The relationship between tree size and tree resource use: Is competition size symmetric or asymmetric?

Relationships between tree size and resource uptake are important when linking patterns observed at the individual level to those at the stand level, and therefore when upscaling from the individual to the stand. They also indicate how plants of different sizes may be competing for resources. Size-uptake relationships were examined and reviewed in the 1990’s e.g. Schwinning and Weiner (1998) and Casper and Jackson (1997). However, these early studies are often misinterpreted as evidence that competition for light is size-asymmetric, such that larger trees acquire more light per unit size than smaller trees, while competition for soil resources (water and nutrients) is size-symmetric, such that all trees acquire a similar amount of resource per unit size. This is despite the authors explaining that these are likely to be oversimplifications.

The objective of this study was to examine relationships between tree size and light absorption or transpiration to examine whether these relationships were as consistent as often assumed.

Main findings

* The choice of size variable is critical when examining these relationships (1)

*It is often assumed that competition for light is size-asymmetric and belowground competition is size-symmetric. However, this appears to be an oversimplification. Asymmetric aboveground competition appears to be based on an assumption that shading by taller trees will cause shorter trees to absorb less light, relative to their size, than taller trees. However, light absorption does not only depend on tree size (e.g. leaf area) and shading, it also depends on crown architecture, leaf angle distributions, leaf morphology and the spatial positioning of taller trees relative to shorter trees. The assumption of size-asymmetric aboveground competition for light may not be appropriate when these vary with tree size or in response to shading, which they sometimes do. Therefore, in contrast to the assumption of size-asymmetric competition for light, there are many empirical examples of size-symmetric aboveground competition for light (1).

*In relation to soil resources, competition is sometimes assumed to be size-symmetric because it is assumed that most roots can access and acquire the same amount of resource. However, belowground competition is not necessarily size-symmetric because soil water and nutrient availability can be very patchy spatially and temporally and the spatial and temporal distribution of water and nutrient uptake can vary between tree sizes. Indeed, size-transpiration relationships sometimes imply size-asymmetric competition (1,2). Therefore, assuming that aboveground competition is usually size-asymmetric and that belowground competition is usually size-symmetric is not consistent with empirical evidence (1,2).

*A related misconception is that size-growth relationships are reliable indicators of the symmetry or asymmetry of competition for resources. However, while size-growth relationships are partly the result of size-uptake relationships, the production ecology equation indicates that size-growth relationships also depend on size-resource use efficiency relationships, which can be affected by many processes that depend on tree size (1). Size-growth relationships are often unreliable proxies for size-uptake and competition (1).

Journal articles related to this project:

1. Forrester, D.I., (2019). Linking forest growth with stand structure: Tree size inequality, tree growth or resource partitioning and the asymmetry of competition. Forest Ecology and Management 447, 139-157. doi:10.1016/j.foreco.2019.05.053

2. Forrester, D.I., Limousin, J-M., Pfautsch, S. (2022). The relationship between tree size and tree water use: Is competition for water size-symmetric or size-asymmetric? Tree Physiology 42, 1916-1927 doi:10.1093/treephys/tpac018