Site Quality Assessment for Eucalyptus camaldulensis Plantations in the Lower Part of Northeastern Thailand

Abstract

The site quality assessment for Eucalyptus camandulensis plantation was conducted over an area of 8.44 million ha in the lower part of northeastern Thailand. The study area was classified geographically into four groups based on digital maps derived from the Land Development Department: upland soil series, lowland soil series, conservation forests and slopes above 35%. This study concentrated only on the upland soil series groups that covered 3.83 million ha or approximately 45.42% of the total area. A sample of 91 plots, each 40 m² x 40 m², was established to measure the age of trees in various classes, diameter at breast height (DBH), total height (H) and soil series groups by age class. Site index curves were calculated and used to evaluate the site quality of eucalypt plantations based on derived data. Soil samples were also collected at three depths: 0-10, 10-30 and 30-60 cm and analyzed in the laboratory. In addition, relationships between yields, soil properties and climatic factors were also investigated.

            Results of the study revealed that the soil series groups in the study area by number (No.) were: No.40, 41, 35, 48, 29, 56, 55, 47, 31, 36, 46, 49, 48, 47 and 45. In particular, soil series group No.40 (Chumpaung and Yangtalad) covered the largest area of 1 466 384 hectares or 17.38%, followed by No.41 (Kambong) and No.35 (Korat, Satuk and Dansai), respectively. Site quality could be classified into five groups each containing different soil series: (1) very good site quality with soil series group No. 38; (2) good site quality with soil series group No. 33, 44 and 46; (3) moderate site quality with soil series group No. 29, 35, 36, 40, 41, 47, 48, 49 and 55; (4) poor site quality with soil series group No.31; and (5) very poor site quality with soil series group No. 56. The expression of the relationship between the yield of eucalypt in terms of total aboveground biomass and soil properties at 10-30 cm depth showed that the yield increased because of increasing quantities of clay particles, available phosphorus, exchangeable potassium as well as a high pH level. In contrast, the yield decreased because of increasing exchangeable calcium, magnesium, sodium and solid phase. Climate did not show any significant relationship to yield, which may have been a result of the fact that the study sites were rather appropriate for growing this tree species. Hence, digital mapping of site quality for eucalypt plantations when it has been completed will provide useful information to help formulate an extension program for eucalypt plantations in both the government and private sectors.

Keywords: Eucalyptus calmaldulensis, site potential, lower part of northeastern region, environmental factors