Abstract
Correlating plant litter decay rates with initial tissue traits (e.g. C, N contents) is common practice, but in woody litter, predictive relationships are often weak. Variability in predicting wood decomposition is partially due to territorial competition among fungal decomposers that, in turn, have a range of nutritional strategies (rot types) and consequences on residues. Given this biotic influence, researchers are increasingly using culture-independent tools in an attempt to link variability more directly to decomposer groups. Our goal was to complement these tools by using certain wood modifications as ‘signatures’ that provide more functional information about decomposer dominance than density loss. Specifically, we used dilute alkali solubility (DAS; higher for brown rot) and lignin:density loss (L:D; higher for white rot) to infer rot type (binary) and fungal nutritional mode (gradient), respectively. We first determined strength of pattern among 29 fungi of known rot type by correlating DAS and L:D with mass loss in birch and pine. Having shown robust relationships for both techniques above a density loss threshold, we then demonstrated and resolved two issues relevant to species consortia and field trials, 1) spatial patchiness creating gravimetric bias (density bias), and 2) brown rot imprints prior or subsequent to white rot replacement (legacy effects). Finally, we field-tested our methods in a New Zealand Pinus radiata plantation in a paired-plot comparison. Overall, results validate these low-cost techniques that measure the collective histories of decomposer dominance in wood. The L:D measure also showed clear potential in classifying ‘rot type’ along a spectrum rather than as a traditional binary type (brown versus white rot), as it places the nutritional strategies of wood-degrading fungi on a scale (L:D=0-5, in this case). These information-rich measures of consequence can provide insight into their biological causes, strengthening the links between traits, structure, and function during wood decomposition.
Highlights
Fungi extract and metabolize wood carbohydrates using a spectrum of nutritional modes [1]
These nutritional strategies range in terms of selectivity for lignin removal [2] but have traditionally been delineated in a binary way as brown or white rot types, respectively [3, 4]
Birch and pine degraded over a 12-wk time series by fungi with a range of wood-degrading nutritional modes showed a robust relationship between rot type and DAS (Fig. 1a and b)
Summary
Fungi extract and metabolize wood carbohydrates using a spectrum of nutritional modes [1]. A third type (soft rot) occurs in certain environments, often in harsh conditions for aerobic fungi (e.g., [5]) These wood-degrading fungi vary along a gradient of lignin selectivity, with some fungi removing little lignin (brown rot), some removing lignin at similar rates to carbohydrate removal (simultaneous white rot) and others removing lignin >4x faster than carbohydrates (selective white rot) [2, 4]. This spectrum of nutritional strategies, regardless of how it is categorized, leaves signatures in wood residue lignin content that offer more insight into decomposer history than density loss can provide, alone [4]. This flexibility leaves forests vulnerable to shifts in rot dominance, with significant potential consequences in boreal forests where ~16.1 Pg C (22.1% of global deadwood C) is stored in the deadwood of relatively few tree species [13]
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