Effects of competition on vegetation structure and fuel characteristics
Abstract
There is growing evidence that vegetation cover is higher – particularly in the mid-storey – in woodlands and open forests of south-eastern Australia than it was prior to European settlement. There is also a concern that higher vegetation cover equates to a higher availability of fire fuel, and that fire behaviour may become more extreme as a result. There are two main explanations for why vegetation cover has increased. The prevailing explanation is that Indigenous Australians used regular low intensity fire to control vegetation cover prior to European settlement, a practice that has since ceased across south-eastern Australia. Implementing regular prescribed burning is considered a way of reintroducing this process of land management. Alternatively, high vegetation cover may be caused by dense regrowth of trees and shrubs in the absence of competition from large trees, which were subject to widespread removal after European settlement. However, as increasing competition from growing trees slowly forces self-thinning, vegetation cover is predicted to decline.
The aim of my research was to test the explanation that the removal of large trees, and subsequent regrowth, is the cause of higher vegetation cover and fire fuel availability, and more extreme fire behaviour. My research addresses two questions: firstly, about the association between tree size and vegetation cover; and secondly, the association between tree size and fire fuel characteristics and, consequently, predicted fire behaviour. I hypothesised that tree size would be negatively associated with mid-storey vegetation cover and fire fuel availability, and that this would result in a reduction in the rate of spread and flame height of a fire.
As vegetation structure is a function of competition, environmental conditions and disturbance history, data representing these three variables were collected in 0.1 ha plots throughout the western slopes and tablelands of south-eastern Australia. Using these data, a statistical examination was undertaken of the association between stand structural metrics, that are indicative of competition, with: vegetation cover; fire fuel characteristics; and predicted fire behaviour. Vegetation cover was represented by recording vegetation cover at different heights; fire fuel characteristics were represented by quantifying fuel arrangements and heights; and predicted fire behaviour was represented by calculating the rate of spread and flame height under ideal prescribed burning conditions, and conditions recorded during catastrophic 2003 Canberra bushfires. I analysed these data using an information theoretic approach, whereby alternative hypothesis, informed by evidence in the literature, were ranked using the second order Akaikes information criterion (AICc).
Vegetation cover and fire fuel metrics for the litter and groundcover strata were not clearly influenced by tree size dependent competition. Because rate of spread was predicted using litter and groundcover fuel metrics, no clear association between tree size and predicted rate of spread was observed either. However, mid-storey vegetation cover and fire fuel metrics were consistently, positively associated with the number of tree stems, and negatively associated with the quadratic mean stem diameter. Decreasing number of stems and increasing stem diameters represent a growing and self-thinning stand, thus these results suggested that mid-storey vegetation cover declines as regrowth stands mature. Predicted flame height – which uses the height of the mid-storey as a predictive parameter – also increased with the number of stems and decreased with the size of stems.
These findings add to a body of scientific research supporting the hypothesis that regrowth stands pass through a state of higher vegetation cover and fire fuel availability, before returning to a pre-disturbance state. This high vegetation cover state occurs due to the absence of large trees, which limit successful tree and shrub regeneration through competition. Consequently, there is an increase in mid-storey cover resulting from tree and shrub establishment and closely spaced stems with low canopies. Fuel reduction burning can provide a short term reduction in fire fuel availability, but is expensive and requires repeated treatment. Additionally, damage to trees exposed to frequent burning may inhibit development and self-thinning, thus prolonging the state of high mid-storey cover. In light of this evidence, reducing the risk of fire associated with high vegetation cover may simply be a matter of allowing stands to develop and mature over time.