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Keywords:

  • climate change;
  • ecosystem model;
  • boreal forest;
  • water budget;
  • evapotranspiration;
  • crown structure;
  • canopy conductance

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION AND CONCLUSIONS
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Norway spruce (Picea abies) is sensitive to decreases in soil water availability. In this study, we simulated how the climate change may influence the hydrological processes with impacts on the growth of Norway spruce in three sites located in southern Finland (61oN). The sites differed from each other in regard to the climate and soil water conditions; however, the initial structure of the tree stands was similar. Under the current climate, the canopy surface evaporation (Ec) increased, whereas the transpiration (Et) remained at a constant rate over the simulation period, as the seedlings in the initial stand grew to full maturity. Under the changing climate, on average, the cumulative of Ec and Eg (evaporation from soil surface) were 16% and 14% higher than those under the current climate, respectively, whereas the cumulative Et was 12% lower. The leaf area index (L) increased constantly on the sites with high (SH) and medium (SM) soil moisture, unlike on the site with low (SL) soil moisture. The canopy conductance (gcs) declined rapidly and earlier on the site SL, which implied the acclimation of stomatal behaviour to drought. The increasing water loss through evaporation decreased the water infiltration into the soil profile, resulting in an increasing soil water deficit (Wd), especially on the site SL. It also occurred earlier in all three sites under the changing climate. It was predicted that climate change will create a suboptimal environment for Norway spruce in southern Finland, particularly on the upland sites with lower groundwater table. Copyright © 2011 John Wiley & Sons, Ltd.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION AND CONCLUSIONS
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Climate change may profoundly change the water/heat dynamics in northern Europe, where Norway spruce (Picea abies) is among the dominant tree species, well adapted to cool and humid conditions (Kellomäki et al., 2005). Forest evapotranspiration plays an important role in controlling the water and energy balance in ecosystems, and has a strong impact on the carbon uptake and the growth of trees (Govind et al., 2011). In Finland, Norway spruce covers about 33% of the total forest area, and is an important timber producing species. In these conditions, even a small change in evapotranspiration can have a significant regional effect because of the abundance of Norway spruce forests (e.g. Barker et al., 2009).

The interaction between water budget and forest growth will be profoundly changed under the changing climate, which is likely to increase the mean annual temperature in Finland by 2–7 °C with a concurrent elevation of CO2 by the end of the 21st century (Carter et al., 2005; Ruosteenoja et al., 2005). At the same time, the annual precipitation will increase by 6%–37%, mainly in wintertime. In summer, the precipitation will probably be similar to current levels or even slightly less in southern and central Finland (60oN–62oN). Consequently, drought episodes may become more frequent owing to increasing evapotranspiration and lower availability of soil water (Kellomäki et al., 2005). The frequency of droughts is also affected by the reduced accumulation of snow (Kellomäki and Väisänen, 1996; Venäläinen et al., 2001), which reduces the recharge of soil water in the spring.

Norway spruce has a long leaf life span (up to 200 years), large leaf area index and leaf-to-sapwood ratio (Whitehead et al., 1984). This is associated with a dense canopy, which extends to lower parts of the stem (Cienciala et al., 1994, 1998). This further enhances the potential water depletion through evaporation and transpiration from the canopy surface. Consequently, less water falls through the canopy (throughfall) onto the soil. On the other hand, the expanding foliages intercept the radiation and affect the diffusive resistance within the stand microclimate with forest age, which influences the evaporation from the soil surface, surface runoff and water infiltration into the soil profile, as well as the water availability in the rooting zone (Radoglou and Jarvis, 1990; El Kohen and Mousseau, 1994; Overdieck et al., 1998).

The main parts of the roots of Norway spruce are in the surface soil, which makes this species vulnerable to low levels of soil moisture (Puhe, 2003). Prolonged dry periods reduce the stomatal conductance and thus limit the carbon uptake and growth (e.g. Phillips et al., 2001). Under the changing climate, the elevated atmospheric CO2 concentration may increase the photosynthetic carbon gain by improving the ratio between carbon assimilation and transpirational water loss, but the higher temperatures will probably reduce the CO2-induced stimulation (Wang et al., 1995; Tjoelker et al., 1998). Moreover, the effect of temperature and CO2 could be counteracted and potentially overwhelmed by the increased drought and nutrient stress (Roberntz and Stockfors, 1998; Phillips et al., 2001; Bergh et al., 2005).

In this context, the aim of this study was to simulate the impacts of climate change (the elevation of temperature and atmospheric CO2 and changes in precipitation) on the evapotranspiration and the availability of soil water in three high-density Norway spruce forests located in southern Finland (61oN), by employing a process-based ecosystem model and FINADAPT climate change scenario provided by the Finnish Meteorological Institute. The simulations, over a 100-year period, concern three sites occupied by Norway spruce in the seedling phase. The sites differed from each other in regard to altitude, mean annual temperature, precipitation and current soil water conditions; however, the initial structure of the tree stands was similar at the beginning of simulations. Thus, the simulations indicated how sensitive the carbon uptake and growth of Norway spruce is to the climate change in varying site conditions.

MATERIAL AND METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION AND CONCLUSIONS
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Outlines for the modelling and processes simulated

The calculations concern the water exchange through the soil–plant-atmosphere continuum based on the classical energy balance (Jansson, 1991a, 1991b), resistance network (Thom, 1975; Monteith and Unsworth, 1990) and conductive processes (Jarvis and McNaughton, 1986; McMurtrie et al., 1990), with the consequent output in terms of availability of soil water. The availability of soil water (Wa) refers to the extractable water for trees as indicated by the balance between water recharge (precipitation), water loss (evaporation and runoff) and soil water storage (Wsoil) (Equation (1)). Total evaporation is calculated as the sum of evaporation from canopy (Ec) and soil surface (Eg). Evapotranspiration (ET) consists of Ec, Eg and water from canopy transpiration through the stomata (Et) (Equation (2)),

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  • display math(2)

where P is the precipitation, and Wrunoff is the total runoff.

In terms of the computation framework (Figure 1), the simulations of hydrological processes focused on the effects of climate change on the leaf-canopy stomatal conductance, diffusivity resistances, crown and stand structure, water and heat fluxes, and the interrelationships among these responses, with the consequent effects on carbon uptake and growth. For this purpose, an integrated process-based model (FinnFor) developed by Kellomäki and Väisänen (1997) was used.

image

Figure 1. Factors controlling the stand water budget.

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Evapotranspiration dynamics

Energy balance

In these calculations, the methodology was adopted from Kellomäki and Wang (1999) and Wang et al. (2004), who applied it for eddy covariance measurements for analysing and simulating the CO2 and H2O fluxed in a boreal coniferous stand. In the calculations, the net solar radiation (Rn) for energy above the canopy is divided into the net radiation absorbed in the canopy (Rnc) and soil surface (Rns). The energy balance in the tree stand can be expressed on an instantaneous time-scale (Equations (3)-(5)),

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where λEc and λEg are the latent heat fluxes on the canopy and soil surface, respectively. Hc and Hg are the sensible heat fluxes on the canopy and soil surface, respectively. Sc is the energy storage within the canopy, Gg is the soil surface heat flux, and δ is a certain limit.

A weather generator, developed by Strandman et al. (1993), was used to generate the hourly weather factors of solar radiation, relative humidity, wind velocity and cloudiness using the Finnish weather statistics for the period 1971–2000 (Drebs et al., 2002).

Evaporation from canopy surface

Rain falling on the canopy is intercepted by the leaves until the intercepted water exceeds the storage capacity of the leaves, and the additional water falls through the canopy (throughfall) onto the soil surface. The intercepted water forms a pool on the canopy surface, where water is evaporated (evaporation from canopy surface, Ec). The amount of water on the foliage surface (Wc) is equal to the daily interception (Id) minus daily evaporation (Kellomäki and Väisänen, 1996, 1997). The water balance between water incoming through precipitation, evaporated from canopy surface and fallen through the canopy is,

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where Ep is the daily potential transpiration, and c is the interception storage capacity of the canopy.

The Penman–Monteith equation is used to compute the potential transpiration and evaporation from the canopy surface based on the net radiation interception in the canopy (Rnc) and the aerodynamic resistances of the canopy, including two eddy diffusivity resistances (rca and rsc) (Kellomäki and Väisänen, 1996, 1997). The eddy diffusivity resistances between the soil surface and mean canopy (zs.1), and mean canopy and reference height (zs.2) are based on Thom (1975) and Monteith and Unsworth (1990),

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where K is the turbulent transport coefficient, u1 and u2 are the wind speeds at the mean canopy height and at the reference height, and u* is the friction velocity.

Evaporation from soil surface

The daily evaporation from the soil surface (Eg) is composed of the evaporation (Es) in the non-snow season and the evaporation from the snow surface (Esnow) in the snow season. The Penman–Monteith equation (Equation (10)) with the variables of aerodynamic (rsc + rca) and soil-surface resistance (rs), is employed considering the vapour pressure deficit (es(Ts) − ea) and net radiation on the soil surface (Rng),

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where kex is the effective extinction coefficient through the canopy (Monteith and Unsworth, 1990). In computing the Rng, the effects of canopy structure of the particular tree cohorts on light and radiation interception are considered (Oker-Blom, 1985, 1986). The leaf area is assumed to be distributed uniformly (randomly) within the crown, and the trees are assumed to be distributed within the stand following the Poisson distribution (Kellomäki and Väisänen, 1997).

Transpiration from tree stand

Hourly values of the whole tree transpiration were simulated using a ‘big-leaf’ model (Jarvis and McNaughton, 1986; Kellomäki and Wang, 2000). Based on the Penman–Monteith equation, the transpiration of a whole tree (Et) can be obtained by summing Esun and Esh, representing the transpiration from the sunlit and shaded foliage per unit ground area (Equation (12)). In the calculations, the leaf area index (L) is divided into sunlit (Lsun) and shaded (Lsh) fractions, and the transpiration from each fraction was calculated separately. The vapour pressure deficit (es(Tc) − ea) and temperature (Tc) on the canopy surface were the same for all the foliages. Furthermore, the boundary layer conductance (gb.m) to water vapour was a function of wind speed,

  • display math(12)
  • display math(13)

where Rnc.sun and Rnc.sh are the net radiation absorbed by the sunlit and shaded foliages per unit ground area, respectively (Kellomäki and Wang, 1999, 2000). Moreover, gcs.sun and gcs.sh are the canopy conductance of the sunlit and shaded foliage per unit ground area, respectively, whereas s is the slope of the saturation vapour pressure curve, ρ is the air density, cp is the specific heat capacity of air at constant pressure, γ is the psychometric constant, and Lt is the total foliage area per tree.

The leaf-level stomatal conductance (gs) is assumed to be controlled by radiation (Rnc), air temperature (Ta), vapour pressure deficit (es − ea), ambient concentration of CO2 (ca), soil water potential (ψ) and soil temperature (Ts) (Equation (14)) (McMurtrie et al., 1990; Kellomäki and Wang, 1999, 2000),

  • display math(14)

where gs.max is the maximum, and gs.min is the minimum values of stomatal conductance.

Moreover, to scale-up from leaf to canopy, the daily dynamics of the sunlit and shaded needle fractions within the canopy and the corresponding differences in canopy conductance (gcs) are calculated (Equation (15)). The contributions from the sunlit (gs.sun) and shaded (gs.sh) fractions were calculated separately (Shuttleworth and Wallace, 1985; Jarvis and McNaughton, 1986).

  • display math(15)

Soil water dynamics

Water flow in the soil profile

The amount of daily water infiltration (Winfil) into the soil profile is calculated as a balance between the incoming and outgoing water flows on the soil surface (see Equation (16)),

  • display math(16)

where Wth is the throughfall and Wr.surf is the surface runoff including rainwater and snowmelt.

The daily water flow into the soil surface pool represents precipitation through the canopy and melting snow. The precipitation is obtained from the weather sub-model in the form of water if the air temperature is more than +1 °C and in the form of snow if the temperature is less than −1 °C. Within this range, the precipitation is a mixture of water and snow in accordance with a linear temperature relationship within the range −1 to +1 °C (Kellomäki and Väisänen, 1997).

To calculate the water storage in the soil profile (Wsoil), the humus and mineral soil are divided into 12 horizontally homogenous layers (no change in soil properties with time). Heat and water conditions are modelled with partial differential equations solved using Euler integration (Jansson, 1991a, 1991b; Kellomäki and Väisänen, 1997). The soil lower boundary conditions for water flow depend on whether the soil is saturated or not by water. If the lower boundary is saturated, the water flow is calculated as a function of hydraulic conductivity (Jansson, 1991a, 1991b). The vertical water flow out of the soil profile is equal to the hydraulic conductivity in the lowest soil layer.

The daily relationship between the Wsoil and soil water potential (ψ) in the soil profile was calculated based on Jansson (1991a, 1991b). Three different cases are distinguished in the computations: (i) if the water potential is greater than the water potential threshold (ψx), the relationship is modelled using a log-linear expression (Equation (17)); (ii) if the linear water potential limit (ψl) is less than the water potential threshold, the relationship is computed using the equation presented by Brooks and Corey (1964) (Equation (18)); and (iii) if the water potential is less than the linear water potential limit, the relationship is computed using the linear expression (Equation (19)).

  • display math(17)
  • display math(18)
  • display math(19)

where ψw is the water potential at the wilting point (Brooks and Corey, 1964), Ww is the wilting point, Wx is the field capacity, ψA is the air entry tension, Wr is the residual water content, p is the porosity, l is the pore size distribution index, and ψa is the air filled porosity limit.

The daily total runoff (Wrunoff) is composed of the surface runoff (Wr.surf) and groundwater outflow (Wout). The values of Wr.surf and Wout are based on an empirical approach by Jansson (1991a, 1991b). In the calculations, the level of groundwater was initialised as 1.5 m beneath the land surface. The horizontal flow of groundwater was defined as a steady-state. The groundwater vertical outflow within the saturated zone was calculated based on the assumption that the water content will only change in the uppermost saturated layer (Jansson, 1991a, 1991b). The excessive water in the lowermost saturated layers was replenished to the increment of groundwater.

Snow dynamics

The daily precipitation is divided into water and snow as described earlier. The dynamics of snow on soil is a function of falling snow and melting–freezing of snow and the depth of the snow layer (Jansson, 1991a, 1991b). The daily snowmelt is a function of temperature, which accounts for the influence of solar radiation and the soil surface heat flow. The snow depth (Zsnow) is the average thickness of accumulative snow, that is, the snow remaining from the previous day (Zsnow.o) calculated with the density of precipitation (ρp) and snow (ρsnow) (Equation (20)),

  • display math(20)

where Zp is the depth of precipitation from the previous day, and ρsnow.o is the density of old snow.

Water availability and potential soil water deficit

The availability of soil water (Wa) during the main growing season (May–September) is calculated as a relative ratio between the available water in the rooting zone (uppermost layers of soil profile where the roots are located) and the water uptake (transpiration demand) water by trees from the rooting zone. The Wa is calculated based on Granier et al. (1999, 2000),

  • display math(21)

where the numerator is the actual extractable water by the tree, and the denominator is the maximum extractable water.

For most coniferous trees (Granier et al., 1999, 2000), the water stress reduces growth (drought effect), when Wa < 0.4 in the rooting zone. Consequently, the amount of monthly deficient water during the main growing season (May–September) is computed as shown in Equation (22),

  • display math(22)

The depth of the root system (rooting zone) is a linear function of tree height (Marklund, 1987, 1988; Kellomäki and Väisänen, 1997), that is, the thickness of the rooting zone will increase along with the height of the trees.

Tree growth and nitrogen availability

In addition to the hydrological processes in forests (see Figure 1), the growth of trees and the development of the forest are based on the sub-models for photosynthesis, allocation of photosynthesis for growth of organs and cycle of nitrogen in the site occupied by the trees. The trees in the stand are divided into cohorts defined by the density of each tree group. The tree groups are defined by the diameter at breast height (Kellomäki and Väisänen 1997, Briceño-Elizondo et al., 2006; Ge et al., 2011a).

Canopy photosynthesis

The hourly photosynthetic rate of the object tree is calculated on the basis of the biochemical model developed by Farquhar et al. (1980). The photosynthetic rate is updated on an hourly basis utilising solar radiation, air temperature and humidity, and CO2 concentration, which affect the stomatal conductance and the biochemical fixation rate of carbon. The photosynthetic rate is also affected by soil moisture and temperature through stomatal conduction. Furthermore, the leaf nitrogen content affects the tree's photosynthetic rate (Kellomäki and Wang 1997, Roberntz and Stockfors 1998; Medlyn et al., 2001, 2002; Roberntz, 2001). Recovery of photosynthesis of boreal conifers was introduced in the calculations by Hänninen and Hari (2002).

Tree growth

The annual allocation of photosynthates for growth of different tree organs (foliage, branch, stem, and coarse and fine root) is based on the dynamic allometric functions of development of height and diameter at breast height with tree growth over time, after excluding the respiration (growth and maintenance respiration) from the gross photosynthesis (Marklund, 1987, 1988). The leaf mass was further converted into leaf area by utilising the values of the specific leaf area (SLA, area of leaf per mass unit) presented by Marklund (1987, 1988).

Available nitrogen

Nitrogen released in the decomposition of soil organic matter (litter and humus) is determined by employing the algorithm developed by Chertov and Komarov (1997) and Chertov et al. (2001). Decomposition rates of different types of litter (foliage, twig, stem, and coarse and fine root) are determined by soil temperature, soil moisture, and nitrogen and ash content of the litter (litter quality). Temperature and moisture of the litter are linear functions of those in the mineral topsoil (Chertov and Komarov, 1997; Chertov et al., 2001). The decomposition works on a monthly basis.

Model validity

The hydrological processes of the model have been parameterised and validated through four main stages: (i) weather simulation in relation to precipitation and snowfall; (ii) response of leaf-canopy stomatal conductance to environmental factors; (iii) water exchange between the boreal stand and the atmosphere; and (iv) water and heat flow through the soil profile. The summary of the validation work is shown in Table 1.

Table 1. Summary of the validation work of hydrological processes carried out for the model.
Processes and outputDescriptionReferences
Weather simulationThe computed values displayed a very close correlation between the observed values for temperature, cloudiness, radiation, precipitation, snowfall, relative humidity and windiness in Finland.Strandman et al., 1993; Kellomäki and Väisänen, 1997; Venäläinen et al., 2001
Leaf-canopy stomatal conductanceThe response of leaf-canopy stomatal conductance to air temperature, soil-water potential, short-wave radiation, ambient CO2 concentration, vapour pressure deficit and soil temperature were carefully parameterised.Roberntz and Stockfors, 1998; Kellomäki and Wang, 2000; Medlyn et al., 2001, 2002
Ecosystem water exchangeThe exchange of water between the atmosphere and a boreal coniferous stand was parameterised satisfactorily on eddy covariance and surface resistance.Kellomäki and Wang, 1999; Wang et al., 2004; Ge et al., 2011b
Soil water and heat flowThe computations of soil temperature and water transfer through the soil profiles in such a way that soil temperature determines whether the soil is frozen or not, and thereby influences the conductivity of water and water conditions. The model had been parameterised for boreal soil types.Jansson, 1991a, 1991b

Regarding the other sub-models, the photosynthesis was simulated using Farquhar's type model and parameterised for Norway spruce (Roberntz and Stockfors 1998; Medlyn et al., 2001, 2002; Roberntz, 2001). The significant processes of water and energy balance including radiation transfer in the canopy, canopy and soil-surface energy balance, heat fluxes in the soil and tree, turbulent transfer (wind profiles and aerodynamic resistances), and the boundary-layer and surface resistances (on canopy and soil surface) were parameterised and validated with the help of 10 years of eddy covariance monitoring and chamber experiments (Kellomäki and Wang, 1999, 2000; Wang et al., 2004; Ge et al., 2011b). The decomposition of soil organic matter and mineralization of nitrogen, and hydrological and nitrogen cycles was validated against the long-term monitoring and laboratory data for boreal forest soils (Chertov and Komarov, 1997; Chertov et al., 2001). Briceño-Elizondo et al. (2006) have done a detailed sensitivity analysis of the model in respect to changes in environmental conditions of temperature, precipitation, atmospheric CO2 concentrations and nitrogen concentration of foliage. Recently, the performance of the model was investigated by comparing the calculated and measured mean volume growth of 1191 sample trees over a 10-year period on the permanent sample plots of the Finnish National Forest Inventory (NFI) (Ge et al., 2011a). The output of the model regarding the growth of Norway spruce is reasonable compared with the measurements in the sample plots.

Furthermore, for validation of simulated leaf area development, we compared the calculated leaf area index (based on FinnFor-type dynamic allometric functions) with the output based on DESPOT-type functions by Buckley and Roberts (2006), who concluded a power function with an exponent between leaf area index (L) and sapwood area (S) with tree development, as L = aSb, where a and b are parameters. As shown in Figure 2, the simulated leaf area index using FinnFor-type original functions on the three study sites (see 2.6. chapter) for the period of 2000–2099 (see Table 2 for initial stand characteristics), correlated reasonably well (r2 = 0.91, n = 300) with the output based on DESPOT-type function between leaf area and sapwood area.

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Figure 2. Comparison of simulated annual mean leaf area index (L) based on FinnFor-type dynamic allometric functions (regular black lines) with the output based on DESPOT-type function between leaf area and sapwood area (bold grey lines) on the three study sites (see Figure 3) for the period of 2000–2099, using the current climate scenario. The DESPOT-type power function was L = 0.441S0.999 (r2 = 0.872) for Norway spruce, fitted to the dataset from the permanent sample plots of the Finnish National Forest Inventory (NFI).

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Table 2. General description of the study sites.
Stand layout and parametersSHSMSL
  1. According to annual precipitation, SH represents the site with the highest soil moisture, SM the medium and SL the lowest soil moisture.

Location61˚19′N, 22˚09′E61˚31′N, 29˚18′E61˚26′N, 25˚09′E
NFI serial numberNo. 35029231No. 40091231No. 32048131
Precipitation (mm)610585512
Elevation (m a.s.l.)30–4090–100170–180
Density (trees ha−1)285830943175
Number of cohorts598
Leaf area index1.341.631.89
Mean diameter (cm)6.095.907.44
Mean height (m)5.646.837.23

Simulations

Study sites

We selected three sites located in southern Finland from west to east at the same latitude (61oN), where Norway spruce dominates in large areas (Figure 3). The sites were selected from the database of the 9th Finnish National Forest Inventory (NFI of 1999–2000). The sites were occupied by young Norway spruce stands with similar high density (~3000 trees ha−1).

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Figure 3. Location of the study sites (SH, SM and SL) including a map of Norway spruce dominated forests distribution (upper left) and altitude gradient in southern Finland (right). A simple relation between groundwater table and altitude on the sites was depicted (lower left).

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The study sites were described in terms of location, annual precipitation (mean values for 1971–2000) and altitude (Table 2). The soil at all sites is sandy loam with lower water holding capacities owing to high porosity and low hydraulic resistance (Aaltonen, 1950). As estimated by the operational watershed model (WSFS) for Finland (Vehviläinen and Huttunen, 2002), the water conditions of the soil profile are determined by the annual/seasonal precipitation, underground catchment, temperature, relative elevation and soil properties (texture). According to the precipitation climate (Table 2), the three sites were located in different zones with varying soil water conditions, that is, SH representing the site with the highest soil moisture, SM the medium and SL the lowest soil moisture.

The three sites were described in terms of density of trees, cohort number, mean diameter at breast height and height, mean density and mean leaf area index. The initial mass of organic matter in the soil was assumed to be 70 Mg ha−1 in each site. The simulations were done without management over 100 years employing the current and changing climate scenarios applicable to Finland (see succeeding details).

Climate scenarios for temperature, atmospheric CO2 and precipitation

The climate and weather inputs used represent two scenarios compiled by the Finnish Meteorological Institute for the FINADAPT project: one for the current climate and another for the changing climate (Carter et al., 2005; Ruosteenoja et al., 2005). The spatial resolution of the grid for the current climate used in the reference simulations is 10× 10 km2, whereas the resolution for the climate change scenario is 50× 50 km2 (Venäläinen et al., 2001; Ruosteenoja et al., 2005). In the simulations for a given site, the algorithm uses the climate for the closest grid point of the climate data. The current climate scenario for the period of 2000–2099 represents the mean data for 1971–2000 repeated over the total simulation period, with a constant CO2 concentration of 351 ppm. The climate change scenario is based on the IPCC SRES A2 emission scenario (Carter et al., 2005; Ruosteenoja et al., 2005). In this scenario, by 2099, the mean temperatures are projected to increase by almost 4 °C in the summer and 6 °C in the winter, whereas the atmospheric concentration of CO2 was 351 ppm at the start of the simulation in 2000 and 840 ppm at the end of the simulation in 2099. The precipitation increases mainly in winter, but changes little in the summer during the simulation period (Carter et al., 2005; Ruosteenoja et al., 2005).

The data for the current and changing climate was given in terms of daily mean values considering the seasonal and inter-annual variation (Figure 4). The values were further decomposed into hourly values by applying the weather generator developed by Strandman et al. (1993) for the calculations of the hydrological and ecological processes.

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Figure 4. Annual variations in mean annual temperature and total precipitation for the current (solid line) and changing (dash line) climate scenarios on the three sites with different current soil moisture conditions, ranging from the high soil moisture (SH) to medium (SM) to the low (SL).

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Outputs of the simulations and data analyses

The output variables of the simulations are the values of (i) water loss including evapotranspiration and total runoff, and (ii) the amount of potential water deficit during the growing season. The dynamics of key variables of physiological response, structural changes and physical processes of the soil-plant-atmosphere are also calculated with regards to the climate scenarios.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION AND CONCLUSIONS
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Effects of climate change on evapotranspiration

Figure 5 (upper panel) shows the annual fluctuation of evaporation from canopy (Ec) and soil surface (Eg) for the study sites. Regardless of climate scenario and site, the Ec was increasing and Eg decreasing as the tree stands grew from the seedling phase up to the phase of full maturity, owing to the development of leaf area index (L) (Figure 5, middle panel) and increasing shading. When the changing climate was applied, the values of Ec and Eg were higher than under the current climate regardless of site, owing to elevated temperature (Figure 4). At the end of the simulation period, the cumulative Ec was 19%, 18% and 10%, and Eg 15%, 14% and 14% higher under the changing climate than that under the current climate on the sites SH, SM and SL, respectively (Table 3).

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Figure 5. Annual amount of evaporation from canopy (Ec) and soil surface (Eg), mean leaf area index (L), and mean aerodynamic resistance (rsc + rca) on the three sites with different current soil moisture conditions, ranging from the high soil moisture (SH) to medium (SM), to the low (SL). The solid line represents the current climate, and the dashed line represents the changing climate. The arrows indicated that the L began to be lower under the changing climate than the current climate.

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Table 3. Ratio of cumulative canopy surface evaporation (Ec/ET), canopy transpiration (Et/ET) and soil surface evaporation (Eg/ET) to evapotranspiration, and the cumulative monthly mean (during growing season of May–September) of soil water deficit (Wd) on the three sites over the 100-year simulation period under the current (Cur) and changing climate (CC). The values in parentheses are the percentage change (%) under changing climate from that under current climate. SH represents the site with the highest, SM the medium and SL the lowest initial soil moisture content.
SitesEc/ET (%)Et/ET (%)Eg/ET (%)Wd (mm)
CurCCCurCCCurCCCurCC
SH3238 (+19%)5346 (−13%)1316 (+15%)130.9559.1
SM3440 (+18%)5145 (−12%)1416 (+14%)208.2669.6
SL4246 (+10%)4338 (−12%)1416 (+14%)507.9973.8

The annual dynamics of L are among the key factors regulating the water fluxes through changes in physiological responses, expansion of canopy and physical processes. Regardless of climate scenario, the L increased constantly on the sites SH and SM along with the stand age, whereas on the site SL, the increase pattern was not as constant (Figure 5 middle panel). When the changing climate was applied, on the sites SH and SM, the values for L were higher during the simulation period of 2000–2080 and 2000–2060 years, respectively, thereafter it was slightly lower compared with the values under the current climate. This only was the case on the site SL during the period 2000–2020 years.

The aerodynamic diffusion resistance (rsc + rca), modified by temperature and relative humidity, is one of the factors influencing evaporation. The computations showed that the rsc + rca decreased by about 6%–7% under the changing climate compared with current climate on the three sites (Figure 5 bottom panel), leading to higher evaporation. The reduction was related to the increasing temperature and reduced air humidity.

Under the current climate, the canopy transpiration (Et) remained at a constant rate throughout the simulation period, regardless of site. When the changing climate was applied, the values of Et were lower than under the current climate, also regardless of site (Figure 6, upper panel). Over the simulation period, the cumulative Et was 12%–13 % lower under the changing climate than that under the current climate on all the sites (Table 3).

image

Figure 6. Annual amount of canopy transpiration (Et), mean canopy conductance (gcs), and mean vapour pressure deficit (es – ea) on the three sites with different current soil moisture conditions, ranging from the high soil moisture (SH), to medium (SM), to the low (SL). The solid line represents the current climate, and the dashed line represents the changing climate. The arrows indicated that the gcs began to be at a low-level.

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Canopy conductance controls the transpirational water flow between plant and atmosphere. As the trees get older, the values of gcs declined, regardless of the climate scenario and site (Figure 6 middle panel). On the sites SH and SM, the reduction in gcs levelled off from the year 40 onwards. The pattern was similar on the site SL, but the levelling-off took place earlier from about year 20 onwards. On all the sites, the values of gcs were lower under the changing climate than under the current climate during most of the simulation period. This was due to the increasing air vapour pressure deficit (es – ea) (Figure 6, bottom panel) related to the increasing temperature.

Effects of climate change on soil water flow

Figure 7 shows the annual amount of water infiltration into the soil profile (Winfil), mean snow depth (Zsnow) and total runoff (Wrunoff) over the simulation period. Regardless of site and climate scenario, the values of Winfil decreased. This was due to the increasing interception of precipitation with the expansion of leaf area and canopy size over the simulation period. Under the changing climate, the increase in temperature further enhanced the evaporation (Ec and Eg), leading to less infiltration of water into the soil profile than under the current climate (Figure 7, upper panel). The increase in temperature also led to a substantial reduction in the depth of the snow layer (Zsnow). This was because the fraction of precipitation in the form of snow was less because of the later snowfall and earlier snowmelt (Figure 7, middle panel). The reduced water input to the soil profile also reduced the Wrunoff regardless of site (Figure 7, bottom panel).

image

Figure 7. Annual amount of water infiltration (Winfil), mean snow depth (Zsnow, total annual accumulation / 365 days) and total runoff (Wrunoff) on the three sites with different current soil moisture conditions, ranging from the high soil moisture (SH) to medium (SM) to the low (SL). The solid line represents the current climate, and the dashed line represents the changing climate.

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Effects of climate change on soil water deficit for tree growth

Figure 8 (upper panel) shows that the annual amount of potential transpiration (Ep) increased along with the stand age and development of leaf area regardless of site, and that the Ep increased much more under the changing climate than under the current climate. The increased transpirational demand under the warmer climate increasingly reduced the availability of water (Wa) during the growing season compared with that under the current climate (Figure 8, middle panel). The values of Wa declined to the low level from about the year 50 onwards on the sites SH and SM, but much earlier on the site SL (since about year 20). Under the current climate, the deficit of soil water (Wd) occurred from the years 65–70 onwards on the sites SH and SM, and from the year 40 onwards on the site SL, whereas the changing climate shifted it earlier, especially on the sites SH and SM. (Figure 8, bottom panel). The cumulative monthly mean of Wd (during the growing season) over the simulation period were higher on all the sites under the changing climate than under the current climate, and the amount of cumulative Wd was much higher on the site SL than that on the sites SH and SM (Table 3).

image

Figure 8. Annual amount of potential transpiration (Ep), and monthly mean of soil water availability (Wa) and water deficit (Wd) during growing season (May–September), on the three sites with different current soil moisture conditions, ranging from the high soil moisture (SH) to medium (SM) to the low (SL). The solid line represents the current climate, and the dashed line represents the changing climate. The arrows indicated that the Wa began to be at a low-level.

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DISCUSSION AND CONCLUSIONS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION AND CONCLUSIONS
  7. ACKNOWLEDGMENTS
  8. REFERENCES

General performance of the model

The aim of this study was to estimate the effects of climate change on stand hydrological processes and soil water availability in three high-density Norway spruce forests located in the southern boreal zone in Finland. These areas are considered to be more sensitive to the climate change compared with the northern parts of the boreal zone (Carter et al., 2005; Ruosteenoja et al., 2005). The simulations, over a 100-year period, were based on the local climate scenarios scaled down to the same scale as the grid of the three sites used in this study. The model used in the simulations integrates physiological and growth responses of trees to climatic and soil factors, water/nutrient uptake of trees and the water vapour exchange between canopy and soil surface and the atmosphere (Kellomäki and Väisänen, 1997; Ge et al., 2011a, 2011b). In the simulations, we excluded the uncertain risks of abiotic (wind, snow, frost and fire) and biotic (insect and fungal pests) damage on the growth and life cycle of trees because the focus of this study was on the pure impacts of changes in the main climatic factors (temperature, precipitation and atmospheric CO2) on hydrological processes in the boreal forest ecosystem and the availability of soil water for growth of trees.

In our simulations, the canopy size (crown surface) played a key role in influencing the microclimate conditions above and below the canopy such as the rainfall and the interception of precipitation as well as irradiance on the soil surface. We used the ‘big-leaf’ model to calculate the canopy conductance based on the accumulative effect of the sunlit and shaded leaves (Jarvis and McNaughton, 1986). The simulated responses of canopy conductance to environmental variation were similar to those found in field measurements representing trees grown under the elevated CO2 and temperature and limited soil water in boreal Sweden (Oren et al., 1998; Ewers et al., 2000; Ewers et al., 2001; Phillips et al., 2001). The increase in temperature and atmospheric CO2 promoted the growth of Norway spruce trees (Roberntz and Stockfors, 1998; Roberntz, 2001; Bergh et al., 2003) as our simulations show. Similarly, the leaf-canopy area expanded under the changing climate. Consequently, the interception of water increased due to the larger canopy surface area. On the other hand, the simulated evaporation from the soil surface was much lower in Norway spruce than that observed previously in Scots pine (Pinus sylvestris) (Kellomäki and Väisänen, 1996). This is because Scots pine is a light demanding species, with a crown representing sparse foliage located in the upper part of the stem. While shade tolerant tree species, such as Norway spruce, generally have a dense canopy and large leaf area index (Whitehead et al., 1984; Cienciala et al., 1994, 1998). Accordingly, spruce-specific parameters of form and growth of trees were employed in the model to obtain a reasonable simulation of the Norway spruce forest.

Responses of water budget to climate and stand variables

In the boreal zone, a moderate increase in temperature and CO2 would most likely lead to increased photosynthesis and tree growth (Bergh et al., 2003). According to our results, the climate change increased the growth of Norway spruce during the first few decades (leaf area development as an indication), regardless of site. This was in agreement with the 10 years of measurements from the Duke FACE (free-air CO2 enrichment) site. This experiment included several coniferous species growing under elevated CO2, the results showed that elevated CO2 led to greater leaf area and plant biomass production compared with those under the ambient conditions (McCarthy et al., 2006, 2010).

The enhancement of carbon uptake and tree growth by the elevated CO2 was not uniform, but rather primarily dependent on the availability of growth resources such as water (McCarthy et al., 2006, 2010). Our simulations showed that the interception of water on the canopy surfaces was enhanced by the climate change owing to the increase in leaf area. The climate change will further create an environment with larger evaporation owing to a higher vapour pressure deficit and lower diffusive resistance. The increased evaporation and reduced water infiltration into the soil profile increased the occurrence of drought periods and decreased the canopy stomatal conductance and tree growth during the latter stages of the simulation period. Our findings are also supported by the long-term field experiments for Norway spruce in southern and central Sweden (Phillips et al., 2001; Roberntz, 2001) and in Finland (Jyske et al., 2010). On the other hand, the soil moisture also strongly influences the decomposition of soil organic matter (Chertov and Komarov, 1997; Chertov et al., 2001). The simulations showed that the amount of available nitrogen (decomposed humus) and the consequent canopy photosynthesis decreased due to the reduced decomposition induced by increased soil water deficit on all the sites (data not presented). McCarthy et al. (2010) have reported that the variation in net primary productivity of a CO2 enriched forest was greatly controlled by nitrogen availability.

As presented by Pumo et al. (2010), the effects of climate change on the water stress for vegetation are strictly dependent on the future seasonal distribution of rainfall and the possible modifications in its frequency and intensity. Based on the FINADAPT climate scenario we used, the future precipitation in summer will be similar to the present levels or even slightly less (Carter et al., 2005; Ruosteenoja et al., 2005). The anticipated higher temperatures will likely lead to a substantial reduction in the snow accumulation owing to a decreased fraction of precipitation as snow and later snowfall and earlier snowmelt (Kellomäki and Väisänen, 1996), which could limit the recharging of soil water in the springtime and early summer.

In a forest ecosystem, transpiration is one of the major water fluxes, and it is mainly controlled by atmospheric and edaphic conditions through stomatal conductance (Small and McConnell, 2008). Previously, Oren et al. (1999) observed in Norway spruce that a moderate increase in temperature makes the stomata open with increased transpiration. However, Norway spruce treated with a long-term elevation of CO2 showed a reduction in stomatal conductance and transpiration (Roberntz and Stockfors, 1998). Moreover, the soil water conditions also play a significant role in regard to the internal water flow and stomatal conductance in Norway spruce (Lu et al., 1995, 1996; Oren et al., 1998; Ewers et al., 2000; Phillips et al., 2001). During drought, the hydraulic conductance of the soil-leaf pathway in Norway spruce is reduced, leading to decreased transpiration (Lu et al., 1995, 1996). Based on our simulations, the increased evaporation and decreased infiltration of water into the soil profile enhanced the soil water deficit, resulting in a reduction in the canopy conductance under the changing climate. Furthermore, a decline in canopy conductance was observed earlier on the site SL with poor soil water conditions, compared with the other two sites (SH and SM) with better soil water conditions. However, the discrepancy in canopy conductance between the current and changing climate scenarios was smaller on the site SL than the sites SH and SM. This implied that the trees growing on the dry sites could acclimate to the continuous drought with continuously low-levels of stomatal conductance; that is, transpiration from the forest is a ‘conservative hydrological process’ as claimed by Roberts (1983).

We found that water stress caused lower tree growth of Norway spruce in southern Finland under the warmer climate. Especially, the intermittent growth retardation in Norway spruce on the site SL was indicated by the retardation of leaf area expansion. A similar performance was found in several field experiments in southern boreal forests (Roberntz, 2001; Bergh et al., 2005; Jyske et al., 2010), where precipitation only partly recharged the soil water. As another resource support, the availability of groundwater may also become a limiting factor for tree growth under the conditions where high water consumption reduces the water availability of the soil profile. The groundwater resources are plentiful in Finland, but the resources are not distributed evenly across the country (Vehviläinen and Huttunen, 2002). According to the hydrological scenarios designed by Govind et al. (2011) for the topographically driven subsurface flow in boreal ecosystem, we assumed a simple relationship between latitude and groundwater level (Figure 2). This implied that the groundwater level on the upland sites SL and SM was substantially lower than on the site SH. Moreover, the depth of root system of mature Norway spruce is generally shallow, which was parameterised in our model. Therefore, the groundwater table did not reach the rooting zone of Norway spruce over the simulation period, leading to the stress on the growth of trees. Nevertheless, on the lowland sites (such as SH) with enough high groundwater table level, Norway spruce might be capable to adapt relatively well to the expected climate change.

Conclusions

In summary, our simulations showed that the drought episodes may become more frequent in the growing season in southern Finland. On some sites where Norway spruce currently grows well, the climate change might create an environment that is suboptimal for the species in cases of high tree density. As a result, its growth may decline in the regions with poor soil water availability. This implies that appropriate site-specific management is needed to adapt Norway spruce to the changing conditions to sustain its growth. Such options might be, for example, preference for the use of more drought-tolerant genotypes in regeneration, wide spacing in planting, and shorter rotation to mitigate the detrimental effects of changes in water availability.

ACKNOWLEDGMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION AND CONCLUSIONS
  7. ACKNOWLEDGMENTS
  8. REFERENCES

This work was funded through the Finland Distinguished Professor Programme (FiDiPro) (2008–2012) of the Academy of Finland (Project No. 127299-A5060-06) and the ‘Motive’ research programme (EU Grant Agreement 226544) of the European Union. Two anonymous reviewers are greatly appreciated for their constructive criticism and comments on a previous version of this paper. We would like to offer our great thanks to Dr. David Gritten for revising the language of the paper.

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  2. ABSTRACT
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  4. MATERIAL AND METHODS
  5. RESULTS
  6. DISCUSSION AND CONCLUSIONS
  7. ACKNOWLEDGMENTS
  8. REFERENCES
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