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.
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.