Understanding Patterns of Water Use in a Dryland Woodland Using Stable Isotopes of Water and High-Resolution Dendrometers

Abstract:

Dryland ecoystems across inland Australia play a key role in the global carbon cycle owing to their extensive distribution and capacity to respond to highly episodic rainfall events. Understanding the pulse-response of vegetation to unpredictable and episodic rainfall also underpins mechanistic models of ecosystem function in dryland regions that can be used to assess vulnerability to altered hydrology, either from groundwater abstraction or climate change. Here, we used high resolution measurements of stem radius to understand diurnal and seasonal patterns of tree water-uptake in response to highly dynamic (cyclone driven) water availability. We also measured leaf water potential and δ²H and δ¹⁸O to validate assumptions about pulse water-use and the spatial and vertical distribution of soil water. We tested the hypotheses that (1) stem radius variation follows daily and seasonal patterns that reflect tree water use, and (2) trees are highly responsive to summer cyclone events that recharge the soil profile. We logged changes in stem radius of 11 Eucalyptus victrix trees every 20 minutes from November 2011 – November 2013 in the Hamersley Ranges of NW Australia. Pre-dawn and midday leaf water potential was also measured every three months between November 2011 and November 2012. We found that stem radius followed diurnal patterns of night-time rehydration and daytime water loss associated with transpiration. Periods with the largest Δψ corresponded with the largest measurements on maximum daily shrinkage (MDS) of the stem. Broader seasonal trends in trunk radius related to temperature, rainfall and VPD were also apparent. Rapid and extended periods (days) of stem expansion were associated with rehydration following cyclonic rainfall followed by prolonged periods (months) of gradual contraction associated with depletion of soil moisture. Cyclonic rainfall events had distinct isotopic composition and could be traced in stem water. Similarly, evaporation of moisture from shallow (< 5m) soils during the dry season was reflected in enrichment of ¹⁸O in stem water. Future work will couple these data with sap flow measurements to develop an integrated understanding of water and carbon fluxes in eucalypt woodlands of the arid northwest of Australia.