Effects of the Spatial Pattern of Defoliation on Regrowth of a Tussock Grass I. Growth Responses

The effects of the spatial pattern of defoliation within a tussock grass, Agropyron desertorum, were investi- gated at a semiarid field site. In the middle of the spring growing season (mid-May), tussocks were clipped in repeat- able defoliation patterns, and the regrowth of foliage was monitored. These clipping patterns involved removal of fo- liage from different locations within the tussock, but the total amount of foliage removed was held constant. Active meristems were left intact in all cases. The spatial pattern of defoliation affected both initial rates of tussock regrowth and total growing-season aboveground biomass produc- tion. When leaves were removed low in the tussock (older leaves), regrowth was greater than after removal of the same quantity of foliage high in the canopy (younger leaves). These differences in regrowth were due to differences in the rate of new tissue production rather than differences in the timing of senescence. The results were consistent over two years even though aboveground production differed considerably between years. The interaction of the spatial pattern and timing of defoliation was also studied by clip- ping additional plants in late May. The timing of defoliation affected the relative influence of different defoliation pat- terns on regrowth. In those defoliation patterns where ac- tive meristems were not removed in the late-May clipping, there were no differences in regrowth of tussocks which had either upper or lower foliage removed. However, be- cause the grass culms had elongated by late May, active meristems were higher and were removed by one of the defoliation pattern treatments (a uniform clipping). This resulted in much less regrowth. Differences in the effects of clipping patterns applied in late May were associated with this removal of active meristems. Whereas, differences among clipping treatments following the earlier mid-May defoliation were probably a result of changes in factors which affected tussock carbon gain (e.g., light interception, foliage age structure). In either case, the spatial pattern of defoliation within a tussock grass clearly had significant effects on aboveground regrowth