Vegetation characteristics at the site
During the experiment a total of 166 species was recorded at the site, 74% of which were annuals. Average percentage plant cover in all the paddocks across the 4-year study was dominated by 10 species, which accounted for almost 75% of total plant cover. These comprised four grasses (37%), two thistles (14%), two legumes (15%), one crucifer (6%) and one forb (3%). About half of the relative plant cover comprised annual species and the other half perennial species, mostly hemicryptophytes. Palatable species (grasses, annual legumes and several dicots) represented close to 55% of the total plant cover. Tall grasses were the main palatable plants (37%) and included the perennial grass Hordeum bulbosum, the most abundant species (23%), and three annual grasses, Avena sterilis (6·7%), Hordeum spontaneum (3·9%) and Triticum dicoccoides (3·5%). Other important, but less palatable, species were the perennial legume Bituminaria bituminosa (9·4%), the annual thistle Scolymus maculatus (5·8%), the perennial thistle Echinops adenocaulos (7·9%) and the annual crucifer Rapistrum rugosum (6·2%). The most common annual legume was the prostrate Trifolium pilulare (6%), while the less palatable Echium spp. were the most abundant annual forbs (3·8%). The main geophyte present at the site was Asphodelus ramosus (0·8%), an unpalatable plant with tuberous roots.
Effects of grazing treatments on species richness
The grazing treatments had a significant effect on species richness in 1994, 1995 and 1996 (F1,5 = 13·3, P = 0·007; F1,5 = 12·9, P = 0·008; F1,5 = 7·33, P = 0·027, respectively; Fig. 3). Results from the repeated measures anova showed significant differences in species richness between treatments over time (F5,5 = 7·73, P = 0·014), with higher values in continuous grazing treatments (CM and CH). Differences among years and their interaction with treatments were also significant (F3,5 = 22·7, P = 0·002; F15,15 = 28·4, P = 0·0019, respectively). Species richness was greater in paddocks under continuous grazing, in contrast with plots with seasonal grazing (F1,5 = 25·3, P = 0·005). Lowest species richness was noted in paddocks with the heavy late (S-HL) treatment, which remained practically ungrazed during the growing season and at the time of the vegetation sampling. In contrast to the differences in species richness among grazing treatments, no pattern of increase or decrease in richness was found within each treatment during the 4-year study (Fig. 3).
Figure 3. The effects of grazing treatments on species richness in continuous (a) and seasonal regimes (b and c). Treatments: continuous moderate, CM (black squares); continuous heavy, CH (white squares); seasonal heavy early, S-HE (white triangles); seasonal heavy late, S-HL (black triangles); seasonal very heavy early, S-VHE (white circles) and seasonal very heavy late, S-VHL (black circles).
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Effects of grazing treatments on functional groups
Tall annual and perennial grasses were the main contributors to the palatable biomass consumed by the cattle, and were similarly affected by the grazing treatments. Tall annual grasses were sensitive to grazing manipulations (Fig. 4a–c and Table 1) and the three main species in this group (Avena sterilis, Hordeum spontaneum and Triticum dicoccoides) had similar patterns of change. Their cover showed a general trend of reduction with increase of grazing intensity (F5,5 = 10·5, P = 0·011), and was affected by differences in growth conditions between years (F3,15 = 11·3, P = 0·001). The reduction of cover by grazing was stronger in rainy years (1995 and 1996; Table 1) than in the dry year (1994) (treatment × year, F15,15 = 4·26, P = 0·004), particularly when compared with the high cover in the S-HL treatment in which the paddocks were not under grazing at the time the vegetation was recorded. Indeed, when contrasting timing (early vs. late grazing), cover was higher in the late treatments (F1,5 = 15·4, P = 0·011), particularly S-HL. When comparing stocking rates (SR) in the seasonal regime (1·1 vs. 2·2 cow ha−1 year−1), a reduction in cover was noted with the increase in grazing pressure (F1,5 = 15·6, P = 0·011). However, the reduction in cover was dependant on the season of grazing (F1,5 = 7·26, P = 0·043), mainly due to the S-HL treatment.
Figure 4. The effects of grazing treatments on functional group cover. Tall annual grasses (a–c); tall perennial grasses (d–f); short annual grasses (g–i). Key for grazing treatments as in Fig. 3.
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Table 1. Results of two-way and repeated measures anovas for functional groups over the 4-year study. Bold numbers indicate significant differences between treatments. Stocking rate (SR), continuous vs. seasonal (C vs. S). Coefficient of variation (CV) in per cent*
|Short annual grasses||Tall annual grasses||Tall perennial grasses||Annual legumes||Perennial legumes||Annual thistles||Perennial thistles||Crucifers||Geophytes||Forbs|
|Treatment × year|| ||15,15||1·91||0·113||4·26||0·004||1·25||0·336||0·65||0·797||5·00||0·002||0·69||0·756||1·33||0·293||0·95||0·535||1·56||0·201||2·14||0·076|
|CV|| || ||83·3|| ||75·1|| ||29·2|| ||66·6|| ||51·1|| ||73·9|| ||49·9|| ||59·3|| ||92·7|| ||40·4|| |
|Timing (early vs. late)|| ||1,5||4·74||0·081||15·4||0·011||4·66||0·083||3·25||0·131||0·67||0·449||54·1||0·001||3·64||0·115||2·31||0·189||0·20||0·675||0·01||0·920|
|SR (seasonal)|| ||1,5||5·15||0·072||15·6||0·011||0·41||0·552||0·53||0·499||0·01||0·963||32·5||0·002||0·96||0·373||7·15||0·044||3·03||0·142||0·25||0·638|
|Timing × SR|| ||1,5||0·03||0·874||7·26||0·043||1·33||0·301||0·02||0·892||0·19||0·681||0·01||0·927||2·53||0·173||13·7||0·014||0·39||0·561||0·01||0·918|
|C vs. S|| ||1,5||1·11||0·341||5·98||0·058||0·84||0·402||0·56||0·468||0·11||0·754||31·8||0·002||0·01||0·973||14·1||0·013||2·04||0·213||0·01||0·932|
|SR × C vs. S|| ||1,5||5·10||0·073||21·4||0·006||5·63||0·064||3·19||0·134||0·08||0·788||26·5||0·004||0·04||0·850||10·8||0·021||2·95||0·146||0·39||0·558|
Cover of tall perennial grasses (mostly Hordeum bulbosum) was consistently higher in paddocks with continuous grazing at lower pressure (moderate vs. heavy) as well as in treatments with late grazing, as found for tall annual grasses (Fig. 4d–f). However, the overall analysis showed that grazing treatments, differences between years and contrasts, had no effect on cover of perennial grasses (Table 1). Cover of tall perennial grasses was significantly affected by the grazing treatment only in 1996 (Table 1), the relatively wet year, with the largest differences occurring between treatments CM and S-VHE.
Short annual grasses showed a small but significant trend of increase in cover from year to year (2·1–6·4%, average for all treatments, F3,5 = 16·3, P = 0·001) but their cover was not influenced by grazing treatments and regimes (Fig. 4g–i and Table 1). In contrast, cover of annual legumes decreased with time, specially in the CH and S-VH treatments (F3,5 = 5·63, P = 0·009). However, no significant differences between grazing treatments were found, as observed for the short annual grasses, even though a trend of higher cover occurred under CH and early grazing treatments (Fig. 5a–c and Table 1).
Figure 5. The effects of grazing treatments on functional group cover. Annual legumes (a–c); perennial legumes (d–f). Key for grazing treatments as in Fig. 3.
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Perennial legumes were represented by a single species, Bituminaria bituminosa. Its cover was not affected by grazing treatments (Fig. 5d–f and Table 1) but increased from year to year, principally in the treatments with continuous and S-HL grazing (F3,5 = 23·5, P = 0·001), while in other treatments cover remained unchanged (treatment × year, F15,15 = 5·00, P = 0·002).
Annual thistles tended to show opposite patterns of response compared with tall grasses, as their cover increased with the severity of the grazing treatment (Fig. 6a–c). Changes in cover were related to yearly conditions and grazing treatment (F3,5 = 11·1, P = 0·001; F5,5 = 24·7, P = 0·002, respectively). In 1996 and 1997, thistle cover values were lower in paddocks that were grazed late than in those that were grazed early in the season at high stocking rates (S-HL vs. S-VHE) (F1,5 = 21·2, P = 0·002; F1,5 = 11·7, P = 0·009, respectively). Indeed, contrast analysis of the seasonal regime showed that early grazing (HE and VHE treatments) increased annual thistle cover (F1,5 = 54·1, P = 0·001), and that this increase was greater at the highest stocking rate (2·2 vs. 1·1 cow ha−1 year−1) (F1,5 = 32·5, P = 0·002). Furthermore, cover of annual thistles was higher in the continuously grazed paddocks when contrasted with the seasonal regime plots, and this trend was dependent on stocking rate intensities (SR × continuous vs. seasonal, F1,5 = 26·5, P = 0·004).
Figure 6. The effects of grazing treatments on functional group cover. Annual thistles (a–c); perennial thistles (d–f); crucifers (g–i). Key for grazing treatments as in Fig. 3.
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Perennial thistle cover varied according to differences in growth conditions between years, with the lowest cover in 1996 (F3,5 = 11·8, P = 0·001), a relatively rainy season in which tall grasses reached a high cover (Fig. 6d–f). In 1995, cover was lower in paddocks that were late grazed compared with those grazed early in the season with high stocking rates (S-HL vs. S-HE, F1,5 = 7·54, P = 0·022). However, the overall analysis showed that cover of perennial thistles was not affected by grazing treatments (Table 1).
Cover of crucifers, all annual species hardly consumed by the cattle, tended to be higher with higher stocking rates (F5,5 = 8·36, P = 0·018; Fig. 6g–i). In the seasonal grazing treatments, the contrast analysis showed that cover was significantly higher when the stocking rate was double (1·1 vs. 2·2 cow ha−1 year−1, F1,5 = 7·15, P = 0·044), particularly when grazing started early in the season (F1,5 = 13·7, P = 0·014). Furthermore, as observed for annual thistles, cover of crucifers in paddocks that were grazed late in the season (S-HL) was lower than in continuously grazed paddocks (CH). Differences between continuous vs. seasonal regimes were dependent on the intensity of grazing (F1,5 = 14·1, P = 0·013; F1,5 = 10·8, P = 0·021, respectively).
Cover of geophytes, mainly the non-palatable Asphodelus ramosus, was low (general mean cover of 0·8%) and showed no significant changes due to grazing treatments and year conditions (Table 1). Furthermore, even the heaviest grazing pressure treatment applied in this study did not increase cover of this unpalatable monocot, commonly dominant in overgrazed Mediterranean environments.
Forbs was a heterogeneous group including low-cover dicots that did not share clear trends of response to the grazing treatments. Most species in this group were less palatable annual composites (2·2%), annual umbellifers (2·3%), perennial umbellifers (e.g. the toxic tall Ferula communis, 0·6%) and the less palatable annual Echium spp. (3·8%), with a total cover close to 9%. Forb cover was affected by differences in growth conditions between years, but not by grazing treatments (Table 1).