Long-term dynamics and drivers of forest structural complexity across temperate old-growth remnants in Slovenia

Forest structural complexity plays an important role in regulating forest functions and biodiversity habitat. A number of variables are known to influence structural complexity within forest stands across space, including tree species diversity and disturbance, yet less is known how such factors influence complexity over time. In temperate, old-growth forests, for example, a long-standing assumption is that continuous gap dynamics maintain relative stability in forest structure over time at stand scales, yet few datasets are available to test how this applies to structural complexity. We use permanent plot data spanning four decades to examine changes in plot-level structural complexity as a function of tree species diversity, disturbance, and tree recruitment, across a network of old-growth forest remnants in Slovenia dominated by mixtures of Fagus sylvatica and Abies alba. We modeled structural complexity as a function of time and plot-level covariates using Bayesian hierarchical generalized additive models. We found pronounced spatial and temporal variability in stand scale structural complexity. The results indicate that moderate severity disturbances play a role in driving temporal instability in structural complexity via mortality of canopy trees and subsequent recruitment of understory stems. We also found evidence linking higher structural complexity with the presence of A. alba in stands otherwise dominated by F. sylvatica. Emulating the large variation in complexity across space and time would require careful silvicultural planning, including prescriptions that mimic both small-scale gap dynamics, and use of variable retention harvesting to emulate moderate severity disturbances.