Vulnerability of plantation carbon stocks to defoliation under current and future climates

Plantation species globally are susceptible to a range of defoliating pests, but pest damage is rarely considered when estimating biomass C sequestered by these forests. We examined the impacts of defoliation on Eucalyptus globulus plantation C stocks under current and future climates using Mycospharella Leaf Disease (MLD) as a case study, hypothesising that biomass C sequestered in plantations would decrease with a warming and drying climate, and that impacts of defoliation would be strongly site dependent. Six E. globulus plantation sites with varying productivity were selected for the study. Current (1961-2005) and future (2030 and 2070) severity and frequency of MLD were estimated for each site using the bioclimatic niche model CLIMEX, and used as inputs to the process-based forest productivity model CABALA. CABALA was used to develop annual estimates of total living and dead biomass for current, 2030 and 2070 climate scenarios. Averaged annual biomass outputs were used to initialise the carbon accounting model FullCAM for calculation of C sequestered in living and dead biomass over a growing cycle. E. globulus plantations were predicted to sequester between 4.8 and 13.4 Mg C·ha-1·year-1 over 10 years under current climatic conditions. While our estimates suggest that overall this is likely to increase slightly under future climates (up to a maximum of 17.2 Mg C·ha-1·year-1 in 2030, and a shift in minimum and maximum values to 7.6 and 17.6 respectively in 2070), we predict considerable between-site variation. Our results suggest that biomass C sequestration will not necessarily be enhanced by future climatic conditions in all locations. We predict that biomass C sequestration may be reduced considerably by defoliation meaning that any gains in C sequestration associated with changing climate may be substantially offset by defoliation. While defoliation has a generally small impact under current climatic conditions in these plantations, the impact is likely to increase in the future, with reductions of up to 40% predicted for some sites under future climates. We conclude that the combined impacts of climate change on pest frequency and severity, and on host responses to defoliation, may reduce biomass C sequestration in E. globulus plantations in the future.