Abstract

Suwannaratana, S. (1994)

The Effects of Irrigation on the Ground Flora of a Deciduous Dipterocarp Forest at Huai Hong Khrai

Soil moisture is one of the most important factors affecting the structure and species composition of forests. Therefore if more water is put into a degraded deciduous forest, the species composition of the forest should change to a more mesic community. The study situdy site for this thesis was at Huai Hong Khrai Royal Development Study Center, Doi Saket, Chiang Mai province, in an area that has been irrigated for 8 years. Eighty permanent quadrats (1m x 1m) were used for a ground flora survey at 4 different sites: irrigated ridge and gully and non-irrigated ridge and gully. All the ground plants rooted in each quadrat were identified and given abundance scores every 3 months during the dry season and every month during the rainy season. On the ridge of both sites another 15 (2m x 2m) quadrats were laid out for a study of biomass and productivity. Every 2 months 1m squared of the above-ground standing crop was harvested. The living and dead standing crop was separated, dried and weighed.

The number of species recorded at both the irrigated and non-irrigated ridges was 39 while in the irrigated and non-irrigated gullies the number was 52 and 64 species respectively. The highest species diversity (Hill's number, N1 and N2) and eveness (Modified Hill's ratio) occurred in the non-irrigated gully (36.2, 31.5 and 0.9 respectively) and lowest in the irrigated ridge (16.3, 11.3 and 0.7 respectively). Perennial species in the irrigated site comprised only 66.67% of the total species, while in the non-irrigated ridge the figure was 76.92%. However in the gully sites the percentage of perennials was similar (86.54% in irrigated and 84.37% in non-irrigated area). The similarity (Sorensen's index) of IG and NIG was 0.74 while between IR and NIR was a little bit lower, 0.62. However all of the species occurred in all study sites were typical to deciduous dipterocarp forest. The study of biomass and productivity showed the effects of the irrigation more clearly. The total biomass at all 4 collection times was higher in the irrigated sites. The living standing crops at both sites were highest in September (155.536 g/m2 in the irrigated site and 84.541 g/m2 in the non-irrigated site). The dead standing crop in the irrigated area was highest in March (239.710 g/m2) but in the non-irrigated area it peaked in June (183.256 g/m2). The limited effects of irrigation may be due to the inefficiency of the irrigation system.