Resilience of stand structure and tree species diversity in subtropical forest degraded by clear logging

Abstract

Subtropical forests in the Ryukyu Islands have been degraded by silvicultural practices, and thus their structural attributes are being shifted to other states dominated by a few tree species. This study clarified the mechanisms of the change, and examined the effect of clear logging on the resilience of a subtropical forest. Sprouting regeneration and typhoon disturbance were introduced into an individual-based model, SEIB-DGVM, for describing stand development and succession. The regeneration dynamics from young secondary to old-growth stands were reproduced fairly well with the model. Sprouting recruitment produced high stem density at the beginning of stand development, which caused a self-thinning trajectory following the −3/2 power law. In the late development stage after 70 years, tree species diversity fluctuated because of the regenerative response of sprouting species and the facilitatory effect of typhoon disturbance on the coexistence of subordinate species. The death of canopy trees because of typhoon disturbances reduced the dominance of Castanopsis sieboldii, and depressed its dominance in the understory. Consequently, the understory species could establish by virtue of fallen canopy trees, and tree species diversity increased at the stand level. Clear logging experiments in the model revealed that species diversity deteriorated, especially in the stand dominated by sprouting species. Resilience of subtropical forests was determined by initial species composition before clear logging. Our simulation results suggest that repeated logging drives subtropical forests with high species diversity to a stand monopolized by C. sieboldii.

Keywords:

• SEIB-DGVM
• Species diversity
• Sprouting
• Typhoon disturbance
• Wood density

Acknowledgments

We would like to express our sincere thanks to Dr Hisashi Sato, Frontier Research Center for Global Change (FRCGC), the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), for his advice and technical support. We thank the staff of University Forest, Faculty of Agriculture, University of the Ryukyus, for providing us with the climate data for the subtropical forest. This study is partly supported by the research project “Interactions between Natural Environment and Human Social Systems in Subtropical Islands” of the Research Institute for Humanity and Nature, and was also funded by the project of Science and Technology Promotion Division of Okinawa Prefecture.