Effects of an extended drought period on physiological properties of grassland species in the field

A very high percentage (around 70%) of the agronomic area in Switzerland is covered by grasslands at various altitudes where environmental conditions, management, community structure and productivity vary widely. As heat waves and drought are predicted to increase in future climate, survival of plant species in grasslands is a major issue of concern in Central Europe. The effect of summer drought on representative grasslands in Switzerland was studied through drought experiments (using rain-out shelters avoiding natural precipitation) to understand the response of predominant species to changed climatic conditions. The physiological performance (gas exchange, leaf water potential) of selected species was investigated at three locations in Switzerland. The pre-dawn leaf water potential of all species was lower (more negative) under the dryer conditions at the three sites. Net photosynthesis and stomatal conductance of forb and legume species did not show major changes under drought, while grass species showed large decreases at the lowland site. These differences between forb-legume and grass species were not observed at the pre-alpine and alpine site. The apparent drought tolerance of the forb-legume species seems to be due—at least partially—to increased water use efficiency under drought condition

Vapor-pressure deficit and extreme climatic variables limit tree growth

Assessing the effect of global warming on forest growth requires a better understanding of species-specific responses to climate change conditions. Norway spruce and European beech are among the dominant tree species in Europe and are largely used by the timber industry. Their sensitivity to changes in climate and extreme climatic events, however, endangers their future sustainability. Identifying the key climatic factors limiting their growth and survival is therefore crucial for assessing the responses of these two species to ongoing climate change. We studied the vulnerability of beech and spruce to warmer and drier conditions by transplanting saplings from the top to the bottom of an elevational gradient in the Jura Mountains in Switzerland. We (1) demonstrated that a longer growing season due to warming could not fully account for the positive growth responses, and the positive effect on sapling productivity was species-dependent, (2) demonstrated that the contrasting growth responses of beech and spruce were mainly due to different sensitivities to elevated vapor-pressure deficits (VPD), (3) determined the species-specific limits to VPD above which growth rate began to decline, and (4) demonstrated that models incorporating extreme climatic events could account for the response of growth to warming better than models using only average values. These results support that the sustainability of forest trees in the coming decades will depend on how extreme climatic events will change, irrespective of the overall warming trend

Loss of testate amoeba functional diversity with increasing frost intensity across a continental gradient reduces microbial activity in peatlands

Soil microbial communities significantly contribute to global fluxes of nutrients and carbon. Their response to climate change, including winter warming, is expected to modify these processes through direct effects on microbial functions through osmotic stress, and changing temperature regimes. Using four European peatlands reflecting different frequencies of frost events, we show that peatland testate amoeba communities diverge among sites with different winter climates, and that this is reflected through contrasting functions. We found that exposure to harder soil frost promoted species β-diversity (species turnover) thus shifting the community composition of testate amoebae. In particular, we found that harder soil frost, and lower water-soluble phenolic compounds, induced functional turnover through the decrease of large species (-68%, > 80 μm) and the increase of small-bodied mixotrophic species (i.e. Archerella flavum; +79%). These results suggest that increased exposure to soil frost could be highly limiting for large species while smaller species are more resistant. Furthermore, we found that β-glucosidase enzymatic activity, in addition to soil temperature, strongly depended (R2 = 0.95, ANOVA) of the functional diversity of testate amoebae. Changing winter conditions can therefore strongly impact peatland decomposition process, though it remains unclear if these changes are carried–over to the growing season

The effect of warmer or cooler winters on European beech phenology carries over the phenophases of next growing season

Phenology of trees of temperate regions has become an important research topic for global change issues over the last two decades because of its major role in regulating ecosystems processes such as carbon cycle and species interactions. While spring phenology has often been studied, autumn phenology is still poorly understood. Most of the studies focus on specific phenophases, such as budburst or flowering, and generally do not consider the carry over effect of a phenophase on following ones during the same annual cycle (e.g. budburst and bud set). Using beech saplings in a reciprocal transplanting experiment between two elevations in the Swiss Jura, we hypothesized that: (i) cooler or warmer winter conditions would affect the following spring and autumn phenophases, as well as the spring phenology of the following year. (ii) These changes could be related to non-structural carbohydrates (NSC) storage and/or to a shift in the dormancy release driven by chilling and forcing temperature requirements. Our results showed that the advance and the delay of budburst timing induced by warmer and cooler winter conditions, respectively, has impacted the following autumn phenology: an advanced spring budburst has led to an earlier autumn bud set. This suggests that the potential delay in senescence processes due to global warming might be smaller than expected and that the assumption of longer growing season does not necessarily apply. The dependency of these two phenophases might be explained by the building up of the NSC storage. Furthermore, no carry over effect was found between the advanced or the delayed budburst timing in 2014 and the spring phenology of 2015, indicating that the following year, the current climatic variables were the dominant drivers of spring phenology. Adapting phenological models to the whole annual phenological cycle would improve predictions of tree phenology under future climate warming conditions

Effects of an Extended Drought Period on Grasslands at Various Altitudes in Switzerland: A Field Study

From recent investigations, it can be concluded that extreme events (e.g. heat waves, extreme drought periods) will become more relevant in Central Europe during the next decades (Schär et al. 2004). The effects of an artificial drought period (installation of rain shelters for 10 weeks) on physiological traits of representative grasslands species was investigated at two altitudes (400 and 1000 m a. s. l.) in Switzerland. The net assimilation rate (Pn) and stomatal conductance (gs) were affected in Phleum pratense and in other gramineae at both altitudes, while these effects were in dicots (Rumex obtusifolius, Trifolium repens) relevant only at the higher altitude. The decline of Pn was parallelled by a decrease of gs , but the intrinsic efficiency of photosystem II was not affected by the treatment. The still high Pn of Rumex in lowland under drought may explain the dominance of this plant at the end of the drought period. Differences in the species composition and differences in reversible and irreversible damages caused by drought in the various species may contribute to the overall response of grasslands in a site-specific manner

Effects of an extended drought period on grasslands at various altitudes in Switzerland

From recent investigations, it can be concluded that extreme events (e.g. heat waves, extreme drought periods) will become more relevant in Central Europe during the next decades (Schär et al. 2004). The effects of an artificial drought period (installation of rain shelters for 10 weeks) on physiological traits of representative grasslands species was investigated at two altitudes (400 and 1000 m a. s. l.) in Switzerland. The net assimilation rate (Pn) and stomatal conductance (gs) were affected in Phleum pratense and in other gramineae at both altitudes, while these effects were in dicots (Rumex obtusifolius, Trifolium repens) relevant only at the higher altitude. The decline of Pn was parallelled by a decrease of gs , but the intrinsic efficiency of photosystem II was not affected by the treatment. The still high Pn of Rumex in lowland under drought may explain the dominance of this plant at the end of the drought period. Differences in the species composition and differences in reversible and irreversible damages caused by drought in the various species may contribute to the overall response of grasslands in a site-specific manner

Non-stomatal limitations of photosynthesis in grassland species under artificial drought in the field

As drought stress is expected to occur more frequently in future climate in central Europe, survival and productivity of grassland species are an important issue. Non-stomatal limitation processes related to the drought-stress inhibition of photosynthesis of selected grassland species were analysed at three locations using leaf gas exchange and chlorophyll fluorescence. The effect of an artificial drought on the non-stomatal limitations differed considerably between species present in the same grassland plot. The maximum efficiency of photosystem II (Fv/Fm), indicator for the intactness of the photosynthetic electron transport, showed only small differences under drought. On the other hand, more pronounced effects were observed for the carboxylation velocity of Rubisco (Vc,max). Vc,max was in Phleum pratense about 20% lower under drought than in control plants, while other species in the same plot were far less affected. The carboxylation velocity of Rubisco is highly sensitive to water deficit and might represent a tool to evaluate the drought response of various species in order to address the performance of grasslands. © 2010 Elsevier B.V