| This study examined the effect of precipitation, the available water capacity of the soil, soil texture, and soil chemistry on the site index (base age 25) of loblolly pine for 100 plots located in east Texas. Spring-summer (March--September) or July-September rainfall was superior to annual or fall-winter (October--February) rainfall. Rainfall levels over the first five year post-planting period were slightly better for explaining site index than levels over the 25 year post-planting period. The available water capacity up to the wilting point (-1500 kPa) better explained site index than the available water capacity to -350 kPa (a level where the literature suggests tree growth is severely affected). The available water capacity in the top one or two feet of soil was superior to available water capacity to lower depths. The available water capacity of the soil was better explained by the percentage of sand in the soil than by the percentage of clay. The percentage of clay, in the top one or two feet of soil was a better predictor of site index than the percentage of sand or the available water capacity to the same depths. The best model, which contained percent clay in the top one foot of soil, average July-September rainfall over the first five year post-planting period, and their interaction explained 18.5% of the variability in site index.;When plots were divided into groups based on soil profile type, the plots having a sandy profile throughout or those with sandy A and E horizons and a clayey B horizon showed no significant relationship between site index and rainfall. The significant effect of rainfall seems to come from its negative effect on soils with a high clay content throughout the profile.;The relationship between dominant height and the same explanatory variables was less significant at ages seven through fifteen, but the relationships at ages sixteen through eighteen tended to be slightly more significant than those for dominant height (site index) at age 25. |
