Wood decay resistance moderates the effects of tree mortality on carbon storage in the indigenous forests of New Zealand

Abstract

The maintenance of carbon (C) storage in indigenous forest is a key component of efforts to manage atmospheric carbon dioxide concentrations. Increased pressures from extreme climatic events and invasive pests and pathogens pose major threats to the future stability of C storage in indigenous forests through elevated canopy-tree mortality. We assessed the potential for interspecific differences in wood decay resistance to moderate decadal-scale net C losses following canopy tree mortality. We recorded tree mortality, growth and recruitment over a period spanning almost 40years in repeatedly surveyed plots spanning a wide range of mortality rates. We combined these survey data with national data on species-specific wood decay resistance (i.e. retention of wood density) to estimate contemporary C lost through decay of trees that died during our study. We also included C losses from CWD contributed by a major synchronous mortality event before the study period (legacy CWD C loss) for a subset of the plots where CWD C storage measurements were available. C flux from live to dead biomass (1.36Mgha−1year−1, s.e. 0.16) was the main factor influencing estimated net contemporary changes in C storage, with the largest net contemporary C losses (−1.5Mgha−1year−1) observed in plots experiencing high mortality. Estimated net contemporary C loss from tree mortality was reduced when the dominant species had highly decay-resistant wood. The ability to predict contemporary changes in C was significantly improved when a plot-level indicator of CWD decay resistance was included in multiple regressions. Mean legacy CWD C loss was 0.39Mgha−1year−1, s.e. 0.16. When legacy losses were incorporated in net C change estimates, both the size of the legacy CWD pool and its interaction with legacy CWD decay resistance explained a significant amount of variation in net C change in multiple regressions. In plots losing large (around 3Mgha−1year−1) amounts of C from live biomass (or with more than 300Mgha−1 C stored in legacy CWD at the start of the study) wood decay resistance altered the net C balance by as much as 1.11Mgha−1year−1, which is a considerable effect given that the mean annual C assimilation rate across plots was 1.38Mgha−1year−1. Thus, our study reveals strong potential for interspecific variation in decay resistance to moderate the impact of canopy tree mortality on C storage in forests. We suggest that research effort on wood decay rates should be prioritised toward areas, such as drought-prone regions of Amazonia, where forests are likely to experience synchronous mortality events more frequently in future.