Shorter species had leaves with higher SLA and/or lower toughness, a similar pattern to that found in other areas (Díaz et al. 1999; Lavorel & McIntyre 1999; Díaz, Noy-Meir & Cabido 2001). These results suggest that short plants have higher quality and growth rate, and are more tolerant to herbivory than taller plants. A trade-off between support and growth could explain these relationships. Taller plants have competitive advantages through prior access to light, but their investment in support tissues is costly. Shorter plants, in turn, could maximize growth by minimizing costs related to support (Westoby et al. 2002). Additionally, the low SLA and/or high toughness could be a way of coping with the harsh climatic conditions (wind, frosts) to which taller plants are more exposed in the Patagonian steppe. The presence of some relatively tall woody species with low SLA but soft leaves (Empetrum rubrum and Chiliotrichum diffusum) weakened the correlations among toughness and the other traits. When calculated for dicotyledons alone, the height–toughness correlation was not significant, while for monocotyledons, needing to have structural strength in their leaves to reach higher stature, the correlation was significant.
As predicted, sheep selected shorter plants over taller ones, indicating that a short stature is not an effective avoidance strategy in relation to sheep, the small mouth and body size of which allow them to eat very close to the ground (Schwartz & Ellis 1981; Hanley 1982; Hodgson et al. 1991). The avoidance of taller plants is probably not related to their height per se; rather, it seems to be caused by the associated low quality of their leaves. For example, Festuca gracillima, which forms a tussock of about 30 cm of height, has lower quality (estimated through nitrogen concentration) than the shorter, non-tussock sward (Posse 1997; Anchorena et al. 2001; Mendoza et al. 2002). The three tallest shrubs (Berberis buxifolia Lam., Chiliotrichum and Empetrum), all with low SLA, are able to grow successfully in poor environments (Collantes, Anchorena & Cingolani 1999), suggesting that they have low intrinsic growth rates and nutrient content in their leaves (Berendse & Elberse 1990; Chapin, Autumn & Pugnaire 1993).
However, direct plant quality indicators (SLA and leaf toughness) were not related to sheep selectivity as expected. SLA was not related at all, and leaf toughness showed a unimodal response. Selectivity by invertebrate herbivores has been found to be negatively related to leaf toughness and positively related to SLA and other direct indicators of high quality (Coley 1983; Herms & Mattson 1992; Cornelissen et al. 1999; Díaz et al. 1999; Pérez-Harguindeguy et al. 2003). As leaf toughness is positively associated with fibre content (Coley 1983), plants with hard leaves have low digestibility and are less preferred. Nevertheless, in this study sheep appeared to respond negatively to toughness only beyond a threshold level. This was the case for the tussock grass Festuca gracillima, similar to other tussock grasses elsewhere that are generally avoided due to their very low digestibility (Hunter 1962; Bakker & Ruyter 1981; Bakker, Leeuw & van Wieren 1983; INTA 1997; Golluscio et al. 1998). Excluding Festuca gracillima, the relation between toughness and selectivity was still quadratic, but mostly positive. This result is unusual and surprising. The selection of tougher plants by sheep could be partially explained by higher amounts of chemical defences in the softer leaves of the perennial dicotyledons of this flora compared with the tougher leaves of grasses (Schwartz & Ellis 1981; Coughenour 1985). A similar inverse relation among leaf toughness and chemical defences could be present within dicotyledons in our study area. Some of the dicotyledons with softer leaves are known to have high concentrations of chemical defences, for instance Empetrum rubrum (Moore & Williams 1970), Chiliotrichum diffusum (K. Braun, personal communication), Rumex acetosella L. and Oxalis enneaphylla Cav. (Moore 1983). It is also possible that the proportion of species with soft leaves in the diet (and hence their selectivity) was underestimated due to the use of microhistological technique to estimate the diet composition (Holechek, Vavra & Pieper 1983).
Contrary to our expectations, response to grazing did not show any relationship with sheep selectivity. The response of each species was related only with two plant traits, SLA and height. Although tall species are not highly selected by sheep, they are consumed in winter when regrowth is impossible (Posse, Anchorena & Collantes 1996). Additionally, trampling contributes to their mechanical destruction (Cingolani 1999; Anchorena et al. 2001; Stoffella 2003). Their low SLA values suggest that a low recovery capacity after damage (low tolerance) is the cause of their decrease, even with moderate grazing intensity. These results were opposite to those obtained by Anderson & Briske (1995), who found that selective herbivory of dominants (or late seral species), rather than a lesser expression of herbivore tolerance, was the main mechanism contributing to herbivore-induced species replacement in mesic grasslands. In our case, short species with high SLA could have benefit from grazing through the elimination of light competition from taller but slower-growing species (Westoby et al. 2002). Compensatory growth also operates (McNaughton 1983). Similar trends in the response to grazing of SLA and height were found by Díaz, Noy-Meir & Cabido (2001) on mesic grasslands in Argentina and Israel, but the opposite was found by Wardle, Bonner & Barker (2002) in New Zealand Nothofagus forests, where browsing clearly favours low-quality species. In contrast, Vesk, Leishmann & Westoby (2004) did not find a consistent response of height and SLA to grazing for semi-arid Australian rangelands. The low R2 for the GRI suggests that there are other factors involved in the grazing response not accounted for here. Additionally, the relationships of grazing response with plant traits and sheep selectivity could be obscured because, in many cases, species do not have a consistent increaser or decreaser response, but rather the response depends on the grazing context (Noy-Meir, Gutman & Kaplan 1989; Westoby, Walker & Noy-Meir 1989; Westoby 1999; Vesk & Westoby 2001).
At the whole community level, the patterns were closer to our expectations. Shorter swards with higher than average SLA supported species that were preferred by sheep compared with taller swards with lower SLA. Additionally, short swards have experienced a greater grazing pressure during the history of domestic grazing in the area. Previous studies have shown that sheep select grazing lawns or more open tussock grasslands over closed tussock grassland and shrublands (Posse, Anchorena & Collantes 2000; Anchorena et al. 2001; Cingolani et al. 2002). The differences between the results obtained at the species and community levels are related to the characteristics of the dominants. Under ungrazed and lightly grazed situations the dominants are Chiliotrichum and Festuca, two avoided but highly susceptible tall species with low SLA (Oliva 1996; Stoffella 2003). Grazing produces a partial replacement of these dominants by several subordinate species, increasing evenness and the average selectivity index, even when many of the replacement species are still avoided (Cingolani 1999; Collantes, Anchorena & Cingolani 1999; Posse, Anchorena & Collantes 2000). These replacement species are functionally different from previous dominants, as judged by their SLA and height values. This fact determined an overall decrease in height and increase in SLA with increasing grazing. As SLA is related to key ecosystem processes, such as litter decomposition and productivity (Cornelissen et al. 1999; Pérez-Harguindeguy et al. 2000; Lavorel & Garnier 2002), grazing in this case markedly changed ecosystem functioning. This change in the functional properties of the community is in contrast with the predictions of Walker, Kinzig & Langridge (1999), who suggested that, due to functional redundancy in ecosystems, disturbance would produce a replacement of dominant species by subordinates with similar functional properties, without strong changes in ecosystem functioning.
The results at the community level support the hypothesis that in environments with high resource availability, tolerance is the most successful strategy to cope with grazing. The greater tolerance of some subdominant species, compared with the lesser tolerance of dominants, seems to be the main mechanism driving grazing-induced species replacement. However, at the species level more complex patterns appeared. The lack of relationship between sheep selectivity and grazing response indicates that not all increaser species are tolerant. On the contrary, many species having traits that apparently indicate a high quality (i.e. high SLA and/or low toughness) are in fact avoided. Chemical defences could be playing a more important role in this area than expected, allowing some short species to achieve high growth rates without incurring the cost of being eaten, and to increase within the grazing lawn community under grazing.
The results obtained in this study have practical implications for range management in the humid Magellanic steppe and similar rangelands. In general, the grazing-induced formation and maintenance of short, productive and palatable grazing lawns at the expense of tall, tough grassland has been considered to favour animal nutrition and production (McNaughton 1984). It has been implicitly assumed that in these cases intensive grazing can only improve animal production and no special care in management is necessary. The results here indicate the potential risk of a shift in composition of grazing lawns towards a dominance of species avoided by sheep. This study highlights the importance of studying in more detail the variation within lawns and possible effects of management once the lawn is formed. In the study area, tussocks hardly return after grazing has eliminated them (Cingolani 1999). Thus, there are potential management tools, such as periodical rests, for improving lawn composition and forage value, which would not favour tussocks but could benefit palatable short species.