 stands and the respective average radiation (or VPD) on those
days (linear regression, P > 0.05), i.e., observed variability in
transpiration among the 15 stands could not be explained by differences in
weather conditions. A further analysis of the water use rates of eight
medium-aged stands with highly variable transpiration rates also gave no
indications of variability being induced by differences in radiation. As for
the leaf- and palm-level water use rates, a Hill function explained the
relationship between stand transpiration and stand age (R2adj = 0.45,
P < 0.01, Appendix Fig. 1d), but the observed scatter was
high, particularly among medium-aged plantations. Overall, stand
transpiration rates increased linearly with increasing stand water
conductive area (R2= 0.42, P= 0.01). On the palm level,
there was a linear relationship between water use and trunk height
(R2= 0.32, P= 0.03), but stand transpiration did not have
a linear relationship with average stand trunk height due to decreasing
stand densities with increasing stand age; instead, as for transpirations
vs. stand age, a Hill function explained the relationship between
transpiration and stand trunk height best (R2adj = 0.44, P < 0.01)
(also see summary in Table 2).
The change of maximum hourly sap flux density (a),
average leaf water use (b), average palm water use (c) and stand
transpiration (d) over stand age. Data of the different levels derived from
simultaneous sap flux measurements on at least 13 leaves per stand; values
of three sunny days averaged.
Diurnal course of vapor pressure deficit (VPD) and
radiation (Rg) (a) and of sap flux density in four oil palm stands (b).
Leaf water use plotted against hourly averages of normalized VPD (c) and Rg
(d). Average water use estimates based on at least 13 leaves measured
simultaneously; average water use rates, VPD and radiation of three sunny
days, each point represents one-hourly observation. Data are from the
locations PA (2 years old, black arrows), BO3 (12 years old, low water use,
red arrows), PTPN6 (12 years old, high water use, blue arrows) and
HAR_old (25 years old, green arrows). Data were normalized by
setting the maximum to one.
Summary table of results for all 15 oil palm stands.
R2 and P values for linear regression and fitting a Hill
function, respectively, are presented to explain variability in water use
characteristics (i.e., maximum sap flux density, leaf water use, palm water
use and stand transpiration) by the stand variables age, trunk height and
sapwood area.
Maximum sap
Leaf water use
Palm water use
Stand
flux density
transpiration
Linear fit
Hill function
Linear fit
Hill function
Linear fit
Hill function
Linear fit
Hill function
Age
n.s.
R2adj = 0.16**
R2= 0.31*
R2adj = 0.61**
n.s.
R2adj = 0.59**
n.s.
R2adj = 0.45**
Trunk height
n.s.
R2adj = 0.15**
R2= 0.37*
R2adj = 0.62**
R2= 0.32*
R2adj = 0.61**
n.s.
R2adj = 0.44**
Sapwood area
n.s.
R2adj = 0.02**
R2= 0.41**
R2adj = 0.60**
R2= 0.39**
R2 adj = 0.61**
R2= 0.42**
R2adj = 0.43**
* For P≤ 0.05, ** for the P≤ 0.01, n.s. for no significant
relationship (P > 0.05).
On comparably sunny days, the stand-level transpiration among the 15 oil
palm stands varied 12-fold, from 0.2 mm day-1 in a 2-year old
to 2.5 mm day-1 in a 12-year old stand. A large part of this spatial variability
was explained by different stand variables when applying the Hill function.
Stand age explained 45 % of the observed spatial variability of stand
transpiration (i.e., R2adj = 0.45 at P < 0.01,
Appendix Fig. 1), and variables correlated to stand age, i.e., by average
stand trunk height and by stand water conductive area, explained 44 and
43 %, respectively (Table 2). Much of the remaining variability in stand
transpiration rates could be explained by varying stand densities
(variations of up to 30 % between stands of similar age, see Table 1).
Thus, when shifting from the stand level to the palm level, up to 60 % of
the spatial variability in palm water use rates could be explained by age
and correlated variables (see Fig. 3c and Table 2). Much of the variability
that remains on the palm level is induced by three stands where palm water
use was much higher (> 150 kg day-1) than in the other 12
stands (< 125 kg day-1); excluding these three stands from the
analysis, 87 % of the spatial variability in palm water use rates could be
explained by age (Table 3).
Summary table of results for 12 oil palm stands, i.e.,
excluding three stands of yet unexplained much higher water use (PTPN6, BO5,
and HO2). R2 and P values for linear regression and fitting a
Hill function, respectively, are presented to explain variability in water
use characteristics (i.e., maximum sap flux density, leaf water use, palm
water use and stand transpiration) by the stand variables age, trunk height
and sapwood area.
Maximum sap
Leaf water use
Palm water use
Stand
flux density
transpiration
Linear model
Hill function
Linear model
Hill function
Linear model
Hill function
Linear model
Hill function
Age
n.s.
R2adj = 0.16**
R2= 0.67**
R2adj = 0.86**
R2= 0.63**
R2adj = 0.87**
n.s.
R2adj = 0.75**
Trunk height
n.s.
R2adj = 0.13**
R2= 0.60**
R2adj = 0.82**
R2= 0.56**
R2adj = 0.86**
n.s.
R2adj = 0.77**
Sapwood area
n.s.
R2adj = 0.01**
R2= 0.68**
R2adj = 0.80**
R2= 0.64**
R2adj = 0.85**
R2= 0.61**
R2adj = 0.69**
* For P≤ 0.05, ** for the P≤ 0.01, n.s. for no significant
relationship (P > 0.05).
Evapotranspiration rates derived from the eddy covariance technique for the
2-year old stand (PA) were 2.8 mm day-1 (average of three sunny days);
the contribution of sap flux derived transpiration was 8 %. For the
12-year old stand (PTPN6), the evapotranspiration estimate was 4.7 mm day-1;
transpiration amounted to about 53 %.
Drivers of oil palm water use
Radiation peaked between 12 and 1 p.m. while vapor pressure deficit peaked at
around 3 p.m.; the diurnal course of sap flux densities on three sunny days
except for the 2-year old stand (PA) showed an early peak of sap flux density
(10 to 11 a.m.), which then decreased throughout the rest of the day (Fig. 4a
and b, respectively).Thus, there was a varying and partly pronounced
hysteresis in the leaf-level response of transpiration to VPD (Fig. 4c). It
was small in the 2-year old stand (PA). In contrast, it was very pronounced
in the 12-year old PTPN6 stand (high water use, commercial plantation),
where a very sensitive increase of water use rates with increasing VPD
during the morning hours was observed, reaching a peak in water use rates at
only about 60 % of maximum daily VPD. After that, water use rates declined
relatively consistently throughout the day, despite further rises in VPD.
The same pattern was observed in most of the stands; we present values for
the oldest stand (HAR_old, 25 years) and another 12-year old
stand (BO3, low water use, smallholder plantation) as further examples. The
hysteresis in the transpiration response to radiation (Fig. 4d) was
generally less pronounced than for VPD.
The day-to-day behavior of oil palm leaf water use rates to environmental
drivers (i.e., VPD, radiation) seemed “buffered”, i.e., already relatively low
VPD and radiation lead to relatively high water use rates (except for in the
2-year old stand), while even strong increases in VPD and radiation only
induced rather small further increases in water use rates (Fig. 5). For the
2-year old stand (PA), leaf water use rates over time were almost constant
(about 0.4 kg day-1), regardless of daily environmental conditions.
Likewise, the water use rates of the remaining stands were relatively
insensitive to increases in VPD, i.e., two-fold increases in VPD only led to
1.1- to 1.2-fold increases in water use rates (Fig. 5). A similarly buffered
water use response to radiation was observed for the 12-year old
small-holder stand (BO3) and the 25-year old stand (HAR_old),
i.e., 1.5- and 1.3-fold increases, respectively, for two-fold increases in
radiation. The water use response to fluctuations in radiation of the
12-year old commercial stand (PTPN6) was more sensitive, i.e., two-fold
increases in radiation induced 1.8-fold increases in water use rates
(Fig. 5). The PTPN6 stand also had the highest absolute water use rates among the
studied stands.
The day-to-day response of leaf water use rates in four
different oil palm stands to changes in average daytime vapor pressure
deficit (VPD) (a) and integrated daily radiation (Rg) (b) taken from the
closest micrometeorological station from the respective plots. Data of at
least 20 days per plot, each point represents 1 day. Leaf water use rates
are from the locations PA (2 years old, black circles), BO3 (12 years old,
low water use, red circles), PTPN6 (12 years old, high water use, blue
circles) and HAR_old (25 years old, green circles).
Significant linear relationships are indicated with solid (P < 0.05)
and dotted (P < 0.1) lines, regression functions are provided in the
figure.
Discussion
Oil palm transpiration over age
Among 13 studied productive oil palm stands (i.e., > 4 years old)
stand transpiration rates varied more than two-fold. The observed range
(1.1–2.5 mm day-1) compares to transpiration rates derived with
similar techniques in a variety of tree-based tropical land-use systems,
e.g., an Acacia mangium plantation on Borneo (2.3 mm day-1 for stands of
relatively low density, Cienciala et al., 2000), cacao monocultures and
agroforests with varying shade tree cover on Sulawesi (0.5–2.2 mm day-1, Köhler et al., 2009, 2013) and reforestation and
agroforestry stands on the Philippines and in Panama (0.6–2.5 mm day-1, Dierick and Hölscher, 2009; Dierick et al., 2010). The
highest observed values for oil palm stands (2.0–2.5 mm day-1,
PTPN6, BO5, and HO2 stands) compare to or even exceed values reported for
tropical forests (1.3–2.6 mm day-1; Calder et al., 1986; Becker,
1996; McJannet et al., 2007), suggesting that oil palms can transpire at
substantial rates under certain, yet unexplained site or management
conditions despite, e.g., a much lower biomass per hectare than in natural
forests (Kotowska et al., 2015).
In the 15 studied oil palm stands, stand-level transpiration rates increased
almost 8-fold from an age of 2 years to a stand age of 5 years; they
then remained relatively constant with further increasing age but were
highly variable among productive stands. In our study region, oil palm
plantations are commonly cleared and replaced at an age of max. 25–30
years due to constrictions in fruit harvest with further increasing palm
height; the oldest studied stand was 25 years old. In contrast to previous
studies for dicot tree mono-cultural stands of varying age we thus did not
find, after a relatively early peak, lower stand transpiration rates with
increasing stand age (e.g., Jayasuriya et al., 1993; Roberts et al., 2001;
Vertessy et al., 2001; Delzon and Loustau, 2005). Asides from the
productivity-related artificial short oil palm lifespan in our studied
stands as opposed to much larger time-scales in studies on tree stands (e.g.,
comparison of 10- and 91-year old stands in Delzon and Loustau, 2005), this
is also related to differences in stand establishment: oil palms are
commonly planted in a fixed, relatively large grid, which results in a less
pronounced reduction of stand density with increasing stand age than in
dicot tree stands, which are often established at much higher stand
densities and consequently show higher density-dependent mortality rates.
After an initial steep rise of transpiration at a very young plantation age,
stand transpiration thus does not seem to vary considerably over the life
span of a certain oil palm plantation, which contrasts with the water use
characteristics of tree plantations.
The observed substantial stand-to-stand variability of transpiration among
the 15 stands, particularly among medium aged plantations, could to 60 %
be explained by the variables stand age and density, and up to 87 % when
excluding three stands with much higher water use. The remaining unexplained
variability as well as the high water use rates in the three mentioned
stands could be related to differences in site and soil characteristics.
However, all studied stands were located in comparable landscape positions
(i.e., upland sites of little or medium inclination) and on similar mineral
soils, i.e., loam or clay Acrisols of generally comparable characteristics
(Allen et al., 2015; Guillaume et al., 2015). Differences in management
intensity could also contribute to the remaining unexplained variability of
stand transpiration rates over age. For example, on P-deficient soils such as the
Acrisols of our study region (Allen et al., 2015), fertilization can greatly
increase oil palm yield (Breure, 1982) and thus total primary productivity,
which could consequently lead to a higher water use of oil palms.
Accordingly, the highest observed transpiration value in our study came from
a stand in an intensively and regularly fertilized, high-yielding commercial
plantation. Thus, there may be a trade-off between management intensity, and
hence yield, on the one hand, and water use of oil palms on the other hand.
This trade-off is of particular interest in the light of the continuing
expansion of oil palm plantations (FAO, 2014) and increasing reports of
water scarcity in oil palm dominated areas (Obidzinski et al., 2012; Larsen
et al., 2014)
Evapotranspiration and the contribution of transpiration
Our eddy-covariance derived evapotranspiration estimates of 2.8 and
4.7 mm day-1 (on sunny days, in 2- and 12-year old stands, respectively)
compare very well to the range reported for oil palms in other studies: For
3–4-year old stands in Malaysia, eddy-covariance derived values of
1.3 and 3.3–3.6 mm day-1 were reported for the dry and rainy
season, respectively (Henson and Harun, 2005). For mature stands, a value of
3.8 mm day-1 was given, derived by the same technique (Henson, 1999).
Micrometeorologically derived values for 4-5 year-old stands in Peninsular
India were 2.0–5.5 mm day-1 during the dry season (Kallarackal et
al., 2004). A catchment-based approach suggested values of
3.3–3.6 mm day-1 for stands in Malaysia between 2 and 9 years old (Yusop et al.,
2008); evapotranspiration rates derived from the Penman-Monteith equation
and published data for various stands were 1.3–2.5 mm day-1 in the
dry season and 3.3–6.5 mm day-1 in the rainy season (Radersma and
Ridder, 1996). The values reported in most available studies as well as our
values overlap in a corridor from about 3 to about 5 mm day-1; this range compares to evapotranspiration rates reported for
rainforests in South East Asia (e.g., Tani et al., 2003a; Kumagai et al.,
2005). Considering that oil palm stands, e.g., have much lower stand densities
and biomass per hectare than natural tropical forests (Kotowska et al.,
2015), this indicates a quite high evapotranspiration from oil palms at both
the individual and the stand level. Additionally to the previously discussed
relatively high water use of oil palms under certain site or management
conditions, the high evapotranspiration from oil palm can be explained by
substantial additional water fluxes to the atmosphere. These fluxes (i.e.,
the differences between evapotranspiration and transpiration estimates) were
substantial in both the 2-year old and the 12-year old oil palm stand, i.e.,
2.6 and 2.2 mm day-1, respectively. In the 2-year old PA stand the
contribution of palm transpiration to total evapotranspiration was very low
(8 %, Fig. 6). The majority of water fluxes to the atmosphere came from
evaporation (e.g., from the soil, interception) and transpiration by other
plants. The spaces between palms (planting distance approx. 8 × 8 m)
were covered by a dense, up to 50 cm high grass layer at the time of
study (approx. 60 % ground cover); transpiration rates from grasslands can
exceed those of forests (e.g., review in McNaughton and Jarvis, 1983;
Kelliher et al., 1992) and could well account for 1–2 mm day-1 to
(partly) explain the observed difference of 2.6 mm day-1 between
evapotranspiration and transpiration estimates in the PA stand. The 2-year
old oil palms were still very small (average trunk height 0.3 m, overall
height 1.8 m) and had a low average number of leaves, 24 as opposed to 37 ± 1
(mean ± SD) between the 15 studied stands; further, leaves
were much smaller than in mature stands. Leaf area index (LAI) of 2-year old
oil palm stands was reported be at least 5-fold lower (LAI < 1) than
in mature stands of similar planting density as our study plots (Henson and
Dolmat, 2003). The very low observed water use of the oil palms in PA (12 kg day-1
per palm compared to approx. 100 kg day-1 per palm in mature
stands) and the consequent very low contribution of palm transpiration to
evapotranspiration thus do not seem contradictory. At PTPN6 (12 years old)
transpiration rates as well as the contribution of transpiration to
evapotranspiration (53 %) were much higher than in the 2-year old stand
(PA); also, total evapotranspiration was almost 70 % higher. The sum of
evaporation (e.g., from the soil) and transpiration by other plants was of
similar magnitude (2.2 mm day-1, i.e., 15 % lower) as in PA. Due to
the intense management, there was very little ground vegetation in
inter-rows present in the PTPN6 stand. However, the abundant trunk epiphytes
in butts of pruned leaf petioles that remain on the trunks of mature oil
palms may contribute significantly to non-palm transpirational water fluxes.
Additionally, oil palm trunks were reported to have a large potential
external water storage capacity (up to 6 mm, Merten et al., 2015) for
stemflow water after precipitation events; the mentioned butts of pruned
leaf petioles constitute “chambers” filled with humus, water and epiphytes,
which can remain moist for several days following rainfall events. On dry,
sunny days of high evaporative demand, the (partial) drying out of these
micro-reservoirs may significantly contribute to water fluxes from
evaporation. This is supported by the diurnal course of all water fluxes
except oil palm transpiration at PTPN6 (calculated by subtracting hourly
transpiration from evapotranspiration rates), which closely followed VPD
until its 3 p.m. peak, but then declined rapidly. Generally, our comparison of
eddy-covariance derived evapotranspiration and sap-flux derived
transpiration suggests significant other water fluxes to the atmosphere than
transpiration (e.g., from evaporation) that are still marginal during the
morning hours, reach their peak at the time VPD peaks and are extremely
sensitive to decreasing VPD in the afternoon. In our study, transpiration
amounted to only 8 and 53 % of evapotranspiration in the 2-year old
and the 12-year old oil palm stand, respectively, which is lower than values
reported, e.g., for mature coconut stands (68 %, Roupsard et al., 2006) and
rainforests in Malaysia (81–86 %, Tani et al., 2003b). The low relative
contribution of palm transpiration to total evapotranspiration in oil palm
stands could be due to relatively high water fluxes from evaporation, e.g.,
after rainfall interception. Interception was reported to be substantially
higher in oil palm stands in the study region (28 %, Merten et al., 2015) than, e.g., in rainforests in Malaysia (12–16 %, Tani et al.,
2003b) and Borneo (18 %, Dykes, 1997). The high water losses from
interception paired with the relatively high water use of oil palms and the
consequent high total evapotranspirational fluxes from oil palm plantations
could contribute to reduced water availability at the landscape level in oil
palm dominated areas, e.g., during pronounced dry periods (Merten et al., 2015).
Normalized diurnal pattern of vapor pressure deficit
(VPD), radiation (Rg), transpiration (T) and evapotranspiration
(ET) in a 2-year old (PA) (a) and a 12-year old (PTPN6) (c) oil palm
stand; absolute hourly values of ET and T in PA (b) and PTPN6 (d). Eddy
covariance and sap flux density measurements were conducted in parallel to
derive evapotranspiration and transpiration rates, respectively. Values of
three sunny days averaged.
Micro-meteorological drivers of oil palm water use
At the diurnal scale, we examined the relationship between water use rates
and VPD and radiation (hourly averages of three sunny days, Fig. 4): in all
examined oil palm stands except the very young stand (PA, 2 years old), under
comparable sunny conditions, the intra-daily transpiration response to the
mentioned environmental drivers was characterized by an early peak
(10–11 a.m.), before radiation (12 a.m.to 1 p.m.) and VPD (2–3 p.m.)
peaked; after this early peak of water use rates, however, they subsequently
declined consistently throughout the day, regardless of further increases of
radiation and VPD (Fig. 4). For most thus far examined dicot tree species,
peaks in water use rates coincide with peaks in radiation (e.g., Zeppel et
al., 2004; Köhler et al., 2009; Dierick et al., 2010; Horna et al.,
2011); however, a similar behavior as in oil palms, i.e., early peaks of
transpiration followed by consistent declines, has been reported, but not yet
explained, for Acer rubrum L. (Johnson et al., 2011) and some
tropical bamboo species (Mei et al., 2015). Due to the early peaks,
considerable hysteresis in the oil palm transpiration response to VPD was
observed in all examined stands except for PA (2 years old). In studies on
tree species, pronounced hysteresis has been reported, e.g., for eucalyptus
trees in Australia during the dry season (O'Grady et al., 1999; Zeppel et
al., 2004) or for popular hybrids on clear, but not on cloudy days (Meinzer et al., 1997). The underlying
eco-hydrological mechanisms remain unexplained; potentially, the development
of water stress (Kelliher et al., 1992), decreasing leaf stomatal conductance
and assimilation rates over the course of a day (Easmus and Cole, 1997;
Williams et al., 1998; Zeppel et al., 2004) or changes in leaf water
potential, soil moisture content or xylem sap abscistic acid content (Prior
et al., 1997; Thomas et al., 2000; Thomas and Easmus, 2002) could play a
role. For oil palms, no eco-physiological studies are available yet to assess
these potential underlying reasons for the observed pronounced diurnal
transpirational hysteresis. A contribution of stem water storage to
transpiration in the morning could be another potential explanation (Waring
and Running, 1978; Waring et al., 1979; Goldstein et al., 1998). It could
explain the early peak followed by a steady decline of transpiration
regardless of VPD and radiation patterns, the decline being the consequence
of eventually depleted trunk water storage reservoirs. Other (palm) species
were reported to have substantial internal trunk water storage capacities
(e.g., Holbrook and Sinclair, 1992; Madurapperuma et al., 2009), which can
contribute to sustain relatively high transpiration rates despite limiting
environmental conditions (e.g., Vanclay, 2009).
At the day-to-day scale, in all 15 oil palm stands, the response of water
use rates particularly to changes in VPD seemed “buffered”, i.e.,
near-maximum daily water use rates were reached at relatively low VPD, but
better environmental conditions for transpiration (i.e., higher VPD) did not
induce strong increases in water use rates (i.e., 1.2-fold increase in water
use for a two-fold increase in VPD). Likewise, for both photosynthesis rates
(Dufrene and Saugier, 1993) and water use rates (Niu et al., 2015) of oil
palm leaves, linear increases with increasing VPD were reported at
relatively low VPD, until a certain threshold (1.5–1.8 kPa) was reached,
after which no further increases in photosynthesis and water use rates,
respectively, occurred. For tropical tree and bamboo species, more sensitive
responses to fluctuations in VPD, i.e., 1.4- to 1.7-fold increases and more
than two-fold increases, respectively, have been reported (e.g., Köhler
et al., 2009; Dierick et al., 2010; Komatsu et al., 2010). However, a
similar “leveling-off” effect of water use rates at higher VPD, as observed
for the oil palm stands in our study, has been reported for Moso bamboo
stands in Japan (in contrast to coniferous forests in the same region, where
water use had a linear relationship with VPD, Komatsu et al., 2010). The
hydraulic limitations “buffering” the day-to-day oil palm water use response
to VPD are yet to be explained. As soil moisture was non-limiting, they are
likely of micrometeorological or eco-physiological nature. The early peaks
of water use rates and the consequent strong hysteresis to VPD on the
intra-daily level, which may point to a depletion of internal trunk water
storage reservoirs early in the day as a possible reason for substantially
reduced oil palm water use rates at the time of diurnally optimal
environmental conditions, give some first indications of the direction that
further studies could take.