Fine-scale patterns in the distribution of semi-arid tree species at Wyperfeld National Park, southeastern Australia – The potential roles of resource gradients vs disturbance

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Abstract

Relatively little is known of the environmental determinants of fine-scale patterns of tree distribution and regeneration on inland dune systems in semi-arid regions. Trees in semi-arid regions can experience extremes of temperature, and resource availability can vary both temporally and spatially. Climate change may disrupt these fine-scale patterns and alter patterns at the landscape level. Here we investigate potential influences of environmental factors and disturbance on fine-scale distribution patterns in four tree species across a predicted aridity gradient along four inland dunes in semi-arid southeastern Australia. The tree species were restricted to narrow topographic ranges, occurring sequentially up dune slopes. Relatively little variation was detected in conditions experienced by each species, except for the contrasting soils of the alluvial flat at the base of the dunes. Fine-scale gradients in resource availability and microclimate, together with variation in site-specific fire and flood history on these dunes, appear to be determining tree distribution patterns. Landscape-level species coexistence and diversity may decline if fine-scale topographic differences and disturbance regimes are disrupted by climate change.

Introduction

Relatively little is known of the factors controlling tree species distribution patterns in arid and semi-arid environments, particularly with respect to the combined effects of resource availability and large-scale disturbance. Climate can be an important determinant of how resources are distributed (James and Richards, 2007) and of the frequency and intensity of disturbances such as fire (Brown and Wu, 2005, Gworek et al., 2007). Climate change, with predicted higher temperatures and lower precipitation in many arid regions, is likely to result in an expansion of arid and semi-arid environments (Breshears et al., 2005, Schlesinger et al., 1990). Thus it is important to understand vegetation processes in these regions to enable predictions regarding future change. In areas with a large gradient in elevation, tree species may potentially migrate up or down slope to more suitable environments as climate change alters environmental conditions. However, where there is little elevational variation, climate change may eliminate suitable local environments for some species. Species migration will be dependent on dispersal ability and the degree of connectivity between habitats in the landscape (Jeltsch et al., 1998, Vetaas, 1992), in addition to the scale over which resources and conditions vary and the responses of species to fine-scale environmental variation.

The distribution, regeneration and growth of plant species occupying arid and semi-arid regions are considered to be controlled primarily by water availability (Ackerly, 2004, Noy-Meir, 1973). However, arid environments are more complex than simply being dry. Evaporative loads can be high, and extremes of heat and cold and low concentrations of soil nutrients can constitute additional stresses (James et al., 2005, Noy-Meir, 1973, O'Brien et al., 1988). Furthermore, rainfall, temperature and nutrient availability can vary across a range of temporal scales (Burton and Bazazz, 1991, Carriera and Niell, 1992, Schade and Hobbie, 2005), fluctuating diurnally, seasonally and between years. Hence plants in these regions must be able to tolerate not only severe conditions, but quite variable and often relatively unpredictable conditions.

The broad geographic patterns that govern the spatial distribution of water in the landscape may grade to finer scale patterns induced by variation in topography and soil type. For example, soil texture directly influences infiltration rates and hydraulic conductivity, and determines the amount of water and the length of time it is available to plant roots (Fernandez-Illescas et al., 2001, Noy-Meir, 1973). Concentrations of soil nutrients can be low in arid and semi-arid environments and nutrient availability may be influenced by water availability and soil pH, in addition to the mineral composition of the soil (Beadle, 1962, James et al., 2005, Prentice et al., 1992;). Therefore, vegetation in arid and semi-arid regions may change at a fine spatial scale in response to topographic gradients of resource availability and aridity, as well as across broader spatial scales.

Disturbance can also be a feature of arid and semi-arid systems and different life-history strategies may evolve in response to these disturbances (Grime, 2001, Lloret et al., 1999). For example, grazing pressure can be extreme because of the restrictions to plant growth that occur in resource-limited environments (Cheal, 1993, De Bello et al., 2007, Huston, 1999, Johnston, 1968) and in some regions semi-arid environments can also be prone to fires (Bradstock et al., 2006, Cheal and Parkes, 1989, Weisberg et al., 2008). The demand for water by agriculture has led to changed hydrological regimes in many arid regions (Jenkins et al., 2005, Sooroshian et al., 2002), intensifying the stresses faced by native vegetation. Therefore, species distributions are likely to be determined by disturbance history in conjunction with gradients of resource availability and conditions.

This study investigates the distribution of four tree species, comprising two gymnosperms (Callitris species) and two angiosperms (Eucalyptus species), in a region of semi-arid vegetation that is known to experience both the typical features of semi-arid environments (e.g. dryness and high summer temperatures) as well as infrequent fires and floods. It focuses on the fine-scale distribution patterns of these species along topographic gradients on four dunes in order to explore the factors controlling species coexistence on a broader landscape scale. A range of soil and water variables were measured and data were collated for each site with respect to disturbance and flood history. The specific aims were to investigate (1) how these species are distributed along a fine-scale topographic gradient (dune slopes), (2) which environmental variables best explain the distribution patterns of these trees, and (3) the potential influence of large-scale disturbance on these patterns, to better understand the controls of tree species regeneration and distribution patterns in these potentially complex environments.

The location chosen for the study was Wyperfeld National Park, in northwestern Victoria, Australia. The region is semi-arid with a highly seasonal climate. The mean daily temperature is 23 °C, with an average of 80 days experiencing temperatures above 30 °C and 5 days below 0 °C. Rainfall averages 330 mm annually, with 67% occurring between late autumn and spring (May–November). Average daily evaporation rates are 1–7 mm across the year (Bureau of Meteorology DSS, 2003). In this region marine sands deposited in the Miocene were mobilised during the Quaternary into a series of northwest–southeast aligned dunes (Cheal and Parkes, 1989, Wasson, 1989). A drainage channel, Outlet Creek, bisects the region, originating at Lake Albacutya to the south and terminating in a series of 17 ephemeral lakes. Historically, Lake Albacutya would fill with a periodicity of c. 25 years and Outlet Creek would carry overflow to progressively fill upstream lakes (Bren and Sandell, 2004). However, since European settlement of the region and subsequent increased demand for water in agriculture, the hydrologic regime has been disrupted. The most northerly lake last filled in 1874 and the most southerly lake last filled in 1976 (NPS, 1996). Bordering the lakes are larger dunes of aeolian sands deposited when lake beds were dry (Wasson, 1989). The dune soils have a deep profile of pale yellow quartzite sands at the dune crests, grading to loamy sands down the dune slopes (Gibbons and Rowan, 1993). They typically have low concentrations of N, P, K and some trace elements (Blackburn and Wright, 1989, Gibbons and Rowan, 1993).

The study region was selected because it contains populations of four semi-arid tree species, the angiosperms Eucalyptus largiflorens F. Muell. and E. incrassata Labill. (Myrtaceae) and the gymnosperms Callitris preissii R.T. Baker and C. verrucosa A. Cunn. ex Endl. (Cupressaceae), each of which has a broad geographic range and occur on these dune systems. Eucalyptus largiflorens is a medium tree reaching 10–20 m in height. It occurs along watercourses in semi-arid regions of southeastern Australia and in intermittently flooded regions of inland eastern Australia (Costermans, 1981) and in the study area borders the ephemeral lakes and Outlet Creek system. Callitris preissii, a tree reaching up to 20 m in height, occurs through semi-arid regions of northwestern Victoria, South Australia and southern New South Wales (Walsh and Entwisle, 1994). Eucalyptus incrassata is a small multi-stemmed (mallee) tree, 3–8 m in height, found through semi-arid regions of Victoria and South Australia (Costermans, 1981, Walsh and Entwisle, 1996). Callitris verrucosa is a shrub or small tree, 2–6 m in height, found in semi-arid regions of South Australia, New South Wales and northwestern Victoria (Costermans, 1981, Walsh and Entwisle, 1994).

Section snippets

Sampling design

Four dunes at Wyperfeld National Park (141° 57′ E, 35°30′ S, c. 70 m asl) were selected in the Outlet Creek system in September 2001, with 20 km between the most southerly and northerly dunes. A transect was positioned from the alluvial flat near the dune base to the crest of each dune (440–580 m in length) (Fig. 1a–d). Quadrats of 20 m × 40 m were established at 40 m intervals up the dune slope, starting from the lake or creek flats, near the mid-point of the range of E. largiflorens. A clinometer was

Species distributions across the topographic gradient

On all four dunes E. largiflorens was restricted to the alluvial flat and lowermost dune slope, terminating at an elevation of c. 1–3 m and a distance of 100–200 m from the transect start (Fig. 1). Tree density was greatest at 0–50 m from the transect start on dunes 1, 2 and 3, and at c.120 m on dune 4 (Fig. 1).

Callitris preissii occurred up-slope of E. largiflorens, grading into stands of E. incrassata at an elevation of 11–16 m above the transect start (Fig. 1). However, its distribution and

Discussion

This study of tree-vegetated inland dunes in a semi-arid climate suggests a complex pattern of environmental gradients in resource availability and disturbance, including both floods and fire. Fine-scale trends in resource availability can be important determinants of species distribution patterns in dune systems (Corre, 1991, Parsons, 1969). However, in many dune systems in semi-arid regions, grass and shrub species are the dominant life-forms (Bowers, 1996, Mangan et al., 2004, Munoz-Reinoso

Acknowledgements

We thank the Department of Sustainability and Environment (Victoria) and Parks Victoria for allowing this study to be undertaken at Wyperfeld National Park, and Vanessa Deveraux and Hunter Morgan for their assistance in the field. TW was supported by an Australian Postgraduate Award.

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