Effects of Irrigation on The Phenology and Seedling Community of a Deciduous Dipterocarp Forest at Huai Hong Khrai

ABSTRACT: An investigation of the effects of irrigation on the phenology and seedling communities of a deciduous dipterocarp forest at Huai Hong Khrai, Doi Saket District, Chiang Mai Province, Thailand, was undertaken from February to December, 1993. Two sites, irrigated and non-irrigated, were selected. Transects were established in both areas, along  ridge tops and gully bottoms, to compare tree seedling communities, the seasonal periodicity of flowering, fruiting, leaf flushing/abscission and insect damage of adult trees. Tree seedling relative growth rates and mortality rates were also recorded and compared between the two sites. The number of species, Modified Hill's index and Hill's diversity index were calculated, to compare species richness, diversity and evenness between the communities.

Species diversity of the seedling communities on the irrigated ridge (IR) and non-irrigated gully (NG) were lower than on the non-irrigated ridge (NR) and in the irrigated gully (IG). On the NR, the proportion of leaf fall was significantly higher than on the IR, but leaf flushing was lower. Flowering and fruiting were not affected by irrigation, but varied amongst species. Seed drop correlated with soil moisture. Insect damage on the NR was higher than on the IR, while it was similar between the irrigated and non-irrigated gullies. Irrigation did not appear to accelerate seedling growth. Individual species showed no consistent response to irrigation. Irrigation significantly decreased the percentage of sand and increased clay and soil moisture at field capacity in the irrigated gully only.

An irrigation system with higher efficiency (e.g. sprinklers, etc.) may increase the effects of irrigation. Alternatively, planting deciduous tree species that are associated with mycorrhizae, in conjunction with irrigation, may also accelerate recovery of degraded deciduous forest.