Data from: Tracking shifts in forest structural complexity through space and time in human-modified tropical landscapes

Habitat structural complexity is an emergent property of ecosystems that
directly shapes their biodiversity, functioning and resilience to
disturbance. Yet despite its importance, we continue to lack consensus on
how best to define structural complexity, nor do we have a generalised
approach to measure habitat complexity across ecosystems. To bridge this
gap, here we adapt a geometric framework developed to quantify the surface
complexity of coral reefs and apply it to the canopies of tropical
rainforests. Using high-resolution, repeat-acquisition airborne laser
scanning data collected over 450 km2 of human-modified tropical landscapes
in Borneo, we generated 3D canopy height models of forests at varying
stages of recovery from logging. We then tested whether the geometric
framework of habitat complexity – which characterises 3D surfaces
according to their height range, rugosity and fractal dimension – was able
to detect how both human and natural disturbances drive variation in
canopy structure through space and time across these landscapes. We found
that together, these three metrics of surface complexity captured major
differences in canopy 3D structure between highly-degraded, selectively
logged and old-growth forests. Moreover, the three metrics were able to
track distinct temporal patterns of structural recovery following logging
and wind disturbance. However, in the process we also uncovered several
important conceptual and methodological limitations with the geometric
framework of habitat complexity. We found that fractal dimension was
highly sensitive to small variations in data inputs and was ecologically
counteractive (e.g., higher fractal dimension in oil palms than old-growth
forests), while rugosity and height range were tightly correlated (r=0.75)
due to their strong dependency on maximum tree height. Our results suggest
that forest structural complexity cannot be summarised using these three
descriptors alone, as they overlook key features of canopy vertical and
horizontal structure that arise from the way trees fill 3D space.