Dieback of stringybark eucalypt forests in the Mount Lofty Ranges

Abstract

Canopy dieback and concerning rates of tree mortality have been noted in iconic forests of the Mount Lofty Ranges (MLR), South Australia, dominated by the stringybark eucalypt species Eucalyptus baxteri (Brown Stringybark) and E. obliqua (Messmate Stringybark). The extent and causes of stringybark forest decline are not yet fully understood, prohibiting evidence-based management strategies. Here, we explore the distribution of MLR populations of the two species and their position in climate space relative to eastern populations. We also conducted field assessments to investigate stand health and dieback aetiology, and analysed existing tree monitoring data. Stringybarks in the MLR are disjunct from eastern populations and occupy a more summer-arid niche. The species are also susceptible to summer water stress and Phytophthora. Periods of drought during 2006-2009 and 2018-2019 may have contributed to observed dieback. However, field assessments suggest a complex landscape syndrome that includes borer infestations and fire impacts among other factors, rather than solely hydraulic failure. Messmate Stringybark has suffered widespread but patchy stand collapse. There is no obvious common pattern of collapsed sites with respect to topography or local water availability (e.g., swamps and ridges equally affected), although northern range-edge sites are heavily affected. Brown Stringybark is less affected but has notable collapse sites. We hope these studies establish a springboard for future investigations and more widespread sampling of MLR stringybark forests. Further investigations should include regional surveys of stringybark sites to record spatial and temporal patterns of tree mortality combined with multi- or hyperspectral analysis of remotely sensed imagery and visual inspection of dieback from very high resolution aerial images and ground-truthing. Our findings confirm the susceptibility of stringybark forests in the MLR to ecosystem collapse and highlight the urgent need to understand the causes and aetiology of the observed dieback.