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- Materials and methods
1. Differences in the competitive ability of plant functional groups at early life-history stages can have important consequences for community structure. In particular, trade-offs in allocation to roots by woody plant seedlings may influence competitive ability with grasses in fire-prone vegetation.
2. We followed post-fire survival of seedlings of facultative resprouter and obligate seeder (fire-killed) shrubs for 3 years in adjacent communities with a grassy/graminoid ground stratum (54 plots, 20 m2) or a non-graminoid ground stratum (54 plots, 20 m2).
3. The competitive effect of a grass (Poa) on seedlings of three congeneric pairs of resprouters and obligate seeder shrubs was tested in a factorial experiment where nutrients and the grass competitor were manipulated. The effects of grass (+,−) and nutrients (+,−) on the growth response, biomass allocation and root carbohydrate storage were measured after harvest at 26 weeks and the relative neighbour effect calculated.
4. Post-fire shrub seedling survival was high with about 50% (2163 seedlings) surviving over 3 years, but this varied between habitats and functional groups. In the grassy/graminoid ground layer communities 27% of shrub seedlings survived, whereas in the habitats with a more open ground stratum 55% of seedlings survived. In grassy habitats, obligate seeder survival was lower (23% survival) than that of resprouter seedlings (35% survival). Similarly, in open habitats, obligate seeder seedling survival was lower (51%) than that of resprouter seedlings (64% survival).
5. Growth of both resprouters and obligate seeders in our manipulative experiment was strongly reduced in the presence of a grass competitor. Moreover, the addition of nutrients increased the relative difference in mass and height between those seedlings exposed to a grass competitor and those grown without a competitor. Resprouter species allocated more to roots under competition and were less affected by grass competition than obligate seeders.
6. Synthesis. The results of seedling survival and of the experiment on the effects of grass competition on woody plant seedlings suggest that early life-history trade-offs in allocation influence seedling survival. Allocation to resprouting appears to enhance the ability of shrub seedlings to survive grass competition. We propose that grass competition across productivity gradients plays an important role in influencing landscape-level distribution patterns of woody resprouters.
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- Materials and methods
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.