Comparative growing space efficiency of four tree species in mixed conifer-hardwood forests

The influence of shade tolerance, canopy position, and tree size on growing space efficiency (GSE) in mixed stands of co-occurring conifer and hardwood species was investigated in hemlock-northem hardwood forests. Three alternative measures of two-dimensional growing space-total crown area (TCA), exposed crown area (ECA), and a projection of the total available growing space (AGS)-were investigated to clarify the comparative importance of shaded and illuminated crown regions and unoccupied space in the forest canopy. GSE was expressed as ratios of stem volume increment and biomass increment per unit of growing space. Late-successional, shade-tolerant species have often been portrayed as slow growing, inefficient users of their growing space; however, hemlock (Tsuga canadensis), which is one of the most shade-tolerant conifers in North America, was the most efficient canopy tree in our sample across all measures of GSE. Likewise, the mid-tolerant yellow birch (Betula alleghaniensis) tended to be less efficient than the more shade-tolerant maples (Acer rubrum and Acer saccharum). For all species, volume increment per unit of growing space increased with increasing tree height and canopy position, but within a given stratum decreased with increasing crown size. The relative efficiency of each species did not appear to be influenced by the measure of growing space employed. In most cases, volume and biomass increments per unit of ECA and AGS were significantly greater (p < 0.05) for intermediate than dominant crown class trees. However, for a given level of ECA or AGS, efficiency did increase with increasing relative height, which suggests that efficiency is influenced by the relative vertical position of growing space in the forest canopy. In general, the shaded area of a crown (i.e., TCA - ECA) was not a significant predictor of volume increment once height and ECA were known, suggesting that once 100% canopy closure is reached, packing trees more tightly may not increase stand-level production. However, mean volume increment per unit of TCA scaled more accurately to the stand-level than mean volume increment per unit of ECA. Potential scaling problems associated with mixed-species stands are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.