Growth and photosynthesis of seedlings of four tree species from a dry tropical afromontane forest

Abstract

Growth, photosynthesis and carbon allocation pattern were studied in four dry afromontane forest tree species (Olea europea subspecies cuspidata, Podocarpus falcatus, Hagenia abyssinica and Juniperus procera) under varied light regimes in a greenhouse. The objective of the study was to assess the potential of the species for growth under forest canopies and to identify their habitat preferences. The light regimes were created using shade cloth and they corresponded with deep forest understoreys (2% of the light in the open gap); moderate shade (10%), slight shade as is found in edges of forest gaps (20%) and 100% representing open gaps.

Seedling biomass was significantly influenced by light regimes for all species as were total leaf area and relative growth rate. H. abyssinica had the highest growth rate but the lowest survival rate in shade. There was a consistent trend for higher specific leaf area (SLA) and leaf area ratio (LAR) at lower growth light regimes in O. europea and P. falcatus. O. europea and P. falcatus had similar allocation patterns whereby between 20-40% of the total biomass was allocated to stem, 20-30% to roots and 50-60% to leaves. In H. abyssinica up to 80% was allocated to leaves and only a small percentage to stem and to roots.

Within species there were significant differences in the light saturated rate of photosynthesis (A_max) per unit area among the plants grown at the lowest light level and in the open. For plants grown in the open there were very little among species differences in A_max per unit area. However, there were significant differences in the A_max of the different species grown at the lowest light level. Chlorophyll fluorescence measurements indicated that open-grown P. falcatus and O. europea experienced moderate photoinhibition. From the growth and photosynthesis results P. falcatus and O. europea appeared to be non-pioneer, shade-tolerant species, while J. procera and to a larger extent H. abyssinica showed pioneer, light-demanding characteristics. The results are discussed in terms of the occurrence of the species in the mosaic climaxes of afromontane forests.