Time since fire and climate interact to shape vegetation structure over seven decades in two semi-arid vegetation types

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Aims: Climate and fire are global drivers of vegetation structure. Fire ecologists face the challenge of understanding post-fire change, how it may be influenced by climate, and its implications for land management. We investigated time-since-fire responses of vegetation structure variables in two vegetation types, each occurring in two climate zones in a semi-arid region.

Location: South-eastern Australia

Methods: We measured 21 structure variables relating to ground, understorey and tree layers, and overall structural diversity, at a chronosequence of 251 sites stratified by climate zone, vegetation type and time since last fire. For each variable, we compared (using AICc values) a time-since-fire model to one that also included climate zone to assess whether post-fire structural changes depend on the aridity level.

Results: In total, 95% of structure variables across both vegetation types showed a significant time-since-fire response, with changes extending over 60+ years post-fire. Different attributes showed different ecological responses through time, the most common being a bell-shaped response, in which the variable typically peaks in magnitude in mid-succession ~30-40 years post-fire. Notably, changes in 70% of structure variables were best described by a combined model (additive or interactive) that included both time-since-fire and climate zone. For eight variables, ecological responses were similar between zones but with differing magnitudes. For 17 variables, the relationship was interactive, with differing responses between climate zones.

Conclusions: In this semi-arid environment, vegetation structure is profoundly influenced by fire. First, because post-fire structural changes extend over decades, inter-fire intervals that are short (e.g. <15-20 years) risk loss of attributes that take decades to develop. Second, the influence of climate zone on time-since-fire responses means that management prescriptions for a given vegetation type will be most effective when tailored to a particular climatic zone. Third, quantifying temporal responses to fire helps determine when vegetation is likely to provide more (or less) suitable habitat for faunal species.