Response of wild wheat populations to grazing in Mediterranean grasslands: the relative influence of defoliation, competition, mulch and genotype

Summary

Livestock grazing intensity is a major factor controlling distribution and abundance of declining populations of wild wheat Triticum dicoccoides, an annual of eastern Mediterranean grasslands. Its large seeds and erect habit give it a competitive advantage over associated annuals, but these attributes and high palatability render it susceptible to grazing. An experiment was undertaken in northern Israel to investigate several processes that may potentially contribute to its decline in intensively grazed grasslands, and also to promote the persistence of wild wheat through appropriate management.

Study area: The experiment was conducted in natural grassland on the Korazim Plateau in the 1991-92 growing season.

Experimental design: Nursery-grown seed of two morphologically distinct wild wheat genotypes (an 'erect' population from an ungrazed rocky area, and a 'geniculate' population from an intensely grazed area whose tillers tended to be short and initially horizontal or inclined, bending upwards to grow vertically only in the reproductive stage) were spring sown. At the end of October 1991, a 25 × 25 m area was fenced to exclude further grazing, within which, 16 replicate blocks were established. Each block was divided into 16, 30 × 30 cm plots, the inner 20 × 20 cm sown with wheat. The plots were allocated combinations of mulch (intended to supress other plants) and clipping treatments (to mimick grazing) and seed genotype, as follows.

a) No mulch; 2 seed sources × 4 clipping treatments:

1) no clipping (control)

2) wheat not clipped, all other plants clipped (reduced competition)

3) wheat plants clipped once, all other plants clipped

4) wheat plants clipped twice, all other plants clipped

(b) Mulch added at two levels; 2 seed sources × 4 mulch + clipping treatments:

1) single mulch amount (120 g/m²) × clipping treatments 0, 1 and 2

2) double mulch amount (240 g/m²) – no clipping

Data collection: Seedling emergence, survival and the number of tillers per plant were recorded on several dates. Near the end of the growing season (26 April), the number of 'emerged' and 'enclosed' wheat ears was recorded. Plant material in each plot was harvested and separated into ears (emerged and enclosed separately), wheat vegetative material, and other plants, when plants were mostly dry (6-15 May 1992). The material was dried and weighed. The number of harvested ears and of spikelets per ear of each type was recorded.

Seeds were extracted from 10 spikelets per ear type in each plot. Seeds were classified and counted by position in the spikelet (basal or terminal) and by 'seed fullness' ('full' or 'thin'). In April 1992, seed germinability was tested; seed  was placed in a Petri dishes containing wet filter paper.  The number of germinating seeds was recorded over 8 days.

Mulch application: Mulch application did not affect wheat seedling emergence or establishment, but did reduce tiller number per plant and ear size compared with unmulched plants. Mulch application reduced wheat biomass to a greater extent than other plant competitors. The negative response indicated that the mulch removal observed by intensive grazing during the dry season was unlikely to contribute to the decline of wild wheat.

Clipping: Vegetative and reproductive performance of wheat increased by 50% in response to a reduction of competition from other species following clipping of neighbouring plants. A single severe clipping of vegetative wheat plants in clipped 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.

A second severe clipping of wheat plants in the reproductive growth phase severely reduced survival to reproduction, and seed quantity and quality in those plants that did become reproductive. Around 50% of the ears initiated following late-season clipping did not emerge from the flag leaf and produced mostly thin seed with low germinability.

Genotypes: Geniculate genotypes exhibited greater grazing tolerance and better reproductive performance than the erect genotypes in response to the second clipping. An increase in the relative abundance of geniculate genotypes in intensively grazed areas may provide an important persistence mechanism for wild wheat populations.

Wild wheat fitness: The estimate of wild wheat fitness was < 1 in plants clipped during the later phase of reproductive growth. This indicates that wheat populations would experience local extinction if this regime (equivalent to intensive grazing at this time in the life cycle) were continued for several successive years.

Conclusions: Based on these results, the authors conclude that management to ensure population persistence of wild wheat must focus on the reduction or deferment of late-season grazing during the reproductive growth phase.


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