Effects of weeds on survival and growth of planted seedlings of native forest tree species during forest restoration in Northern Thailand

Abstract

Herbaceous weeds often hinder forest restoration projects by reducing establishment of planted tree seedlings. The study, presented here, compared survival and health of tree seedlings in the presence and absence of weeds, from 1.5 to 2 years after planting, to restore montane evergreen forest (1,300 m above sea level) from abandoned agricultural land in Chiang Mai Province. The grass Panicum maximum Jacq. was the dominant weed species. In addition, root collar relative growth rates (RGR-RCD) of four target native species in weedy plots were compared with those in weed-removal plots: Bauhinia variegata L., Bischofia javanica Blume, Hovenia dulcis Thunb, and Prunus cerasoides Buch.-Ham. ex D.Don. Ten 5-m-radius circular plots (78.5 m² each) were established randomly across the site. Five plots were cleared of weeds every two months and five were not. Alive planted tree seedings were counted in the plots before the weeding treatment and at approximate six months after the start of the experiment. The root collar diameters (RCD) of all surviving seedlings were measured and each was assigned a health score. Over the study period, no significant differences in the survival, health and RGR-RCD of the planted seedlings were detected, between the weeded and non-weeded plots. Larger seedlings survived better than smaller ones. Consequently, planted seedlings, which had already survived for 1.5 years in the restoration site (i.e. to the end of the second rainy season after planting), had apparently grown robust enough to withstand the competition from surrounding weeds. RGR-RCD’s of the four intensively studied species were (in descending order) 83.1% per year (P. cerasoides), 40.4% per year (H. dulcis), 25.3% per year (B. javanica) and 15.5% per year (B. variegata).