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Whether global change will drive changing forests from net carbon (C) sinks to sources relates to how quickly deadwood decomposes. Because complete wood mineralization takes years, most experiments focus on how traits, environments and decomposer communities interact as wood decay begins. Few experiments last long enough to test whether drivers change with decay rates through time, with unknown consequences for scaling short‐term results up to long‐term forest ecosystem projections. Using a 7 year experiment that captured complete mineralization among 21 temperate tree species, we demonstrate that trait effects fade with advancing decay. However, wood density and vessel diameter, which may influence permeability, control how decay rates change through time. Denser wood loses mass more slowly at first but more quickly with advancing decay, which resolves ambiguity about the after‐life consequences of this key plant functional trait by demonstrating that its effect on decay depends on experiment duration and sampling frequency. Only long‐term data and a time‐varying model yielded accurate predictions of both mass loss in a concurrent experiment and naturally recruited deadwood structure in a 32‐year‐old forest plot. Given the importance of forests in the carbon cycle, and the pivotal role for wood decay, accurate ecosystem projections are critical and they require experiments that go beyond enumerating potential mechanisms by identifying the temporal scale for their effects. [ABSTRACT FROM AUTHOR]

Pest outbreaks are driving tree dieback and major influxes of deadwood into forest ecosystems. Understanding how pulses of deadwood impact the climate system requires understanding which factors influence greenhouse gas production during wood decay. Recent analyses identify stem diameter as an important control, but report effects that vary in magnitude and direction. This complexity may reflect interacting effects of soil contact, geometry and variable tissue properties. To dissect these effects, we implemented a three-way factorial experiment in Fraxinus americana, (white ash), an iconic North American species threatened by an invasive beetle. Soil contact accelerated decay rates by an order of magnitude with an effect that varied with stem diameter, not bark presence. After experimentally controlling surface area-to-volume ratio, half-buried wide stems decayed more slowly than half-buried narrow stems but more quickly than the aggregate decay rate of buried and suspended stems. These results closely matched variation in moisture content within and among samples, suggesting that limited vertical conduction of soil moisture through deadwood mediates the effect of stem diameter on wood decay. Soil contact also influenced greenhouse gas concentrations reinforcing recent evidence that deadwood acts as a source for CO2 and CH4 while acting as a sink for N2O. Our results suggest that managing tree species affected by pest outbreaks, including F. americana, for biomass salvage and greenhouse gas mitigation requires understanding traits that mediate wood permeability and diffusivity to soil moisture and greenhouse gases. [ABSTRACT FROM AUTHOR]

Conspecific negative density dependence is thought to maintain diversity by limiting abundances of common species. Yet the extent to which this mechanism can explain patterns of species diversity across environmental gradients is largely unknown. We examined density-dependent recruitment of seedlings and saplings and changes in local species diversity across a soil-resource gradient for 38 woody-plant species in a temperate forest. At both life stages, the strength of negative density dependence increased with resource availability, becoming relatively stronger for rare species during seedling recruitment, but stronger for common species during sapling recruitment. Moreover, negative density dependence appeared to reduce diversity when stronger for rare than common species, but increase diversity when stronger for common species. Our results suggest that negative density dependence is stronger in resource-rich environments and can either decrease or maintain diversity depending on its relative strength among common and rare species. [ABSTRACT FROM AUTHOR]

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