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Disturbance and the possibility of damage are ubiquitous aspects of the life of plants. Damage can in many cases be avoided, often referred to as resistance in the herbivory literature (van der Meijden et al. 1988; Mauricio et al. 1997). Resprouting is a key strategy for plants faced with unavoidable disturbances causing the loss of most above-ground biomass. As a species trait, resprouting is part of a complex demographic trade-off and potentially can vary with disturbance type and intensity, and environmental productivity, among other factors (Bellingham & Sparrow 2000; Bond & Midgley 2001, 2003; Del Tredici 2001; Vesk & Westoby 2004b). In the spirit of simplicity, Bond & Midgley (2001, 2003) suggested focusing on the resprouting capacity of mature plants for the contribution to persistence, as resprouting by seedlings can be considered to be part of the regeneration niche (Grubb 1977). The questions that arise are whether the resprouting abilities of seedling and mature stages are generally related across species, and if this is the case, how? Here, I address these questions drawing on the published literature on resprouting following clipping or burning.
Hodgkinson (1998) suggested that within-species resprouting ability could be adequately described by three stages (based on shrubs of semi-arid eastern Australia): (1) established seedling stage (height ∼5 cm); (2) the stage of first acquiring maximum resprouting ability (25–60 cm); and (3) at the onset of maturity. Bond & van Wilgen (1996) identified four curve types for survivorship of fire through ontogeny: (1) early increase of survivorship, thereafter maintained; (2) a linear increase; (3) an initial increase followed by a decline; and (4) perpetual low survivorship. However, survivorship is a function of avoidance of mortality risks by size and defence (Jackson et al. 1999), as well as resprouting ability. If plants can avoid stem-kill (loss of above-ground biomass, also known as top-kill) then there is no need for resprouting ability (Hoffmann & Solbrig 2003).
The probability of stem-kill by disturbance such as browsing or low-intensity fire is reduced in larger plants (Burrows 1985; Hoffmann & Solbrig 2003), in the sense both of larger individuals of a given species and of species with potentially large mature size. This has at least two components. First, bark thickness, and thus cambial insulation, increase with stem diameter (Gill & Ashton 1968; Vines 1968; Ryan & Reinhardt 1988; Uhl & Kauffman 1990). Second, greater height means that buds can be elevated beyond the reach of browsing herbivores, or lethal heating from low-intensity fire (Gill 1981; Morrison & Renwick 2000). The potential size of the plant can thus influence avoidance as, for instance, ground plants have limited opportunity to avoid fire or severe herbivory through size. Yet, given a disturbance that results in stem-kill, resprouting ability may actually decline as plants become larger (Burrows 1985). It is important to note that for some long-return disturbances, such as crown fires or catastrophic windthrow, size may not increase damage avoidance (Peterson & Pickett 1991; Peterson & Rebertus 1997).
Thus, we may expect that the potential size of the plant, the ontogenetic stage and the definition of disturbance intensity all interact to influence damage avoidance, survivorship and the need for resprouting. In this paper, I use a literature dataset to investigate the distribution of resprouting ability between species after clipping and burning in different ontogenetic stages. Specifically, I assess whether there is evidence for interaction between these factors, and how the ontogenetic pattern of resprouting ability and survival may differ among growth forms (as a proxy for potential size). Phylogenetic analyses are not reported here (however, see Vesk & Westoby 2004b) because previous studies have demonstrated that resprouting is a phylogenetically widespread and labile trait across broad phylogenetic trees (Vesk & Westoby 2004b) and within particular clades (Schwilk & Ackerly 2001; Bond & Midgley 2003).
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This study has shown that plant resprouting ability may change through ontogeny. At least some of the changes can be explained by exposure to, and possibility of avoiding, damage. Yet considerable variation exists between species with similar exposure and avoidance ability. Previous studies have demonstrated an ontogenetic change in resprouting ability, but only in single species or relatively small cross-species datasets (Kayll & Gimingham 1965; Burrows 1985; Keeley 1986; Bell & Pate 1996; Bond & van Wilgen 1996; Hodgkinson 1998; Jackson et al. 1999). The current study brings a generalized perspective to this issue.
As seedlings, plants are highly exposed to damage from herbivory and other sources, and thus there is an imperative for resprouting ability at seedling stages. Seedlings were broadly able to resprout following clipping whereas fewer resprouted after fire, although this pattern was strongest in shrubs. This is consistent with herbivory being a major hazard to seedlings, but for fire to be relatively less important (Leishman et al. 2000). Resprouting ability of seedlings (indeed all stages) does vary widely between species (see, for example, Figs, 1, 2 and 4), and will probably be driven by reserves both in cotyledons (Armstrong & Westoby 1993; Harms & Dalling 1997) and post-establishment storage (Klimešová & Klimeš 2003; Walters et al. 2005). For ground plants, escape from the reach of terrestrial herbivores and fires is not an option. Thus, resprouting ability throughout the lifetime is at a premium. Trees are able to avoid many disturbances by being large, in particular tall, and by defences (Loehle 1988; Jackson et al. 1999). In this case, resprouting ability may be less necessary. Results from disturbances that cannot be avoided, such as when a tree is sawn down or the stem cambium irredeemably burned (as opposed to a blackened trunk), suggest that many species that survive fire by resprouting do so epicormically and that basal resprouting is relatively rare in trees (but see Del Tredici 2001), although this may well differ with site productivity (see below). Broadly, this study suggests that if damage cannot be avoided through growth, resprouting ability should be maintained throughout life. But where growth allows damage to be avoided, then resprouting ability may decline with increased size. The mechanism responsible for declines in sprouting ability is not clear and may include a number of causes. Buds may senesce over time and it is conceivable that buds of different species differ in their longevity (Vesk & Westoby 2004a). Buds may become trapped in bark (Cremer 1972; Fink 1983, 1984). Poor vascular connections between sprouts and roots following resprouting may prevent successful sprouting despite sufficient buds and reserves (Midgley 1996; Del Tredici 2001). Additionally, ongoing respiratory demands of large established root systems following stem kill may prevent buds from successful resprouting if the carbon assimilated through photosynthesis by sprouts is insufficient (Vesk & Westoby 2004a).
This investigation also points to the issue of defining disturbance intensity. Using extrinsic measures of intensity (e.g. Byram's fire intensity or, less well defined, ‘crown-fire’) as explanatory variables will confound avoidance and resprouting ability. However, intrinsic measures such as leaf scorch or stem-kill are dependent upon species attributes such as size.
The use of Bayesian modelling allowed between-species variation to be incorporated into predictions (Clark 2005). The standard credible intervals (analogous to classical confidence intervals) represent uncertainty about the between-species mean. By incorporating the species random effect, which cannot be achieved in a classical analysis, it is possible to appreciate the variation between species within classes and that, in some species, patterns very different from those indicated by the mean can be expected. This demonstrated the low specificity of prediction; this is unsurprising, given that resprouters and non-sprouters are found in most clades, vegetation types and disturbance classes (Figs 1 and 2; Vesk & Westoby 2004b).
Analyses can only be as good as the data on which they are based. There are many limitations in these data, assembled from studies of multiple vegetation types, growth forms and clades; observation error will also be associated with size and disturbance definition. For example, previous work has found that resprouting ability is mixed within growth forms (Bellingham et al. 1994; Hodgkinson 1998; Vesk et al. 2004), although some growth forms are less variable, e.g. grasses tend to be resprouters and subshrubs from the Chenopodiaceae tend to be non-resprouters (Vesk et al. 2004). However, these data do represent the current state of knowledge. The patterns here warrant testing in a more controlled dataset, which may result in predictions with higher specificity; the Bayesian framework enables incorporation of the current results as prior information.