Setting the benchmark: Patterns of forest structure after wildfire in temperate, montane forests across a multi-century chronosequence

Many forest types are shaped by wildfire, which can reset ecological
succession and modify species trajectories. Novel fire regimes are
increasing globally, posing a major threat to forests. Understanding the
successional dynamics of forests after wildfire is critical to develop
evidence-based ‘benchmarks’ of forest structure to inform appropriate
ecological management and restoration. However, this understanding is
limited in intact forests over long periods, including those where old
growth forest is sometimes used as a reference state. Several large,
high-severity wildfires burned significant areas of the Mountain Ash
(Eucalyptus regnans) forests in south-eastern Australia ≤1900, and in
1926/1939 and 2009. Here, we take advantage of this fire history to
implement a natural experiment to better understand the patterns and
environmental drivers of variation in forest structure at 84 sites across
a multi-century chronosequence. We explored inter-specific differences
across major plant lifeforms in relation to stem size and density, and
their relative interactions over time. Our findings indicate that: 1.
Early-successional forests ≤10 years after wildfire in 2009 are
characterised by a high, but variable, density of homogeneous-sized stems
(≥7000 stems ha-1). In contrast, old-growth stands (≤1900 stand age) were
characterised by the lowest number of stems, that were the least variable
in density, but had the greatest variability in stem size, relative to
other stand ages. 2. Environmental factors including slope and elevation
influenced patterns of stem density, with the direction of the influence
varying across lifeforms. 3. Interspecific relationships between
individual plant lifeforms varied across the chronosequence, and were more
negative in early-succession (2009 stand age), relative to
late-succession. Synthesis: Our findings demonstrate that rates of
self-thinning in forests following wildfire (that occur within historical
fire-regimes), may result in a ~50-60% decline in stem count in the
several decades between early-successional forests and mature (1926/1939
stand age) forests, and then again between mature and old-growth forests.
Self-thinning is a critical forest function, and represents one pathway
through which structurally complex old-growth forests develop, providing
significant ecological values. These empirical insights provide a
benchmark to guide evidence-based restoration of montane forests in the
face of predicted novel fire regimes.