Morphological Canalization Drives the Environmental Action Scale (EAS): Cascading Effects on Tree–Fallen Wood Spatial Associations Across Ontogenetic Stages in a Montane Conifer Forest, China

Fallen wood is widely recognized as an important micro-habitat that facilitates forest regeneration in certain forest environments, yet how its facilitative role varies across ontogenetic stages and what constraints govern its spatial effectiveness throughout the tree life cycle remain poorly understood. This study investigated the spatial associations between Picea schrenkiana (Fisch. & C.A.Mey.) populations and fallen wood across four ontogenetic stages in a 6-ha (6 hm2) permanent plot in the West Tianshan Mountains, China. Our results revealed that fallen wood serves as an irreplaceable regeneration substrate throughout the entire life history: 99.6% of seedlings were established on fallen wood, and this dependency remained remarkably high (>78%) even at the adult stage. Spatial point pattern analyses demonstrated stage-specific shifts in tree–fallen wood associations: strong positive correlations at the seedling stage, significant negative correlations at the sapling stage driven by light limitation, multiscale fluctuating associations at the sub-adult stage, and positive correlations at the adult stage—indicating a functional transition from micro-habitat sheltering to historical legacy. Furthermore, after accounting for environmental heterogeneity using heterogeneous Poisson process models, we identified an environmental action scale (EAS)—the critical spatial distance at which environmental filtering overrides the fallen-wood micro-habitat effect—which expanded systematically with ontogeny: from 3.94 m at the seedling stage to 4.3–8.61 m at the sapling stage and to approximately 16 m at both the sub-adult and adult stages, reflecting a cascading control mechanism in which local environmental filters and fallen wood micro-habitat interact across ontogenetic stages, driving enhanced physiological tolerance and crown expansion that increase the EAS and ultimately shape population spatial patterns. Notably, EAS exhibited a strict proportional scaling relationship with maximum crown diameter (R2 = 0.9839), revealing that population spatial structure is intrinsically constrained by species-specific morphological conservatism. These findings advance our understanding of fallen wood dynamics beyond early recruitment and provide a mechanistic framework for assessing regeneration resilience in montane coniferous forests under climate change.