The relationship between rates of lignin and cellulose decay in aboveground forest litter

We examined the rates of lignin decay (dC3/dt) and holocellulose decay (dC2/dt) in aboveground leaf litter of predominately northern conifer forests to test our hypothesis that the rate of lignin decay is a linear function of the lignocellulose index (LCI = lignin/[lignin + holocellulose]). We proposed that lignin decays only when LCI > 0.4 and that LCI cannot exceed 0.7, approximating humus. These constraints suggest that holocellulose content of litter is a control on lignin decay. We evaluated this hypothesis with a set of 112 field studies in two ways, by: (1) comparing simulations from a decomposition model to observations across the full range of litter decay and (2) analyzing the relative rate of lignin decay ([dC3/dt]/[dC2/dt + dC3/dt]) as a function of litter LCI. Simulated dynamics of LCI in decaying litter were highly correlated to observed patterns, particularly when litter fractions of water and ethanol solubles from model output were pooled with holocellulose fractions (mean R2 = 0.87 ± 0.02, P < 0.01). More detailed analyses of 64 of these studies yielded variable relationships between lignin decay rate and litter LCI; a regression based on pooled data (N = 385; total number of observations) from these studies produced a slope and an intercept that were not significantly different from predicted (slope = 2.33, intercept = −0.93). Both site and litter characteristics had significant effects on the proposed LCI threshold for lignin decay (LCI ∼ 0.4), but no effects on slope or intercept, suggesting that the proposed lignocellulose control hypothesis is relatively robust across a range of litter and forest types used in this study.