Associate Professor University of Idaho Moscow, Idaho, United States
Abstract: The increasing frequency and severity of future drought events will profoundly impact the ability of forest ecosystems to sequester atmospheric carbon dioxide. In the southern hemisphere, eucalypt forests account for more than 75% of total native Australian forests and provide a long-term sink for up to 860 Megagrams of carbon per hectare. Some of the largest extant Eucalyptus spp. trees exist in Tasmania, Australia, where atmospheric aridity, temperature, and soil water deficit are all expected to increase dramatically by the end of the 21st century. Here, we investigate how historical and contemporary carbon dynamics differ among wet ( > 1,000 mm yr-1) and dry (< 1,000 mm yr-1) eucalypt forests dominated by one of the most commonly occurring Eucalyptus tree species, messmate stringybark (Eucalyptus obliqua, hereafter EUOB), in northern Tasmania. To do this, we collected 120 tree cores from mature EUOB trees across three wet and three dry study locations (n = 20 cores site-1) from which we developed chronologies of growth and annual carbon increment. We performed total carbon inventories on live wood (including saplings), dead wood (including snags), and coarse woody debris at each site to examine how contemporary carbon density differs among stands experiencing contrasting climatic conditions. We further quantify future impacts to long-term carbon sequestration potential of EUOB trees in both stand climate-types using a process-based Earth system model parameterized using data from our sites under a range of scenarios (SSP4-3.4, SSP4-6.0, SSP5-8.5) expected by the end of the 21st century. On average, we found wet eucalypt forests contain 19% greater carbon density than adjacent dry eucalypt forests, primarily contributed to by a given EUOB tree containing ca. 26% carbon in aboveground stocks. We also found wet sites contained a slightly greater density of dead wood and coarse woody debris. Model predictions indicate those EUOB trees in dry sites are less sensitive to future climate variability, exhibiting nearly 8% lower reductions in growth under the most severe drought years, particularly under SSP5-8.5, but sequester less carbon under the most extreme future climate conditions. Overall, our findings highlight while those EUOB trees in dry sites currently store less carbon, their growth may be less susceptible to future climate extremes, and therefore will continue to be important future carbon sinks.