Pine growth, soil properties and succession in Pinus kesiya plantations, and influence of fragmented forests on reforestation in Boakaew highland watershed, Chiang Mai province

Abstract

Studies on pine growth, soil properties and succession in pine (Pinus kesiya) plantations, and influence of fragmented forests on reforestation in Boakaew highland watershed, Chiang Mai province, had been carried out in a series of plantations including 21 age classes (14 to 34 years old) and fifteen adjacent fragmented montane forests. The objectives were: (1) to study pine growth and wood production (2) to assess plant succession in a series of pine plantations and potential roles of adjacent fragmented forests on succession, and (3) to investigate changes of soil properties under a series of pine plantations. Three sampling plots, 40 x 40 m2 in size, were used in each of 21 age-class plantations, and one sampling plot was made in each of fifteen fragmented forests and three for abandoned lands after shifting cultivation. In each plot, stem girth and height of all tree species with > 1.5 m in height were measured for calculating growths and quantitative characteristics. Two small plots, 5 x 5 m2 in size, were placed inside the big plot and used for studying natural regeneration. Soil study was taken in selected five pine plantations of 17, 21, 25, 29 and 33 years old. The soil samples were collected along soil profiles and later analyzed for soil physical and chemical properties in laboratory. The soil study was also carried out in five sites of fragmented forests and one site for abandoned land. Average growth increments of pine trees in 21 age-class plantations were determined: height, 0.82 m/yr; diameter at breast height, 1.28 cm/yr; and tree volume, 6.23 m3/ha/yr. There were 17 to 72 species of succession broad-leaved tree species in these plantations with different densities, 540-2,688 trees/ha, and tree volume increment of 2.63 m3/ha/yr. The densities of pine varied between 75-429 trees/ha whereas the other species had greatly different densities among plantations, 131-2,331 trees/ha. Shannon-Wiener indexes of species diversity were varied among plantations, 1.57-4.65. Most succession broad-leaved tree species in plantations were belong to the families of Fagaceae, Euphorbiaceae, Juglandaceae, Lauraceae, and Myrtaceae, etc. The adjacent fragmented forests were lower montane forest. The species richness in the whole fragmented forests was 103 tree species (82 genus and 44 families). Fagaceae was the most dominant family. The majority of dominant trees in the forest were Pinus kesiya and Castanopsis acuminatissima, and some fragmented forests were dominated by Schima wallichii and C. diversifolia. Tree densities were greatly different among the forests, 556-1,769 trees/ha. Total stem basal areas were varied between 17.46-36.58 m2/ha that implied to different forest conditions, from degraded to good forests. The Shannon-Wiener Index of species diversity was high as 5.28. Morphology of fruit/seed has influenced on spatial distribution of plant species. Trees in the families of Pinaceae, Juglandaceae and Bignoniaceae have seed dispersal by wind and maybe distributed with long distance from mother trees. Those species of heavy seeds in Fagaceae, Lauraceae and Proteaceae usually have seed dispersal around mother trees by gravity force. The seeds maybe also distributed more longer distance by animal or moved down along sloping area by surface runoff water. Variable seed/fruit morphology in fragmented forests have the potential roles on plant succession in surrounding pine plantations. The natural succession in plantations will be resulted in development of plantation forests to be a natural lower montane forest. However, it needs more many decade to change forest structure to be montane forest. Reforestation on the highland watershed through Pinus kesiya plantations could change soil properties particularly soil physical and chemical properties. Bulk densities had a trend of decrease with plantation ages. Soil pHvaried from moderately acid to very strongly acid. The surface soils were rich in organic matter and tended to be increased with plantation ages. This trend was the same as those of total carbon and nitrogen, available phosphorus, extractable calcium and magnesium. Cation exchange capacity and base saturation percentage tended to be higher with plantation ages. Assessment of the soil fertility level revealed that all surface soils were moderately fertile and had a trend of increasing with plantation ages. Succession broad-leaved tree species played important roles on soil properties through litterfall. Accelerating natural succession and replanting of broad-leaved tree species in pine plantations are thought to be the good technique of reforestation on highland watershed to make the shortage of development from plantation ecosystems to be abundant montane forest.