Tree biodiversity in northern forests shows temporal stability over 35 years at different scales, levels and dimensions

Abstract

1. At global scales, species richness is declining. However, at local scales, understanding exactly how, where and why biodiversity is changing becomes challenging since researchers have assessed biodiversity trends using different indicators, data sources and methods (e.g. repeated measurements at the same site over time vs. space-for-time substitutions).2. In this study, we present a multifaceted analysis of biodiversity change by assessing how tree diversity in Québec, Canada changed between two sampling periods (1970-1977 and 2005-2016), in regards to different: levels of diversity (alpha diversity, temporal turnover and spatial beta diversity), dimensions of diversity (taxonomic, functional and phylogenetic), metrics of diversity (presenceabsence and abundance based), and spatial scales of analysis (plot, 50, 100 and 200 km). We then assess how well potential drivers of biodiversity change (climate change and land cover change) explain the observed changes in alpha diversity. Since the data came from plots that remained forested over the course of the study, we used historical land cover change data and scenario analyses to test whether results from forest plots were likely to be representative of the broader landscape.3. Across all levels, dimensions, metrics and spatial scales of analysis, we found either increases or no net change in diversity over time, with wide distributions of values around the mean. Presence-absence metrics often indicated increases in diversity over time, while abundance-based metrics were more likely to show no net change. Potential drivers such as climate change and land cover change explained only a small fraction of the variation in alpha diversity change (i.e. why particular sites experienced positive vs. negative change) at the plot scale (adjusted R 2 ≈ 0.03), but a greater fraction at coarser spatial scales (adjusted R 2 of ~0.10 to ~0.50). Results from these forest plots are likely representative of the diversity change within the study region, since estimates of alpha diversity change only became negative under scenarios with the most extreme disturbance impacts.4. Synthesis. None of our indicators showed evidence of declines in alpha or beta diversity of trees in temperate and boreal forests in Quebec (except for simulations with extremely high forest loss), but we did find temporal turnover in

Recent tree diversity increase in NE Iberian forests following intense management release: A task for animal‐dispersed and drought‐tolerant species

“…Overall, the passive increase in tree species diversity reported here is in line not only with the signs of recovery of mixed forests reported in other studies varying in their spatial and temporal scale (Sánchez de Dios et al., 2023; Vayreda et al., 2013, 2016 for the Iberian Peninsula) but also in other biomes (e.g. Crockett et al., 2022 for northern forests), a pattern that has been attributed to both climate and forest management changes. In our region, the fact that the higher increase in tree species diversity occurred mostly in long‐established, non‐recently managed forests and those that initially exhibited lower values of basal area (Tables S4–S9; Figure 3) reflects a trend towards the recovery of tree diversity, after a well‐known history of extensive land‐use changes and intense forest management that resulted in the impoverishment of tree diversity and the homogenization of forest composition (Cervera et al., 2019; Sánchez de Dios et al., 2023).…”

“…R 2 < 20% in Basnou et al, 2016; R 2 < 15% in Cruz-Alonso et al, 2021; R 2 < 5% in García-Valdés et al, 2021). This suggests the relevance that stochastic community processes (Crockett et al, 2022) or filters imposed by past disturbances events (Espelta et al, 2020) may have in shaping tree species diversity at small spatial scales, in comparison with more F I G U R E 4 Recruitment events (%) of tree species from inside or outside the plots for the tree (a) and regeneration (b) layers for the subset of plots where tree species richness increased. Species were classified according to their dispersal mode (animal vs. other methods) and drought and shade tolerance (Figure S2).…”

“…We then performed a permutational analysis of variance to test for systematic temporal turnover (i.e. overall directional shifts in composition) between both time periods for the abundance and the presence/absence of tree species composition, for the 3141 plots sampled (see Crockett et al., 2022 for a similar approach).…”

“…See Figure 4 for cluster codes. evident changes at the landscape level (Crockett et al, 2022;Cruz-Alonso et al, 2021;Espelta et al, 2020). In addition, the fact that the moderate diversity increase was mostly driven by changes in species richness, rather than by changes in evenness (i.e.…”

“…Due to the growing interest in the management and environmental factors shaping tree species diversity and the availability of databases resampling the same stands through long periods of time (e.g. National Forest Inventories), an increasing number of studies have examined the trends in tree species diversity through time at regional scales (Crockett et al, 2022;Sánchez de Dios et al, 2023;Vayreda et al, 2013Vayreda et al, , 2016. Interestingly, although they cover different biogeographic regions, all these studies point out towards an increase in tree species diversity during the last decades of the 20th century onwards, reporting from moderate (Crockett et al, 2022) to rapid shifts (Vayreda et al, 2016), mainly driven by the effects of climate warming and land-use changes.…”