Abstract

Phonesavanh, B. (1994)

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

An investigation of the effects of irrigation on the phenology and seedling communities in 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 the ridge tops and gully bottoms to compare the seedling communities, the seasonal periodicity of flowers, fruits, and leaf production, and insect damage of adult trees. For the seedling communities, the relative growth rates and mortality rates were also studied 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 as well as diversity and evenness between the communities.

The species diversity of the seedling community in the irrigated ridge (IR) and non-irrigated gully (NG) was 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. According to their natural phenologies 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 increase the growth rate of seedlings and 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 irrigatioin system with higher efficiency (e.g. sprinklers, etc.) may increase the effects of irrigation. Alternatively, planting of deciduous tree species, associated with mycorrhizae in conjunction with irrigation may also accelerate recovery of degraded deciduous forest.