Data from: Hollows in living trees develop slowly but considerably influence the estimate of forest biomass

The decomposition of wood inside living tree hollows influences forest structure and processes. Although the decomposition rate controls the formation of hollows, it has not previously been measured. In an old-growth subtropical montane evergreen broad-leaved forest in south-west China, we measured respiration rates of decaying wood inside living tree hollows, logs (downed tree trunks) and snags (standing dead trees) using infrared CO2 analysis. We compared stem radial growth rates to the horizontal expansion rates of hollows. We also estimated the proportion of hollows to total stem volume and the time required to form nesting hollows. Overall, decaying wood inside living tree hollows had significantly higher wood density but lower a moisture content and respiration rate than logs. Wood moisture content and wood density significantly affected the respiration rates of wood inside tree hollows and logs and snags. The average respiration rate of wood inside tree hollows (0·16 ± 0·02 μmol kg−1 s−1) was 49% lower than in logs and 34% lower than in snags. The proportion of stem volume occupied by hollows was 8·0% in this forest. The radial expansion rate of tree hollows was 2·0 mm year−1, which was slightly less than the stem radial growth rate. The rate of vertical expansion of hollows was eight times higher than radial expansion. At these rates, it would require 50–100 years for the formation of hollows large enough for honeybees Apis cerana cerana to nest. We conclude that wood decomposition inside tree hollows is more strongly limited by moisture content than in logs, and a very long time is required to form nesting hollows by decomposition. However, the decomposition of wood inside living tree hollows can considerably influence estimates of stem biomass in old-growth forests.

活树树洞内部的木材分解过程会影响森林结构与生态过程。尽管分解速率决定了树洞的形成，但此前尚未有相关测量。本研究在中国西南部一处亚热带山地常绿阔叶林原始林内开展，采用红外CO₂分析法，测定了活树树洞、倒木（downed tree trunks）以及枯立木（standing dead trees）内部腐朽木材的呼吸速率。我们将树干径向生长速率与树洞的水平扩张速率进行了对比，同时估算了树洞占树干总体积的比例，以及形成筑巢树洞所需的时间。总体而言，活树树洞内部的腐朽木材相较于倒木，具有显著更高的木材密度，但含水率与呼吸速率更低。木材含水率与木材密度对树洞、倒木及枯立木内部木材的呼吸速率均存在显著影响。活树树洞内部木材的平均呼吸速率为(0.16±0.02) μmol·kg⁻¹·s⁻¹，较倒木低49%，较枯立木低34%。该林分中树洞占树干体积的比例为8.0%。树洞的径向扩张速率为2.0 mm·a⁻¹，略低于树干径向生长速率；而树洞的垂直扩张速率是径向扩张速率的8倍。按照该扩张速率，形成足够中华蜜蜂（Apis cerana cerana）筑巢的树洞需要50至100年。本研究表明，相较于倒木，活树树洞内部的木材分解更易受含水率限制，且通过分解形成可供筑巢的树洞需要极长的时间。然而，活树树洞内部的木材分解会显著影响原始林树干生物量的估算结果。