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Recently, Díaz, Noy-Meir & Cabido (2001) demonstrated that grazing responses could be predicted from simple measurable traits of herbaceous plant species from subhumid Argentine and Israeli grasslands (Argentina, c. 880 mm mean annual rainfall, c. 10 °C mean annual temperature; Israel, c. 500 mm mean annual rainfall, 21 °C mean annual temperature). This is a major step because it suggests that at a new site with different species, grazing responses could be predicted by measuring traits of plants rather than by carrying out a new grazing trial. This could potentially save significant research effort and better direct management. The critical assumption is that the relationships between species’ traits and species’ responses to grazing hold in the new site. One way that this assumption could fail would be for species to respond inconsistently to grazing (Noy-Meir, Gutman & Kaplan 1989; Vesk & Westoby 2001); another would be if the relationships between traits and responses were context-specific. This second possibility was the objective of the present study: to test if the relationships identified by Díaz, Noy-Meir & Cabido (2001) hold for semi-arid and arid shrublands and woodlands, the most extensive rangelands in Australia.
The major findings of Díaz, Noy-Meir & Cabido (2001) that served as hypotheses for this study were that plant species that decrease in abundance under increased grazing pressure (decreasers) are characterized (in decreasing order of importance) by tall height, perennial life history, large (and thus, all else being equal, heavy) leaves and low specific leaf area (SLA), compared with neutral species and species that increase under increased grazing pressure (increasers). We need to know under what conditions these relationships between traits and grazing responses hold and, for situations where they do not hold, why this is. There are a number of possibilities for why these relationships may or may not hold in semi-arid and arid rangelands, which we outline below.
Height is hypothesized to decrease under increased grazing pressure because tall species receive most grazing pressure and short species are protected from grazing by tall species. This is based on the assumption that the groundstorey vegetation (c. ≤ 1 m) is dense and that grazers take bites from above. However, in situations where grazers can take bites from the side and where short species are not protected by taller species, there may be no short species advantage. Such a situation may be encountered where vegetation is sparse, as in semi-arid and arid shrublands.
Leaf size is hypothesized to decrease under grazing pressure (Noy-Meir, Gutman & Kaplan 1989; Díaz, Noy-Meir & Cabido 2001) because larger leaves provide better bites for grazers, and smaller leaves require either more bites for a given leaf area (and mass) or include more stem material if taken in the same number of bites. This hypothesis should hold across all vegetation types.
Westoby (1999) suggested that SLA might be related to grazing responses in a different manner at low and high grazing intensity. At low grazing intensities, selective grazing on high SLA (thin, soft leaves) species may lead to their decreasing relative to low SLA species. However, under intense, non-selective grazing intensity, all species are grazed and high SLA species may be advantaged by faster regrowth, linked to faster leaf turnover and higher rate of return per gram of carbon invested in leaf tissue. Under conditions of set stocking in semi-arid and arid rangelands, grazing intensity and specificity may be affected by the amount of herbage available, which varies as a result of past rainfall (Westoby 1974; Landsberg et al. 2002). Thus the relationship between SLA and grazing response may be context-specific.
It is hypothesized that seed mass will decrease with grazing. This is because grazing removes above-ground biomass, creating open space. Grazers may also physically disturb soil by feeding and walking. Both physical disturbance and open space create opportunities for colonization. Species able to take advantage of increased opportunities for recruitment are likely to be those that produce the greatest quantity of seed per unit biomass. As seed mass is inversely proportional to seed output per m2 of canopy (Henery & Westoby 2001), smaller seeded species will have more colonization opportunities than large seeded species and so may increase in abundance under grazing (Westoby 1999). This hypothesis is likely to be true for vegetation types that are site-limited, such as dense vegetation. Semi-arid and arid shrublands have an open canopy and are not site-limited; instead recruitment occurs episodically in response to rainfall.
Annual species have been shown to increase at the expense of perennials under grazing pressure in many systems (Whalley, Robinson & Taylor 1978; Noy-Meir, Gutman & Kaplan 1989; McIntyre & Lavorel 2001). Under year-round grazing and relatively fixed growth seasons, annuals may allocate all resources to reproduction and avoid grazing consequences over unfavourable periods. In contrast, perennials maintain vegetative biomass and remain exposed to grazing during unfavourable periods (Bazzaz, Ackerly & Reekie 2000). Annuals also have greater reproductive allocation than perennials, which, all else being equal, means greater seed output (Bazzaz et al. 1987). Short generation times allow rapid exploitation of colonization opportunities due to grazing disturbance (Grime 1977). As for seed size (above), site limitation is not expected to operate in these open vegetation types. Also, growth seasons are fairly indeterminate for the majority of the sites studied here. Thus, annuals may not have strong advantages over perennials under grazing in these semi-arid and arid shrublands.
In this study we aimed to broaden empirical knowledge of how traits may be used to predict grazing responses under different conditions. We present tests of the above hypotheses on grazing responses for species of arid and semi-arid shrublands and woodlands of Australia. Previously published grazing studies were combined with trait data from previous surveys of plant traits (Leishman & Westoby 1992, 1994) and newly collected data to assess relationships between traits and grazing responses at several shrubland and woodland sites.
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This study has shown that traits that have been shown to predict grazing responses in Argentine and Israeli subhumid grasslands (Díaz, Noy-Meir & Cabido 2001) broadly failed to explain grazing responses in Australian semi-arid and arid shrublands and woodlands. There were no effects of plant height or leaf size on grazing responses. Annuals were more likely to be increasers in both the pooled analysis and meta-analysis. Forbs and high SLA species were more likely to increase in the pooled analyses, but not in the meta-analyses. The conservative interpretation would be to have greater confidence in the meta-analyses, as the pooled analyses had the characteristics of single species contributing multiple and often conflicting responses. Analyses of traits within growth forms (McIntyre et al. 1999) were limited in power but provided little evidence for relationships between traits and responses beyond annual grasses, which have high SLA leaves, being increasers.
We do not think that the difference in the analyses between the current study and that of Díaz, Noy-Meir & Cabido (2001) is the reason for discrepancies between the findings. While the details differed, if relationships with an explanatory power (r2) ranging from 5% to 30% for single traits, were operating (as found by Díaz, Noy-Meir & Cabido 2001) the present analyses should have detected them. We believe that the important difference between the present study and the study by Díaz, Noy-Meir & Cabido (2001) is the vegetation structure. The Argentine and Israeli data were comprised solely of herbaceous species from productive grasslands bearing a continuous sward. Vegetation cover was > 90% for the Argentine sites (Díaz, Cabido & Casanoves 1998), and the Israeli sites achieved 100% cover each spring (Noy-Meir, Gutman & Kaplan 1989). Above-ground net primary production (ANPP) was c. 300 g dry matter (DM) m−2 year−1 at Argentine and Israeli sites (Díaz, Noy-Meir & Cabido 2001). In such a situation, grazers eat from top down and so, most importantly, tall species receive most grazing pressure and short species benefit by avoiding leaf loss and through relaxed competition (Díaz, Noy-Meir & Cabido 2001).
In contrast, in the Australian semi-arid and arid rangeland studies analysed here, more than 20% of responses were from shrubs and subshrubs, bare ground was frequent and ANPP was lower. Fine-tissue ANPP was estimated to be < 80 g DM m−2 year−1 for nine of the 11 sites in this study (Appendix S2; Barrett 2002). Bare ground and litter covered at least 50% of the ground in all eight grazing gradients studied by Landsberg et al. (1997) analysed here, and more than 90% in several gradients. As a result of these three factors (low productivity, growth form diversity and bare ground) grazers can move through vegetation and taller species do not necessarily receive greater grazing pressure. In addition, ground-layer species are more exposed to grazers as there are fewer tall species in the way. Hence, height is not so likely to be important as a trait, and simple traits in general are less likely to be good predictors of grazing response in semi-arid shrublands and woodlands (Landsberg, Lavorel & Stol 1999).
A previous detailed trait analysis of understorey species from two arid shrublands also studied here identified associations between increased grazing and small plant size, small leaves, high fecundity and plasticity of growth form (Landsberg, Lavorel & Stol 1999). However, Landsberg, Lavorel & Stol (1999) pointed out the overall lack of clear patterns and, as explanation, cited the complexity of grazing effects (selection, defoliation, altered recruitment opportunities) and lack of evolutionary history of ungulate grazing to provide selective pressure for clearly grazing-related traits (Landsberg, Lavorel & Stol 1999).
Unfortunately, we do not have data for comparable grassland sites to test whether the vegetation structure was the cause of the different results from Díaz, Noy-Meir & Cabido (2001). However, other work from subhumid Australian temperate grasslands (c. 780 mm, 400–600 g DM m−2 year−1 New South Wales northern tablelands; McIntyre, Lavorel & Tremont 1995) and subtropical open woodlands (c. 710 mm, ∼300 g DM m−2 year−1 south-east Queensland; McIntyre & Lavorel 2001) has identified associations with increased grazing pressure for low height, small seeds and annual life history. This supports our idea that it is not the continent but the vegetation structure that is responsible for the differences between the results of this study and that of Díaz, Noy-Meir & Cabido (2001).
While we cannot discount possible explanations for the differing results being the dissimilar evolutionary history of grazing or that Australia is a ‘special case’, the most parsimonious explanation appears to be openness of vegetation due largely to low rainfall. Comparing different vegetation types are suggested as an avenue for further research.
do increasers and decreasers have opposite traits, with neutral species intermediate?
In most traits, increasers were more distinct from neutral species than decreasers. In several cases, decreasers were intermediate between neutral species and increasers. This was surprising, as Díaz, Noy-Meir & Cabido (2001) had pooled neutral and increaser species for their analyses, and the original intention here was to do the same. What is the cause of the apparent similarity in trait values of increasers and decreasers, and why were increasers more distinct as a group than decreasers?
One explanation is that there are many ways to be a decreaser, and fewer of being an increaser. For instance, long-lived palatable shrubs are targeted during dry periods when little other forage is available and thus they become decreasers near watering points, where grazers congregate. During good seasons, short-lived palatable species produce abundant nutritious foliage and are targeted when grazers are most selective in their diet and can forage farther from water (Landsberg et al. 2002). In contrast, increasers need to take advantage of open space and establishment opportunities resulting from the activity of grazers. It is not enough to avoid or tolerate grazing. This would lead to a species being recorded as neutral. Species that reach most establishment sites, grow rapidly and reproduce early will tend to increase in abundance. In the shrublands and woodlands, annuals were more likely to be increasers (Noy-Meir, Gutman & Kaplan 1989; Belsky 1992).
Clearly, however, not all annuals were increasers. Annuals were unlikely to be neutral but no less likely than longer-lived species to decrease. Short generation times result in rapid increases (or decreases). Because of compounding effects over time, population-size shifts in short-lived species can become large and hence noticeable within a few years. In contrast, most long-lived species, especially shrubs, did not respond. As long as grazing does not hasten the death of mature plants, then decrease may not be detected by studies until individuals senesce without being replaced by recruiting juveniles (Andrew & Lange 1986). Thus on the time scales of these studies, persisting (or neutral) and increasing are different. For at least four of the studies for which we have data, grazing has had c. 100 years to exert effects on populations (Appendix S2).
Pooling traits of increasers and neutral species still seems intuitively reasonable. To increase in abundance, a species must maintain its population, i.e. not decrease. However, if species are actually decreasing over long time scales, but do not respond within the time frame and power of the study, then lumping increasers and neutrals will obscure relationships between traits and grazing responses. Increasers’ traits may also differ from those of neutral species if they are invaders. Altered conditions for establishment and competitive interactions due to grazing provide opportunities for non-resident species to invade. Invaders do not need to be able to maintain a population at the site in the absence of grazing. As a result, invading species’ traits may be distinct from neutral resident species.
These results should not diminish the enthusiasm for trait-based understanding of species’ responses to grazing. A possible way forward is further analyses based on the same trait sets in situations that can be contrasted on a few fundamental axes, with annual rainfall and evolutionary history of grazing being primary ones (Milchunas & Lauenroth 1993).