Banksia Woodland Fire Ecology Biology Essay

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Abstract

Whiteman Park's Banksia Woodland was used as a location to examine the influence of fire on ecological processes. Compiled data in the form of transects and quadrats suggest that fire has a destructive influence on density and a minor influence on diversity. Two main regenerative capacities were assessed in both burnt and unburnt populations. Two Banksia species, B.Attenuata and B.Menziesii were examples of resprouters, and a species of Calytrix as a reseeder. Measures of density and mortality in resprouters indicated high densities of older individuals (>20cm) and a lower number of young representatives. Calytrix showed significant seedling success following fire. Fire disturbance was considered a necessary precursor for diversity and fire management strategies were proposed.

Introduction

In South-West Western Australia fire has been an evolutionary force of change for 2.5 million years (Burrows 2006). The regions shift to a Mediterranean climate and hot, dry summers saw an increase in the resiliency of native flora to fire (Burrows 2006) (Bell 2001). Many species are heavily or completely dependent on fire to facilitate reproduction, and fire regimes have therefore become integral in community diversity (Burrows 2006) (Lamont & Markey 1995).

The consequences of fire can be readily observed in Whiteman Park by comparing two geographically close sites separated by a fire break. One area has been recovering over two years from an intense wildfire, and the other has been untouched by fire as a disturbance. By utilising transects and quadrats to establish community and species density, diversity and age between the sites, a representation of fire as a necessary or unnecessary disturbance will emerge.

Plant species are classed as reseeders or resprouters depending on their adaptive responses to fire (Bell 2001). In this study these two strategies will be compared. Seeder species are usually killed by fire and are wholly reliant on their seed after fire to establish a new population (Bell 2001). Resprouting species are considerably more versatile and if damaged, can renew themselves from potent stores of biomass such as in aerial epicormic buds or in subterranean lignotubers (Knox et al. 2005).

Banksia attenuata and Banksia menziesii are both predominantly resprouters, whereas Calytrix fraseri is an obligate reseeder. The population dynamic s and success of these three species in response to fire will be analysed over a broad range of data.

For this field study a range of hypotheses is necessary. (i) Due to the versatility and increased survivability of resprouting species they will be present in higher numbers than reseeding species in the burnt area, with older plants surviving in greater numbers. (ii) Both attenuata and menziesii will respond to fire fairly similarly. (iii) The floral biodiversity at the burnt site will be higher than the unburnt site. (ix) There will be considerably more density at the unburnt site. (x) Reseeding populations in burnt areas will be largely immature.

Methods

(Refer to Fieldwork Handbook for primary methodology)

Task 1: Observations

1. Observe and compare the two sides of the firebreak and make notes on the apparent differences in vegetation.

Task 2: Transects

1. At 0.5m intervals along a 10 metre transect record the abundance of plants at each point of sampling.

2. Count the number of species occurring along the transect within a proximity of 1 metre from the measuring tape.

3. At the burnt site compare the approximate percentage of plant cover which utilizes resprouting or reseeding strategies. Similarly, estimate what proportion of the species belongs to each strategy.

Task 3: Quadrats

1. In the burnt and unburnt areas plot a 20x20 metre quadrat and establish the abundance of all individuals belonging to Banksia attenuata and Banksia menziesii. Record whether the plant is dead or alive and ascertain its diameter at 1.3m high. (Plants greater than 3cm in diameter require a tape measure, for plants less than 3cm a ruler can be used).

2. Within the 400m squared plot, randomly allocate three 2x2m quadrats. Record the presence and diameter of Calytrix plants.

Results

Transects

Upon immediate observation of Figure 1 it can be surmised that fire has had a noteworthy impact on the abundance and density of living plants. The unburnt site averages more than double the burnt site in the number of mean hits per sampling point in the transects, detailing a significant difference in abundance across a broad range of quantitative data.

There is little disparity between the sites in the level of biodiversity. Figure 2 illustrates that the burnt site featured just one less species per transect than the unburnt site, a comparatively minimal gap compared to the difference in density.

Homogeneity

There existed homogeneity between the burnt and unburnt areas in terms of the presence of similar species, however the abundance was dissimilar.

Condition

In the burnt section major scorching was found up to 4 metres high on many up to grass trees and species of Banksia indicating intense crown fire. Most banksias were in decent condition in the burnt area, however some adult individuals had been completely destroyed. The unburnt area was generally home to older, vibrant growth.

There were small areas of bare soil present in the unburnt section, with a relatively dense understorey featuring a covering of organic matter. The burnt section had extensive areas of bare leached soil with an inconsistent understorey.

Resprouters vs. Reseeders

In the burnt community, individuals whom employed resprouting as a primary means of regeneration were considerably more plentiful than reseeders in relation to ground cover (36.5% vs. 63.5%). Likewise, more species were considered resprouters in the community (67%), with just 33% of reported species being reseeders.

SEEDERS % of species

33

RESPROUTERS % of species

67

SEEDERS % of cover

36.5

RESPROUTERS % of cover

63.5

Table 1. Resprouters vs. Reseeders (%Cover and Species)

Quadrats

Individuals of Banksia Menziesii were present in high numbers (Figure 3). The unburnt area had a higher proportion of younger plants, and there is a dramatic drop in number at 15cm and then a steady progression downwards in quantity as size and age increases. The highest numbers of living menziesii in the unburnt section are between 5 and 10 cm, whereas the burnt section peaks at 15cm and then regresses, further indicating an age gap. The burnt data is fairly erratic and fails to show an infallible trend, nevertheless it can be seen that the burnt area had a higher average diameter, inferring that older trees were abundant. The density of individuals in the unburnt section was higher than in the burnt section.

Banksia Attenuata presented some similar trends to Menziesii. Figure 4 illustrates that the unburnt section contained a higher number of younger, smaller plants in the 5-10cm range than at larger parameters. Interestingly, while there is a higher level of older, larger plants in the burnt section, a regression in number can be seen with an increase in diameter. Likewise, both areas have their highest quantity of plants in the 10cm range and which precedes a fairly dramatically decrease in number at 15cm. Generally then, while there is a larger amount of younger plants in the unburnt section, and a higher amount of older plants in the burnt section, there is a downwards movement in number with increasing age/diameter in both cases. Once again living density was higher in the unburnt area.

There is a noticeably high mortality in smaller, younger plants in the burnt section; this drops profoundly at the 15cm parameter and generally continues to do so (Figure 5). Conversely, significant mortality in the unburnt section is largely restricted to plants 15cm and above in diameter. This point is demonstrated at the 10cm and 25cm markers where there is a massive incongruity in abundance due to differences in disturbance between burnt and unburnt areas. An outlier at 50cm exists in the unburnt section.

Demonstrated in figure 6 is the immense number of Calytrix individuals in the burnt section that are primarily less than 0.5cm. 431 specimens occur in the 0.25cm range and 126 in the 0.5cm range. With the exception of 2cm, where 61 individuals are present, there is a continuing decline in numbers as diameter increases. With the unburnt area, the highest number of individuals belongs to 2cm with 39 specimens. Dissimilar to the burnt area, the unburnt area features individuals with diameters greater than 5cm. The burnt area exhibits appreciably higher densities of Calytrix, until 3cm and above where the unburnt area reveals very minor densities.

Discussion

Transects

Transects were used to determine the density, diversity and fire response of plants over a large and varied geographical distance.

Effectively illustrates the comparing densities of flora between the unburnt and burnt population. The unburnt site exhibits more than double the density of the burnt site, sustaining the hypothesis; this can be attributed to a variety of factors. Firstly, the soil was relatively infertile and inhospitable at the burnt site, suggesting the intensity of the fire. This would exacerbate local competition for nutrients, decreasing abundance (Knox et al. 2005). The extent of the fire would have eliminated the understorey flora, and while it would be expected there would have been significant regrowth from 2008, it is possible many seeds experienced lethality and were likely exposed to unusual extremes of temperature and drought, rendering them unviable (Burrows 2006) (Enright & Lamont 1989).

The denser unburnt population had obviously avoided catastrophic disturbance, and therefore plant growth was limited only by biological influence such as the carrying capacity of the soil, light availability and the presence of herbivores.

Both sites featured remarkably similar levels of species diversity considering the difference in density. Speculatively, the burnt site probably had a higher number of species than the unburnt site shortly after the fire (Bell 2001), with the fire providing an opportunity for enhanced understorey diversity by clearing the canopy and enabling light to filter through, assisting the germination of reseeding species (Enright & Lamont 1989). The slightly higher level of diversity in the unburnt site may be because of enhanced fertility, a comparative lack of selection pressures and apt opportunities for establishment. However, reseeder senescence in this population may account for a future decline in diversity (Lamont & Markey 1995). It can be assumed that the regenerative mechanisms of the plants in the burnt area are responsible for the upkeep of diversity (Bell 2001). The higher diversity at the unburnt site invalidates the hypothesis.

Resprouters vs. Reseeders

For the species in the Mediterranean climate of the South-West, two main regenerative strategies exist. Resprouting individuals primarily depend on their own survival to maintain their genetic presence, and renew themselves from stores of biomass including epicormic buds and lignotubers (Bell 2001) (Knox et al. 2005). Reseeders are typically killed by fire, and reinstate their population post-fire through the mass germination of seeds (Bell 2001).

In the burnt population major disparity in ground cover and number of species in the favour of resprouters over seeders, suggests that resprouters exhibit higher environmental tolerance (Lamont & Markey 1995). Resprouters have distinct advantages that allow them to maintain abundance (Bell 2001) (Lamont & Markey 1995). Resprouters are able to maintain their presence and rejuvenate themselves from dormant stores of energy when fire intervals and climatic conditions are non-ideal for seed production or seedling recruitment (Lamont & Markey 1995).

Massive life spans and survivability means resprouters have a lesser need to reproduce, and minimal success in reproduction is adequate to maintain or enhance the population (Lamont & Markey 1995). Similarly, resprouters can easily outcompete establishing reseeders, as they have higher reserves of biomass than competing reseeders (Bell 2001).

Seeders are generally restricted to narrow climatic parameters if they are to reproduce successfully (Enright & Lamont 1989). Overwhelming reliance on seedling recruitment is a relatively dangerous strategy as localised extinction can ensue if seeds fail (Bell 2001). Some species' seeds rapidly lose viability if exposed to aridity and high temperatures (Lamont & Markey 1995) (Enright & Lamont 1989). Therefore it can be assumed that resprouters can tolerate a wider breadth of fire intensities and climatic conditions, meaning they are higher in number and diversity at the burnt site justifying the hypothesis.

Quadrats

Quadrats were extensively employed as a measure of community composition and age structure in three species in response to considerable or no fire disturbance. Plant diameter was used as a broad indicator of age. With both B. Menziesii and B. Attenuata the unburnt area was dominated by younger Banksias between the 5 and 10cm constraints. Likewise in both cases high densities of older individuals (>20cm) was more prevalent in the burnt community, thus supporting the hypothesis. Supporting the ‘living' data is the measures of mortality. The lower number of young living representatives of both species in the burnt quadrats corresponds to higher mortality in the 5-10cm range. A noteworthy drop in mortality at 15cm interestingly correlates to an equally significant spike in living menziesii at the same diameter, likely indicating age of fire tolerance. This shows that fire has had an undeniably significant affect on population structure.

The high mortality of 5-10cm banksias indicates that living individuals of the same range have emerged after the fire. While small in quantity compared to the unburnt site, the number of young banksias existing in the burnt section is notable and perhaps alerts to the post fire recruitment success of the banksias (Enright & Lamont 1989). In a study by Enright & Lamont (1989) both Attenuata and menziesii delayed their seed release to avoid summer heat, assisting the viability of seeds into winter, similarly, both had >80% follicle success. The high density of living older individuals in the burnt community correlates to decreased mortality rates. The decline in death rates is conditional on fire tolerance, this is realised over a large expanse of time and is conditional on age (diameter), explaining the survivability of older specimens and the sensitivity of younger specimens (Lamont & Markey 1995). The thick bark which prevents damage to underlying tissues is possibly undeveloped in smaller individuals (Knox et al. 2005). The diameter where fire tolerance is attained is most likely between 15 and 20cm.

The high density of young individuals in the unburnt population and the low density/high mortality of older individuals is somewhat enigmatic. As resprouters generally utilise fire intervals to initialise germination, the amount of young plants can probably be attributed to enhanced soil bioavailability or other biotic influences rather than disturbance. It may be that mortality is largely density dependent and for older plants it is possible that senescence is occurring earlier in response to minor disturbance and consequently diminished renewability.

Conceivably, fire had the most dramatic influence on the population breadth and structure of Calytrix. As a reseeder, calytrix is solely reliant on fire to permit the germination and recruitment of seedlings. 557 individuals, most of which are in the 0.25cm range, indicate the absolute necessity of fire in assisting the viability of future generations of Calytrix and uphold the hypothesis. Maintaining this idea is the comparatively marginal densities of calytrix in the unburnt plots. There is a risk of local extinction as a result of senescence and non-disturbance (Lamont & Markey 1995). Also demonstrated is the enhanced viability and size of seed banks in reseeding species, as equal post-fire success in seedling recruitment was not enjoyed by either resprouter (Enright and Lamont 1989). Resource allocation and thermal tolerance of seed from reseeding species is also superior to resprouters, as fitness is not conditional on consistent and frequent recruitment (Bell 2001).

Our data suggests that fire has a destructive influence on density, and a marginal, yet negative influence on levels of diversity. However as exceptions to both statements exist in our data it cannot be considered absolute. The transects suggested that resprouters were more successful than reseeders in the burnt area, but as has been ascertained, there is a wide variability in fire tolerance amongst co-existing species (Burrows 2006), therein representing a management problem. Erratic or frequent fire regimes will likely favour resprouters because of their propensity to survive, but similar regimes with short or long intervals may prevent reseeders completing their life cycle or reaching senescence respectively (Lamont & Markey 1995). Fire regimes should therefore utilize variation in intensities and intervals to compensate for different resiliencies across the population while maximising diversity (Bell 2001). By studying data from both burnt and unburnt populations quantifiable patterns in abundance, biodiversity, age and adaptive strategy have been examined, therefore relating community structure to fire. The data asserts that fire can both threaten and benefit populations, however this is interlaced with a host of important biological considerations.

References

Bell, D.T. 2001. Ecological Response Syndromes in the Flora of Southwestern Western Australia: Fire Resprouters versus Reseeders. THE BOTANICAL REVIEW VOL. 67. Department of Botany, University of Western Australia.

(Bell 2001)

Burrows, N.D. 2006. Linking fire ecology and fire management in south-west Australian forest landscapes. Department of Environment and Conservation, Western Australia, 6983, Australia.

(Burrows 2006)

Enright, N.J., Lamont, B.B. 1989. Seed Banks, Fire Season, Safe Sites and Seedling Recruitment in Five Co-Occurring Banksia. Journal of Ecology, Vol. 77, No. 4, British Ecological Society.

(Enright & Lamont 1989)

Knox, B., Ladiges, P., Evans, B. & Saint, R. 2005, Biology: An Australian Focus Third Edition, McGraw Hill, Sydney.

(Knox et al. 2005)

Lamont, B.B., Markey, A. 1995. Biogeography of Fire-killed and Resprouting Banksia Species in South-western Australia. Aust. J. Bot., 43. School of Environmental Biology, Curtin University of Technology, Perth.

( Lamont & Markey 1995)

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