Multi-century changes in vegetation structure and fuel availability in fire-sensitive eucalypt woodlands

Abstract

Understanding how communities change with time since fire is critical for identifying appropriate fire return intervals for biodiversity conservation. In infrequently-burnt communities, vegetation structure, habitat features and fuel availability can change over time-scales much longer than can be measured using contemporary remote-sensing approaches, creating challenges for conservation and fire management. To characterize longer-term patterns of vegetation structural change, we measured vegetation cover, ground cover, tree density and stand basal area across a multi-century time-since-fire sequence derived from growth ring-size relationships in fire-sensitive Eucalyptus salubris woodlands of south-western Australia. We hypothesized that: (i) vegetation structural components reflecting fuel availability increase with time since fire; (ii) recovery of vegetation structural components with time since fire requires long time-frames; and (iii) vegetation components indicating senescence are more evident in mature than intermediate fire-age classes. All vegetation structural components showed significant differences between time-since-fire classes (termed ‘young’, ‘intermediate’ and ‘mature’), and to a lesser extent between years of sampling. The two vegetation structural components with the highest covers overall, and hence likely greatest contributors to fuel availability, were vegetation 4–10m high and ground fuel. These two layers showed non-monotonic changes indicating a peak at intermediate times since fire (∼35–150 or 35–250years; depending on the model used to estimate stand age), conflicting with the common assumption that fuel availability increases with time since fire. Total stand basal area increased rapidly after fire then appeared to stabilize beyond about 100years, with competition likely mediating density-dependent thinning such that declining plant density offset increasing trunk size. There was little evidence for an increase in standing dead vegetation in mature woodlands such as would suggest significant senescence when long-unburnt. Replacement of mature woodlands with intermediate time-since-fire woodlands with greater cover and connectivity of key fuel layers potentially instigates a self-reinforcing fire regime shift favouring larger and/or more uniform fires. If such changes eventuate, substantial losses in conservation values in E. salubris woodlands are likely. Elucidating these changes in vegetation structure and implications for conservation management only became feasible due to the development of methods to estimate the time since fire of vegetation not burnt for hundreds of years.