Competitive asymmetry in a forest composed of a shade-tolerant species depends on gap formation

Abstract

Competitive asymmetry is one of the most important determinants of size structure in plant communities. Light competition was believed to be size-asymmetric, as taller individuals preempt light. In fact, many studies indicated that light competition is size-asymmetric in monospecific stands of herbaceous plants. However, competition is size-symmetric in multispecific stands because shorter, shade-adapted species compensate for the low light availability through higher leaf area per aboveground mass (leaf area ratio; LAR) or higher light use efficiency for growth (LUE). Then, a question arises on whether light competition is asymmetric or symmetric if the stand is occupied by a shade-tolerant species. Moreover, it is also interesting how gap dynamics affect competitive asymmetry, given the fact that gap formation can improve light availability in lower layers of the canopy. We investigated the plant size, growth rate, and three-dimensional distribution of foliage and light availability in a natural forest dominated by a shade-tolerant tree, Fagus crenata. We found that competitive asymmetry varied dynamically across the forest. In closed canopies, the relative growth rate (RGR) was higher in taller trees, indicating size-asymmetric growth. Shorter trees partly compensated for their light interception by having higher LAR but did not achieve comparative RGR to taller trees. Conversely, in gaps, the RGR of shorter trees was equivalent to those of taller trees because of the improved light availability for smaller trees, indicating size-symmetric growth. These results suggest that changes in competitive asymmetry driven by gap dynamics can influence the spatiotemporal diversity of size structures in forests.