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).