Prediction of tree age distribution based on survival analysis in natural forests: a case study of preserved permanent plots in the University of Tokyo Hokkaido Forest, northern Japan

Abstract

In forests, tree mortality is strongly determined by complex interactions between multiple biotic and abiotic factors, and analysis of tree mortality is widely implemented in forest management. However, age-based tree mortality remains poorly evaluated quantitatively at the stand scale of the uneven-aged forests. The objective of this study is to predict the age distribution of living and dead trees based on survival analyses. We used a combination of tree-ring and census data from the two preserved permanent plots in the University of Tokyo Hokkaido Forest in panmixed and sub-boreal natural forests, Hokkaido, northern Japan, to derive site-specific survival models. All the living trees (diameter at breast height ≥ 5 cm in 2009) were targeted to identify tree ages using a RESISTOGRAPH, a semi non-destructive device. Periodical tree age data with a 10year age class were used during the observation periods of 2009-2019, and all the changes (i.e., death and new in-growth) during the periods were recorded. We found the time stabilities of survival functions between periods in advance. The results showed that the parametric survival analysis with Weibull distribution successfully yielded the mortality rate, mortality probability, and survival probability in each plot. Finally, we predicted the future age class distribution of living and dead trees of each plot based on the survival analyses results and discussed its management implications. We recommended that the estimated mean lifetime was facilitated to make decisions on the selection of harvesting trees in the uneven forest management based on selective cutting.