The influence of light environment on indigenous tree sedlings in Kenya

Abstract

Attempts to restock large areas of exploited forests in Kenya have been hampered by inadequate understanding on the ecological requirements of indigenous tree species. In particular, very little is known about the responses of these species to different levels of light. This study, therefore, examined the significance of shade and light in regeneration of some important tree species. The species studied were Cordia afYicana Lam., Vitex keniensis Turill, Markhamia lutea (Benth.) K. Schum. and Olea capensis L. Artificial shading experiments were carried out in the nursery using seedlings of C. africana and V keniensis. The effects of different shade levels were compared to full sunlight under low and high nutrient regimes. The response of seedlings transferred among different light environments was also examined. The effects of different levels of light quality (R:FR ratios) were tested. In a forest clearing, seedlings of the four species were grown under artificial shading and their responses were compared to those grown under full sunlight. The influence of deep canopy shade on survival and growth in seedlings of the four species was also determined. In the nursery, seedlings of C. africana and V keniensis maintained positive relative growth rates at irradiance level of 19% of full sun and their growth was enhanced by increasing irradiance. The supply of nutrients at high level also enhanced the growth. Seedlings transferred between light environments acclimated within a month. Acclimation to increase in light availability was faster than acclimation to decrease. Although acclimation was mainly phy~iological, biomass was allocated in favour of shoots and roots when seedlings were shaded and exposed respectively. The supply of nutrients at higher level had no effects on the acclimation process. Seedlings of these two species showed increased stem elongation under very low R:FR ratio of 0.02, but not at R:FR ratios of 0.36 to 0.65. Seedlings grown at very low R:FR ratio and irradiance level of 12% of full sun showed high mortality when exposed to full sun, but those grown under irradiance level of 19-52% of full sun did not. In the clearing, growth in height and dry matter production were generally higher under moderate shade in all species except C. africana. The latter had highest growth in height and biomass production under full sunlight. High mortality was observed in seedlings of C. africana and V keniensis grown under the canopy with irradiance level of 1.5% of full sunlight, but those of M lutea and 0. capensis persisted for 15 months. Under the canopy growth in height was relatively slow in all the four species. Dry matter production was also low. The results show that C. africana is a typical pioneer species, but the others are intermediate in shade tolerance and are mainly associated with canopy gaps. These results are relevant for rehabilitation of degraded montane tropical forests in Kenya.