Phenotypic differentiation within a foundation grass species correlates with species richness in a subalpine community

The effects of species loss on ecosystems depend on the community’s functional diversity (FD). However, how FD responds to environmental changes is poorly understood. This applies particularly to higher trophic levels, which regulate many ecosystem processes and are strongly affected by human-induced environmental changes. We analyzed how functional richness (FRic), evenness (FEve), and divergence (FDiv) of important generalist predators—epigeic spiders—are affected by changes in woody plant species richness, plant phylogenetic diversity, and stand age in highly diverse subtropical forests in China. FEve and FDiv of spiders increased with plant richness and stand age. FRic remained on a constant level despite decreasing spider species richness with increasing plant species richness. Plant phylogenetic diversity had no consistent effect on spider FD. The results contrast with the negative effect of diversity on spider species richness and suggest that functional redundancy among spiders decreased with increasing plant richness through non-random species loss. Moreover, increasing functional dissimilarity within spider assemblages with increasing plant richness indicates that the abundance distribution of predators in functional trait space affects ecological functions independent of predator species richness or the available trait space. While plant diversity is generally hypothesized to positively affect predators, our results only support this hypothesis for FD—and here particularly for trait distributions within the overall functional trait space—and not for patterns in species richness. Understanding the way predator assemblages affect ecosystem functions in such highly diverse, natural ecosystems thus requires explicit consideration of FD and its relationship with species richness.     

The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes

Questions: 1. Is there a primary role of disturbance at local scale and of environmental stress at regional scale? 2. Does disturbance increase or decrease environmental stress at local scale? Location: The Atlantic coastal dune system of the Aquitaine Region (France). Methods: Species biomass and 16 environmental variables were sampled in 128 quadrats along a local beach‐inland gradient and a regional North‐South gradient. Environmental data were analysed with ANOVAs and vegetation‐environment relationships with Canonical Correspondence Analysis. Results: At the local scale community composition was primarily driven by disturbance due to sand burial, whereas water and nutrient stress better explained regional differences. However, random biogeographical events are very likely to also affect community composition at the largest scale. The main interaction between environmental stress and disturbance was the mitigation of nutrient stress induced by disturbance at a local scale. This was due to a positive direct effect of sand burial and a positive indirect effect of wind (decrease in VPD by ocean spray). Although wind had also a significant effect on soil conductivity and pH, there was no evidence that these factors had any role in community composition. Conclusions: Our results support the hypothesis that disturbance had a primary role at local scale and environmental stress at regional scale but further research is needed to separate the effect of stress from that of dispersal at regional scale. We also demonstrated that environmental stress in primary succession may not always decline with decreasing disturbance.     

Regional Variability and Drivers of Below Ice CO<Subscript>2</Subscript> in Boreal and Subarctic Lakes

Northern lakes are ice-covered for considerable portions of the year, where carbon dioxide (CO₂) can accumulate below ice, subsequently leading to high CO₂ emissions at ice-melt. Current knowledge on the regional control and variability of below ice partial pressure of carbon dioxide (pCO₂) is lacking, creating a gap in our understanding of how ice cover dynamics affect the CO₂ accumulation below ice and therefore CO₂ emissions from inland waters during the ice-melt period. To narrow this gap, we identified the drivers of below ice pCO₂ variation across 506 Swedish and Finnish lakes using water chemistry, lake morphometry, catchment characteristics, lake position, and climate variables. We found that lake depth and trophic status were the most important variables explaining variations in below ice pCO₂ across the 506 lakes_. Together, lake morphometry and water chemistry explained 53% of the site-to-site variation in below ice pCO₂. Regional climate (including ice cover duration) and latitude only explained 7% of the variation in below ice pCO₂. Thus, our results suggest that on a regional scale a shortening of the ice cover period on lakes may not directly affect the accumulation of CO₂ below ice but rather indirectly through increased mobility of nutrients and carbon loading to lakes. Thus, given that climate-induced changes are most evident in northern ecosystems, adequately predicting the consequences of a changing climate on future CO₂ emission estimates from northern lakes involves monitoring changes not only to ice cover but also to changes in the trophic status of lakes.     

Importance,but not intensity of plant interactions relates to species diversity under the interplay of stress and disturbance

The lack of clarity on how the intensity and importance of plant interactions change under the co‐occurrence of stress and disturbance strongly impedes assessing the relative importance of plant interactions for species diversity. We addressed this issue in subalpine grasslands of the French Pyrenees. A natural soil moisture gradient further experimentally stretched at both ends was used and a mowing disturbance treatment was applied at each position along the soil moisture gradient. Changes in intensity and importance of plant interactions were assessed by a neighbour removal experiment using four target ecotypes. A structural equation modelling approach was used to assess the relative impact of stress, disturbance, the intensity and importance of plant interactions on diversity at both the neighbourhood and community scales. Without mowing, changes in intensity and importance of plant interactions only diverged in the dry part of the soil moisture gradient. The intensity of plant interactions linearly shifted from competition to facilitation with increasing stress, while the importance followed a hump‐shaped relationship. Species diversity components were tightly related to the importance of plant interactions only, both the neighbourhood and community scales. Mowing disturbance strongly reduced the importance of facilitation along the soil moisture gradient, and suppressed the relationship between the importance of plant interactions and diversity components. Together, our results highlight that 1) the importance is the best predictor of variations in species diversity in this subalpine herbaceous system, and 2) that fine‐scale processes such as plant interactions can affect the entire plant communities. Finally, our results suggest that high level of constraints due to co‐occurring stress and disturbance can inhibit the effects of plant interactions on species diversity, highlighting their potential role in regulating diversity and the maintenance/extinction of plant communities. Synthesis How plant interactions change along environmental gradients is an unsolved debate, particularly when both stress and disturbance interact. This lack of clarity explains why the relative impact of plant interactions (intensity and importance) on species diversity has been rarely assessed. Using an experimental approach, we found that the importance of plant interactions highly contributed to variation in species diversity, confirming that neighbourhood scale processes such as plant interactions can affect the entire plant communities. The co‐occurrence of stress and disturbance inhibited the effects of plant interactions, highlighting that plant interactions may regulate drops of diversity and the maintenance/extinction of plant communities.     

The interplay of stress and mowing disturbance for the intensity and importance of plant interactions in dry calcareous grasslands

Background and Aims There is still debate regarding the direction and strength of plant interactions under intermediate to high levels of stress. Furthermore, little is known on how disturbance may interact with physical stress in unproductive environments, although recent theory and models have shown that this interplay may induce a collapse of plant interactions and diversity. The few studies assessing such questions have considered the intensity of biotic interactions but not their importance, although this latter concept has been shown to be very useful for understanding the role of interactions in plant communities. The objective of this study was to assess the interplay between stress and disturbance for plant interactions in dry calcareous grasslands. Methods A field experiment was set up in the Dordogne, southern France, where the importance and intensity of biotic interactions undergone by four species were measured along a water stress gradient, and with and without mowing disturbance. Key Results The importance and intensity of interactions varied in a very similar way along treatments. Under undisturbed conditions, plant interactions switched from competition to neutral with increasing water stress for three of the four species, whereas the fourth species was not subject to any significant biotic interaction along the gradient. Responses to disturbance were more species-specific; for two species, competition disappeared with mowing in the wettest conditions, whereas for the two other species, competition switched to facilitation with mowing. Finally, there were no significant interactions for any species in the disturbed and driest conditions. Conclusions At very high levels of stress, plant performances become too weak to allow either competition or facilitation and disturbance may accelerate the collapse of interactions in dry conditions. The results suggest that the importance and direction of interactions are more likely to be positively related in stressful environments.