Site productivity evaluation of some yemane (Gmelina arborea Roxb.) plantations in the Philippines

Growth performance of yemane plantations grown in Cebu and Nueva Vizcaya [Philippines] was evaluated based on measurable physiographic, climatic and soil factors. Physiographic factors included aspect, elevation, plot position and slope or gradient; climatic factors were annual rainfall, relative humidity, temperature and number of rainy days; and soil chemical and physical properties were nitrogen, phosporus, potassium, calcium, magnesium, sodium, organic matter, pH, base saturation, cation exchange capacity, water holding capacity, soil depth and soil texture. The climatic factors obtained significant correlations with some tree growth parameters such as diameter at breast height, merchantable height, basal area, volume and site index indicating that growth of yemane plantations was significantly affected by these factors. However, only rainfall had positive correlations. Temperature and relative humidity were inversely related to tree growth. Comparatively, inverse relationships between tree growth parameters and all physiographic variables were obtained which imply that steeper slopes, from lower to upper slopes and ridgetops and from lower to upper elevations may result to a concomitant decrease in growth or site quality of yemane plantations. Analysis of soil chemical and physical properties showed that calcium content of soil, base saturation, soil depth, magnesium, cation exchange capacity, sodium and clay content of soil were all significant explanatory variables for estimating or predicting growth and site index of yemane plantations. Nitrogen, phosphorus, potassium, organic matter and percent clay content of soil are all positively correlated with tree growth indicators while calcium, magnesium, cation exchange capacity, base saturation, pH, percent silt and percent sand had negative correlations. Eight growth prediction equations were developed from among the significant components of site factors.