Anke Jentsch

A new generation of climate change experiments : Events, not trends

Intensification of weather extremes is currently emerging as one of the most important facets of climate change. Research on extreme events (“event-focused” in contrast to “trend-focused”) has increased in recent years and, in 2004, accounted for one-fifth of the experimental climate-change studies published. Numerous examples, ranging from microbiology and soil science to biogeography, demonstrate how extreme weather events can accelerate shifts in species composition and distribution, thereby facilitating changes in ecosystem functioning. However, assessing the importance of extreme events for ecological processes poses a major challenge because of the very nature of such events: their effects are out of proportion to their short duration. We propose that extreme events can be characterized by statistical extremity, timing, and abruptness relative to the life cycles of the organisms affected. To test system response to changing magnitude and frequency of weather events, controlled experiments are useful tools. These experiments provide essential insights for science and for societies that must develop coping strategies for such events. Here, we discuss future research needs for climate-change experiments in ecology. For illustration, we describe an experimental plan showing how to meet the challenge posed by changes in the frequency or magnitude of extreme events.

The Search for Generality in Studies of Disturbance and Ecosystem Dynamics

Studies of disturbance have a long tradition in vegetation ecology (Cooper 1926; Raup 1941; White 1979) and have increased lly during the last 30 years (Dayton 1971; Heinselman 1973; Levin and Paine 1974; Borman and Likens 1979; Sousa 1979a,b, 1984; Pickett 1980; Pickett and White 1985; Van der Maarel 1993; Bornette and Amoros 1996; Paine et al. 1998; Freiich and Reich 1999; White et al. 1999). We have learned a tremendous amount about the significance of disturbance as an ecological factor in various habitats and communities (Knapp 1974; Grubb 1977; Miles 1979; Oliver 1981; Pickett and White 1985; Goldberg 1988; Frelich and Lorimer 1991; Milton et al. 1997), about disturbance regimes (Romme 1982; Turner et al. 1993; White et al. 1999), about functional adaptations of plants (Garcia-Mora et al. 1999; Walker et al. 1999), about responses of ecosystems (Bornette and Amoros 1996; Johnson et al. 1998; Engelmark et al. 1999) and about restoring disturbance as an ecosystem process (White and Walker 1997; Covington et al. 1999). During this period, a few theories and synthetic concepts have been proposed, but we do not yet have an inclusive general paradigm for this important body of work.

Biodiversity Increases the Resistance of Ecosystem Productivity to Climate Extremes

It remains unclear whether biodiversity buffers ecosystems against climate extremes, which are becoming increasingly frequent worldwide. Early results suggested that the ecosystem productivity of diverse grassland plant communities was more resistant, changing less during drought, and more resilient, recovering more quickly after drought, than that of depauperate communities. However, subsequent experimental tests produced mixed results. Here we use data from 46 experiments that manipulated grassland plant diversity to test whether biodiversity provides resistance during and resilience after climate events. We show that biodiversity increased ecosystem resistance for a broad range of climate events, including wet or dry, moderate or extreme, and brief or prolonged events. Across all studies and climate events, the productivity of low-diversity communities with one or two species changed by approximately 50% during climate events, whereas that of high-diversity communities with 16–32 species was more resistant, changing by only approximately 25%. By a year after each climate event, ecosystem productivity had often fully recovered, or overshot, normal levels of productivity in both high- and low-diversity communities, leading to no detectable dependence of ecosystem resilience on biodiversity. Our results suggest that biodiversity mainly stabilizes ecosystem productivity, and productivity-dependent ecosystem services, by increasing resistance to climate events. Anthropogenic environmental changes that drive biodiversity loss thus seem likely to decrease ecosystem stability, and restoration of biodiversity to increase it, mainly by changing the resistance of ecosystem productivity to climate events.

Worldwide evidence of a unimodal relationship between productivity and plant species richness

Grassland diversity and ecosystem productivity The relationship between plant species diversity and ecosystem productivity is controversial. The debate concerns whether diversity peaks at intermediate levels of productivity—the so-called humped-back model—or whether there is no clear predictable relationship. Fraser et al. used a large, standardized, and geographically diverse sample of grasslands from six continents to confirm the validity and generality of the humped-back model. Their findings pave the way for a more mechanistic understanding of the factors controlling species diversity. Science, this issue p. 302 The humped-back model of plant species diversity is confirmed by a global grassland survey. The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.

Effects of Extreme Weather Events on Plant Productivity and Tissue Die-Back are Modified by Community Composition

Extreme weather events are expected to increase in frequency and magnitude due to climate change. Their effects on vegetation are widely unknown. Here, experimental grassland and heath communities in Central Europe were exposed either to a simulated single drought or to a prolonged heavy rainfall event. The magnitude of manipulations imitated the local 100-year weather extreme according to extreme value statistics. Overall productivity of both plant communities remained stable in the face of drought and heavy rainfall, despite significant effects on tissue die-back. Grassland communities were more resistant against the extreme weather events than heath communities. Furthermore, effects of extreme weather events on community tissue die-back were modified by functional diversity, even though conclusiveness in this part is limited by the fact that only one species composition was available per diversity level within this case study. More diverse grassland communities exhibited less tissue die-back than less complex grassland communities. On the other side, more diverse heath communities were more vulnerable to extreme weather events compared to less complex heath communities. Furthermore, legumes did not effectively contribute to the buffering against extreme weather events in both vegetation types. Tissue die-back proved a strong stress response in plant communities exposed to 100-year extreme weather events, even though one important ecosystem function, namely productivity, remained surprisingly stable in this experiment. Theories and concepts on biodiversity and ecosystem functioning (insurance hypothesis, redundancy hypothesis) may have to be revisited when extreme weather conditions are considered.

Stochastic trajectories of succession initiated by extreme climatic events

Deterministic or rule-based succession is expected under homogeneous biotic and abiotic starting conditions. Effects of extreme climatic events such as drought, however, may alter these assembly rules by adding stochastic elements. We monitored the succession of species composition of 30 twin grassland communities with identical biotic and abiotic starting conditions in an initially sown diversity gradient between 1 and 16 species over 13 years. The stochasticity of succession, measured as the synchrony in the development of the species compositions of the twin plots, was strongly altered by the extreme warm and dry summer of 2003. Moreover, it was independent from past and present plant diversity and neighbourhood species compositions. Extreme climatic events can induce stochastic effects in community development and therefore impair predictability even under homogeneous abiotic conditions. Stochastic events may result in lasting shifts of community composition, as well as adverse and unforeseeable effects on the stability of ecological services.

Soil biotic processes remain remarkably stable after 100-year extreme weather events in experimental grassland and heath

Climate change will increase the recurrence of extreme weather events such as drought and heavy rainfall. Evidence suggests that extreme weather events pose threats to ecosystem functioning, particularly to nutrient cycling and biomass production. These ecosystem functions depend strongly on below-ground biotic processes, including the activity and interactions among plants, soil fauna, and micro-organisms. Here, experimental grassland and heath communities of three phytodiversity levels were exposed either to a simulated single drought or to a heavy rainfall event. Both weather manipulations were repeated for two consecutive years. The magnitude of manipulations imitated the local 100-year extreme weather event. Heavy rainfall events increased below-ground plant biomass and stimulated soil enzyme activities as well as decomposition rates for both plant communities. In contrast, extreme drought did not reduce below-ground plant biomass and root length, soil enzyme activities, and cellulose decomposition rate. The low responsiveness of the measured ecosystem properties in face of the applied weather manipulations rendered the detection of significant interactions between weather events and phytodiversity impossible. Our data indicate on the one hand the close interaction between below ground plant parameters and microbial turnover processes in soil; on the other hand it shows that the plant–soil system can buffer against extreme drought events, at last for the period of investigation.

The challenge of plant regeneration after fire in the Mediterranean Basin: scientific gaps in our knowledge on plant strategies and evolution of traits

Though observations on re-colonisation of post-fire sites in the Mediterranean Basin are plentiful, there still is an ongoing debate on the interrelation of fire regimes and species traits related to fire adaptation. Most of the studies found are restricted to particular species or claim to present community attributes. Therefore they often lack information for the evaluation of evolutionary evidence and historical contingency of the local fire regime and other abiotic conditions, which may act as selective pressure for plant regeneration strategies. Indeed, knowledge about the success of regeneration mechanisms and their interrelation with ecological factors is essential for the interpretation of the high spatio-temporal variability found in post-fire species performance. Such knowledge would be necessary to assess the potential of different regeneration mechanisms to cope with ongoing land-use and climate change—a crucial scientific challenge.A summary is given of the knowledge about the limits and potential of plant regeneration mechanisms after fire in the Mediterranean Basin, along with corresponding studies conducted in other parts of the world with similar climatic conditions in order to present the fullest possible picture. Moreover, the positive or negative impacts of particular parameters of a fire regime on different regeneration strategies (post-fire seeders, resprouters, and facultative resprouters) are explained and discussed in the light of published literature. To conclude, reference is made to scientific gaps that need to be filled in order to analyse species resistance and community resilience absorbing possible climate or land use changes.

Opposite metabolic responses of shoots and roots to drought

Shoots and roots are autotrophic and heterotrophic organs of plants with different physiological functions. Do they have different metabolomes? Do their metabolisms respond differently to environmental changes such as drought? We used metabolomics and elemental analyses to answer these questions. First, we show that shoots and roots have different metabolomes and nutrient and elemental stoichiometries. Second, we show that the shoot metabolome is much more variable among species and seasons than is the root metabolome. Third, we show that the metabolic response of shoots to drought contrasts with that of roots; shoots decrease their growth metabolism (lower concentrations of sugars, amino acids, nucleosides, N, P, and K), and roots increase it in a mirrored response. Shoots are metabolically deactivated during drought to reduce the consumption of water and nutrients, whereas roots are metabolically activated to enhance the uptake of water and nutrients, together buffering the effects of drought, at least at the short term.

Late frost sensitivity of juvenile Fagus sylvatica L. differs between southern Germany and Bulgaria and depends on preceding air temperature

Fagus sylvatica, the dominant native forest tree species of Central Europe, is sensitive to late frost events. Advanced leaf flushing due to climate warming may lead to more frequent frost damage in the future. Here, we explore local adaptation to late frost events at both continental and regional scales and test how moderate climate warming (+1.5°C) affects late frost sensitivity. Short-term leaf injury and height growth after a late frost event were quantified in a common garden experiment with 2-year-old F. sylvatica seedlings. The fully crossed three-factorial design consisted of a late frost manipulation, a continuous warming manipulation and selected provenances (three provenances from western Bulgaria and three from southern Germany). Late frost led to leaf injury and reduced height growth (−7%). Provenances differed in their late frost sensitivity at the regional scale, and local adaptation was detected. At the larger scale, the Bulgarian provenances showed reduced height growth (−17%), while the German provenances did not exhibit growth reduction. The warming treatment prevented late frost damage, while height growth declined by 19% in the reference temperature treatment. This surprising finding was attributed to advanced leaf maturity in the warming treatment. The impact of late frost events on F. sylvatica in a warmer world will depend on timing. An event that damages leaves immediately after leaf flushing appears negligible a few days earlier or later, thereby complicating projections. Local adaptation to late frost is evident at a regional scale. Management strategies should aim at maximizing genetic diversity to adapt to climate change.