Does fire deplete the physical and biological resources of tall-tussock (Chionochloa) grasslands? The latest attempt at some answers
Ian J. Payton1 ( email@example.com ) and Grant Pearce2
2 Forest Research, PO Box 29-237, Fendalton, Christchurch, New Zealand
Abstract: The deliberate use of fire in the pastoral management of tall-tussock (Chionochloa) grasslands has long been a contentious issue. Environmental advocates portray fire as damaging native biota, increasing opportunities for weed invasion and promoting soil erosion, while pastoralists see it as a means of improving access for stock, promoting palatable regrowth, and removing ‘woody weeds’. This paper describes the development of a study that aims to provide answers to three questions central to this debate, (i) Does fire cause long-term damage to the physical and biological resources of tall-tussock grasslands, (ii) Are accidental summer fires more damaging than prescribed burns in late winter or early spring and (iii) Are fuel availability (plant biomass) and local weather conditions good predictors of fire behaviour in tall-tussock grasslands?
Fire and herbivory have been powerful selective forces in the development, maintenance, and in more recent times the degradation of New Zealand’s native grasslands. Below timberline, these communities are largely a seral vegetation, and in the absence of periodic disturbance will revert to the woody growth forms that characterised montane and lowland landscapes in pre-human times (Molloy et al. 1963).
In the eastern South Island, extensive tracts of tall-tussock (Chionochloa) grassland present at the time of European settlement have their origins in early Polynesian fires that allowed downslope migration of subalpine Chionochloa species (reviewed in McGlone 2001). The advent of pastoralism in the 1850s increased fire frequency, introduced mammalian herbivory, and precipitated widespread and dramatic changes in the stature and composition of these fire-induced grasslands (Connor 1964, O’Connor 1982). Change has been greatest in areas of lower rainfall, where the displacement of the dominant tall-tussocks by shorter tussock (Festuca, Poa) or mat (Hieracium, Raoulia) growth forms is exacerbated by a range of alien plant (e.g., Hieracium, Rosa) and animal (e.g., rabbit) pests. As the native forage resource declined, many montane grasslands were oversown with adventive pasture grasses and legumes, and since the introduction of aerial topdressing some have received intermittent applications of phosphate-based fertilisers (O’Connor 1987, O’Connor & Harris 1991). The net result is an extensive mosaic of modified grassland communities that retain varying degrees of native dominance and biodiversity.
The deliberate use of fire has long been a contentious issue in the South Island high country. While environmentalists frequently portray fire as damaging native flora and fauna, increasing opportunites for invasive weeds and promoting soil erosion, pastoralists see it as a means of improving access for stock, promoting palatable regrowth, and reducing or removing what are seen as ‘woody weeds’. Over recent decades, the retirement of pastoral leasehold land and predictions of reduced rainfall have raised concerns in some rural communities that the increased biomass (fuel load) of grasslands that are no longer intentionally burned or grazed may pose an increased fire risk during the dry summer and autumn months. From a conservation perspective there is also the issue of whether deliberate burning under damp spring conditions (the current pastoral management practice) is less damaging to the native biota than an accidental fire when weather conditions are hot and dry, and seasonal plant growth is well advanced.
Over the last 50 years a large number of studies have described the impacts of burning and grazing on tall-tussock grasslands, and have documented the effect of fire on the growth and forage quality of the dominant tussock species (reviewed in Basher et al. 1990, McKendry & O’Connor 1990). Most of these studies have examined only single aspects of the tall-tussock ecosystem, and none have established a pre-burn baseline and characterised the severity of the fire. In this paper we discuss the development of a study that aims to quantify the extent to which fire degrades the fertility of tall-tussock grasslands and depletes the native biota, and present some initial results on biomass and nutrient pools and fire behaviour.
Development of the project proposal
In 1995/96 a consortium of three government agencies (Forest Research, Landcare Research, and the Department of Conservation) began developing a proposal to compare the effects of fire on red tussock and snow tussock grasslands. By early 1997 the group had failed to find areas of red tussock grassland (a relatively uncommon type of tall-tussock grassland in Otago) that were able to be burned safely, and were meeting opposition to using Conservation Land for their experimental sites. These issues came to a head at an April 1997 meeting of interested parties from conservation (Department of Conservation, Royal Forest & Bird Protection Society), farming (Federated Farmers), regional government (Otago Regional Council), university (Botany Department, Otago University) and government science (Forest Research, Landcare Research) agencies. That meeting changed the focus of the project to a comparison of fire effects in snow tussock grasslands from coastal (maritime) and inland (subcontinental) environments in Otago, and committed the researchers to seeking experimental sites outside the Conservation Estate.
The project is now a collaborative venture between the Department of Conservation and three government science agencies (AgResearch, Forest Research, and Landcare Research), and receives funding from the Foundation for Research, Science, and Technology, the Department of Conservation, the Hellaby Indigenous Grasslands Research Trust, and the National Rural Fire Authority.
Landcare Research and AgResearch staff are determining changes to the vegetation composition, plant biomass and nutrient pools. The quantities of plant material and nutrients are measured using quadrat harvests and soil cores (Figure 1). The plant material is sorted by species or species-group, and subsamples are analysed for their nutrient content.
Figure 1. Pre-burn biomass sampling of tall-tussock (Chionochloa rigida) grassland at Deep Stream, near Dunedin.
AgResearch staff and graduate students from the University of Otago are using turf samples and soil cores to determine changes to litter and soil dwelling groups of invertebrates. The former are extracted using Berlese funnels, and the soil cores are hand sorted to provide estimates of earthworm numbers and the soil dwelling stages of insects such as cicadas and scarab beetles.
Fire research staff from Forest Research determine the fire behaviour of each burn, in particular the rate of fire spread and fuel consumption. This provides a measure of the intensity of the fire and, in conjunction with measurements of fuel moisture content and fire temperature, allows the severity of each burn to be described. Maximum temperatures reached during each fire are determined using metal plates coated with heat-sensitive paints (Figure 2), which are placed in the soil, at ground level, and 1m above the ground. These data will also be used to help develop predictive models of fire behaviour based on parameters such as slope, fuel availability and weather conditions. The models are a necessary first step for developing safe and effective strategies for fighting tussock wildfires, and for defining appropriate conditions for prescribed burning or the imposition of fire restrictions.
Fire safety at the experimental burns is the responsibility of Department of Conservation fire crews, supported by rural fire staff from local authorities (Dunedin City Council, Central Otago District Council) and forestry companies.
Figure 2. Metal plates coated with heat sensitive paints used to determine the temperatures generated by the fire
Biomass and nutrient pools in the unburned grasslands
Table 1. Pre-burn assessment of plant biomass and nutrient pools in snow tussock grasslands at Deep Stream and Mt Benger. Values (mean ± SE) are expressed as kg.ha-1, and are the mean of 9 replicates.
* Carex wakatipu, Chionochloa rigida, Poa colensoi, Uncinia spp., misc. fine grasses
Biomass losses associated with spring and summer burnsBecause of the three year delay between the initial biomass harvests (1997/98) and the imposition of the first fire treatments, plots were resampled before and again immediately after each fire (Figure 3). The spring burns at Mt Benger consumed an average of 32% of the aboveground biomass. They do not appear to have killed mature tussocks, and they left much of the prostrate shrub, herb and lower plant (Lycopodium, moss, lichen) biomass intact.
Figure 3. Sampling post-burn biomass residues after a spring fire in tall-tussock grassland at Mt Benger.
Figure 4. Tall-tussock grassland at Deep Stream six weeks after a summer burn. Note the rapid recovery of the golden spaniard (Aciphylla aurea), and high proportion of exposed soil.
By contrast the summer fires at Deep Stream burned an average of 74% of the above-ground plant material, including most (89%) of the shrub, herb and lower plant biomass. Tussock tiller mortality was much higher than that after the spring burns, and some mature tussocks have been killed. The plant species least affected by the summer fires is the golden spaniard (Aciphylla aurea), which 6 weeks after the fires was regrowing vigorously from its blackened stumps (Figure 4).
In the absence of grazing by stock, tussock cover on the spring burn plots appears to be recovering well (Figure 5). Tussock recovery after the summer burns was much less advanced at the end of autumn, and the plots have gone into winter with little tussock cover and a high proportion of exposed soil (Figure 4).
Figure 5. Tall-tussock grassland at Mt Benger six months after a spring burn. This site has not been grazed by sheep since the burn.
Fire weather conditions were characterised using onsite climate data, and the fuel moisture codes of the Fire Weather Index (FWI) System1 (Van Wagner 1987). The fuel moisture codes provide a measure of the dryness of available fuels and soil organic layers, based on the cumulative effects of temperature, humidity and rainfall. These data were supplemented by measurements of vegetation (height, cover, biomass, moisture) and soil (moisture) parameters (Tables 1&2).
The spring burns at Mt Benger were carried out in early November 2000. While this is later than pastoral burns are permitted, weather conditions were still characteristic of the spring-burning period. There was a good crisp frost on the morning of the burns, and snow blanketed the site two days later. The Deep Stream plots were burned in early March 2001, which was the first time the site had crossed our threshold for summer burns (FFMC > 75, DMC 10-30, DC > 200, BUI 20-50).
Table 2. Weather conditions, fire weather indicies, fuel characteristics, and fire behaviour for tall-tussock grassland fires at Mt Benger (spring burns) and Deep Stream (summer burns).
1Values are the mean of 16 measurements
Among the fuel characteristics, the primary feature distinguishing spring and summer burns was the moisture content of the tussock bases and the upper soil layers (Table 2). Other parameters did not vary significantly between the seasonsMaximum fire temperatures measured using heat-sensitive paints also showed little variation between seasons. The highest temperatures were observed at the ground surface (> 1010 ºC), and at 1 m above-ground (>500 ºC).
Soil temperatures at depths of 2.5 and 5.0cm did not exceed 69 ºC (the temperature at which the first heat sensitive paint strip melts) during any of the spring or summer burns. Rates of fire spread (Figure 6, Table 2) for individual burns were influenced by wind speed, fuel moisture content and slope, and generally increased during the afternoon as wind speeds increased and fuels dried out. Despite quite different seasonal conditions, rates of fire spread were similar for spring and summer burns.
Figure 6. Recording the rate of fire spread for a spring burn at Mt Benger.
Establishing a study that crosses institutional, sectoral and science discipline boundaries against a background of declining government funding for high country research hasn’t been easy. Nevertheless it is our belief that if we are to raise the quality of the debate over the management of these grassland ecosystems, multi-disciplinary studies of the type described here are essential. Wherever possible these studies should include participation from (or at least consultation with) interested parties spanning sectoral interests, and the free exchange of research data. If you have a research question you think our experimental set-up could be used to answer, or would like access to the data from this study we invite you to contact us at the addresses given at the beginning of this paper.
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