From this two-year study in Utah, Gold and Caldwell quantified the effects of different spatial patterns of defoliation on regrowth rates, peak standing biomass, and aboveground biomass production of a semi-arid-region tussock grass, Agropyron desertorum. Tussocks which had older leaves (lower in the canopy) removed during rapid growth in mid-May quickly exhibited higher regrowth rates and were able to fully compensate aboveground for this defoliation by the time of peak standing biomass. Tussocks from which younger foliage (higher in the canopy) was removed had lower regrowth rates and were only able to partially compensate for this defoliation. These differences in regrowth were due to differences in the rate of new tissue production rather than differences in the timing of senescence. Uniform defoliation in late May removed all apical and intercalary meristems and resulted in no compensation in aboveground biomass production relative to control plants. In the late-May clipping, the impact of the spatial pattern of defoliation on plant meristems had greater consequences for tussock regrowth than changes in foliar structure. Active meristems were left intact in the two leaf-blade removal patterns in both mid- and late-May defoliations. Although the compensatory growth response was different in lower and upper plants after clipping in mid-May, it was equal in those two treatments following late-May defoliation. This could have been due to factors such as changes in the developmental state of the intercalary meristems or smaller differences in photosynthetic characteristics of different-aged foliage in late May.
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