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Fire severity effects on resprouting of subtropical dune thicket of the Cape Floristic Region.

ABSTRACT: It has been hypothesised that high-intensity fires prevent fire-dependent fynbos from being replaced by fire-avoiding subtropical thicket on dune landscapes of the Cape Floristic Region (CFR). Recent extensive fires provided an opportunity to test this hypothesis. We posit that (1) fire-related thicket shrub mortality would be size dependent, with smaller individuals suffering higher mortality than larger ones; and (2) that survival and resprouting vigour of thicket shrubs would be negatively correlated with fire severity. We assessed survival and resprouting vigour post-fire in relation to fire severity and pre-fire shrub size at two dune landscapes in the CFR. Fire severity was scored at the base of the shrub and categorised into four levels. Pre-fire size was quantified as an index of lignotuber diameter and stem count of each shrub. Resprouting vigour consisted of two variables; resprouting shoot count and resprouting canopy volume. A total of 29 species were surveyed. Post-fire survival of thicket was high (83-85%). We found that smaller shrubs did have a lower probability of post-fire survival than larger individuals but could detect no consistent relationship between shrub mortality and fire severity. Fire severity had a positive effect on resprouting shoot count but a variable effect on resprouting volume. Pre-fire size was positively related to survival and both measures of resprouting vigour. We conclude that thicket is resilient to high-severity fires but may be vulnerable to frequent fires. Prescribed high-intensity fires in dune landscapes are unlikely to reduce the extent of thicket and promote fynbos expansion.

SUBMITTER: Strydom T

PROVIDER: S-EPMC7293192 | biostudies-literature | 2020

REPOSITORIES: biostudies-literature

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Fire severity effects on resprouting of subtropical dune thicket of the Cape Floristic Region.

Strydom Tiaan T Kraaij Tineke T Difford Mark M Cowling Richard M RM

PeerJ 20200610

It has been hypothesised that high-intensity fires prevent fire-dependent fynbos from being replaced by fire-avoiding subtropical thicket on dune landscapes of the Cape Floristic Region (CFR). Recent extensive fires provided an opportunity to test this hypothesis. We posit that (1) fire-related thicket shrub mortality would be size dependent, with smaller individuals suffering higher mortality than larger ones; and (2) that survival and resprouting vigour of thicket shrubs would be negatively co ...[more]

PMID: 32566395

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Project description:BackgroundGlobally, and in the Cape Floristic Region of South Africa, extreme fires have become more common in recent years. Such fires pose societal and ecological threats and have inter alia been attributed to climate change and modification of fuels due to alien plant invasions. Understanding the flammability of different types of indigenous and invasive alien vegetation is essential to develop fire risk prevention and mitigation strategies. We assessed the flammability of 30 species of indigenous and invasive alien plants commonly occurring in coastal fynbos and thicket shrublands in relation to varying fire weather conditions.MethodsFresh plant shoots were sampled and burnt experimentally across diverse fire weather conditions to measure flammability in relation to fire weather conditions, live fuel moisture, fuel load and vegetation grouping (fynbos, thicket and invasive alien plants). Flammability measures considered were: burn intensity, completeness of burn, time-to-ignition, and the likelihood of spontaneous ignition. We also investigated whether the drying of plant shoots (simulating drought conditions) differentially affected the flammability of vegetation groups.ResultsFire weather conditions enhanced all measures of flammability, whereas live fuel moisture reduced burn intensity and completeness of burn. Live fuel moisture was not significantly correlated with fire weather, suggesting that the mechanism through which fire weather enhances flammability is not live fuel moisture. It furthermore implies that the importance of live fuel moisture for flammability of evergreen shrublands rests on inter-specific and inter-vegetation type differences in fuel moisture, rather than short-term intra-specific fluctuation in live fuel moisture in response to weather conditions. Fuel load significantly increased burn intensity, while reducing ignitability. Although fire weather, live fuel moisture, and fuel load had significant effects on flammability measures, vegetation and species differences accounted for most of the variation. Flammability was generally highest in invasive alien plants, intermediate in fynbos, and lowest in thicket. Fynbos ignited rapidly and burnt completely, whereas thicket was slow to ignite and burnt incompletely. Invasive alien plants were slow to ignite, but burnt with the highest intensity, potentially due to volatile organic composition. The drying of samples resulted in increases in all measures of flammability that were comparable among vegetation groups. Flammability, and by implication fire risk, should thus not increase disproportionately in one vegetation group compared to another under drought conditions-unless the production of dead fuels is disproportionate among vegetation groups. Thus, we suggest that the dead:live fuel ratio is a potentially useful indicator of flammability of evergreen shrublands and that proxies for this ratio need to be investigated for incorporation into fire danger indices.

Project description:In June 2017, extreme fires along the southern Cape coast of South Africa burnt native fynbos and thicket vegetation and caused extensive damage to plantations and residential properties. Invasive alien plants (IAPs) occur commonly in the area and were thought to have changed the behaviour of these fires through their modification of fuel properties relative to that of native vegetation. This study experimentally compared various measures of flammability across groups of native and alien invasive shrub species in relation to their fuel traits. Live plant shoots of 30 species (10 species each of native fynbos, native thicket, and IAPs) were sampled to measure live fuel moisture, dry biomass, fuel bed porosity and the proportions of fine-, coarse- and dead fuels. These shoots were burnt experimentally, and flammability measured in terms of maximum temperature (combustibility), completeness of burn (consumability), and time-to-ignition (ignitability). Multiple regression models were used to assess the relationships between flammability responses and fuel traits, while the Kruskal-Wallis H test was used to establish if differences existed in flammability measures and fuel traits among the vegetation groups. Dry biomass significantly enhanced, while live fuel moisture significantly reduced, maximum temperature, whereas the proportion of fine fuels significantly increased completeness of burn. Unlike other similar studies, the proportion of dead fuels and fuel bed porosity were not retained by any of the models to account for variation in flammability. Species of fynbos and IAPs generally exhibited greater flammability in the form of higher completeness of burn and more rapid ignition than species of thicket. Little distinction in flammability and fuel traits could be made between species of fynbos and IAPs, except that fynbos species had a greater proportion of fine fuels. Thicket species had higher proportions of coarse fuels and greater dry biomass (~fuel loading) than species of fynbos and IAPs. Live fuel moisture did not differ among the vegetation groups, contrary to the literature often ascribing variation in flammability to fuel moisture differences. The fuel traits investigated only explained 21-53% of the variation in flammability and large variation was evident among species within vegetation groups suggesting that species-specific and in situ community-level investigations are warranted, particularly in regard fuel moisture and chemical contents.

Project description:In Mediterranean-Climate Ecosystems (MCEs), Holocene coastal dunes comprise small, fragmented and dynamic features which have nutritionally imbalanced and excessively drained, droughty, sandy soils. These characteristics, along with summer drought and salt-laden winds, pose many challenges for plant colonization and persistence. Consequently, MCE dune floras are likely to be distinctive with a high proportion of habitat specialists and strong convergence in growth form mixes. Very little research has compared the species traits of dune floras within and across MCEs. This paper contributes to filling that gap. Here, we analyze the taxonomic, biological and geographical traits for all 402 species in a flora from a dune landscape (Cape St Francis) in the southeastern Cape Floristic Region (CFR) and compare patterns with the trait profiles of other dune floras at a regional (CFR) and global (MCE) scale. Within the CFR, the southeastern (all-year-rainfall) flora at Cape St Francis had a similar trait profile to western (winter-rainfall) dune floras, except for having a lower representation of species belonging to CFR-endemic clades, and higher number of species associated with tropical lineages. The St Francis flora, in common with other CFR and MCE floras, was dominated by members of the Asteraceae, Fabaceae and Poaceae. Some 40% of the St Francis flora was endemic to the CFR, typical of the high rate of MCE-level endemism elsewhere in the CFR, and in other MCEs. About 30% of the flora was confined to calcareous sand, a value typical for many other MCE sites. The St Francis flora, as well as other CFR dune floras, differs from those of other MCEs by having many species associated with shrubby lineages, and by the relatively high incidence of species associated with tropical lineages. The growth form profile of the St Francis and other CFR floras shows strongest similarity with that of Australian MCE dunes in that in both regions, evergreen hemicryptophytes and shrubs share dominance, and annuals are floristically and ecologically subordinate. The least similar of MCEs to the St Francis trait profile is the Mediterranean Basin where annuals are the most frequent growth form while shrubs are subordinate. California and Chile dune floras appear to occupy an intermediate position, in terms of growth form mix, between the Cape and Australia on the one hand, where dune floras have retained features typical of nutrient-poor soils, and the Mediterranean Basin, where dwarf, deciduous shrubs and annuals dominate the life form spectrum. All MCE dunes are threatened by alien plants, infrastructure development, tourism demands and rising sea levels. The high incidence of species of conservation concern in CFR dune floras underestimates the exponentially increasing threats to their habitats, which are already historically at a much-reduced extent. All remaining coastal dune habitat in the CFR, and probably in other MCEs, should be conserved in their entirety.

Project description:The Cape Floristic Region (CFR) is globally recognized as a hotspot of plant diversity and endemism. Much of this diversity stems from radiations associated with infertile acid sands derived from sandstones of the geologically ancient Cape Fold Belt. These ancient montane floras acted as the source for most subsequent radiations on the Cape lowlands during the Oligocene (on silcretes) and Mio-Pliocene (on shales). The geomorphic evolution of the CFR during the Plio-Pleistocene led to the first large-scale occurrence of calcareous substrata (coastal dunes and calcarenites) along the Cape coast, providing novel habitats for plant colonization and ensuing evolution of the Cape coastal flora-the most recent diversification event in the Cape. Few studies have investigated the CFR's dune and calcarenite floras, and fewer still have done so in an evolutionary context. Here, we present a unified flora of these coastal calcareous habitats of the CFR and analyze the taxonomic, biological and geographical traits of its component species to gain insights into its assembly. The Cape coastal flora, comprising 1,365 species, is taxonomically dominated by the Asteraceae, Fabaceae and Iridaceae, with Erica, Aspalathus and Agathosma being the most speciose genera. In terms of growth-form mix, there is a roughly equal split between herbaceous and woody species, the former dominated by geophytes and forbs, the latter by dwarf and low shrubs. Species associated with the Fynbos biome constitute the bulk of the flora, while the Subtropical Thicket and Wetland biomes also house a substantial number of species. The Cape coastal flora is a distinctly southern African assemblage, with 61% of species belonging to southern African lineages (including 35% of species with Cape affinity) and 59% being endemic to the CFR. Unique among floras from the Cape and coastal Mediterranean-climate regions is the relatively high proportion of species associated with tropical lineages, several of which are restricted to calcareous substrata of the CFR. The endemic, calcicolous component of the flora, constituting 40% of species, represents 6% of the Cape's regional plant diversity-high tallies compared to other biodiversity hotspots. Most coastal-flora endemics emerged during the Plio-Pleistocene as a product of ecological speciation upon the colonization of calcareous substrata, with the calcifugous fynbos floras of montane acid substrata being the most significant source of this diversification, especially on the typically shallow soils of calcarenite landscapes. On the other hand, renosterveld floras, associated with edaphically benign soils that are widespread on the CFR lowlands, have not been a major source of lineages to the coastal flora. Our findings suggest that, over and above the strong pH gradient that exists on calcareous substrata, soil depth and texture may act as important edaphic filters to incorporating lineages from floras on juxtaposed substrata in the CFR.

Dataset Information

Fire severity effects on resprouting of subtropical dune thicket of the Cape Floristic Region.

Publications

Fire severity effects on resprouting of subtropical dune thicket of the Cape Floristic Region.

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