What Drives the Phenology of Carbon Exchange in an Australian Temperate Woodland?

Abstract:

Temperate, broadleaved evergreen woodlands dominated by Eucalyptus species are adapted to a wide range of moisture conditions. However, these ecosystems can be susceptible to extremes of environmental stress, including droughts and heat waves. We evaluated climatic drivers of carbon and water exchange using eddy covariance techniques for over two years in a dry sclerophyll woodland near Sydney, Australia. We found that the strongest net C uptake by this ecosystem occurred during the winter months (June through August), and that precipitation and minimum air temperature were the most important environmental drivers of net uptake. Ecosystem respiration was highest during summer as soil drought was alleviated by frequent thunderstorm events, and lowest during winter due to drier soil and cooler temperatures. Gross primary production was independent of surface soil moisture but was constrained by high VPD during summer. Highest water and light use efficiencies for GPP were observed during winter. This study demonstrates the importance of strong stomatal regulation of dry Eucalyptus woodlands in limiting summer uptake, and warm-season rain in enhancing soil organic matter decomposition, leading to net C losses during summer. The temporal patterns of CO₂ fluxes in this mild temperate forest are unusual in comparison to other temperate forests. By persisting through unfavourable conditions and growing in response to favourable conditions during any season, the phenology of C dynamics in temperate sclerophyll woodlands can resemble that of drier ecosystems. The climatic drivers of net C uptake by these woodlands should be considered for evaluating vulnerability to extreme climate events that might limit their productivity as well as ecosystem C storage.