Ontogeny influences tree growth response to soil fertility and neighborhood crowding in an old-growth temperate forest.

BACKGROUND AND AIMS
Abiotic and biotic factors simultaneously affect tree growth and thus shape community structure and dynamics. In particular, trees of different size classes showed different growth response to soil nutrients and neighborhood crowding, but our understanding of how species joint responses to these factors vary between size classes remains limited in multi-storied temperate forests. Here, we investigated size class differences in tree growth response to soil gradients and neighborhood crowding in an old-growth temperate forest.


METHODS
We combined growth data over 15 years from 38,902 individuals of 42 tree species with trait data in a 25-ha temperate forest plot in northeast China. We built hierarchical Bayesian models of tree growth to examine the effects of soil gradients and neighborhood crowding between size classes and canopy types.


KEY RESULTS
We found that soil and neighbors mainly acted separately in shaping tree growth in small and large trees. Soil total nitrogen and phosphorus increased tree growth in small trees of understory species, but decreased that of large trees of canopy species. Neighbors reduced tree growth in both tree size classes, with stronger effects on large than small trees, and on canopy than understory species. Furthermore, small trees with higher specific leaf area grew faster in fertile soils, and small trees with less seed mass grew faster in crowded neighbor. Large trees with higher specific leaf area, specific root length and less seed mass grew faster in crowded environments, while traits have limited influence on tree growth responded to soil gradients.


CONCLUSIONS
Our study highlights the importance of size class in modulating the response of tree growth to soil and neighbors, and the differential role of species canopy types and functional traits in capturing these effects in large vs. small trees.