| Popis: |
While the light regimes of closed forest strands and open pasture have been quantitatively described and modelled, agroforestry involves greater complexity and spatial variability. This research involved intensive measurement of quantum flux density (QFD) above and below re-spaced tree strands of Sitka spruce (Picea sitchensis [Bong.] Carr.) and hybrid larch (Larix x eurolepis Henry) in conjunction with measurement of tree growth. QFD was also measured above and below pasture growing in boxes in imported soil sunk to ground level beneath the trees. The Monteith hypothesis that crop growth in unstressed conditions is linearly related to the amount of QFD intercepted by its canopy was found to hold for above ground growth of trees at agroforestry spacings. The overall dry matter: QFD quotient (e) for Picea sitchensis across the range of tree frequencies was 0.32 g mol-1 which is lower than typical values reported for agricultural crops, young container grown trees of Salix viminalis and Populus trichocarpa and closed-canopy Picea sitchensis, but comparable with previous estimates for a range of mature tree stands. e was unaffected by tree spacing, but was significantly higher for the tall tree stand which had been pruned. The mean annual QFD transmitted to the understorey varied from 39% to 96% of that in the open across the agroforestry treatments. The mean transmittance was similar but spatial variability of QFD was significantly higher in tree stands with crown to ground level when compared with stands having similar crown dimensions but which had been pruned to approximately 1.3 m height. More than 90% of seasonal pasture growth could be explained by a regression of growth on incident QFD. |
