The relationship of the community composition of forest vegetation and soil
nutrients were studied near the Sokli phosphate ore deposit in northern Finland.
Simultaneously, the effects of the dominant species and the age of trees,
rock parent material and soil layer on these nutrients were examined. For
this purpose, 16 study plots were established at different distances from
the phosphate ore along four transects. Phosphate mining may take place in
Sokli in the future, and the vegetation surveys and soil sampling conducted
at the plots can be used as a baseline status for following the possible
changes that the mining may cause in the surrounding ecosystem. The total
phosphorus (P) and nitrogen (N) contents of the soil humus layer were
positively related with species number and abundance of the understorey
vegetation, and the correlation was slightly higher with P than N. This is
interesting, as N usually has the most important growth-limiting role in
boreal ecosystems. The spatial variation in the content of soil elements was
high both between and within plots, emphasizing the heterogeneity of the
soil. Dominant tree species and the soil layer were the most important
environmental variables affecting soil nutrient content. High contents of P
in the humus layer (maximum 2.60 g kg
Climate and availability of soil nutrients are important factors controlling the species composition of tree stand and understorey vegetation in boreal forests (Cajander, 1909, 1949; Kuusipalo, 1985; Økland and Eilertsen, 1996). High-latitude forest ecosystems are characteristically cold, have a short growing season and are nutrient poor. Organic matter decomposition and nutrient release are usually slow in cold climates (Hobbie et al., 2002). The edaphic conditions are reflected in the growth and chemical composition of plant species, as well as in species composition of vegetation (Vinton and Burke, 1995; Salemaa et al., 2008). In addition, tree cover affects the species composition and abundance in the understorey by shading (Verheven et al., 2012; Tonteri et al., 2016) and regulating nutrient input in throughfall precipitation (Salemaa et al., 2019) and litterfall (Ukonmaanaho et al., 2008).
Nitrogen (N) and phosphorus (P) are generally the main growth-limiting
nutrients for plants (Koerselman and Meuleman, 1996). Boreal forests are
mostly N limited (Tamm, 1991), and fertilization with N usually speeds up
forest growth (Saarsalmi and Mälkönen, 2001). Nitrogen is bound in
organic material, and only a little is directly available for plants as
inorganic ammonium (
In this study, we analysed whether plant species composition and nutrient
levels of tree leaves indicate soil total N and P at a northern boreal
(Hämet-Ahti, 1981) research site in Sokli, Finland. At this site, the
soil contains naturally large variations in P content. In Sokli, there is a
large deposit of phosphate rock, a carbonatite complex mainly consisting of
apatite (
The general aim of this study was to determine the undisturbed baseline status of the forest ecosystem in terms of soil, understorey vegetation and tree layers in the Sokli area in case there is a need to monitor the effects of phosphate mining. Phosphate mining can cause, for instance, aerial deposition of heavy metals and phosphate onto the surroundings of the mine (Reta et al., 2018), which can lead to changes in the abundance and species composition of the understorey. Vegetation, soil and foliage chemistry surveys provide data on the current state of the ecosystem (from the year 2015) that can be used as a reference level for the changes. Our specific aim was to identify which factors in the soil and tree layer explain the composition and abundance of plant species. In addition, we studied which environmental variables could explain soil nutrient contents, especially total P content.
We hypothesize that there are positive relationships between the following factors:
N and P contents of the soil humus layer and the abundance and species
composition of the understory vegetation, N and P contents in the topmost soil layers and the N and P contents of
needle and leaf biomass, N and P contents in the topmost soil layers and the occurrence of birch
trees in the research plots.
We established 16 study plots along four transects (A–D) around the planned
Sokli mining district (67
Geological map of the research area. Plots are marked with black
stars. The easternmost plot is located at the SMEAR 1 Station. (source: Hakku
Service,
Meteorological parameters from the years of data collection and for the
climatological normal period of 1980–2010 are presented in Appendix A. The
wind blows almost equally from the south-west and north-east during spring
and summer, whereas in winter and autumn the prevailing wind direction is
south-west (Ruuskanen et al., 2003). The growing season, when daily average
temperature exceeds 5
Tree species composition of the research plots (dbh
The distance between two plots depended on the topography and existing
roads, but generally it was about 2 km. A plot consisted of four clusters,
each including three 1 m
Set-up of each research plot with clusters and subplots within
clusters. Trees were measured from the whole 30 m
Altogether, 256 soil samples were collected from 16 plots using a soil
corer (inner diameter 5 cm) in June 2015. The soil was sampled within a 1 m
distance from the subplots (see Liski, 1995). The soil cores were separated
by visual criteria into four soil horizons: the top layer, which is a
mixture of litter and decomposing organic layer (F); the humus layer (O);
the eluvial layer (A); and the illuvial layer (B) (see Köster et al., 2014). The rocky soil and shallow humus layer made it impossible to sample
the mineral soil layers in some clusters. The soil samples from each horizon
were combined into composite samples in each cluster in the field. The
composite samples were air-dried, except for the organic F and O horizons,
which were dried at 60
Five pines and five spruces per plot were chosen for needle sampling in
September 2015, when the needle growth had ended. If less than five trees
per species were present, all of them were chosen. Three branches (length
approximately 50 cm) were taken from the upper third of the canopy using a
branch saw. We took only second-order branches because cutting of first-order branches would have been too destructive to the trees (see Helmisaari,
1990). Needle age classes (C
We sampled green birch leaves in July 2015 and sampled leaf litter in September 2015.
Approximately 10 green leaves from 10 different trees were picked and
combined (Rautio et al., 2010). Only mature, undamaged leaves were chosen.
Birch litter was collected under the same tree canopies from which the green leaves had been taken and
in approximately the same number as the green leaf samples. We aimed to take
litter leaves shed in the current year, so that they were decomposed as
little as possible. Green and litter leaves were dried at 65
Soil total phosphorus (P) contents of the research plots
(based on dominant tree species) in different soil horizons. Species 1
Total element contents of potassium (K) and P were analysed from soil and
foliar samples by inductively coupled plasma optical emission spectrometry.
For this analysis, the samples were first wet combusted. A 1 g amount of mineral
soil sample and 0.3 g of organic sample were combusted with 1 mL of
Total carbon (C) and N were analysed directly from dried and milled foliar samples and from the F and O soil layers. Samples of 2–3 mg were measured and analysed with an element analyser, which uses a high-temperature combustion method with subsequent gas analysis of CN (VarioMax, Elementar Analysensysteme GmbH, Germany). Soil pH was measured from two O layer samples per plot, and their average value was used. A total of 20 mg of dried sample was mixed together with ultrapure water (50 mL). The suspension was covered and left standing for 24 h, and pH was measured with a glass electrode.
We used one-way analysis of variance (ANOVA) and Tukey's honest significant difference post hoc test for analysing the plot-wise differences in needle element contents. Plot-averages of needle elemental contents were calculated across all needle age classes and both conifer species. One-way ANOVA was also used in analysing differences between the needle age classes. We grouped the plots based on their dominant tree species into pine, birch and spruce plots and calculated the average soil nutrient contents in each horizon in these plots. We then compared the nutrient contents in each soil horizon with one-way ANOVA.
We tested the effects of environmental variables on soil total P and N
contents and
We calculated plot-wise averages from the percentage covers of the plant species in the subplots. We ordinated this vegetation data by global non-metric multidimensional scaling (Minchin, 1987) using the vegan package (Oksanen et al., 2018) in R programme 3.4.3 (R Development Core Team, 2017). Ordination pattern of the plots based on the Bray–Curtis dissimilarity indices in floristic composition was analysed to find the main environmental gradients behind the vegetation variation. We analysed the data in three-dimensional space but present the results in one vs. two and one vs. three dimensions (the results in two vs. three dimensions did not give any new information). We then fit the plot-wise data of soil elements (O horizon), needle element contents, volume of birch (percent of total tree volume), species cover (percent of the surface area) and plot distance from the phosphate ore as linear vectors to the ordination pattern of the plots. The correlation between the environmental variables and the ordination was calculated by a linear vector procedure (envfit in vegan). The total P in the soil O horizon was also fitted as a smooth surface to the ordination pattern in order to analyse the form of the relationship (linear or non-linear). The fit was done by a generalized additive model (Gaussian distribution error).
Soil total N content
Correlation (Pearson) including soil elements (K, P,
The outlying points in Fig. 3. and the high standard deviations of P
showed rather high variation between and within plots (Table C1, Fig. C2 in Appendix).
Birch-dominated plots had the highest P and N contents and lowest
Average foliar element contents (g kg
Needle P contents were highest in the C needles and significantly different
from other age classes in both pine and spruce (Table D1). Against our
expectations, the needle P contents of both conifer species were rather
similar across plots (Table D2). On the other hand, N and C contents, as
well as the
Ordination pattern of the research plots in dimensions 1
and 2
We used linear mixed-effect models for determining which environmental
factors can best explain soil total P and N contents and the
Ordination pattern with smooth surface fit and linear vector fit
of soil phosphorus (P) in the O layer
The closer the plots were to each other in the ordination space, the more
similar their vegetation was (Fig. 6a, b). Plots positioned more on the
left-hand side of each panel had a higher number of forbs and grasses
growing on them than the plots positioned on the right-hand side of each panel (Fig. 6a, b). Species such as
The vector arrows fitted to the ordination space (Fig. 7c, d) depict the
maximum correlations between environmental variables and plot ordination.
The length of an arrow indicates the magnitude and direction of the polarity
(plus–minus) of the correlation. The highest correlations occurred between
the plot-wise average P content of the soil O horizon and the ordination
pattern of the plots (Table C4). The isocline gradient of soil P in relation
to the ordination pattern was almost linear (Fig. 7a). Vectors of soil pH, N
and P content all increased towards the more fertile plots, but the vectors
of soil
All the plant species growing in the study plots were common forest species
in Finland (e.g. Reinikainen et al., 2000, Finnish Biodiversity Information
Facility
Our second hypothesis stated that the N and P contents of the topmost soil
layers correlate with the N and P contents of foliar biomass, but our
results (Fig. 5, Table D2) did not support this hypothesis. The reason could
be that we measured total contents of N and P in soil instead of the plant-available contents of these nutrients. The plant-available contents of these
nutrients might have given different results. Perhaps plant species
composition in ground vegetation is sensitive to even small additions of
available N and P in the upper soil layers where the roots occur, whereas
higher contents of these elements are required for there to be any effect on the
needles. The P and N levels of our needle samples were similar to those
previously measured in Finland (Helmisaari, 1990; Merilä and Derome, 2008;
Moilanen et al., 2013). The higher P contents of C needles compared with
older needles is common for conifers and occurs because the dry weight in
recently matured needles increases faster than the transportation of P to
the needles (Helmisaari, 1990). The N contents of both green birch leaves and
leaf litter agreed with those reported by Ferm and Markkola (1985). The P
contents of the green leaves were higher than measured in that study (approximately
2.0 g kg
In general, the spatial variation in soil element contents between plots was
high, emphasizing the heterogeneity of soil fertility level (Figs. C2 and
C3). As our results showed, this heterogeneity can partially be explained by
the dominant tree species of the research plot, which especially affects the
topmost soil layers. According to the nutrient-uplifting hypothesis
(Jobbágy and Jackson, 2004), trees and other vegetation can transport
minerals such as P and K from the deep soil layers to the surface of soils.
The P contents of soil samples (Table C1) in our study (1.80–2.60 g kg
Our study area does not represent typical northern boreal forest, as it was located near the phosphate massif, the effect of which needs to be considered. Talvitie (1979), who used remote sensing for a geobotanical survey of the Sokli massif, found that the density of occurrence of birch, juniper and grass species increased when carbonatite was the underlying rock material. The surveys related to Natura 2000 (Standard Data Form FI1301512 and FI1301513) stated that the Törmäoja and Yli-Nuortti areas, where plots B1–B3 and A1–A2 were, have a high occurrence of grass species and a sparse birch-dominated tree cover due to carbonatite in the soil. According to the geological map (Fig. 1), only small parts in the western ends of both the Törmäoja and Yli-Nuortti Natura areas are located on top of carbonatite rock. Similarly, the map shows that those of our plots where the vegetation community was reminiscent of Sokli have something other than carbonatite as the rock parent material. However, all of our plots have metamorphic (tonalitic migmatite and amphibolite) or igneous (mafic volcanic and ultramafic) rock as the parent material, and phosphate mineral apatite can occur in such rocks (Walker and Syers, 1976). It is likely that these types of rock materials leach more phosphate than other types of bedrock (Arnesen et al., 2007). Thus, the rocks outside of the carbonatite massif may also have locally high P content, which affects the P content of the soil. The mixed-effect model factor “geology” did not consider this, which could be the reason why it was not important in explaining soil P content.
The baseline status and the current vegetation composition of our research site was worth studying for several reasons. We conducted our study in a region which has for decades been under more or less heated discussion related to whether mining activities will begin or not. The site is very remote and the plan is to move the material from the mine to locations of further production by trucks (Pöyry Environment, 2009). In addition to the aerial deposition from the mine, this could increase the dust and pollution caused by transportation, the amount of which is currently minimal. The effects of mining on the surrounding ecosystem and its vegetation composition can be unpredictable when combined with the changes caused by climate change. High-latitude regions are considered more vulnerable to climate change than more southern regions (Hartmann et al., 2013). Soil microbial activity may change due to a warmer climate, and therefore N may become more available from organic sources (Rustad et al., 2001). This, together with high soil P, may induce growth and affect vegetation dynamics. Climate change has already caused variation in the vegetation at high latitudes, as deciduous shrub coverage has expanded in the Arctic region (Sturm et al., 2001; Park et al., 2016). Greater deciduous shrub cover causes increased leaf litter input, which in turn may bring more nutrients that are recyclable to the ecosystem.
We found that the total P content of the soil humus layer was an important factor explaining the community composition of forest understorey vegetation near the Sokli phosphate ore in Finnish Lapland. The plots with high soil total P in the humus layer had birch as the dominating tree species. As green birch leaves and leaf litter also had high contents of P, we suggest that the litter caused the high total P contents in the humus layer. As climate change and the possible mining activities may affect the nutrient and vegetation dynamics in the studied region, the research that we carried out has an important role in both clarifying the current situation and forming a baseline for evaluating the magnitude of changes in the future.
Meteorological parameters from Värriö. Values
for the climatological normal period are from Pirinen et al. (2012). Growing
degree day sum was calculated as the average daily temperature (average of
daily maximum and minimum temperatures) above the 5
Average coverage (percentage of surface area) of understorey plant species per plot.
The fitted values vs. residuals:
Soil total P content within plots in different soil layers.
Soil total N content and
Average total contents of elements (g kg
Statistical differences of soil elements in each soil layer of
different plots, grouped by their dominant tree species. Levels of
significance:
Results from the mixed-effect models, testing the
effects of environmental variables on soil total P and N content and
Linear correlations of element contents of soil, needles, and
leaves; number of species in different vegetation layers; and plot distance
from Sokli with the non-metric multidimensional scaling ordination pattern.
The group of “grasses and sedges” includes forb, grass, and sedge species and
“d. shrubs and trees” includes dwarf shrubs and tree seedlings. Levels of
significance:
Statistically significant differences between needle age
group by species (C
Statistically significant differences of needle nutrient contents
between plots,
We have made all data used in the analyses publicly available. All data can be
downloaded from
LM and JB planned the study set-up; LM conducted all fieldwork, laboratory and statistical analyses and led the writing process; MS had a substantial role in guiding the ordination analyses and the writing process; and all authors contributed to the writing.
The authors declare that they have no conflict of interest.
We thank the staff at the Värriö Subarctic Research Station for providing full board during the fieldwork, Marjut Wallner for guidance in the laboratory, Jarkko Isotalo for commenting on the statistical analyses, Jukka Pumpanen and Kajar Köster for advice and equipment for the soil sampling, and Olli Peltola for helping with field-work. We also thank the two anonymous reviewers for their helpful comments.
The research has been supported by the Maj and Tor Nessling Foundation and Finnish Centre of Excellence (grant nos. 272041, 307331). Open-access funding provided by Helsinki University Library.
This paper was edited by Yakov Kuzyakov and reviewed by two anonymous referees.