The effects of the spatial pattern of defoliation on regrowth of a tussock grass : II. Canopy gas exchange.

The effect of different defoliation, patterns within a tussock grass on CO2 and water vapor exchange of entire tussocks was assessed and compared with differences in regrowth behavior. During rapid spring growth, 60% of the green foliage area was removed from Agropyron desertorum tussocks in different spatial patterns. Compensatory growth responses of defoliated tussocks corresponded well with increases in integrated daytime CO2 exchange rate of tussocks per unit foliage area (CERd) immediately following clipping. When leaves were removed from low in the tussock (older leaves), both regrowth and CERd were greater than after removal of foliage located high in the canopy (younger leaves). Water vapor conductance (gw) increased similarly for all clipped plants, regardless of defoliation pattern. Thus, the differential responses of CERd to the defoliation patterns could not be fully attributed to changes in gw. The increases in gw of all clipped plants resulted in large increses in tussock daytime water loss (Ed) and decreases in tussock water-use efficiency (CERd/Ed), even in the case where mostly older, shaded foliage was removed. Despite increases in CERd, whole-tussock CO2 fixation (net CO2 exchange per tussock) declined immediately after clipping because of the removal of considerable foliage. Whole-tussock CO2 uptake recovered more rapidly in tussocks which had lower leaf blades removed than in tussocks which had upper foliage removed because of differences in the rate of new foliage production.