Competitive asymmetry in a monospecific stand of the shade-tolerant tree Fagus crenata depends on gap formation

Abstract Competitive asymmetry is one of the most important determinants of size structure in plant communities. Many studies indicated that light competition is size-asymmetric in monospecific stands of herbaceous plants, as taller individuals preempt light. However, competition is size-symmetric in multispecific stands because shorter species adapted to shade. Then, a question arises on whether light competition is asymmetric or symmetric if a shade-tolerant species occupies the stand, such as climax forests. 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. In cool temperate and natural climax forest of northern Japan, dominated by a shade-tolerant tree, Fagus crenata , we investigated the plant size, growth rate, and three-dimensional distribution of foliage and light availability. 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 leaf area per aboveground mass (leaf area ratio; 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.