Neighborhood uniformity increases growth of individual Eucalyptus trees

Abstract

Competition is a crucial factor in determining stand structure and productivity. Competition entails complex interactions that depend on spatial arrangement of trees, resource supplies, and the efficiency of using resources. The net outcome of competition may be characterized by the sizes and distances of trees in a neighborhood around a focal tree. We tested a hypothesis that higher variability in tree sizes within the neighborhood (=low uniformity) would directly reduce growth of individual trees. Neighborhood models tested the influence of focal tree size and neighborhood competition on focal tree growth, and whether further inclusion of a uniformity measure would improve model performance. We modeled growth of 8800 focal clonal trees in a 9ha operational, clonal plantation of Eucalyptus grandis x urophylla to test our hypothesis by estimating the effects of size, and neighborhood competition and uniformity. The growth of a focal tree was strongly related to the tree’s size and to the neighborhood competition index that combined the sizes and distances of neighboring trees (within 8m of the focal tree). For a given size focal tree and a given level of neighborhood competition, the uniformity of neighborhood trees influenced potential growth by −2% (very uniform neighborhoods) to −10% (for heterogeneous neighborhoods), for an overall reduction in potential stand growth of 4.3%. Higher growth of larger trees did not compensate for lower growth of smaller trees, and silviculture systems that maximize stand uniformity may lead to measurable increases in stand-level growth.