Physiographic Controls on Maximum Stand Density Index of Three Major Conifer Species in the Inland Northwest, USA.

Forest productivity and health is optimized when management density maximizes stand growth and minimizes inter-tree competition for limited and dynamic site resources. Precise knowledge of site characteristics controlling forest stand density is not well understood due to complex interactions among various climatic, topographic and edaphic components.  To address these complex interactions, numerous tree records from private, state and federal forestland management organizations across the Inland Northwest, USA were geo-referenced and associated with stand level mensurational and physiographic factors.  Three species of ecologic and economic importance to this region were selected for defining physiographic controls on forest stand carrying capacity: Pinus ponderosa, Pseudotsuga menziesii var. glauca and Abies grandis var. idahoensis.  The relationship between diameter and density were assessed using stochastic frontier analysis to define a tree species maximum stand density boundary in relation to various site factors, as well as target species proportion of stand composition.  Frontier analysis indicated that target species proportion of basal area, elevation, aspect, soil parent material, mean temperature in the coldest month, frost free period, degree days >5C, annual dryness index and summer/spring precipitation balance were significant density controlling factors.  For example, increasing elevation typically decreased tree species carrying capacity, while northerly aspects showed a positive relationship.  Soil parent material analysis indicated that fine textured andic soils significantly increased carrying capacity; whereas soils derived from metasedimentary or igneous intrusive rocks showed a reduction.  Interestingly, the sign of significant site factors often switched between positive and negative depending on a tree species physiologic temperature and moisture growth requirements.  These results will be useful for generating geospatial site carrying capacity projections of important tree species across the Inland Northwest, USA.