Habitat insularity and fire response traits: evidence from a sclerophyll archipelago

Abstract

Rock outcrops are landscape features that may form habitat islands in a matrix of more widespread vegetation. The patterns of floristics, reproduction, gender, life span, growth forms, and fire response traits were compared between rock outcrops and matrix sclerophyll vegetation to test for insularity in taxon composition and functional traits. The outcrops and matrix had similar reproduction, gender, life span, and growth form traits, being dominated by co-sexual sclerophyll shrubs. The outcrops, however, were dissimilar in species composition and functional traits forming an archipelago of habitat islands in a forest matrix. Rank abundance curves were less even on rock outcrops than in adjacent forests, being dominated by shrubs that were killed by fire (obligate seeders). The ratio of shrubs killed by fire (obligate seeders) to resprouters was 70:30 on the outcrops compared with 38:62 in the matrix. Evidence for functional convergence in fire response traits comes from 27 genera, in 17 families, which have congeners in each habitat. Most shrub congeners on or near rocky outcrops were killed by fire whereas related taxa in the forests resprout after fire. Functional convergence can be related to disturbance frequency and/or differences in regeneration niche among habitats. A resprouting response appears to be related to more frequent fires in the matrix as outcrops experience fires less often. The dominance of obligate seeding shrubs on high rainfall outcrops may also be related to better resources in an environment where allocation to growth rather than storage could be advantageous. In drier and shadier habitats, however, resprouting may be promoted over seedling recruitment as the risks of recruitment failure are higher.

Differences in Functional Trait Distribution between Inselberg and Adjacent Matrix Floras

“…Resprouting/clonality traits were more commonly expressed within matrix species where nutrient and water resources were higher and climate more ameliorated. This provides support for Halassy et al [30] who suggested that such species were favored in resource rich mesic environments with high competition and is at odds with Benwell [28] and Clarke [29] who proposed that such strategies were more likely to occur in locations of high stress and high recruitment risk.…”

“…However other suites of functional traits are not consistent (smaller seeds, fruits, lack of dispersal, no vegetative spread/clonality, and no underground storage organs) [4,5,28,29,32,49].…”

“…Species may persist in situ by increasing the longevity of above ground parts, by increasing the longevity of below ground parts or via dormancy mechanisms of diaspores [5,7]. Benwell [28] and Clarke [29] suggested that vegetative reproduction (resprouting and clonality) was a stress tolerant regeneration strategy compensating for high-risk recruitment and such strategies should be more common in drought affected environments and thus favored on inselbergs. In contrast, Halassy et al [30] provided evidence that species that put greater resources into vegetative reproduction and/or storage organs were favored under mesic, resource rich, and competitive environments.…”

Are Trade-Offs in Plant Resprouting Manifested in Community Seed Banks

“…(a) Probability of resprouter presence increasing with probability of crown fire (after Bellingham andSparrow 2000, Bond andMidgley 2003). crown fires, and adjacent rocky outcrops, which act as fire refugia and are less frequently burned (see Clarke 2002). (b) Probability of regeneration by seedling recruitment decreasing with increased resprouting ability because of less allocation to seed.…”

Habitat and fire heterogeneity explain the co-occurrence of congeneric resprouter and reseeder Hypericum spp. along a Florida pine savanna ecocline

“…Reseeders are hypothesized to be eliminated from a site if fires occur repeatedly at intervals shorter than those required for maturation and seed set (Keeley and Zedler 1978;Wooller et al 2002). In other frequently burned habitats, woody reseeders tend be associated with rock crevices, open microsites, or waterlogged depressions that are less likely to burn than adjacent areas, which are dominated by resprouters (Menges and Kimmich 1996;Clarke 2002;Virgona et al 2006;Williams and Clarke 2006). Gradients in fire frequency should generate patches of burned and unburned areas if they occasionally spread down slopes, perhaps once out of every 5-10 fires (10-30 year frequency).…”