How do disturbance-generated landscape patterns influence the spatial dynamics of ecosystem processes?
Principal Investigator
Monica G. Turner
Department of Zoology, Birge Hall
University of Wisconsin
Madison, WI 53706
Co-Prinicipal Investigators
William H. Romme
Department of Biology
Fort Lewis College
Durango, CO 81301
Daniel B. Tinker
Department of Geosciences and Natural Resources Management
Western Carolina University
Cullowhee, NC 28723
Duration: 1 January 2001 – 30 December 2005 (5 years)
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Introduction
Understanding the patterns and causes of spatial heterogeneity in ecosystem function—the processes associated with energy and nutrient flow in ecosystems—remains at the frontier of ecosystem and landscape ecology (Schimel et al. 1997, Turner and Carpenter 1999, Turner et al. in press). Despite tremendous advances in understanding ecosystem processes over relatively small spatial extents, there exists very little theory for predicting variability in ecosystem processes across heterogeneous landscapes. The library of empirical data from a variety of regions is slowly building, but we have no general answers to such questions as: Just how spatially variable are ecosystem processes? How do the controls on processes and rates operate across space? Are regional processes simply the area-weighted sum of the processes measured in component ecosystems? Are there critical thresholds in spatial patterns that are important for ecosystem processes? How do disturbance-generated patterns influence spatial dynamics of ecosystem processes? Do simple scaling rules work when we move from ecosystems to landscapes? Ecologists have long recognized that the abiotic template is a powerful constraint on ecosystem function (e.g., Barnes et al. 1998). However, spatial processes such as land use, natural disturbance, and the activities of organisms also influence the rates and patterns of ecosystem processes. A more synthetic understanding of spatial heterogeneity in ecosystem processes remains an important research need.
Studies of the effects of forest disturbances on biogeochemical processes have yielded a wealth of information about watershed dynamics, energy flow, and nutrient cycling in natural and managed systems (e.g., Bormann and Likens 1979, Swank and Crossley 1988, Johnson and Van
Hook 1989, Likens and Bormann 1995). More recently, ecosystem-level research has expanded to consider spatial heterogeneity, e.g., topographic or regional variation in nutrient cycling processes (e.g., Zak et al. 1987, 1989; Groffman et al. 1993; Benning and Seastedt 1995; Burke et al. 1995; Gross et al. 1995; C. L. Turner et al. 1997); spatial variation across landscapes in evapotranspiration, leaf area (Running et al. 1987,1989; Spanner et al. 1990; Band et al. 1991;
Nemani et al. 1993; Pierce et al. 1994), biomass, productivity (Host et al. 1988, Sala et al. 1988,
White et al. 1998, Brown and Schroeder 1999), and decomposition (Murphy et al. 1998); coarse woody debris abundance and decomposition following disturbance (Bragg 1997, Sturtevant et al.
1997, Wei et al. 1997, Tinker and Knight 2000); the transport of materials between ecosystems
(e.g., Peterjohn and Correll 1984, Shaver et al. 1991, Soranno et al. 1996); and spatial interactions among the plant community, large herbivores, and nutrient cycling (Coughenour
1991, Pastor and Naiman 1992). However, none of these studies has addressed the effects of disturbance-generated landscape patterns or variability in successional communities on ecosystem processes. Forest fire is a well-studied disturbance (e.g., Johnson 1992), yet little is known about the long-term implications of a fire-generated landscape mosaic for ecosystem processes. Opportunities for doing this type of broad-scale research are not common.
The landscape of Yellowstone National Park (YNP) offers many opportunities to investigate general relationships between broad-scale ecological pattern and process. The 1988 Yellowstone fires affected >250,000 ha, creating a mosaic of burn severities across the landscape (Christensen et al. 1989, Turner et al. 1994). Such large fires are rare events, occurring at intervals of perhaps
100 to 300 years (Romme 1982, Romme and Despain 1989, Millspaugh et al. 2000), but major and long-lasting ecological consequences may result. In addition to the very large fires in 1988,
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moderate-sized fires (ca. 1,000 ha) occurred in 1996 and 2000. The relative similarity of substrate and climate across a large, relatively flat area allows the effects of disturbance and postfire succession to be studied without the confounding influences of highly variable environmental conditions that characterize most forested landscapes in the West. The 500,000ha subalpine plateau is characterized by gently rolling topography and similar vegetation dominated by lodgepole pine ( Pinus contorta var. latifolia ) (Despain 1990). Most of YNP is managed as a wilderness area, where postfire responses have been little affected by human actions.
Our studies following the 1988 Yellowstone fires demonstrated that succession was surprisingly more variable in space and time than even current theory would have suggested, and that initial spatial patterns of disturbance may persist to produce long-lasting changes in vegetation (Turner et al. 1997, 1998, 2000a; Romme et al. 1998). Our focus now is on explaining the spatial and temporal patterns of succession and understanding how these patterns influence ecosystem function. The most interesting new questions revolve around the degree to which the spatial variation in postfire vegetation—in particular, the six orders of magnitude variation in pine sapling density, ranging from 0 to >500,000 saplings/ha—controls the spatial variability in ecosystem processes across the landscape. As we presented at the 2000 Ecological Society of
America meeting, we found strong influences of postfire tree density on aboveground net primary production and leaf area index, but we do not know whether and how other processes
(e.g., decomposition, nitrogen mineralization, etc.) are responding to the variation in the plant community (Turner et al. 2000b). This is the direction we wish to explore next, as reflected in the following interrelated questions. In addition, we will utilize the fires that burned in the
Greater Yellowstone Area during summer 2000 to complement our understanding of how ecosystem function changes in space and time shortly following fire.
Questions
We propose four major questions that address the potential influence of the landscape mosaic of postfire vegetation and coarse woody debris on variation in ecosystem processes in space and time:
Question 1. Do the enormous differences in postfire tree density produce differences in carbon and nitrogen availability across the landscape? Or, is nutrient availability governed largely by broad-scale (i.e., 10’s of km) abiotic gradients (e.g., climate, substrate) and/or fine-scale (i.e., <
10 cm) heterogeneity in resources or the microbial community, such that nutrient variability is not sensitive to the spatial variation in plant community structure?
Rationale: We hypothesize that broad-scale abiotic gradients constrain the rates of decomposition, and that decomposition of the same types of litter will be similar across the landscape. More specifically, we expect that herbaceous litter will decompose more rapidly than conifer litter, because of the high concentration of lignins and other recalcitrant materials in conifer litter. We also expect that decomposition will be slower on drier sites, because water stress limits the activity of microbial decomposers (e.g., Murphy et al. 1998). We hypothesize
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that decomposition rates for a particular type of material (e.g., pine vs. herbaceous litter) are controlled mainly by gradients in climate and soils, which are deterministic factors, i.e., constant properties of the environment that are uninfluenced by stochastic processes like fire behavior or stand history. In contrast, we hypothesize that the relative production of different kinds of litter is controlled by sapling density, which is a contingent effect of fire size, fire severity, and prefire serotiny rather than abiotic gradients. We do not know how nutrient availability varies with production of (1) aboveground litter (which will vary spatially in total amount as well as the relative proportions of herbaceous material and pine litter), and/or (2) belowground sources and the turnover of fine roots. Above- and belowground biomass are both likely important sources of nutrient availability. In addition, we do not know whether differences in nutrient availability are reflected in foliar nutrient concentrations, and whether these differ spatially with pine sapling density.
Approach: We will address this question by measuring carbon and nitrogen availability in 2002,
2003 and 2004 in replicated sites that span the range of observed densities of post-1988 lodgepole pine saplings (4 density levels x 3 replicates = 12 sites). We will also measure rates of litter production and decomposition for different types of litter (e.g., pine needles, graminoids, and forbs.) At a subset of sites, nutrient availability will be measured across a set of nested spatial scales so that we can estimate the scales at which nutrient availability varies. In addition, foliar nitrogen will be sampled in 2004 within a subset of stands that span the range of nitrogen mineralization and decomposition rates measured during 2002 and 2003.
Question 2. Does the disturbance-created mosaic leave a persistent functional legacy? What mechanisms in vegetation development may contribute to convergence (or divergence) in ecosystem structure and function across the landscape as succession proceeds?
Rationale: Romme and Despain (1989) found mature lodgepole pine stands (> 100 yr old) of the same age but with very different densities, which suggests that the structural and functional differences that have developed within the first decade after the 1988 fires may persist for a very long time. We also have noted that the highest pine density observed in any mature stand is an order of magnitude lower than the highest densities now present in post-1988 stands – which means that substantial sapling mortality must occur at some time, possibly soon, in very dense stands. We hypothesize that pine seedling recruitment will continue to occur slowly in burned areas that currently have low sapling densities, whereas sapling mortality will begin to occur in areas of very high sapling density. We are interested in the spatio-temporal dynamics of these changes in community structure, as well as identifying the underlying mechanisms. Mortality may be caused by competition for water and/or nutrients. The variation we expect to find in ecosystem processes (question 1) may exert positive or negative feedbacks on tree density.
These changes in stand structure may also strongly influence ecosystem processes. For example, maximum aboveground net primary production (ANPP) and leaf area index (LAI) may have already been achieved in the high-density stands (e.g., > 100,000 saplings/ha), but ANPP and
LAI may continue to increase in lower-density stands (Reed et al. 1999).
Approach: This question will be addressed in two ways. First, we will develop a simulation model to predict rates of tree mortality for moderate to high density stands of pine saplings under
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alternative scenarios of limitation by nitrogen, carbon, or water. The field data obtained under question 1 will be used to parameterize the model and then predict changes in pine sapling density. Second, field studies will be conducted in 2004 to test the predictions of the model for sapling density and to determine whether there are as yet any indications of convergence in
ANPP and LAI among stands. ANPP and LAI were measured in 1999 at 90 widely distributed points spanning the range of pine sapling densities in the burned YNP landscape, and we will remeasure these parameters in the same stands during the summer of 2004. Changes in lodgepole pine sapling density will be quantified to estimate mortality and new tree establishment during the 5-yr interval. The density of recently dead saplings (which will probably persist for several years) and young pine seedlings (< 5 years old) will be measured directly.
Question 3. How does the spatial pattern of coarse woody debris vary across the post-1988 landscape, and what is the importance of this variation for ecosystem function? Are patterns of coarse woody debris abundance related to both prefire stand structure and postfire sapling density?
Rationale: Coarse woody debris (CWD), produced as trees killed by the1988 fall to the ground, is an increasingly important – but variable – structural feature in the burned Yellowstone landscape (Tinker and Knight 2000). For example, along one of our permanent transects for measuring long-term vegetation change, some sampling points had no CWD on the ground in the summer of 2000, whereas other points had fallen logs piled a meter deep. Treefall contributes to long-term carbon dynamics and soil formation (Hart 1999, Jurgensen et al. 1997), but we do not know how the abundance and spatial patterning of CWD varies across the landscape, and how this variation influences current rates of ecological processes. Wood decomposition is quite slow in Yellowstone, thus the CWD will be an important structural feature of these forests for decades. We hypothesize that large fallen trees, especially where they are several layers deep, cause reduced soil temperatures, reduced sunlight, and increased moisture availability, thereby influencing rates of soil processes including nitrogen mineralization and decomposition.
Approach: The abundance and spatial distribution of CWD will be measured in several ways.
Color infrared photographs that we had flown during August 1998 will be used to generate maps of the abundance of CWD at selected locations across the YNP landscape. The photos are
1:30,000 scale, and downed wood is visible with magnification of the photos. Field sampling will be used to ground-truth the photos and to make fine-scale maps of CWD at study sites with varying densities of post-1988 saplings. In addition, the density, species, and size of standing dead trees will be remeasured at 300 sampling points sampled originally in 1989 (see Turner et al. 2000). The difference in standing dead trees from 1989 to 2002 will provide an estimate of the input of new CWD in the first 14 years following stand-replacing fire. To examine the effects of CWD on ecosystem function, we will stratify the 2002-2004 sampling described above for question 1 such that samples are obtained at positions both under CWD and in the open. We can then determine the relative importance of CWD on ecosystem processes.
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Question 4. Does the spatial heterogeneity of processes such as ANPP, nitrogen mineralization, and decomposition change with time since fire? How quickly do spatial patterns in processes develop following a large fire?
Rationale. Our initial studies following the 1988 fires focused on plant community dynamics rather than ecosystem function. The fires currently burning within the Greater Yellowstone Area provide new opportunities for studying the spatial and temporal variation in ecosystem processes soon after the disturbances and how these patterns develop and change through time. We hypothesize that ecosystem processes are relatively homogenous immediately following fire, but become more heterogeneous as the stands develop during the first few years of succession. By comparing spatial and temporal heterogeneity in ecosystem processes in similar sites that burned in 1988, 1996, and 2000, we can determine whether the postfire patterns of tree saplings increase the spatial heterogeneity in process rates.
Approach. Nitrogen and carbon availability will be measured in 2001 at sites in the Greater
Yellowstone Area that burned during the summer of 2000. Several fires burned in southern YNP and the northern portion of Grand Teton National Park in areas similar to sites burned in 1988.
We will initiate sampling at sites where we expect postfire stands to be of high- and low-density lodgepole pine. We will predict postfire pine density by measuring the density of serotinous cone bearing trees in the burned stands (Tinker et al. 1994). We will use a scaled sampling design designed to detect the spatial scale of variation in nutrient availability, and remeasure these sites through successional time. Changes in spatial patterns and in mean rates will be tracked through time by repeating the measurements in 2002, 2003 and 2004. In addition, we will collect streamwater samples during snowmelt to determine whether nitrate is being lost from the uplands during the first four years of succession. Finally, we will estimate herbaceous ANPP and LAI in these recently burned stands because we expect the rate of recovery of these processes to influence nitrogen and carbon availability.
Significance
The ability to predict broad-scale patterns of ecosystem processes requires understanding the variability within and among ecosystems and the consequences of disturbance for ecosystem function. Our work will directly address these needs. Our study also will contribute to understanding potential effects of fire in natural areas of the northern Rocky Mountains. Finally, our research will provide the first integrated study of spatial variability in postfire succession and the consequences of this variability for a variety of ecosystem processes across a large heterogeneous landscape. It will contribute more generally to understanding causes and consequences of spatial heterogeneity for ecosystem function and to integrating ecosystem and landscape ecology.
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