Structural, compositional, and historical factors alter tree-to-region scaling
The shifting prevalence between resource and sink limitation along our temperate-to-alpine gradient interacts with an array of additional locally-variable factors, altering how tree-level constraints scale to regional productivity. At this broader scale, a negative association emerges between mean synchrony and mean BAI, implying a net reduction in forest productivity with increased climate stress. Stem density negatively influenced both BAI and synchrony, further implying that distributing resources over more individuals reduces growth and climate responsiveness. The detected positive associations between basal area, BAI and synchrony imply heightened productivity in plots with a larger proportion of dominant stems, and may further reflect the tendency for density-dependent processes to thin stands as dominant trees increase in size, reducing competitive interactions (Després et al. 2017).Similarly, growth synchrony increases with the severity of each location’s maximum disturbance and time since this most extreme event, suggesting that more developmentally advanced forests have thinned intensely interacting trees, and mean BAI was higher in more recently and more severely disturbed plots.
Species diversity had a negative effect on plot-level growth synchrony, reflecting how species with different niches diverge in their responses to seasonal climate variability (e.g., Jucker et al. 2016, del Rio et al. 2020). Thus, diversity likely buffers forest productivity against stressful climate extremes. However, even though mixing Picea andFagus in plantations can often lead to positive diversity effects via complementarity (Pretzsch et al. 2010), niche differences in climate sensitivity did not translate into an apparent effect of diversity on mean growth at our Carpathian sites. Throughout Europe, positive diversity-productivity effects emerge on the most climatically stressful locations, with weakly negative diversity effects previously detected in temperate Carpathian forests (Jucker et al. 2016). In the Carpathians, the most diverse plots occur at the range limits of the two most abundant species, where competitive suppression of Picea is especially pronounced and Fagus is rather climatically stressed, highlighting the importance of considering underlying assembly processes when estimating diversity effects (Mori et al. 2018).