Abstract
Plantations of exotic trees on areas where tropical forest has been clear-felled have been reported to facilitate regrowth of indigenous tree species. This so-called nurse- or shelter tree effect was investigated in a montane semihumid site in southern Ethiopia where plantations of Pinus patula and Eucalyptus saligna grow in close vicinity to the natural Podocarpus falcatus mixed forest. Physiological performance of P. falcatus saplings growing in the exotic and the natural forests was investigated over the year. Compared with the natural forest, photosynthetic carbon gain and growth of the saplings were significantly enhanced under Pinus patula while likewise retarded under Eucalyptus saligna. Diverging effects of the differently dense shelter canopies on the saplings could be traced to differences in the sub-canopy microclimates and particularly to the intensities and temporal distribution of photosynthetic active radiation (PAR). Moisture also played an important role for photosynthetic carbon gain: while the morning patterns of CO2 uptake were significantly correlated with the patterns of sunflecks, stomatal conductance was the determinant factor in the afternoon. Analysis of the photosynthetic efficiency of sunflecks revealed a lower quantum yield than the basic diffuse PAR intensity. Compared with a theoretically even distribution of the daily PAR, CO2 uptake under the real light climate was 70% under Pinus and in the natural forest, and 59% under Eucalyptus. Relating growth rates of Podocarpus saplings to photosynthesis the microclimate under Pinus was 2.5 times as effective as that in the natural forest and five times more effective than under Eucalyptus.
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