Individual study: The effects of continuous and rotational pony grazing regimes on sapling damage and density of naturally regenerating pedunculate oak Quercus robur in a Montery pine Pinus radiata plantation forest in Galicia, Spain
McEvoy P.M., McAdam J.H., Mosquera-Losada M.R. & Rigueiro-Rodriguez A. (2006) Tree regeneration and sapling damage of pedunculate oak Quercus robur in a grazed forest in Galicia, NW Spain: a comparison of continuous and rotational grazing systems. Agroforestry Systems, 66, 85-92
Concern for natural regeneration of native forest trees in Europe has led to studies looking at the effects of large herbivores on growth and survival of tree seedlings and saplings. In the absence of, or where there is limited grazing due too population declines in native large herbivores, livestock grazing may be useful in forest conservation if sympathetic grazing regimes are introduced. This study investigated sapling damage and density of natural regeneration of pedunculate oak Quercus robur in a pony-grazed, non-native Monterey pine Pinus radiata plantation forest in northwest Spain under continuous and rotational grazing regimes.
Study site: The study was undertaken at Monte San Breixo, Galicia (7º48' W, 43º09' N), northwest Spain, in a 321 ha, 30 year-old Monterey pine plantation. The forest is grazed by free-ranging ponies to reduce combustible understorey vegetation thus lowering fire risk.
Experimental design: In June 2000, a 24 ha experimental area was divided into four 6-ha paddocks, with two replicates of continuous and rotational grazing. Stocking density was 0.33 ponies/ha (2 ponies/paddock). In rotational grazing paddocks, ponies grazed for 30 out of 120 days to give 90 days rest between grazing periods.
Oak seedling and sapling counts: In three adjacent ungrazed 'control' areas in July and August 2001, seedlings and saplings less than 1 m tall were counted, and measurements of sapling dimensions were made, in randomly placed quadrats. The effect of five dominant ground vegetation types present (grass sward, bramble Rubus ulmifolius, gorse Ulex europaeus, western gorse U.gallii and bracken Pteridium aquilinum) on oak regeneration was assessed.
Oak sapling damage: In July 2001, measurements of c.30 oak trees were made along transects in each vegetation type in the control areas, and within the paddocks, recording stem diameter, height, and canopy diameter.
More oak seedlings and saplings were recorded in areas of grass sward (average 2.9 saplings/10 m²) than areas dominated by taller ground vegetation: bramble, gorse and western gorse, had similar densities (averages 1.9, 1.5 and 1.8, respectively); in bracken, regeneration was almost zero.
Height and average canopy diameter of similar-aged saplings was greater in the controls than the paddocks; fresh shoots of saplings were browsed by ponies. Saplings in control plots had the greatest canopy diameter to height ratio (0.6), rotationally browsed saplings were intermediate (0.45), and continuously grazed lowest (0.3) reflecting the greater browsing pressure.
Whilst these results might be as predicted, sapling damage was similar between continuously and rotationally grazed paddocks. Differences in severity of damage (some saplings untouched) appeared related to uneven habitat use by ponies.
Conclusions: Some studies have demonstrated that, in certain cases, grazing can encourage establishment of grasses over woody shrubs, thus grazing may indirectly increase tree regeneration. This would be offset to a greater or lesser degree by browsing damage, but in balance, at appropriate low intensities, livestock grazing may be beneficial to native tree regeneration.
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