The influence of dispersed trees on microclimatic conditions gas exchange and productivity of maize (Zea mays L.) in a Grevillea robusta-based agroforestry system in semi-arid Kenya was examined to test the hypothesis that the benefits of shade seen in savannah ecosystems may be outweighed by competition for below-ground resources. Meristem temperature cumulative thermal time intercepted radiation spatial distribution of shade and gas exchange were determined for maize grown as sole crops in an agroforestry system or under shade nets providing 25 or 50% reductions in incident radiation to discriminate between effects of shade and below-ground competition. The major benefit of shade was to reduce exposure to the supra-optimal temperatures experienced in many tropical regions and which are predicted to become increasingly common by climate change models. However although trees decreased photosynthetic photon flux density (PPFD) incident on understorey maize by ca. 30% the yield reduction was much greater than in the 25% shade net treatment in four seasons providing contrasting rainfall. Maize yield was unaffected by 50% artificial shade in the driest season (168 mm) but decreased with increasing shade when rainfall was high (628 mm). Shade reduced meristem temperature and delayed flowering by 5-24 days depending on treatment and seasonal rainfall. Thermal time to flowering in the agroforestry system doubled from 600 to 1200 āC day as rainfall decreased. Photosynthetic and transpiration rates for understorey maize were similar to the 25 and 50% shade treatments when rainfall was high but were ca. 10% of those for unshaded sole maize in dry seasons. PPFD-saturated photosynthetic rate was initially similar in all treatments but fell sharply in the agroforestry system as the season progressed. Radiation conversion coefficients did not differ between unshaded sole and understorey maize. The results suggest that the ameliorative influence of tree shade was greater in low rainfall seasons as in savannah systems but that potential benefits were outweighed by below-ground competition. This may be managed by root-pruning trees.
DOI:
http://dx.doi.org/10.1016/j.agrformet.2009.02.002Altmetric score:
Dimensions Citation Count:
Publication year
2009
Authors
Lott, J.E.; Ong, C.K.; Black, C.R.
Keywords
Agroforestry, Gas exchange, Grevillea robusta, Maize, Microclimate
