Different intra- and interspecific facilitation mechanisms between two Mediterranean trees under a climate change scenario

Revisiting the classical biodiversity-ecosystem functioning and stability relationships in microbial microcosms

The question of how biodiversity influences ecosystem functioning and stability has been a central focus in ecological research. Yet, this question remains unresolved, primarily because of the widely divergent definitions of functioning, stability, and diversity. Consequently, forecasts of ecosystem services will remain speculative until we can establish more precise and comprehensive definitions for these concepts than previously. Here, we investigated how the maximum specific growth rate, productivity, mortality rate, and species interaction in microbial communities vary with a diversity gradient ranging from 1 to 16 species under control conditions, starvation, or saline stress. We found that diversity played a critical role in maintaining community growth and stability under control conditions, with higher diversity associated with increased maximum specific growth rate and decreased mortality rate. However, higher diversity was associated with an increased mortality rate under starvation, while diversity did not affect the mortality rate under saline stress. Diversity stabilized microbial productivity only under control conditions, defying the "diversity begets stability" hypothesis under stress. Beneficial interactions among species were prevalent in most samples, but species interaction increased mortality rates under starvation. Our findings suggest that while biodiversity is crucial for preserving ecosystem functioning and stability, the presence of multiple definitions and contextual dependence on environmental conditions argues against any general relationship between diversity and ecosystem functioning/stability. Furthermore, we provide new insights into the longstanding debate surrounding the "diversity begets stability" hypothesis and the "diversity destabilizes ecosystem" hypothesis in that diversity begets stability under control conditions but destabilizes ecosystems under severe stress.

Both Mature Patches and Expanding Areas of Juniperus thurifera Forests Are Vulnerable to Climate Change But for Different Reasons

Research Highlights: Water use efficiency (WUE) varied along a gradient of Juniperus thurifera (L.) forest expansion, being higher in recently colonised areas. Background and Objectives: WUE is a classic physiological process of plants that reflects the compromise between carbon assimilation and water loss and has a profound influence on their performance in water-limited environments. Forest expansion in Mediterranean regions associated with land abandonment can influence the WUE of plants due to the existence of two opposing gradients: one of favourable–unfavourable environmental conditions and another one of increased–decreased intraspecific competition, the former increasing and the latter decreasing towards the expanding front. The main objective of this study was to elucidate how the WUE of Juniperus thurifera varied along the stages of forest expansion and to provide insight on how this variation is influenced by intraspecific competition and abiotic factors. Materials and Methods: Seventeen plots at different distances from the mature forest core were selected at three sites located in the centre of the Iberian Peninsula. For 30 individuals within each plot, we measured biometric characteristics, age, tree vigour, and C/N ratio in leaves, and the leaf carbon isotope signature (δ13C (‰)) as a proxy for WUE. Around each individual, we scored the percentage cover of bare soil, stoniness, conspecifics, and other woody species. Results: WUE of J. thurifera individuals varied along the forest expansion gradient, being greater for the individuals at the expanding front than for those at the mature forest. WUE was influenced by the cover of conspecifics, tree age, and C/N ratio in leaves. This pattern reveals that less favourable environmental conditions (i.e., rocky soils and higher radiation due to lower vegetation cover) and younger trees at the expanding front are associated with increased WUE. The increased cover of conspecifics decreases irradiance at the mature forest, involving milder stress conditions than at the expanding front. Conclusions: Lower WUE in mature forests due to more favourable conditions and higher WUE due to abiotic stress at expanding fronts revealed high constraints on water economy of this tree species in these two contrasting situations. Climate change scenarios bringing increased aridity are a serious threat to Juniperus thurifera forests, affecting both mature and juvenile populations although in different ways, which deserve further research to fully unveil.