Modelling dominant height growth: effect of stand density.

In many timber species, height growth of dominant trees (100 largest trees per ha) in even-aged stands is usually assumed to remain unchanged over a wide range of stand density. This assumption allows us to use the stand dominant height (mean height of the 100 largest trees per ha) at a specified reference age as an index of site quality. A "tree - distance independent" growth model was developed for Corsican pine (Pinus nigra Arn. ssp. laricio (Poir.) Maire) in France (Meredieu, 1998) to describe individual growth and mortality according to tree size (diameter at breast height) but not to tree location within the stand, silviculture (modifying stand density and structure) and site quality. The five relationships were: stand dominant height growth, tree diameter growth ("potential x modifiers" form), mortality, stem profile and a static height-diameter function. Data analyses evidenced the density-dependence of height growth even for dominant trees. Therefore, the dominant height growth relation supports an original feature: a stand density effect is included in addition to age at breast height and site index effects. This result was then evaluated using a wider range of experimental stands, namely twenty-seven experimental plots of Corsican pines planted in the region Centre (France) and managed with different thinning regimes. Annual height increments since planting were measured (non-destructive method) for each tree that had been dominant at least once since thinning (plots had been measured every two years). This independent data set allowed us to: 1) determine more precisely the influence of stand density on the dominant tree population; 2) improve the density-related function for dominant height growth. Lastly, we suggest the use of a "potential dominant height increment" (i.e. corrected for stand density effect) as the "potential" growth component in the diameter growth function.