• Energy Research

Fine roots (diameter ≤ 2 mm) contribute significantly to the forest carbon cycle and are essential for resource acquisition from the soil. We conducted a study to assess the relationships between tree and ground vegetation fine root biomass and tree species diversity (monocultures compared to 2–5 species mixtures), conifer proportion and other site factors (stand basal area, soil carbon stocks and C:N ratio) in the six major European forest types, boreal forest in Finland, temperate forests in Poland, Germany and Romania, thermophilous deciduous forests in Italy, and Mediterranean forests in Spain. We sampled the fine roots of trees and ground vegetation to the depth of 20 cm in the mineral soil and allocated the fine root biomass to individual tree species using near-infrared reflectance spectroscopy (NIRS). We did not find any general positive effects of tree species diversity on the fine root biomass of trees or ground vegetation across the forest types and tree species combinations. However, our results suggest that tree fine root biomass increases with tree species diversity in pure broadleaf forests, but not in pure conifer forests. Species diversity explained 7% of the variation in tree fine root biomass in the broadleaf stands. The narrow tree species diversity gradient (1–2 species) in the conifer forests compared to the broadleaf forests (1−4) may have decreased the probability of conifer species combinations with below-ground functional traits conducive to over-yielding. Some evidence of diversity-mediated changes in the vertical rooting patterns of broadleaf trees and ground vegetation were found within the entire organic and 0–20 cm mineral soil layer although the weighted mean depth of fine root biomass was not affected. Negative diversity effects were found in the organic layer and positive diversity effects in the 0–10 cm mineral soil layer for broadleaf tree fine root biomass. Diversity effects were negative for ground vegetation fine root biomass in the 0–10 cm mineral soil layer. There was a general positive effect of conifer proportion on total fine root biomass in the organic layer, but not in the mineral soil layers. In general conifer proportion and site factors explained more of the variation in tree fine root biomass than tree species diversity. More research covering the annual variation in fine root biomass and deeper soil layers is needed before recommending managing species-rich forest for increasing below-ground biomass and carbon pools. The research received funding from the European Union Seventh Framework Program (FP7/2007–2013) under grant agreement n° 265171, FunDivEUROPE. Peer Reviewed

Abstract Winter precipitation and soil freeze–thaw events have been predicted to increase in boreal regions with climate change. This may expose tree roots to waterlogging (WL) and soil freezing (Fr) more than in the current climate and therefore affect tree growth and survival. Using a whole-tree approach, we studied the responses of silver birch (Betula pendula Roth.) saplings, growing in mineral soil, to 6-week Fr and WL in factorial combinations during dormancy, with accompanying changes in soil gas concentrations. Physiological activation (dark-acclimated chlorophyll fluorescence and chlorophyll content index) and growth of leaves and shoot elongation and stem diameter growth started earlier in Fr than NoFr (soil not frozen). The starch content of leaves was temporarily higher in Fr than NoFr in the latter part of the growing season. Short and long root production and longevity decreased, and mortality increased by soil Fr, while there were no significant effects of WL. Increased fine root damage was followed by increased compensatory root growth. At the beginning of the growing season, stem sap flow increased fastest in Fr + WL, with some delay in both NoWL (without WL) treatments. At the end of the follow-up growing season, the hydraulic conductance and impedance loss factor of roots were higher in Fr than in NoFr, but there were no differences in above- and belowground biomasses. The concentration of soil carbon dioxide increased and methane decreased by soil Fr at the end of dormancy. At the beginning of the growing season, the concentration of nitrous oxide was higher in WL than in NoWL and higher in Fr than in NoFr. In general, soil Fr had more consistent effects on soil greenhouse gas concentrations than WL. To conclude, winter-time WL alone is not as harmful for roots as WL during the growing season.

Air temperatures and precipitation are predicted to increase in the future, especially at high latitudes and particularly so during winter. In contrast to air temperatures, changes in soil temperatures are more difficult to predict, as the fate of the insulating snow cover is crucial in this respect. Soil conditions can also be affected by rain-on-snow events and warm spells during winter, resulting in freeze-thaw cycles, compacted snow, ice encasement and local flooding. These adverse conditions during winter could counteract the otherwise positive effects of climate change on forest growth and productivity. For studying the effects of different winter and snow conditions on young Downy birch (Betula pubescens Ehrh.) seedlings, we carried out a laboratory experiment with birch seedlings subjected to four different winter scenarios: snow covering the seedlings (SNOW), compressed snow and ice encasement (ICE), flooded and frozen soil (FLOOD) and no snow at all (NO SNOW). After the winter treatments we simulated a spring and early summer period of 9.5 weeks, and monitored the growth by measuring shoot and root biomass of the seedlings, and starch and soluble sugar concentrations. We also assessed the stress experienced by the seedlings by measuring leaf chlorophyll fluorescence and gas exchange. Although no difference in mortality was observed between the treatments, the seedlings in the SNOW and ICE treatments had significantly higher shoot and root biomass compared with those in the FLOOD and NO SNOW treatments. We found higher starch concentrations in roots of the seedlings in the SNOW and ICE treatments, compared with those in the FLOOD and NO SNOW treatments, although photosynthesis did not differ. Our results suggest a malfunction of carbohydrate distribution in the seedlings of the FLOOD and NO SNOW treatments, probably resulting from decreased sinks. The results underline the importance of an insulating and protecting snow cover for small tree seedlings, and that future winters with changed snow pattern might affect the growth of tree seedlings and thus possibly species composition and forest productivity.

Future climate scenarios predict increased air temperatures and precipitation, particularly at high latitudes, and especially so during winter. Soil temperatures, however, are more difficult to predict, since they depend strongly on the fate of the insulating snow cover. 'Rain-on-snow' events and warm spells during winter can lead to thaw-freeze cycles, compacted snow and ice encasement, as well as local flooding. These adverse conditions could counteract the otherwise positive effects of climatic changes on forest seedling growth. In order to study the effects of different winter and snow conditions on young Scots pine (Pinus sylvestris L.) seedlings, we conducted a laboratory experiment in which 80 1-year-old Scots pine seedlings were distributed between four winter treatments in dasotrons: ambient snow cover (SNOW), compressed snow and ice encasement (ICE), flooded and frozen soil (FLOOD) and no snow (NO SNOW). During the winter treatment period and a 1.5-month simulated spring/early summer phase, we monitored the needle, stem and root biomass of the seedlings, and determined their starch and soluble sugar concentrations. In addition, we assessed the stress experienced by the seedlings by measuring chlorophyll fluorescence, electric impedance and photosynthesis of the previous-year needles. Compared with the SNOW treatment, carbohydrate concentrations were lower in the FLOOD and NO SNOW treatments where the seedlings had almost died before the end of the experiment, presumably due to frost desiccation of aboveground parts during the winter treatments. The seedlings of the ICE treatment showed dead needles and stems only above the snow and ice cover. The results emphasize the importance of an insulating and protecting snow cover for small forest tree seedlings, and that future winters with changed snow patterns might affect the survival of tree seedlings and thus forest productivity.

Tree roots comprise a huge carbon pool. Their dynamics are driven by environmental factors and thereby affected by climate change. We studied the effects of soil temperature on root and shoot phenology and their linkages in Norway spruce (Picea abies (L.) Karst.). Saplings were grown in controlled-environment rooms for three simulated growing seasons (GS1, GS2, and GS3). Soil-temperature treatments of 9, 13, 18, and 21 °C were applied during GS2. Root growth was monitored with minirhizotrons and commenced in all treatments simultaneously. Temporal growth patterns of short and long roots were usually bimodal. Root growth was very low during the coldest treatment of GS2 but increased during GS3 as an aftereffect. During GS3, growth of short roots continued later after colder treatments than warmer treatments. Reduced sink strength of roots and increased carbohydrate accumulation into needles at 9 °C during GS2 probably enabled compensatory root growth under restored temperatures during GS3. Soil temperature did not affect shoot phenology, and root and shoot phenology varied between growing seasons; thus, the linkage of root and shoot phenology was inconsistent. In warmer soil, root longevity was shorter and turnover rate was higher than in colder soil. This can further affect soil carbon dynamics and ecosystem carbon cycling in boreal forest ecosystems.

AbstractThe importance of biodiversity in supporting ecosystem functioning is generally well accepted. However, most evidence comes from small‐scale studies, and scaling‐up patterns of biodiversity–ecosystem functioning (B‐EF) remains challenging, in part because the importance of environmental factors in shaping B‐EF relations is poorly understood. Using a forest research platform in which 26 ecosystem functions were measured along gradients of tree species richness in six regions across Europe, we investigated the extent and the potential drivers of context dependency of B‐EF relations. Despite considerable variation in species richness effects across the continent, we found a tendency for stronger B‐EF relations in drier climates as well as in areas with longer growing seasons and more functionally diverse tree species. The importance of water availability in driving context dependency suggests that as water limitation increases under climate change, biodiversity may become even more important to support high levels of functioning in European forests.