Plot-Scale Deadwood Volume and Litter Depth as Correlates of Scots Pine (Pinus sylvestris L.) Regeneration in Semi-Arid Forests

Deadwood is widely recognized as an important structural component of forest ecosystems, yet its relationship with natural regeneration remains highly context-dependent, particularly in semi-arid forests. This study evaluated associations of plot-scale downed deadwood volume, litter depth, and stand basal area with Scots pine (Pinus sylvestris L.) seedling and sapling abundance in central Türkiye. Field measurements were conducted in 60 circular plots distributed across five naturally regenerated stands. Relationships were explored using Spearman rank correlations and evaluated using four unified zero-inflated negative binomial generalized linear mixed models (ZINB-GLMMs) that accounted for stand differences and paired seedling–sapling observations within plots. Downed deadwood was spatially heterogeneous and dominated by coarse woody debris, whereas fine woody debris contributed little to the total volume. Neither coarse woody debris, fine woody debris, total downed deadwood volume, nor basal area was significantly correlated with seedling or sapling abundance. In a supplementary unadjusted comparison, regeneration abundance did not differ between plots with and without deadwood (Wilcoxon tests, p > 0.05). Spearman correlations showed opposite associations between litter depth and seedlings (ρ = −0.312, p = 0.015) and saplings (ρ = 0.260, p = 0.045). However, AICc model comparison favored the baseline model containing the regeneration stage and stand identity (AICc = 517.26, Akaike weight = 0.672), and the unified model did not detect a significant regeneration stage × litter depth interaction (β = 0.138, p = 0.666). These findings indicate that contrasting bivariate litter-depth associations were not confirmed as a robust stage-dependent response after accounting for stand effects, zero inflation, and within-plot pairing. At the plot scale, total deadwood volume alone did not explain Scots pine regeneration abundance in the studied semi-arid forests.