Using warming tolerances to predict understory plant responses to climate change.

Climate change is pushing species towards and potentially beyond their critical thermal limits. The extent to which species can cope with temperatures exceeding their critical thermal limits is still uncertain. To better assess species' responses to warming, we compute the warming tolerance (ΔTniche ) as a thermal vulnerability index, using species' upper thermal limits (the temperature at the warm limit of their distribution range) minus the local habitat temperature actually experienced at a given location. This metric is useful to predict how much more warming species can tolerate before negative impacts are expected to occur. Here we set up a cross-continental transplant experiment involving five regions distributed along a latitudinal gradient across Europe (43° N-61° N). Transplant sites were located in dense and open forests stands, and at forest edges and in interiors. We estimated the warming tolerance for 12 understory plant species common in European temperate forests. During 3 years, we examined the effects of the warming tolerance of each species across all transplanted locations on local plant performance, in terms of survival, height, ground cover, flowering probabilities and flower number. We found that the warming tolerance (ΔTniche ) of the 12 studied understory species was significantly different across Europe and varied by up to 8°C. In general, ΔTniche were smaller (less positive) towards the forest edge and in open stands. Plant performance (growth and reproduction) increased with increasing ΔTniche across all 12 species. Our study demonstrated that ΔTniche of understory plant species varied with macroclimatic differences among regions across Europe, as well as in response to forest microclimates, albeit to a lesser extent. Our findings support the hypothesis that plant performance across species decreases in terms of growth and reproduction as local temperature conditions reach or exceed the warm limit of the focal species.

Forest understorey communities respond strongly to light in interaction with forest structure, but not to microclimate warming.

Forests harbour large spatiotemporal heterogeneity in canopy structure. This variation drives the microclimate and light availability at the forest floor. So far, we do not know how light availability and sub-canopy temperature interactively mediate the impact of macroclimate warming on understorey communities. We therefore assessed the functional response of understorey plant communities to warming and light addition in a full factorial experiment installed in temperate deciduous forests across Europe along natural microclimate, light and macroclimate gradients. Furthermore, we related these functional responses to the species' life-history syndromes and thermal niches. We found no significant community responses to the warming treatment. The light treatment, however, had a stronger impact on communities, mainly due to responses by fast-colonizing generalists and not by slow-colonizing forest specialists. The forest structure strongly mediated the response to light addition and also had a clear impact on functional traits and total plant cover. The effects of short-term experimental warming were small and suggest a time-lag in the response of understorey species to climate change. Canopy disturbance, for instance due to drought, pests or logging, has a strong and immediate impact and particularly favours generalists in the understorey in structurally complex forests.

Drivers of a riparian forest specialist (Carex remota, Cyperaceae): it is not only a matter of soil moisture.

UNLABELLED: • PREMISE OF THE STUDY: Plants respond to the prevailing conditions in the surrounding environment, but since they are dynamic systems this response may vary during their life. Thus, the identification of key aspects for the maintenance of plant populations requires the consideration of plant performance across environmental gradients and along life stages. This study examines how abiotic conditions and biotic interactions and processes determine the spatial distribution of two life-story stages that play a key role in the functioning of a representative population of Carex remota.• METHODS: We used structural equation modeling (SEM) to test for direct and indirect influences of abiotic and biotic factors on seedlings and adults of Carex remota. The variables used in the analysis were number of seedlings, cover of adults, soil moisture, leaf litter cover, relative light, and topographic position.• KEY RESULTS: Population patterns partially depend on direct and indirect effects of abiotic conditions. Whereas adult individuals were only affected by topsoil moisture, seedling emergence was largely affected by multiple environmental conditions. The number of seedlings increased with high topsoil moisture, low leaf-litter values, high light values as well as in low parts of the study area. The importance of adult individuals in determining seedling success is also highlighted: higher abundance provides seed rain in the surroundings and modifies the microenvironmental conditions favoring high seedling establishment.• CONCLUSIONS: As hypothesized, adults and seedlings responded to the environmental conditions differently. Seedling emergence was a critical aspect in C. remota performance, and abrupt changes in the environment during this stage may strongly influence population performance.