Response of wild wheat populations to grazing in Mediterranean grasslands: the relative influence of defoliation, competition, mulch and genotype.
Abstract
Grassland management must be based on an understanding of key species' responses to various grazing regimes to achieve both production and conservation objectives. An experiment was designed to investigate several population processes that may potentially (i) contribute to the decline of Triticum dicoccoides (wild wheat) in intensively grazed grasslands, and (ii) promote the persistence of wild wheat in these grazing regimes. The experiment was conducted in natural Mediterranean grassland on the Korazim Plateau in northern Israel in the 1991-92 growing season. Nursery-grown seed of two morphologically distinct wild wheat genotypes were sown in plots with defined mulch applications and clipping regimes. Mulch application did not affect seedling emergence or establishment, but it did reduce tiller number per plant and ear size compared with plants grown without mulch. The detrimental effects of mulch on plant performance throughout the growing season indicated that both radiation and nitrogen limitations may have contributed to growth suppression. Mulch application reduced wheat biomass to a greater extent than that of interspecific competitors. The negative response indicated that mulch removal by intensive grazing during the dry season was unlikely to contribute to the decline of wild wheat in response to intensive livestock grazing. Both vegetative and reproductive performance of wild wheat increased by 50% in response to a reduction of interspecific competition following defoliation of neighbouring plants. A single severe clipping of vegetative wheat plants in defoliated neighbourhoods did not affect plant survival or tiller number, but did reduce ear and spikelet numbers and vegetative and reproductive biomass, compared with unclipped wheat plants. The positive wheat response to the reduction of interspecific competition almost exactly compensated for the negative effect of direct clipping on wheat fitness, and may thus contribute to the persistence of wheat populations. A second severe clipping of wheat plants in the reproductive growth phase severely reduced plant survival to reproduction, reproductive biomass, and seed quantity and quality in those plants that did become reproductive. One-half of the ears initiated following late-season clipping did not emerge from the flag leaf and produced mostly thin seed with reduced germinability. Geniculate genotypes exhibited greater grazing tolerance and reproductive performance than the erect genotypes in response to the second severe clipping. An increase in the relative abundance of geniculate genotypes in intensively grazed communities may provide an important persistence mechanism for wild wheat populations. An integrated estimate of wild wheat fitness, calculated as the mean reproductive output per seed sown, was <1 in plants clipped during the later phase of reproductive growth. This indicates that wheat populations would experience local extinction if this defoliation regime were continued for several successive years. Management prescriptions to conserve this key annual species must focus on the reduction or deferment of late-season grazing during the reproductive growth phase to ensure population persistence.