The effects of topographic variation and the fire regime on coarse woody debris: Insights from a large wildfire
Introduction
Fire shapes the composition of ecosystems through its effects on vegetation structure (Bond et al., 2005, Bowman et al., 2009), which in-turn affects the distribution of fauna (Fox, 1982, Friend, 1993). The immediate and longer-term effects of fire on faunal habitat depend on the fire regime: fire severity, fire frequency, time-since-fire, fire interval and the season of fire (Gill and McCarthy, 1998, Smucker et al., 2005, Haslem et al., 2012). Fire regimes can vary within relatively small areas, because even large, intense fires create a mosaic of severities at multiple scales (Turner et al., 1994, Román-Cuesta et al., 2009, Leonard et al., 2014).
Coarse woody debris (CWD: here defined as logs and dead trees) is a common component of many terrestrial ecosystems (Harmon et al., 1986, Jonsson and Kruys, 2001, Lohr et al., 2002). It has an important role in nutrient cycling and carbon storage, and provides habitat for plants and animals (Harmon et al., 1986, Lindenmayer and Franklin, 2002). The dynamics of CWD are driven by the interaction of long-term processes, such as senescence and decay, with shorter-term disturbance processes, such as timber harvesting and fire (Harmon et al., 1986, Haslem et al., 2011).
Fire is integral to the dynamics of CWD, as it both consumes existing debris and generates new material through its influence on tree death and collapse (Harmon et al., 1986, Tinker and Knight, 2000). Diverse responses to aspects of the fire regime have been observed. For example, the effects of time-since-fire on the abundance of logs ranges from a post-fire increase (Monsanto and Agee, 2008), to a peak at intermediate fire ages (Roccaforte et al., 2012), or no detectable effect (Pedlar et al., 2002, Eyre et al., 2010). Such diverse relationships suggest that responses to fire vary between, and potentially within, ecosystems. However, such variable effects of time-since fire could also be influenced by failing to account for other aspects of the fire regime, both spatial and temporal. Fire severity (e.g. Smucker et al., 2005) and fire interval (e.g. Haslem et al., 2012) are known to strongly influence habitat structure, but are rarely accounted for in fire ecology studies, including those on CWD (but see Collins et al., 2012b).
Topographic variation influences fire behaviour, as moist gullies often repeatedly escape fire, or burn less severely than the surrounding landscape (Pettit and Naiman, 2007, Bradstock et al., 2010, Leonard et al., 2014). When gully vegetation does burn at high intensity, for example during extreme fire conditions (Leonard et al., 2014), the vegetation may recover more quickly due to the protected aspect and high soil moisture (Romme and Knight, 1981, Segura and Snook, 1992). Thus, topographic variation may interact with fire regimes to determine the dynamics of CWD.
Research on the post-fire dynamics of CWD has been conducted largely in forests that experience stand-replacing fires, such as the boreal forests of North America and Europe (Harmon et al., 1986, Tinker and Knight, 2000, Pedlar et al., 2002, Monsanto and Agee, 2008), and tall wet eucalypt forests of south-eastern Australia (Lindenmayer et al., 1999). In other forests, such as the mixed Eucalyptus species foothill forests that cover some 7.9 million ha of south-eastern Australia, trees often survive severe fires through epicormic sprouting. Despite the complex role of fire in structuring these ecosystems (Gill, 2012), and the key role that CWD plays within them (Lindenmayer et al., 2006), understanding of the drivers of CWD is limited, particularly in relation to fire regimes.
Here, we explore the role of multiple components of the fire regime and topographic variation on the dynamics of CWD in a foothill forest ecosystem following the 2009 ‘Black Saturday’ wildfires in central Victoria, Australia, which burnt 228,000 ha of forest. We had four primary objectives: (1) to develop a conceptual model of the effects of wildfire on CWD over time; (2) to determine the effects of fire severity and fire history on the relative abundance of CWD (logs and dead standing trees); (3) to examine whether the effects of the fire regime are modified by topographic position (i.e. damp gullies vs. drier slopes); and (4) to determine whether the size of logs and dead trees influences how they are affected by the fire regime.
We developed a conceptual model of the post-fire dynamics of logs in forest ecosystems in which trees often survive severe fire (Fig. 1). There are four main sources of logs following fire. First, at least part of the existing log resource is likely to remain post-fire. Second, trees not killed by fire may drop branches, resulting in a pulse of smaller logs. Third, some trees are damaged at the stem base and are killed by fire, and either fall shortly after the fire or remain as standing dead trees for many years before collapsing. Finally, trees that regenerate in gaps created by fire will contribute to the log resource in the longer term.
The magnitude and rate of log consumption, tree death, tree collapse and tree regeneration will depend on several aspects of the fire regime, including fire severity and fire history. More severe fires will result in the consumption of more logs and kill more trees, but may obscure the effects of previous fire on CWD. Characteristics of logs, including their size, moisture content and level of decay, will affect their flammability; while the death and collapse of trees will be influenced by the composition of tree species, tree health and the (non-fire) disturbance history of the forest. Moisture differentials associated with topographic position will influence the abundance of logs and dead trees, as well as their decay rate. Gullies, with their moister and more sheltered microclimate, experience longer fire intervals than drier slopes, allowing more time for logs to accumulate. These conditions allow growth of larger trees and, therefore, the potential production of larger logs, but also promote more rapid decay. The moist conditions and higher topographic relief may also mitigate the effects of fire on CWD.
We used our conceptual model as a base to predict how topography, fire severity, and fire history will interact to affect the availability of CWD ∼3.5 years after wildfire. We predict that:
- 1.
Gullies will support a greater volume of large logs and greater abundance of large dead trees than slopes.
- 2.
Sites burnt in the 2009 wildfire will experience a reduction in the volume of logs, especially small logs, and an increase in the abundance of dead standing trees compared to sites not burnt in the wildfire. The magnitude of change will be greater at severely burnt than understorey burnt sites, and on slopes compared to gullies.
- 3.
Unburnt sites with a short fire history (time-since fire ⩽3 years) will have fewer logs, and more dead trees, compared to unburnt sites with a long fire history (time-since-fire ⩾20 years).
- 4.
Sites in which only the understorey was burnt by wildfire will have fewer logs, and more dead trees, when the fire history was short (interval ⩽3 years) compared to long (interval ⩾20 years), however differences will be small.
- 5.
Severe wildfire will obscure the effects of previous fires, and there will be no detectable effect of fire history.
Section snippets
Study area
On ‘Black Saturday’, 7 February 2009, two wildfires in central Victoria, Australia, joined to form the Kilmore–Murrindindi fire complex. The study area includes private land, townships, and several State Forest and National Park reserves. Approximately half of the 228,000 ha area burnt was foothill forest, a topographically diverse forest system consisting of damp gullies and drier slopes. Our study was undertaken in these foothill forests, which range in elevation from 153 to 937 m and have a
Results
Following initial modelling to determine whether topography and fire severity interacted to influence CWD availability (see Table A.1 for parameter coefficients and model fits), we developed separate model sets for gullies and slopes to test the relative influence of fire regime components and timber harvesting on the volume of small and large logs and the abundance of small and large dead trees in gullies and on slopes. A single ‘best model’ was evident for all CWD components except small logs
Discussion
Despite the important role of coarse woody debris in ecosystem function (Harmon et al., 1986, Tinker and Knight, 2000), the effects of multiple fire regime components on CWD have rarely been studied concurrently. Here, we have demonstrated that the availability of CWD is influenced by two components of the fire regime – fire severity and fire history – and that the response of structural components to fire depends both on their size and topographic location.
Implications and conclusions
Our results suggest that, while coarse woody debris in foothill forests is relatively resilient to fire, both wildfire severity and fire history are important determinants of CWD dynamics. Damp gullies, which support the greatest abundance of CWD in this system and many others (e.g. Webster and Jenkins, 2005, Collins et al., 2012a), may be particularly vulnerable to changed fire regimes (Bradstock et al., 2010). Frequent fires in gullies, particularly severe wildfires, will reduce the existing
Acknowledgements
This research was conducted as part of the Faunal Refuges Project, with funding provided by the Land and Fire Management Division of the Department of Environment and Primary Industries (Vic.). We thank Natasha Robinson for assistance with mapping, site selection and project discussions, and volunteers who assisted with fieldwork. We thank two anonymous reviewers whose comments helped improve the manuscript. MB and EKC were supported by Australian postgraduate awards. Research was undertaken in
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Current address: Department of Ecology, Environment and Evolution, La Trobe University, VIC 3086, Australia.