Quantifying minimum site occupancy requirements of common forest tree species in northern New England, USA: Implications for stocking assessment

Abstract

Minimum stand densities necessary to achieve crown closure is an important aspect of stocking assessment that merits more attention. Managers concerned with optimizing individual-tree and stand-level responses will benefit from improved guidance on minimum densities when designing intermediate silvicultural treatments. To do so we generated a dataset of crown projection areas using previously published crown width models and considered various assumptions about crown packing and the spatial arrangement of stems. Implied stem densities based on predicted crown sizes were used to determine the minimum stand density index (SDIMIN) by species. These values were integrated with published information about maximum stand density index (SDIMAX) providing estimates of minimum relative density (RDMIN=SDIMIN/SDIMAX). Mixed-effects models were used to predict RDMIN and determine the slope of the minimum size-density relationship (mSDR) for common softwood and hardwood species in northern New England, USA. Large differences in crown-stem allometry were revealed between open- and forest-grown trees (ΔRD ∼0.2). Hardwoods occupied more growing space than softwoods (ΔRD ∼0.05), a finding primarily attributed to differences in crown architecture, though species shade-tolerance may also play a role. Minimum stem densities capable of achieving crown closure span a wide range determined by species composition and stand history. Importantly, the implied slope of mSDR (∼ −1.0) differed substantially from that of the benchmark maximum size-density relationship (MSDR; −1.605), corresponding to lower values of RDMIN for smaller-sized stems and earlier in stand development than when stems are larger and older. Our models and resulting estimates of RDMIN can be used to locate the lower boundary of stocking space on size-density diagrams.