Quantifying woody plant health and its implications for estimates of aboveground biomass in rangeland ecosystems

Rangelands cover vast areas and are subject to episodic droughts, regular fires and grazing pressure, all of which affect woody plant health. Consequently, standing dead and senescent woody plants are key components of these ecosystems. Stand-level, woody aboveground biomass (AGB), an important component of the terrestrial carbon budget, is typically scaled using individual-based allometric relationships with predictor variables such as stem diameter (D) or crown area (CA) measured through on-ground inventories of woody plants. Current data are lacking to allow assessment of the influence of woody plant condition on allometry and subsequent scaling of AGB. To address this, we undertook field measurements across 431 Australian rangeland sites to improve understanding of the variation in the condition of woody plants across rangelands, including how condition of different plant functional types (PFTs) affects overall stand condition, how stem diameter-crown area allometry varies with condition and PFT, and the sensitivity of allometry-predicted AGB to variance in condition. Field measurements included stem diameter, crown width and vigour, and health scores of live and standing dead woody plants. Over one-quarter of individual woody plants were either dead or senescing across all sites. D–CA allometric relationships differed among PFTs, with those for trees differing from those of shrubs and multi-stemmed acacias. For a given stem diameter, allometry-predicted crown area declined as health score decreased. Further, across all sites, allometry-predicted AGB was reduced when corrections were applied to account for standing dead and large senescing eucalypt trees relative to existing allometric equations. Our findings suggest that if traditional allometric relationships developed for live, healthy woody plants are applied to predict AGB in these ecosystems, substantial over-estimations may result, particularly for stands with a high proportion of large woody plants of poor condition. Results will inform ongoing improvements to the accuracy of stand-level biomass estimates in rangelands.