Divergent trajectories of regeneration in early-successional forests after logging and wildfire

Increases in forest disturbances have altered global patterns of forest
demography, with many regions now characterized by extensive areas of
early-successional forest. Heterogeneity in the structure, diversity and
composition of early-successional forests is often overlooked in
ecological research. However, these sources of variability in
early-successional forests can influence their inherent ecological values
now and in later successional stages, including values for biodiversity
and carbon storage. Here, using 14 years of longitudinal data, we describe
temporal patterns in the structure, diversity, and composition of
early-successional forests subject to different disturbance origins
(clearcut logging, salvage logging and wildfire) and disturbance histories
(short, medium and long disturbance intervals) in the Mountain Ash
(Eucalyptus regnans) and Alpine Ash (Eucalyptus delegatensis) forests of
south-eastern Australia. Our analyses revealed several key differences
between early-successional forests that regenerated from wildfire versus
anthropogenic perturbations. Most ash-type plant communities were
resilient to wildfire within historical fire-regimes (75-150 years),
exhibiting temporal trends of recovery within the first decade. In
contrast, the diversity and abundance of some plant lifeforms and
life-history traits were negatively associated with clearcut logging and
salvage logging, relative to forests disturbed by wildfire alone. Taxa
most affected were resprouting species, such as tree-ferns and ground
ferns. However, some onsite-seeding species, including Acacia were more
abundant after clearcut logging than after wildfire. Our findings also
provide evidence of the pronounced and pervasive impacts of salvage
logging on early-successional plant communities. Alarmingly, we observed
several negative temporal responses in plant diversity after salvage
logging. That is, diversity declined over time after salvage logging,
rather than increased as occurred following other disturbance types.
Early-successional forests provide the template from which mature forests
develop. Therefore, altered patterns of recovery in early-successional
forests with different disturbance origins will likely shape the structure
and function of later-successional stages. Forest management should seek
to limit future anthropogenic perturbations, especially salvage logging,
to preserve total diversity of plant communities in ash-type forests. This
is pertinent with predicted increases in wildfire likely to increase the
extent of early-successional fire-prone forests in many already
regrowth-dominated ecosystems, which risk falling into positive feedbacks.