Forest developmental stages override canopy gap attributes in shaping natural regeneration in Hyrcanian beech forests

Technical Summary: Forest Developmental Stages Override Canopy Gap Attributes in Shaping Natural Regeneration in Hyrcanian Beech Forests

Problem Statement
Natural regeneration is the fundamental ecological process determining the long-term structure, composition, and resilience of forest ecosystems. While canopy gaps are widely recognized as key drivers of regeneration dynamics, the relative importance of specific gap characteristics (such as area and gap-maker abundance) versus the broader forest developmental context remains poorly understood, particularly in the Hyrcanian temperate forests of northern Iran. Existing literature suggests that regeneration responses cannot be adequately explained by gap characteristics alone, yet the extent to which developmental stages act as higher-order ecological filters mediating these effects requires empirical validation in near-natural, old-growth systems.

Methodology
The study was conducted in the Gorazbon compartment of the Kheyrud Forest, a control parcel within the Hyrcanian forests dominated by Oriental beech (Fagus orientalis Lipsky). The research utilized a 25-hectare permanent research plot, systematically divided into twenty-five 1-hectare subplots, where no silvicultural interventions had occurred for decades.

• Developmental Staging: Six distinct forest developmental stages were identified based on stand structural characteristics, canopy conditions, and disturbance history: pre-forest, young forest, canopy closure, mature forest, old-growth, and disturbance and legacy creation.
• Gap Inventory: Twenty-six naturally formed canopy gaps were mapped and measured using the FieldMap® system. Gap area was calculated based on geometry (elliptical, circular, or polygonal), and gap-maker abundance (standing or fallen dead trees responsible for the gap) was recorded.
• Regeneration Sampling: Within each gap, nine 2×2 m microplots were systematically established to capture spatial variation from the center to the edge. A total of 9,460 regeneration observations were recorded, quantifying abundance by species, height, and root-collar diameter.
• Statistical Analysis: Generalized linear mixed-effects models (GLMMs) with a negative binomial error structure were employed to evaluate the relative contributions of developmental stage, gap area, and gap-maker abundance. The models accounted for the hierarchical sampling design by including plot identity and gap identity nested within plots as random effects. Pairwise comparisons were adjusted using Tukey's method.

Key Results
The analysis revealed that forest developmental stage is the strongest predictor of regeneration abundance, significantly overriding local gap attributes.

• Developmental Stage Effects: Regeneration abundance varied significantly among stages. Mature forests exhibited the lowest regeneration abundance (mean ~26 individuals), whereas canopy closure, old-growth, and disturbance and legacy creation stages supported substantially higher densities (means ranging from ~44 to 58 individuals). Mature forests were significantly lower in regeneration abundance compared to canopy closure, disturbance/legacy, and old-growth stages.
• Gap Attributes: Neither canopy gap area nor the number of gap-making dead trees exerted a statistically significant effect on regeneration abundance once developmental stage was included in the model.
• Model Performance: Marginal and conditional R² values indicated that developmental-stage-related variation explained a substantial proportion of the observed regeneration patterns. Random-effect estimates showed that local-scale heterogeneity (variation among gaps nested within plots) contributed more to unexplained variability than plot-level variation.
• Gap Size Distribution: While gap area differed significantly across developmental stages overall, pairwise comparisons did not reveal significant differences between individual stages, suggesting that gap size alone provides an incomplete explanation of regeneration dynamics.

Key Contributions
This study challenges traditional gap-centered interpretations of forest regeneration by demonstrating that in Hyrcanian beech forests, regeneration patterns are governed primarily by stand-level structural and ecological conditions associated with developmental stages rather than by individual gap attributes.

• Contextual Dependency: The findings support the hypothesis that developmental stages function as higher-order ecological filters. The ecological context in which a gap occurs (e.g., the structural complexity of an old-growth stand versus the homogeneous canopy of a mature stand) is more critical to regeneration success than the physical dimensions of the gap itself.
• Mature Forest Bottlenecks: The study identifies mature forests as a developmental phase characterized by a "regeneration bottleneck," likely due to prolonged canopy closure, reduced spatial heterogeneity, and intense competition, which restricts seedling establishment.
• Old-Growth Resilience: Conversely, old-growth forests demonstrated high regeneration abundance, attributed to structural complexity, multi-layered canopies, and abundant deadwood that create diverse regeneration niches, facilitating forest renewal independent of large gap formation.

Significance and Claims
The paper claims that its findings provide new evidence supporting the integration of developmental-stage concepts into close-to-nature forest management and conservation strategies in temperate old-growth forests.

• Management Implications: From a silvicultural perspective, the results suggest that maintaining developmental heterogeneity across forest landscapes may be more effective for sustaining regeneration resilience than focusing exclusively on canopy gap characteristics or attempting to replicate specific gap sizes.
• Theoretical Shift: The study posits that regeneration dynamics in natural temperate forests should be interpreted within a developmental-stage framework rather than a purely gap-centered perspective. It argues that while canopy openings are important structural components, their ecological significance is contingent upon the developmental context.
• Limitations: The authors acknowledge limitations, including a relatively small number of canopy gaps which may have reduced statistical power for pairwise comparisons, and a focus on regeneration abundance rather than species-specific responses or environmental variables like soil properties and browsing pressure.

In conclusion, the study asserts that regeneration patterns in Hyrcanian beech forests are driven by the cumulative effects of developmental context—integrating disturbance history, structural complexity, and biological legacies—rather than by isolated gap attributes.