Effects of Irrigation on the Ground Flora of a Deciduous Dipterocarp Forest

ABSTRACT: 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 site for this thesis was Huai Hong Khrai Royal Development Study Center, Doi Saket, Chiang Mai Province, in an area that had 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 study of biomass and productivity. Every 2 months, 1 sq m of the above-ground standing crop was harvested. Living and dead standing crops were separated, dried and weighed.

Thirty-nine species were recorded at both the irrigated and non-irrigated ridges, while in the irrigated and non-irrigated gullies, the corresponding numbers were 52 and 64 species respectively. All species were typical of deciduous dipterocarp forest. The highest species diversity (Hill's number, N1 and N2) and evenness (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%. In the gully sites, the percentage of perennials was similar (86.54% in irrigated and 84.37% in non-irrigated area). Similarity (Sorensen's index) of IG with NIG was 0.74, while between IR and NIR it was slightly lower - 0.62.

Total biomass at all 4 collection times was higher in the irrigated sites than in non-irrigated sites. The living standing crops at both sites were highest in September (155.536 g/sq m in the irrigated site and 84.541 g/sq m in the non-irrigated site). The dead standing crop in the irrigated area was highest in March (239.710 g/sq m) but in the non-irrigated area it peaked in June (183.256 g/sq m).