Can Current Moisture Responses Predict Soil CO2 Efflux Under Altered Precipitation Regimes? A Synthesis of Manipulation Experiments

As a key component of the carbon cycle, soil CO2 efflux (SCE) is being increasingly studied to improve our mechanistic understanding of this important carbon flux. Predicting ecosystem responses to climate change often depends on extrapolation of current relationships between ecosystem processes and their climatic drivers to conditions not yet experienced by the ecosystem. This raises the question to what extent these relationships remain unaltered beyond the current climatic window for which observations are available to constrain the relationships. Here, we evaluate whether current responses of SCE to fluctuations in soil temperature and soil water content can be used to predict SCE under altered rainfall patterns. Of the 58 experiments for which we gathered SCE data, 20 were discarded because either too few data were available, or inconsistencies precluded their incorporation in the analyses. The 38 remaining experiments were used to test the hypothesis that a model parameterized with data from the control plots (using soil temperature and water content as predictor variables) could adequately predict SCE measured in the manipulated treatment. Only for seven of these 38 experiments, this hypothesis was rejected. Importantly, these were the experiments with the most reliable datasets, i.e., those providing high-frequency measurements of SCE. Accordingly, regression tree analysis demonstrated that measurement frequency was crucial; our hypothesis could be rejected only for experiments with measurement intervals of less than 11 days, and was not rejected for any of the 24 experiments with larger measurement intervals. This highlights the importance of high-frequency measurements when studying effects of altered precipitation on SCE, probably because infrequent measurement schemes have insufficient capacity to detect shifts in the climate-dependencies of SCE. We strongly recommend that future experiments focus more strongly on establishing response functions across a broader range of precipitation regimes and soil moisture conditions. Such experiments should make accurate measurements of water availability, they require high-frequency SCE measurements and they should consider both instantaneous responses and the potential legacy effects of climate extremes. This is important, because we demonstrated that at least for some ecosystems, current moisture responses cannot be extrapolated to predict SCE under altered rainfall

Shifts in the regeneration niche of an endangered tree (Acer opalus ssp. granatense) during ontogeny: Using an ecological concept for application

Most of our knowledge regarding ontogenetic niche shifts in plants has been derived from studies involving only two or unconnected life stages. Approaches covering a broader range of different life stages are still needed to fully understand the implications of ontogenetic niche shifts for plant regeneration dynamics. We investigated ontogenetic shifts in the endangered Mediterranean tree species Acer opalus ssp. granatense (A. opalus) comparing the environmental characteristics of individuals of different ages with those of a random sample of available microsites. In addition, since herbivory could be a limiting factor, herbivory damage was quantified. Differences in environmental characteristics between locations of individual plants and randomly selected points became larger with plant age, suggesting that the regeneration niche of A. opalus shifts during ontogeny, undergoing a contraction. The presence of shrubs and adult trees, the depth of the litter layer, and herbivory were the main factors influencing these changes. A. opalus can germinate in all available microhabitats its seeds can reach, but saplings establish and grow only in a subset of microhabitats, which represent a change in tolerance to extrinsic factors. Old saplings establish under the canopy of shrubs, far away from tree cover that could block light required in the oldest stage. Therefore, temporal changes in the nature and strength of plant¿plant interactions are also occurring. The ecological concept of niche shifts reveals the microsites with higher probabilities of long-term sapling survival of A. opalus: shrub cover involves an array of environmental changes crucial for the successful establishment of A. opalus saplings under stressful Mediterranean conditions, from microhabitat amelioration to herbivory protection

Biocrusts mitigate the impact of aridity on soil microbial communities in drylands: evidence from observations across three continents.

Data from "M. Delgado-Baquerizo, F.T. Maestre, D.J. Eldridge, M.A. Bowker, T.C. Jeffries, B.K. Singh. Biocrusts mitigate the impact of aridity on soil microbial communities in drylands: evidence from observations across three continents.". The spreadsheet "Dataset" contains information on the microbial community composition, diversity and functions for two microsites across 39 locations from three continents. The spreadsheet "Metadata" contains the associated metadata, where a description of all the variables and units can be found

Do biotic interactions shape both sides of the humped-back model of species richness in plant communities?

A humped-back relationship between species richness and community biomass has frequently been observed in plant communities, at both local and regional scales, although often improperly called a productivity–diversity relationship. Explanations for this relationship have emphasized the role of competitive exclusion, probably because at the time when the relationship was first examined, competition was considered to be the significant biotic filter structuring plant communities. However, over the last 15 years there has been a renewed interest in facilitation and this research has shown a clear link between the role of facilitation in structuring communities and both community biomass and the severity of the environment. Although facilitation may enlarge the realized niche of species and increase community richness in stressful environments, there has only been one previous attempt to revisit the humped-back model of species richness and to include facilitative processes. However, to date, no model has explored whether biotic interactions can potentially shape both sides of the humped-back model for species richness commonly detected in plant communities. Here, we propose a revision of Grime's original model that incorporates a new understanding of the role of facilitative interactions in plant communities. In this revised model, facilitation promotes diversity at medium to high environmental severity levels, by expanding the realized niche of stress-intolerant competitive species into harsh physical conditions. However, when environmental conditions become extremely severe the positive effects of the benefactors wane (as supported by recent research on facilitative interactions in extremely severe environments) and diversity is reduced. Conversely, with decreasing stress along the biomass gradient, facilitation decreases because stress-intolerant species become able to exist away from the canopy of the stress-tolerant species (as proposed by facilitation theory). At the same time competition increases for stress-tolerant species, reducing diversity in the most benign conditions (as proposed by models of competition theory). In this way our inclusion of facilitation into the classic model of plant species diversity and community biomass generates a more powerful and richer predictive framework for understanding the role of plant interactions in changing diversity. We then use our revised model to explain both the observed discrepancies between natural patterns of species richness and community biomass and the results of experimental studies of the impact of biodiversity on the productivity of herbaceous communities. It is clear that explicit consideration of concurrent changes in stress-tolerant and competitive species enhances our capacity to explain and interpret patterns in plant community diversity with respect to environmental severit

Can we infer plant facilitation from remote sensing? A test across global drylands

Facilitation is a major force shaping the structure and diversity of plant communities in terrestrial ecosystems. Detecting positive plant-plant interactions relies on the combination of field experimentation and the demonstration of spatial association between neighboring plants. This has often restricted the study of facilitation to particular sites, limiting the development of systematic assessments of facilitation over regional and global scales. Here we explore whether the frequency of plant spatial associations detected from high-resolution remotely-sensed images can be used to infer plant facilitation at the community level in drylands around the globe. We correlated the information from remotely-sensed images freely available through Google EarthTM with detailed field assessments, and used a simple individual-based model to generate patch-size distributions using different assumptions about the type and strength of plant-plant interactions. Most of the patterns found from the remotely-sensed images were more right-skewed than the patterns from the null model simulating a random distribution. This suggests that the plants in the studied drylands show stronger spatial clustering than expected by chance. We found that positive plant co-occurrence, as measured in the field, was significantly related to the skewness of vegetation patch-size distribution measured using Google EarthTM images. Our findings suggest that the relative frequency of facilitation may be inferred from spatial pattern signals measured from remotely-sensed images, since facilitation often determines positive co-occurrence among neighboring plants. They pave the road for a systematic global assessment of the role of facilitation in terrestrial ecosystems