Tree hazards compounded by successive climate extremes after masting in a small endemic tree, Distylium lepidotum, on subtropical islands in Japan.

Ongoing global warming increases the frequency and severity of tropical typhoons and prolonged drought, leading to forest degradation. Simultaneous and/or successive masting events and climatic extremes may thus occur frequently in the near future. If these climatic extremes occur immediately after mass seed reproduction, their effects on individual trees are expected to be very severe because mass reproduction decreases carbohydrate reserves. While the effects of either a single climate extreme or masting alone on tree resilience/growth have received past research attention, understanding the cumulative effects of such multiple events remains challenging and is crucial for predicting future forest changes. Here, we report tree hazards compound by two successive climate extremes, a tropical typhoon and prolonged drought, after mass reproduction in an endemic tree species (Distylium lepidotum Nakai) on oceanic islands. Across individual trees, the starch stored within the sapwood of branchlets significantly decreased with reproductive efforts (fruit mass/shoot mass ratio). Typhoon damage significantly decreased not only the total leaf area of apical shoots but also the maximum photosynthetic rates. During the 5-month period after the typhoon, the mortality of large branchlets (8-10-mm diameter) increased with decreasing stored starch when the typhoon hit. During the prolonged summer drought in the next year, the recovery of total leaf area, stored starch, and hydraulic conductivity was negatively correlated with the stored starch at the typhoon. These data indicate that the level of stored starch within branchlets is the driving factor determining tree regrowth or dieback, and the restoration of carbohydrates after mass reproduction is synergistically delayed by such climate extremes. Stored carbohydrates are the major cumulative factor affecting individual tree resilience, resulting in their historical effects. Because of highly variable carbohydrate levels among individual trees, the resultant impacts of such successive events on forest dieback will be fundamentally different among trees.