ABSTRACT
Ephemeral wetlands in the Antarctic dry valleys alternate as sites of in situ net primary production when inundated and sources of organic matter for eolian export when desiccated. Evidence of this switch was obtained from observations made in two field seasons (2001 and 2003), coincidentally separated by a season of exceptionally high water yield from nearby glaciers (2002). In 2001, organic matter on soil surfaces adjacent to several ponds in Taylor Valley often exceeded 500 g C/m2. One pond had a total stock of approximately 1,388 kg organic C on soils within 20 m of the shoreline. These materials formed concentric rings around the pond, suggesting historically higher water levels, consistent with aerial photographs taken a decade earlier (1993). In 2003, these materials were submerged by water apparently received during the intervening year. Also in 2003, pitfall traps were placed along the edges of exposed organic matter adjacent to another pond to evaluate organic material erosion by wind. Over 4.5 days, traps collected 0.22 ± 0.12 to 2.91 ± 1.68 g C/m2, both confirming and quantifying wind transport. These results indicate that organic matter production and movement, driven by seasonal and decadal patterns of inundation/desiccation of small, ephemeral wetlands, overlay longer term (centuries and millennia) and larger scale (landscape) patterns of production and distribution driven by regional fluctuations in hydrological balance between glaciers and large freshwater lakes.
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INTRODUCTION
Variations in conditions and resources favorable to biological activity have important impacts on the structure and function of ecosystems, and typically occur over many scales of resolution (Kolasa 1989). For example, environmental perturbations often occur frequently at small spatial scales but less frequently at larger spatial scales (for example, Pickett and White 1985; Romme and others 1998). Although infrequent, large-scale perturbations, such as hurricanes, have obvious impacts on ecosystems (Boose and others 1994), Swetnam and Betancourt (1998) clearly elucidated a continuum of interacting temporal × spatial linkages between micro-, meso- and mega-scale variations affecting ecosystems. The polar deserts in the dry valleys of Antarctica are no exception to these patterns, with recent studies suggesting that decadal to millennial scale fluctuations in climate and hydrology have important ramifications to ecosystem structure and function (Lyons and others 1997, 1998; McKnight and others 1998; Moorhead and others 1999, 2003; Doran and others 2002b; Foreman and others 2004).
The Antarctic dry valleys are among the most extreme environments on earth, but biological communities inhabit the soils, stones, ice, streams, ponds and lakes of these desert ecosystems (Vincent 1988; Priscu and others 1998). The availability of water appears to be the most pervasive limitation to life in the dry valleys (Kennedy 1993; Moorhead and Priscu 1998), and this limitation is defined in part by energy regime. Small differences in the local energy budget often determine the phase shift between ice and water, because ambient temperatures hover near freezing during the summer (Fountain and others 1999). This combination of extreme aridity and low temperature creates a conundrum for biota in that energy regimes that melt ice also drive evaporation (Moorhead and others 2003). Topographic factors also influence the potential availability of water by defining glacier flow paths, stream channels, pond basins and accumulation of windblown snow. For these reasons, physical conditions defining water availability impose strong controls on where, when and for how long biological processes can proceed, and are apparent across many scales of temporal and spatial resolution.
Long-term and broad-scale variations in climate and hydrology of the dry valleys have been discussed elsewhere (Denton and others 1989; Lyons and others 1997, 1998; Hall and others 2001). In brief, large changes in the mass balance of glaciers during cycles of glacial advance and retreat often generate large changes in the hydrology of the valleys. In some cases, freshwater lakes on valley floors expanded to many times their current size during glacial maxima and diminished or even disappeared during interglacial periods (Lyons and others 1998). Ecological legacies of these past conditions can influence modern communities. For example, soil organic matter at low elevations in Taylor Valley, Antarctica, is in part derived from the larger glacier lakes that once filled the valley lowlands (Moorhead and others 1999; Burkins and others 2000). The dynamics of such legacies operate over broad temporal and spatial scales.
Within this broad-scale context, recent syntheses show that climate and hydrology of the McMurdo Dry Valleys have undergone smaller, but ecologically significant variation over the past century (Chinn 1993; Doran and others 2002b). For example, Bomblies and others (2001) found that current climate and hydrological regime could not explain changes in the level of Lake Bonney, Taylor Valley, since it was first recorded in 1908 (Chinn 1993). Doran and others (2002b) further demonstrated that the valley has become cooler and drier over the past decade, with measurable impacts on biological communities in soils, streams and lakes. These effects on biota in predominantly arid soils and hydrated lakes appear to be gradual, occurring over decades as soils progressively dry and lake levels slowly fall.
In contrast to the much larger dry valley lakes and extensive areas of predominately arid soil, smaller streams and ponds undergo abrupt changes in response to relatively minor fluctuations in hydrological regimes. For example, streams in the McMurdo Dry Valleys are fed exclusively by melting glaciers and thus flow only during the Austral summer. They freeze and desiccate each winter, yet the resident microbial mats respond rapidly to inundation after lying dormant between seasons and even across many years (Vincent and Howard-Williams 1989; McKnight and others 1999). Intra- and inter-annual variations in hydrology have large impacts on the biological activities in these small, ephemeral systems.
Ponds and other ephemeral wetlands are common throughout the dry valleys but have received little study to date. Their small size, spatial isolation and seasonally limited period of activity seem to imply little potential contribution to ecosystem dynamics of the larger dry valley landscape. However, Moorhead and others (2003) found concentrations of organic matter associated with several upland (>300 m asl) ponds and streams in Taylor Valley that rivaled amounts reported for larger streams and permanent lakes (Burkins and others 2001). A detailed investigation of one pond revealed approximately 1,388 kg C in organic matter lying on the surface of soils within 20 m of shore. Organic C concentrations in underlying mineral soils as far as 90 m from the pond were twice the average value reported for soils throughout the valley (Burkins and others 2001). Moreover, tardigrades, rotifers and the nematode, Plectus antarcticus, were found only in moist soils within a few meters of the shore, similar to their distribution patterns near stream margins at lower elevations (Treonis and others 1999). These observations led Moorhead and others (2003) to speculate that upland wetlands serve both as oases of biological diversity and as sources of organic matter to adjacent soils. Polis and others (1997) describe similar spatial subsidies of organic matter to terrestrial ecosystems from aquatic ecosystems, but allochthonous flows are generally larger and more common in the opposite direction.
In January 2003, wetlands examined by Moorhead and others (2003) were revisited to measure changes in levels of inundation and organic matter concentrations since January 2001. A warming event during the intervening austral summer generated record stream discharge in Taylor Valley (Foreman and others 2004), which inundated nearly all the organic matter around ponds described by Moorhead and others (2003). In some places, additional organic matter had accumulated on shores beyond the newly submerged areas. The focus of this study shifted to (1) quantifying the new accumulations of organic matter on soil surfaces around previously examined ponds, (2) quantifying organic matter on soil surfaces around additional ponds in Taylor Valley and (3) quantifying wind erosion of organic matter from accumulations on soil surfaces around a pond in lower Taylor Valley.
METHODS
Organic matter on soil surfaces adjacent to four ponds was examined in this study (Table 1). This included two ponds that were examined in 2001 (Marrs-2 and Marrs-3 Ponds; Moorhead and others 2003), one located at lower elevation on the south side of Taylor Valley (ca. 100 m asl) and one located at high elevation (ca. 600 m asl) along the north side of the valley. These latter ponds do not have names, and are referred herein as Lowland Pond and Northside Pond, respectively. Organic matter samples also were collected from the surfaces of benthic sediments in a few of many shallow basins of water (<1 m depth) that were found 100–200 m asl on the south side of lower Taylor Valley (Table 1). These pools did not exist in 2001.
The organic matter on soil surfaces adjacent to ponds was examined with line transects extending perpendicular from shoreline to the extent of visible organic matter. Line transects were placed 5 m apart along Marrs-2, Marrs-3 and Northside Ponds, and 10 m apart along Lowland Pond. The cover of organic matter on soil surfaces was calculated as the fraction of the distance that materials intercepted each line. Samples of organic matter were collected from along each transect with a 0.83 cm diameter steel corer, and taken to the Crary Laboratory, McMurdo Station, Antarctica for mass determinations. Samples were dried at 60°C for 24 h, dry weights were recorded, dried samples were combusted at 500°C for 6 h, and ash-free dry weights calculated. Organic C content was assumed to represent 50% of the ash-free dry mass. The mass of organic matter on the soil surface around each pond was calculated as the product of the fraction cover and average mass of organic materials along each transect, assuming that these values were representative of the organic matter on soil surfaces between transects.
Erosion of organic matter from surface accumulations was examined along the shore of Lowland Pond. Paper cups were used as pitfall traps to capture materials eroding from surface organic matter. These cups were 88-mm tall with a bottom diameter of 49 mm and top diameter of 71 mm. They were buried so that their tops were flush with the surface of the soil and half filled with glass beads (ca. 7–8 mm diameter), to protect particles falling into the cups from wind currents. Beads were washed with distilled water before being placed in cups. Four cups were buried at 1-m intervals along a line oriented approximately parallel to the pond shoreline, 1 m from the inland edge of these surface organic materials. A total of seven sites were selected for study, around the entire perimeter of the pond.
Cups were deployed on 20 January 2003 and collected 4–5 days later. The top of each cup was sealed and all cups were taken to Crary Laboratory for analyses. Distilled water was used to rinse materials from each cup onto a preashed filter. Samples were dried at 60°C for 24 h, combusted at 500°C for 6 h and organic C content estimated as 50% of the ash-free dry mass. Wind speed and direction were recorded at 15-min intervals throughout the erosion study at a meteorological station located at Lake Fryxell, less than 5 km from Lowland Pond (http://www.huey.edu/LTER).
RESULTS
Several of the ponds examined in 2001 were deeper (not measured) and had much larger surface areas in 2003 (Table 1). Also, most of the organic matter reported by Moorhead and others (2003) was inundated in 2003, with little or no material on soils adjacent to the subsequently deeper and larger Marrs-1, Marrs-4 and Highland Ponds. Organic matter was present along the south to west arc of Marrs-3 and south to east arc of Marrs-2 in 2003, mostly falling within scattered bands on the soil surface at 10–20 cm above current water levels and representing much smaller amounts than reported in 2001. The organic matter on soil surfaces around ponds also differed in density between years. For example, organic matter around Marrs-3 averaged 533 g C/m2 (n = 10) in 2001 and only 223 g C/m2 (n = 10) in 2003. Materials in 2003 lacked the firmer, consolidated structure of materials sampled in 2001. These observations suggest two different means of organic matter deposition on soil surfaces. The more rigid, thicker and continuous layer of materials found firmly attached to the soil surface in 2003 often had a laminar structure similar to contemporary benthic microbial mats commonly found in streams, ponds and lakes of Taylor Valley. In fact, these materials were often contiguous with inundated mats in shallow water at the pond margins. Thus these organic materials appeared to have been formed in situ by benthic microbial communities when these sites were previously inundated, and subsequently exposed by falling water levels. In contrast, materials observed in 2003 were being deposited by wave action during the study (personal observation), apparently from sites of current production by benthic microbial communities within the inundated regions of the ponds. These latter materials were more fragile, discontinuous and easily removed from the underlying soil surface. They were spatially disconnected from microbial mats existing below the water line.
Substantial amounts of organic matter also were found in 2003, at locations that were not examined in 2001. Northside Pond exists at a comparable elevation to the Marrs Ponds (>700 m asl) and was mostly frozen in 2003, except for a narrow moat around much of the east, south and west sides. Organic matter averaged 578 g C/m2 on adjacent soil surfaces, where present, and was similar in amount and appearance of organic materials reported in 2001 for Marrs-3 Pond. This material had a firm, consolidated structure and appeared to have been produced in situ by benthic microbial communities when previously inundated. Several small pools examined in 2003 were located on a flat surface, 100–200 m downstream from the upper stream gage on Von Guerrard Stream (77°38′, 163°18′), and did not exist in 2001. They were less than 30 cm deep and averaged less than 100 g C/m2 organic matter for benthic mats on sediment surfaces, but together represented approximately 135 kg organic carbon in a small area that is topographically exposed to both wind and water erosion.
Lowland Pond was not sampled in 2001 and may not have been inundated at the time. In 2003, it had a roughly semicircular shape but with irregular shoreline and sufficient thawed areas to make direct measurement of its dimensions impossible. However, it had a total perimeter of about 1,440 m, a rough diameter of 306 m and total area of about 73,339 m2. This estimate of area is generous, given the irregular shape. Organic matter was present on the soil surface along most of the shoreline, but covered with snow and ice in several locations, making estimates of total standing stocks difficult (Table 1). Only materials visible along line transects were included in calculations; samples ranged from 32–1,366 g C/m2 (mean = 289 ± 228 g C/m2; n = 104).
The erosion study was conducted at seven sites located on different sides of Lowland Pond. All sites had substantial accumulations of organic matter on soil surfaces (Table 2). The size, shape and cover of materials on these sites varied, but in all cases the average mass of organic carbon exceeded 500 g C/m2, where present, and appeared likely to have been produced in situ. Together, these sites contained about 75% of all the organic matter found around Lowland Pond (Table 1). The total quantities of organic matter collected in pitfall cups over 4–5 days varied between 0.22 and 2.91 g C/m2 (per cup area), for an overall average of 0.34 ± 0.31 g C m−2 d−1 among sites (Table 2), and showed no relationship to the amount of organic matter exposed on adjacent soil surfaces. Pitfall traps located on the northern and eastern sides of the pond accumulated greater amounts of organic matter than traps located along the southern and western sides, suggesting that winds moving in a down-valley direction eroded more materials than winds moving up-valley (see below).
Wind speed and direction varied considerably over the period of time during which pitfall cups were deployed (20–25 January 2003; Table 3). The meteorological station at Lake Fryxell recorded a mean wind speed of 3.62 ± 2.28 m/s, with a maximum value of 10.3 m/s (n = 577). However, wind direction was bimodal, with highest frequencies falling between 30° and 60° (n = 219) and 210°–240° (n = 152), or roughly along the long axis of the valley. Average speed was 5.39 ± 2.38 m/s (maximum = 10.30) for winds descending from the polar plateau (southwest) and 3.16 ± 1.23 m/s (maximum = 7.50) for winds moving up-valley from the Ross Sea (northeast).
DISCUSSION
From their initial study of organic matter distributions around dry valley ponds, Moorhead and others (2003) developed a simple conceptual model of the role of ephemeral wetlands in organic matter dynamics of the larger dry valley landscape. They proposed that decadal-scale cycles of inundation and desiccation drove cycles of organic matter production, when these wetlands were hydrated, and wind-borne losses of organic matter, when they were dry. The most important result of the current study was confirmation that winds erode organic materials exposed on soil surfaces adjacent to dry valley ponds. Although the quantities of organic matter collected in pitfall cups were modest, averaging 0.34 g C m−2 d−1 (Table 2), and collected within 1 m of the surface materials, they suggest a potentially substantial, cumulative export over time. If this rate were constant, it would represent approximately 124 g C m−2 y−1, which is much larger than rates of net primary production for common Nostoc and Phormidium-dominated microbial mats in streams and lakes of the dry valleys, that is, 10.1–24.6 g C m−2 y−1, respectively (Moorhead and others 1997, 2005; Hawes and others 2001).
The quantitative importance of these pitfall trap data is difficult to assess. Although many authors have speculated about the importance of eolian transport of organic matter in the dry valleys (see above), few direct measurements have been taken. Lancaster (2002) recently reported eolian flux of sediment to the surface ice of Lake Bonney (Taylor Valley) at 110.53 g m−2 y−1, but did not report organic matter content. In contrast, Priscu and Fritsen (1996) previously reported eolian flux of particulate organic carbon to the surface ice of the same lake at 43 mg C m−2 y−1, but did not report total sediment flux. Together, these observations suggest a POC concentration less than 0.04% in windblown sediments to the ice cover of Lake Bonney. This estimate falls within the range of organic carbon content reported for mineral soils of Taylor Valley, for example, 0.034% (Powers and others 1998) to 0.08% (Freckman and Virginia 1997), and has implications to the interpretation of data from Lowland Pond.
The sources of organic matter that accumulated in pitfall traps around Lowland Pond are uncertain, but over 90% of the non-glaciated surface area of Taylor Valley consists of bare soil and rock (Freckman and Virginia 1997). Visually detectable quantities of organic matter on soil surfaces are rare, and limited to moist locations or desiccated wetland features (streams, ponds, etc.). The nearest of these sources to Lowland Pond is at the margins of alpine glaciers in the Kukri Hills (over 1 km south) or Lake Fryxell (over 2 km northwest). Alternatively, Burkins and others (2001) reported organic matter concentrations of approximately 0.030% in the top 0–10 cm of soil in the vicinity of Lowland Pond. The materials collected in sediment traps around Lowland Pond lost about 1.6 ± 0.6% dry weight on ignition (n = 28), suggesting an approximate organic carbon content of 0.8%, many times the concentration in surrounding soils. Despite widespread distributions of algal cells and chlorophyll in soils of Taylor Valley, there are no reported values of in situ photosynthesis. Indeed, Wynn-Williams and others (1997) found no evidence of microalgae colonization of soils and concluded that their activity was restricted to the endolithic habit in Taylor Valley. For these reasons, the simplest conclusion about the source of organic matter collected in pitfall traps around Lowland Pond is that it originated from Lowland Pond.
Aside from the source of organic matter to the pitfall traps in this study, few insights to eolian processes in Taylor Valley are offered by such a brief period of observation (4.5 days) around a single pond. Erosion is driven by many non-linear interactions between such factors as topography, wind velocity, surface roughness and soil texture (Lyles 1988; Witman and others 2004). Indeed, Lancaster (2004) found that the roughness characteristics of these rocky soils could be used to predict sediment transport similar to the roughness characteristics imparted by terrestrial vegetation in other ecosystems. Moreover, Doran and others (2002a) reported high variation in wind direction and speed in the valley, both over time and between locations. Given variations in wind, topography, soil texture and surface rockiness in Taylor Valley, it is not surprising that Lancaster (2002) found high spatial variation in eolian sediment deposition in the valley. In this context, the data from pitfall traps around Lowland Pond suggest the likelihood of organic matter moving from the pond as well as impacts of wind speed and direction on spatial patterns of collection around the pond (see below).
There is little doubt that wind moves organic matter between locations within the dry valleys (Wilson 1965; Parker and others 1982; Squyres and others 1991; Nienow and Friedmann 1993; Adams and others 1998; Fritsen and Priscu 1998; Fritsen and others 2000; Burkins and others 2000, 2001), and that consistent patterns in wind speed and direction exist (Doran and others 2002a). It is reasonable to assume that erosion and wind patterns are related. In general, winds travel primarily along the long axis of Taylor Valley, either in the northeast down-valley direction or southwest in the up-valley direction (Doran and others 2002a). Higher speeds are generally reported for winds traveling toward the northeast, especially during katabatic events, when föhn winds descend from the polar plateau and travel down-valley. Not surprisingly, pitfall traps located north and east (down-valley) of surface organic matter accumulations around Lowland Pond collected more organic materials than traps located in most other directions (Table 2), suggesting that the stronger down-valley winds occurring during this study likely moved more materials in that direction. This is consistent with reports (for example, Fritsen and others 2000; Burkins and others 2000, 2001) of higher concentrations of chlorophyll, organic carbon and nitrogen in soils at some locations in Taylor Valley, which lay downhill and/or downwind from upland wetlands described by Moorhead and others (2003). Wind speed and direction undoubtedly interact with wetland location and production to influence organic matter distributions in the dry valleys, consistent with the “spatial subsidies” model proposed by Witman and others (2004) for other ecosystems.
The central premise of the “pulse-reserve” model for dry valley wetlands proposed by Moorhead and others (2003) was that changes in water levels have effects on dry valley wetlands analogous to the effects of episodic rainfall in warm deserts. In brief, Noy-Meir (1973) proposed that pulses of rainfall stimulated biological activities in warm deserts that slowly declined as water reserves dwindled. This “pulse-reserve” paradigm was more appropriate for dry valley wetlands than originally anticipated. The concentric rings of organic matter around Highland Pond reported by Moorhead and others (2003) strongly suggested a long period of slowly receding water levels. The extremely high input of glacial melt-waters during January 2002 (Foreman and others 2004) provided a rapid recharge of Highland Pond within a single season that will take many years to progressively desiccate. Thus the pulse-reserve model explains how small, ephemeral wetlands may serve as organic matter pumps to the larger dry valley landscape over decadal-scale time spans (Moorhead and others 2003).
The overall importance of ephemeral wetlands to organic matter dynamics of the dry valleys is difficult to assess because wetland distributions and activities vary in time and space. Moorhead and others (2003) documented substantial amounts of organic matter on soil surfaces adjacent to several ponds and in intermittent streams in 2001, much of which was subsequently inundated in 2002. Additional sites examined in the present study also had organic matter that was either exposed on soil surfaces or covering benthic sediments in shallow pools that would soon be exposed by falling water levels (Table 1). Not surprisingly, sites near melting glaciers or accumulations of snow tended to have more organic matter than many of the same stream channels at greater distance from sources of water. Also, some of the shallow pools found in 2003 may have received water in the previous year of high glacier melt, with unknown frequency and duration of inundation. Finally, other ephemeral sources of organic matter are also widespread in Taylor Valley, including cryoconite pools on glaciers (Porazinska and others 2004; Tranter and others 2004), communities entrained in permanent ice covers on the lakes (Fritsen and Priscu 1998), and the many seasonal streams (McKnight and others 1999). Resident biota can rapidly become metabolically active following long periods of desiccation (Vincent and Howard-Williams 1989) and may be moved between locations by wind and water. Hence the net contributions of ephemeral wetlands to organic matter dynamics in the dry valleys cannot yet be quantified.
In a broader context, this simple model with an approximately decadal-scale, temporal dimension describes a spatial link between localized organic matter production in scattered wetlands, and eolian distribution across the larger dry valley landscape, consistent with the spatial subsidies model described by Witman and others (2004). This model is driven by valley hydrology (McKnight and others 1999; Foreman and others 2004), which in turn is driven by regional climate (Doran and others 2002a, b). This model also bridges larger and smaller models of dry valley ecosystems, in which water again serves as the primary control on biological activities across different scales of time and space (for example, Kennedy 1993; Moorhead and Priscu 1998; Bomblies and others 2001). For example, the “legacies” model proposes that geological-scale cycles of glacier advance resulted in greater inundation of Taylor Valley by glacier lakes, which in turn served as a source of the lacustrine organic matter found in modern soils (Moorhead and others 1999; Burkins and others 2000). Thus larger lakes resulting from geological-scale pulses of water input produced pulses of organic matter input to sediments that later became a source of organic matter to communities in dry valley soils (Moorhead and others 1999; Burkins and others 2000, 2001). In contrast to such large-scale patterns, the typical annual cycle of stream recharge and desiccation drives pulses of biological activity in benthic mat communities, as well as concomitant export of organic matter to recipient lakes at seasonal time scales (Howard-Williams and others 1989; Moorhead and others 1997; McKnight and others 1999). Between these extremes of large and small-scale patterns of biological activities lay a range of system dynamics controlled by shifting hydrological boundaries, including the pulse event examined in this paper.
In conclusion, the extreme spatial-temporal heterogeneity of biological activities is an important characteristic defining the structure and function of dry valley ecosystems. The lack of water severely limits biological activities most of the time at most locations. Within this inhospitable matrix, hotspots of biological activity occur where and when conditions permit, with the primary threshold for activity defined by water. However, fluctuations in hydrological regimes blur otherwise apparently sharp boundaries separating these islands of biological activity. For example, deeper lakes during past glacial maxima superimposed a lacustrine environment over what is now a highly heterogeneous distribution of hydro-biological systems in lower Taylor Valley. Moreover, the depths of these glacial lakes may have fluctuated rapidly, further complicating spatial patterns of “legacy” carbon in valley soils (Hall and others 2001). On a decadal time scale, occasional years of high stream flow within a valley (Foreman and others 2004), can hydrate pools, streams and ponds that may have been dry for many years, as well as redistribute organic matter along flow paths. Such complexities pose challenges to understanding even fundamental features of dry valley ecosystems. For example, any broad assessment of organic matter dynamics in Taylor Valley must necessarily include an integration of biological activities across time and space, physiologically disconnected by water limitations and spatially linked through physical redistribution by wind and water (Polis and others 1997; Witman and others 2004).
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Financial support for this work was provided by a grant from the United States National Science Foundation OPP-0096250.
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Moorhead, D.L. Mesoscale Dynamics of Ephemeral Wetlands in the Antarctic Dry Valleys: Implications to Production and Distribution of Organic Matter. Ecosystems 10, 87–95 (2007). https://doi.org/10.1007/s10021-006-9005-8
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DOI: https://doi.org/10.1007/s10021-006-9005-8