Abstract
Abstract. Fungal decay of heart wood creates hollows and areas of reduced wood density within the stems of living trees known as stem rot. Although stem rot is acknowledged as a source of error in forest aboveground biomass (AGB) estimates, there are few data sets available to evaluate the controls over stem rot infection and severity in tropical forests. Using legacy and recent data from 3180 drilled, felled, and cored stems in mixed dipterocarp forests in Sarawak, Malaysian Borneo, we quantified the frequency and severity of stem rot in a total of 339 tree species, and related variation in stem rot with tree size, wood density, taxonomy, and species' soil association, as well as edaphic conditions. Predicted stem rot frequency for a 50 cm tree was 53 % of felled, 39 % of drilled, and 28 % of cored stems, demonstrating differences among methods in rot detection ability. The percent stem volume infected by rot, or stem rot severity, ranged widely among trees with stem rot infection (0.1–82.8 %) and averaged 9 % across all trees felled. Tree taxonomy explained the greatest proportion of variance in both stem rot frequency and severity among the predictors evaluated in our models. Stem rot frequency, but not severity, increased sharply with tree diameter, ranging from 13 % in trees 10–30 cm DBH to 54 % in stems ≥ 50 cm DBH across all data sets. The frequency of stem rot increased significantly in soils with low pH and cation concentrations in topsoil, and stem rot was more common in tree species associated with dystrophic sandy soils than with nutrient-rich clays. When scaled to forest stands, the maximum percent of stem biomass lost to stem rot varied significantly with soil properties, and we estimate that stem rot reduces total forest AGB estimates by up to 7 % relative to what would be predicted assuming all stems are composed strictly of intact wood. This study demonstrates not only that stem rot is likely to be a significant source of error in forest AGB estimation, but also that it strongly covaries with tree size, taxonomy, habitat association, and soil resources, underscoring the need to account for tree community composition and edaphic variation in estimating carbon storage in tropical forests.
Highlights
Fungal rot of secondary xylem causes hollows and regions of reduced wood density in tree stems
The discrepancy in the prevalence of stem rot between the central Sarawak and Lambir data sets may be caused in part by differences in the tree sizes sampled: at Lambir 78 % of the cored trees were < 30 cm diameter at breast height (DBH), whereas 96 % of trees drilled or felled in Central Sarawak were ≥ 30 cm DBH (Fig. 2; Fig. S4)
We investigated these possibilities by sub-setting the data to include the same size range (DBH ≥ 30 cm) across all data sets and testing for differences between data sets in the probability of a tree having stem rot, while accounting for DBH
Summary
Fungal rot of secondary xylem causes hollows and regions of reduced wood density in tree stems. This type of fungal infection, commonly referred to as stem rot, is important for the structure, dynamics, and functioning of forests, given that it may increase tree mortality The effect of stem rot on aboveground biomass is of particular importance for efforts to map carbon storage in tropical regions as part of global conservation and climate change mitigation strategies (Saatchi et al, 2011). Because stem rot is difficult to detect by non-destructive means, we understand little about what controls its frequency and severity, especially in tropical forests
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