SHORT RANGE SOIL VARIABILITY AND ITS EFFECT ON RED OAK GROWTH (QUERCUS RUBRA L.)

The objective of this study is to document the effect of soil short-range variability on Red Oak growth and mortality. The spatial variability of soil properties was investigated in a 1.93-ha plot of 15-year-old monospecific Red Oak (Quercus rubra L.) located in the Dordogne valley in southwest France. The location and the individual height of live trees were systematically noted. Soils were sampled on a regular 10 × 10-m grid. Additional sampling on 2 × 2-m spacing was conducted in four directions around 10 points of the grid. The organization of soil layers indicated that the variability in Red Oak growth could be attributed mainly to short-range variability in two of the main soil properties studied: depth to bedrock and depth to carbonates. A geostatistical approach was used to analyze the spatial structure of these two soil characteristics. At this stand age and in this climate we found that mortality was greater than 50% for trees in a soil thickness less than 40 cm. Soils deeper than 80 cm appeared to enable good growth conditions if the other soil conditions (e.g., drainage, fertility, stoniness, and structure) were favorable. Below this threshold value, soil thickness had a significant effect on tree height growth. It was difficult to quantify carbonate effects on growth and mortality. However, mortality was greater and tree height was less in the calcareous sub-area than in the non-calcareous part of the plot. These findings raise interesting issues relative to tree growth modeling.