Interactions between grasses and woody vegetation have been studied extensively, but the effects of variation in life history (i.e. resprouting ability) in woody plants have rarely been examined in the context of grass competition. Trade-offs in resource allocation that favour persistence (resprouting) over seed-based regeneration (adults killed by fire) occur in woody plants (Bond & Midgley 2001), and this can have a profound influence on community dynamics (Clarke & Dorji 2008). Taxa that resprout after fire allocate resources to structures that enhance their ability to survive the next fire, whereas those that are killed by fire allocate resources to ensure they are reproductively mature before the next disturbance (Bell 2001; Bond & Midgley 2003; Lamont & Wiens 2003). Many studies have found that resprouting woody species allocate more resources to root mass and have higher levels of non-structural carbohydrate in the roots than those species that are killed by fire (e.g. Pate et al. 1990; Bell & Ojeda 1999; Verdaguer & Ojeda 2002; Knox & Clarke 2005; Schwilk & Ackerly 2005). In addition, experimental manipulations have shown that both biomass allocation and carbohydrate accumulation are also under strong environmental control in resprouting species, but less so in species killed by fire (Knox & Clarke 2005). As a consequence, seedlings of resprouters are predicted to be better at capturing below-ground resources, whilst seedlings of obligate seeders are predicted to be better at capturing above-ground resources. If allocation trade-offs appear early in ontogeny, they should also lead to different competitive responses to neighbours (sensuGoldberg 1990) between seedlings of resprouters and obligate seeders.
Woody species are generally regarded as ineffective competitors for below-ground resources when establishing. In particular, grass neighbours are known to induce large reductions in woody seedling growth (Aerts, Boot, & van der Aart 1991; Wilson 1998; Nano & Clarke 2009), and increasing soil fertility reinforces the competitive superiority of grass vegetation (Aerts, Boot, & van der Aart 1991; Bloor, Barthes, & Leadley 2008). In fire-prone ecosystems, seedlings of both facultative resprouters and non-resprouting species initially escape the competitive effect of neighbours by rapid germination and establishment. This occurs because fires remove the herbaceous above-ground biomass, release nutrients and stimulate seed dispersal and/or germination. Predicting the subsequent outcome of woody–grass interactions at the seedling stage is complex because resource allocation in woody plants differs with resprouting ability (resprouters vs. obligate seeders) and with soil fertility (Knox & Clarke 2005). Landscape-scale studies have shown that resprouting shrubs are more common in grassy landscapes such as savanna woodlands, temperate grassy woodlands and cerrado (Frost 1985; Hoffmann 2000; Clarke et al. 2005; Bond 2008; Hoffmann et al. 2009). In contrast, obligate seeders are often more common in nutrient-poor communities such as chaparral, fynbos and sclerophyllous woodlands, where grasses rarely dominate the ground stratum (Keeley 2000; Pausas et al. 2004; Clarke et al. 2005; but conversely see Kruger, Midgley, & Cowling 1997; Bellingham & Sparrow 2000; Bell 2001). These landscape patterns suggest that the interactive effects of resprouting ability, grass competition and soil fertility may influence the composition and evolution of woody plants in fire-prone landscapes. More broadly, these trait patterns align with the theory of allocation trade-offs along productivity gradients (Tilman 1988).
In this study, we examined the post-fire survival of seedlings of resprouter (facultative resprouters) and obligate seeder (fire-killed) shrubs for 3 years in adjacent sclerophyllous communities with ground strata dominated by either graminoids (grasses and sedges) or non-graminoids. We then used a glasshouse experiment to assess how grass competition and nutrient availability interacted to affect growth, biomass allocation and investment in root carbohydrates in congeners with resprouter versus obligate seeder life histories.