Astigarraga, J. | bioRxiv Science

bioRxiv2024

TOP-DOWN CLIMATIC PROCESSES MODULATE BIODIVERSITY-FUNCTIONING RELATIONSHIPS ACROSS NORTH AMERICAN FORESTS

Experimental evidence indicates that more diverse communities sustain higher levels of ecosystem functioning. Generally, bottom-up processes accounting for community selection and complementarity effects on biodiversity are used to explain biodiversity effects on ecosystem functioning. However, top-down macroecological processes can also influence the distribution of ecosystem functions across large environmental gradients and, importantly, biodiversity patterns. Here, we tested whether past climate instability, current climate, and climate-driven species abundance explain tree richness gradients, and therefore, determine the relationship between species richness and ecosystem functioning using 137,808 forest inventory plots across continental extents including forests of Canada, the US, and Mexico. Consistent with previous research, we found a positive monotonic relationship between species richness and biomass (measured as level stand basal area) across North American forests. However, structural equation models revealed that the indirect effect of past climate instability, current climate conditions, and climate-driven individuals abundance underlie species richness patterns and the positive covariation between species richness and biomass. The importance of top-down climatic processes varied from boreal to tropical regions, yet their significant effects were pervasive across all forest types, indicating that climate strongly modulates the strength of the relationships between species richness and basal area. Our results help to understand the assembly processes driving the ubiquitous positive monotonic relationship between species richness and basal area across continental extents. This knowledge, obtained from adopting a macroecological perspective, helps understanding biodiversity-functioning relationships, which are usually explained by mechanisms at the community level. Significance statementMore biodiverse communities sustain higher levels of ecosystem functions (BEF hypothesis). The role of biotic interactions among trees within ecological communities determining BEF has been widely studied, but it is not well known how past climatic variability, current climate, and species abundance determine BEF. Using 137,808 forest inventory plots across North American forests located in Canada, the US, and Mexico, we document a positive relationship between tree species richness and stand basal area. Detailed analysis revealed that the positive BEF relationship is underlined by a strong and indirect effect of past and current climate. Our results highlight the importance of top-down climate processes operating at macroecological scales to understand biodiversity-functioning relationships.

ecology↗

bioRxiv2023

Abundance of Northern Hemisphere tree species declines in the warm and arid regions of their climatic niches

Climate change is expected to drive species towards colder and wetter regions of their distribution with alternative processes such as forest management having the potential to alter species displacements. Here, using data from more than two million monitored trees from 73 widely-distributed species, we quantify changes in tree species abundance across Northern Hemisphere forests and find a widespread decline in abundance across the whole of species climatic niches. Yet, our analysis revealed that this decline is heavily influenced by alterations at the stand-level and consequent stand development. Remarkably, when accounting for stand development, our findings show a consistent trend of species abundance optimum shifting towards cold and wet regions within their climatic niches. We provide species-specific information on the direction and magnitude of climate-driven changes in abundance that should be taken into account when designing conservation, management and restoration plans in an era of unprecedented human-caused environmental change.

ecology↗

bioRxiv2023

Forest structural diversity determines tree growth synchrony in response to climate change

After centuries of deforestation, many industrialised countries are experiencing an increase in forest area and biomass due to changes in land- and forest-use since the mid-20th century. At the same time, the impacts of climate change on forests are aggravating, but the interplay between past land- and forest-use (i.e. land- and forest-use legacies) and climate change in forest functioning remains elusive. Here using network theory and linear mixed models, we quantified how land- and forest-use legacies modulate tree growth synchrony in response to climate change. We analysed tree growth data from European beech (Fagus sylvatica L.) stands with different histories of forest management at the species rear edge. We found that tree growth synchrony increased following heatwaves, late spring frosts, and reduced precipitation. Interestingly, the greatest tree growth synchrony occurred in recently-established forests, while stands containing large trees and heterogeneous tree sizes showed much lower growth synchrony. Our results highlight the importance of maintaining large trees and structurally heterogeneous forests to mitigate the negative effects of climate change on forest productivity, and thereby, increase forest resilience to future forest climate risks.

ecology↗