The effects of the spatial pattern of defoliation on regrowth of a tussock grass : III. Photosynthesis, canopy structure and light interception.

The spatial pattern of foliage removal from a tussock grass can influence regrowth through effects on daily carbon gain (CERd). This field study examined the extent to which tussock photosynthetic responses to different defoliation patterns were associated with changes in whole-canopy attributes (e.g., foliage age structure, canopy light microclimate). During the spring growing season, 60% of the green foliage area was removed from individual Agropyron desertorum tussocks with scissors in different spatial patterns. These patterns represented extremes of defoliation patterns that might be inflicted by natural herbivores. Tussock photosynthesis (per unit foliage area) at high light (2000 μmol photons m-2 s-1 between 400 and 700 nm; P2000) increased following clipping with all defoliation patterns. The increases in P2000 were greater when leaves were removed from low in the tussock (older leaves) than if leaves high in the canopy (younger leaves) were removed. These relative changes of P2000 among clipping patterns paralleled the responses of CERd and regrowth from an earlier study. Furthermore, the changes in P2000 corresponded with increases in the proportion of foliage within the tussocks that was directly illuminated at midday. The greater photosynthesis of tussocks after lower-leaf removal was directly related to a higher proportion of younger foliage and a smaller fraction of foliage shaded within the tussock. In a dense canopy, such as these grass tussocks, the influence of defoliation on whole-canopy attributes may be of primary importance to whole-plant photosynthetic responses.