Showing papers on "Ecosystem published in 1993"

Terrestrial ecosystem production: A process model based on global satellite and surface data

Abstract: This paper presents a modeling approach aimed at seasonal resolution of global climatic and edaphic controls on patterns of terrestrial ecosystem production and soil microbial respiration. We use satellite imagery (Advanced Very High Resolution Radiometer and International Satellite Cloud Climatology Project solar radiation), along with historical climate (monthly temperature and precipitation) and soil attributes (texture, C and N contents) from global (1°) data sets as model inputs. The Carnegie-Ames-Stanford approach (CASA) Biosphere model runs on a monthly time interval to simulate seasonal patterns in net plant carbon fixation, biomass and nutrient allocation, litterfall, soil nitrogen mineralization, and microbial CO2 production. The model estimate of global terrestrial net primary production is 48 Pg C yr−1 with a maximum light use efficiency of 0.39 g C MJ−1PAR. Over 70% of terrestrial net production takes place between 30°N and 30°S latitude. Steady state pools of standing litter represent global storage of around 174 Pg C (94 and 80 Pg C in nonwoody and woody pools, respectively), whereas the pool of soil C in the top 0.3 m that is turning over on decadal time scales comprises 300 Pg C. Seasonal variations in atmospheric CO2 concentrations from three stations in the Geophysical Monitoring for Climate Change Flask Sampling Network correlate significantly with estimated net ecosystem production values averaged over 50°–80° N, 10°–30° N, and 0°–10° N.

Global climate change and terrestrial net primary production

Abstract: A process-based model was used to estimate global patterns of net primary production and soil nitrogen cycling for contemporary climate conditions and current atmospheric CO2 concentration. Over half of the global annual net primary production was estimated to occur in the tropics, with most of the production attributable to tropical evergreen forest. The effects of CO2 doubling and associated climate changes were also explored. The responses in tropical and dry temperate ecosystems were dominated by CO2, but those in northern and moist temperate ecosystems reflected the effects of temperature on nitrogen availability.

The Trophic Cascade in Lakes

Abstract: 1. Cascading trophic interactions 2. Experimental lakes, manipulations and measurements 3. Statistical analysis of the ecosystem experiments 4. The fish populations 5. Fish behavioral and community responses to manipulation 6. Roles of fish predation: piscivory and planktivory 7. Dynamics of the phantom midge: implications for zooplankton 8. Zooplankton community dynamics 9. Effects of predators and food supply and diel vertical migration of Daphnia 10. Zooplankton biomass and body size 11. Phytoplankton community dynamics 12. Metalimnetic phytoplankton 13. Primary production and its interactions with nutrients and light transmission 14. Heterotrophic microbial processes 15. Annual fossil record of food-web manipulation 16. Simulation models of the trophic cascade: predictions and evaluations 17. Synthesis and new directions Index.

Patterns in decomposition rates among photosynthetic organisms: the importance of detritus C:N:P content

Abstract: The strength and generality of the relationship between decomposition rates and detritus carbon, nitrogen, and phosphorus concentrations was assessed by comparing published reports of decomposition rates of detritus of photosynthetic organisms, from unicellular algae to trees. The results obtained demonstrated the existence of a general positive, linear relationship between plant decomposition rates and nitrogen and phosphorus concentrations. Differences in the carbon, nitrogen, and phosphorus concentrations of plant detritus accounted for 89% of the variance in plant decomposition rates of detritus orginating from photosynthetic organisms ranging from unicellular microalgae to trees. The results also demonstrate that moist plant material decomposes substantially faster than dry material with similar nutrient concentrations. Consideration of lignin, instead of carbon, concentrations did not improve the relationships obtained. These results reflect the coupling of phosphorus and nitrogen in the basic biochemical processes of both plants and their microbial decomposers, and stress the importance of this coupling for carbon and nutrient flow in ecosystems.

Primary production control of methane emission from wetlands

Abstract: Based on simultaneous measurements of CO2 and CH4 exchange in wetlands extending from subarctic peatlands to subtropical marshes, a positive correlation between CH4 emission and net ecosystem production is reported. It is suggested that net ecosystem production is a master variable integrating many factors which control CH4 emission in vegetated wetlands. It is found that about 3 percent of the daily net ecosystem production is emitted back to the atmosphere as CH4. With projected stimulation of primary production and soil microbial activity in wetlands associated with elevated atmospheric CO2 concentration, the potential for increasing CH4 emission from inundated wetlands, further enhancing the greenhouse effect, is examined.

Arctic ecosystems in a changing climate : an ecophysiological perspective

Abstract: F.S. Chapin III, R.L. Jefferies, J.F. Reynolds, G.R. Shaver, and J. Svoboda, Arctic Plant Physiological Ecology: A Challenge for the Future. The Arctic System: B. Maxwell, Arctic Climate: Potential for Change under Global Warming. D.L. Kane, L.D. Hinzman, M. Woo, and K.R. Everett, Arctic Hydrology and Climate Change. L.C. Bliss and N.V. Matveyeva, Circumpolar Arctic Vegetation. W.D. Billings, Phytogeographic and Evolutionary Potential for the Arctic Flora and Vegetation in a Changing Climate. L.C. Bliss and K.M. Peterson, Plant Succession, Competition, and the Physiological Constraints of Species in the Arctic. Carbon Balance: W.C. Oechel and W.D. Billings, Effects of Global Change on the Carbon Balance of Arctic Plants and Ecosystems. O.A. Semikhatova, T.V. Gerasimenko, and T.I. Ivanova, Photosynthesis, Respiration, and Growth of Plants in the Soviet Arctic. G.R. Shaver and J. Kummerow, Phenology, Resource Allocation, and Growth of Arctic Vascular Plants. J.D. Tenhunen, O.L. Lange, S. Hahn, R. Siegwolf, and S.F. Oberbauer, The Ecosystem Role of Poikilohydric Tundra Plants. B. Sveinbj~adornsson, Arctic Tree Line in a Changing Climate. Water and Nutrient Balance: S.F. Oberbauer and T.E. Dawson, Water Relations of Arctic Vascular Plants. K.J. Nadelhoffer, A.E. Giblin, G.R. Shaver, and A.E. Linkins, Microbial Processes and Plant Nutrient Availability in Arctic Soils. D.M. Chapin and C.S. Bledsoe, Nitrogen Fixation in Arctic Plant Communities. K. Kielland and F.S. Chapin III, Nutrient Absorption and Accumulation in Arctic Plants. F. Berendse and S. Jonasson, Nutrient Use and Nutrient Cycling in Northern Ecosystems. Interactions: J.B. McGraw and N. Fetcher, Response of Tundra Plant Populations to Climatic Change. J.P. Bryant and P.B. Reichardt, Controls over Secondary Metabolite Production by Arctic Woody Plants. R.L. Jefferies, J. Svoboda, G. Henry, M. Raillard, and R. Ruess, Tundra Grazing Systems and Climatic Change. J.F. Reynolds and P.W. Leadley, Modeling the Response of Arctic Plants to Changing Climate. F.S. Chapin III, R.L. Jefferies, J.F. Reynolds, G.R. Shaver, and J. Svoboda, Arctic Plant Physiological Ecology in an Ecosystem Context. Index.

A Hierarchical Model for Decomposition in Terrestrial Ecosystems: Application to Soils of the Humid Tropics

Abstract: A general model is presented in which the dynamics of decomposition in terrestrial ecosystems are determined by a set of hierarchically organized factors which regulate microbial activity at decreasing scales of time and space in the following order: climate - clay mineralogy + nutrient status of soil - quality of decomposing resources - effect of macroorganisms (i.e., roots and invertebrates). At the lower scale of determination, biological systems of regulation based on mutualistic relationships between macro- and microorganisms ultimately determine the rates and pathways of decomposition. Four such systems are defined, i.e., the litter and surface roots system, the rhizosphere, the drilosphere and the termitosphere in which the regulating macroorganisms are respectively litter arthropods and surface roots, live subterranean roots, endogeic earthworms, and termites. In the humid tropics, this general model is often altered because climatic and edaphic constraints are in many cases not important and because high temperature and moisture conditions greatly enhance the activity of mutualistic biological systems of regulation which exert a much stronger control on litter and soil organic matter dynamics. This general hypothesis is considered in the light of available information from tropical rain forests and humid savannas. Theoretical and practical implications regarding the biodiversity issue and management practices are further discussed. It is concluded that biodiversity is probably determined, at least partly, by soil biological processes as a consequence of enhanced mutualistic interactions, which enlarge the resource base available to plants. It is also concluded that any effort to restore or rehabilitate degraded soils in the humid tropics is promised to fail unless optimum levels of root and invertebrate activities are promoted and the resulting regulation effects operate in the four abovedescribed biological systems of regulation. Research required to substantiate and adequately test the present set of concepts and hypotheses are expressed.

Magnitude and patterns of herbivory in aquatic and terrestrial ecosystems

Abstract: HERBIVORES can consume a sufficiently large proportion of primary production to regulate plant biomass in some environments1–3. Little is known, however, about how rates of herbivory vary among ecosystems and how herbivores influence the global distribution of vegetation. Patterns of herbivory in terrestrial ecosystems have been summarized recently4,5, but comparisons with aquatic systems are uncertain because past generalizations about herbivory in aquatic systems are based on surprisingly few data6–8. Herbivory is thought to be higher in aquatic than in terrestrial ecosystems9–11 and the impact of herbivores in aquatic systems is believed to decrease with increasing primary productivity12–15, a pattern opposite to that in terrestrial systems4,5. Here we examine these hypotheses using data from 44 aquatic sites. Herbivore biomass and herbivory rates increase at similar rates with increasing primary productivity in aquatic and in terrestrial systems. For a given level of primary productivity, aquatic and terrestrial herbivores reach similar biomass, but aquatic herbivores remove on average 51% of annual primary production, three times more than terrestrial herbivores. Mass-specific rates of herbivory are greater in aquatic than in terrestrial systems.

A history of the ecosystem concept in ecology : more than the sum of the parts

Abstract: The ecosystem concept-the idea that flora and fauna interact with the environment to form an ecological complex-has long been central to the public perception of ecology and to increasing awareness of environmental degradation. In this book an eminent ecologist explains the ecosystem concept, tracing its evolution, describing how numerous American and European researchers contributed to its evolution, and discussing the explosive growth of ecosystem studies. Golley surveys the development of the ecosystem concept in the late nineteenth and early twentieth centuries and discusses the coining of the term ecosystem by the English ecologist Sir Arthur George Tansley in 1935. He then reviews how the American ecologist Raymond Lindeman applied the concept to a small lake in Minnesota and showed how the biota and the environment of the lake interacted through the exchange of energy. Golley describes how a seminal textbook on ecology written by Eugene P. Odum helped to popularize the ecosystem concept and how numerous other scientists investigated its principles and published their results. He relates how ecosystem studies dominated ecology in the 1960s and became a key element of the International Biological Program biome studies in the United States-a program aimed at "the betterment of mankind" specifically through conservation, human genetics, and improvements in the use of natural resources; how a study of watershed ecosystems in Hubbard Brook, New Hampshire, blazed new paths in ecosystem research by defining the limits of the system in a natural way; and how current research uses the ecosystem concept. Throughout Golley shows how the ecosystem concept has been shaped internationally by both developments in other disciplines and by personalities and politics.

A break in the nitrogen cycle in aridlands? Evidence from δp15N of soils.

Abstract: We examined the content and isotopic composition of nitrogen within soils of a juniper woodland and found that a cryptobiotic crust composed of cyanobacteria, lichens, and mosses was the predominant source of nitrogen for this ecosystem. Disturbance of the crust has resulted in considerable spatial variability in soil nitrogen content and isotopic composition; intercanopy soils were significantly depleted in nitrogen and had greater abundance of 15N compared to intra-canopy soils. Variations in the 15N/14N ratio for inter- and intra-canopy locations followed similar Rayleigh distillation curves, indicating that the greater 15N/14N ratios for inter-canopy soils were due to relatively greater net nitrogen loss. Coverage of cryptobiotic crusts has been reduced by anthropogenic activities during the past century, and our results suggest that destruction of the cryptobiotic crust may ultimately result in ecosystem degradation through elimination of the predominant source of nitrogen input.

Long-Term Nutrient Accumulation Rates in the Everglades

Abstract: Anthropogenic nutrient inputs to the northern Everglades of Florida during the last three decades have resulted in alteration of vegetation and soil nutrient storage. Due to the nutrient-limited status of this ecosystem, increased loading may have altered the capacity for long-term nutrient accumulation. Our study was conducted to determine the potential long-term nutrient accumulation rates for this ecosystem along a gradient of nutrient loading. Accumulation rates were calculated using the vertical peat accretion rates, as determined by Cs dating, and nutrient concentration profiles. Intact soil cores were obtained along a 15-km transect and evaluated as a function of distance from the inflow structure. Soil cores were sectioned into 1-cmdepth increments and analyzed for '-"Cs, P, N, C, and selected cations. Vertical accretion rates of peat decreased logarithmically with distance from the inflow, with rates of 1.1 cm yr~' at 0.3 km from the inflow to about 0.25 cm yr' in unimpacted sawgrass (Cladium jamaicense Crantz)-dominated areas. Phosphorus, N, and C accumulation rates in soil and floodwater total P concentrations also showed similar relationships. The P accumulation rates ranged from 0.54 to 1.14 g P myr~' in cattail (Typha spp.)-dominated areas, and 0.11 to 0.25 g P myr~' in sawgrass-dominated areas. The C/P and N/P accumulation ratios increased with distance from the inflow, suggesting that a greater proportion of P accumulated in the system, compared with C and N. Similar P retention coefficients were obtained when calculated using either changes in surface water total P concentration, or the long-term P accretion rates. These findings suggest that P was either directly adsorbed by soil or precipitated with Ca in the water column and deposited on the soil surface. This hypothesis was further supported by a highly significant correlation between P and Ca accretion rates, suggesting that Ca-bound P controls equilibrium concentrations in this ecosystem. L NUTRIENT ACCUMULATION in wetland ecosystems is determined by the balance between inputs and outputs. Nutrients in wetlands undergo several biogeochemical transformations, some resulting in the loss of certain nutrients as gaseous end products or through leaching and discharge to outflow, while K.R. Reddy and W.F. DeBusk, Soil and Water Science Dep., Institute of Food and Agricultural Sciences, 106 Newell Hall, Univ. of Florida, Gainesville, FL 32611; R.D. DeLaune, Laboratory for Wetland Soils and Sediments, Louisiana State Univ., Baton Rouge, LA 70803; and M.S. Koch, Dep. of Everglades Systems Research, South Florida Water Management District, P.O. Box 24680, West Palm Beach, FL 33416. Florida Agric. Exp. Stn. Journal Series no. R-02740. Received 13 May 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:1147-1155 (1993). others result in nutrient accumulation within the ecosystem. Nutrient accumulation can occur through sedimentation or organic matter accumulation. In peatdominated wetlands, a major portion of the nutrients is stored in live and detrital plant tissue, microbial biomass, and stabilized soil organic matter. Nutrients stored in vegetation and microbial biomass can be readily released through natural die-off and decomposition (Davis, 1991). In herbaceous wetlands, nutrient storage in vegetation is usually short term (Reddy and DeBusk, 1987), while in forested wetlands incorporation of nutrients into woody tissue of trees can result in long-term storage (Richardson and Davis, 1987). As C and N are cycled through a wetland, a portion can be lost as gaseous end products. For example, organic C is converted to CO2 and CH4, and is lost from the system. This process is influenced by the hydrologic regime of the system, with frequent wet and dry cycles increasing decomposition rates and loss of C (Reddy and Patrick, 1975). In the EAA, oxidation of organic matter under drained conditions accounted for soil loss of about 3 cm yr (Snyder et al., 1978). However, CO2 fixation by vegetation and accumulation of detrital material in many wetlands usually offsets decomposition, resulting in a net C accumulation. Similarly, organic N is mineralized to NH4-N, which is subsequently lost through nitrification-denitrification and NH3 volatilization reactions (Reddy and Patrick, 1984). However, P released during decomposition is usually retained by the wetland through sorption and precipitation reactions (HowardWilliams, 1985). Nutrients added to a wetland are rapidly incorporated into living and detrital plant material, and eventually incorporated into soil organic matter (Puriveth, 1980; Day, 1982; Davis and van der Valk, 1983; DeBusk and Reddy, 1987). Long-term nutrient retention by soil organic matter is affected by environmental factors such as temperature, hydroperiod and fire. Nutrient accumulation rates have been estimated for many wetland ecosystems using Cs as a marker (DeLaune et al., 1978; Hatton et al., 1983; Kadlec and Robbins, 1984; Patrick and DeLaune, 1990). Peat Abbreviations: EAA, Everglades Agricultural Area; WCA, Water Conservation Area; ENP, Everglades National Park; SRP, soluble reactive P; TKN, total Kjeldahl N. 1148 SOIL SCI. SOC. AM. J., VOL. 57, JULY-AUGUST 1993

Nutrient and energy transport between estuaries and coastal marine ecosystems by fish migration

Abstract: Biomass accumulation and changes in body energy and nutrient (carbon, nitrogen, and phosphorus) composition were evaluated relative to the migration pattern of gulf menhaden (Brevoortia patronus) t...

Consequences of nonequilibrium resource availability across multiple time scales: the transient maxima hypothesis.

Abstract: Nonequilibrium biotic responses to changes in resource limitation dominate the behavior of tallgrass prairie ecosystems. Rates of leaf photosynthesis on a time scale of minutes, amounts of annual plant productivity, patterns in the productivity of certain consumer groups, and amounts of soil organic matter accumulation over millennia all reflect biotic responses to frequent and recurring shifts in limiting resources. Productivity is higher during a transition period when the relative importance of an essential resource is changing than during an equilibrium interval generated by single resource limitation. These "transient maxima" are both characteristic and easily measurable in the tallgrass prairie because of the unpredictable climate and ecological constraints such as grazing and recurrent fires that modify water, nitrogen, and light availability. Such diverse phenomena as overcompensation for herbivory, the intermediate disturbance hypothesis, maximum levels of productivity observed in successional ec...

Changes in the microbial biomass and metabolic quotient during leaf litter succession in some New Zealand forest and scrubland ecosystems.

Abstract: The soil microbial biomass and microbial metabolic quotient (respiration: biomass ratio) was measured in 16 forest and scrubland ecosystems throughout New Zealand, on materials representing successional stages of plant litter and its subsequent incorporation into the F-H and mineral soil layers. Microbial biomass usually peaked in the L 1 layer and then declined. The microbial carbon:organic carbon ratio decreased sharply between the F-H layer and the underlying mineral layer, indicating that a stress factor (possibly pH) reduced the proportion of organic matter immobilized in the microbial biomass at this stage. The microbial respiration: biomass ratio declined between the L 1 and F-H stages

Relationships Between Litter Quality and Nitrogen Availability in Rocky Mountain Forests

Abstract: We assessed the relationships between net nitrogen mineralization, litter quality, and litter decomposition rates in 30 Colorado forests to test three hypotheses of the LINKAGES forest ecosystem mo...

Influence of Productivity on the Stability of Real and Model Ecosystems

Abstract: The lengths of food chains within ecosystems have been thought to be limited either by the productivity of the ecosystem or by the resilience of that ecosystem after perturbation. Models based on ecological energetics that follow the form of Lotka-Volterra equations and equations that include material (detritus) recycling show that productivity and resilience are inextricably interrelated. The models were initialized with data from 5-to 10-year studies of actual soil food webs. Estimates indicate that most ecological production worldwide is from ecosystems that are themselves sufficiently productive to recover from minor perturbations.

A Physiology‐Based Gap Model of Forest Dynamics

Abstract: A computer model of forest growth and ecosystem processes is presented. The model, HYBRID, is derived from a forest gap model, an ecosystem process model, and a photosynthesis model. In HYBRID individual trees fix and respire carbon, and lose water daily; carbon partitioning occurs at the end of each year. HYBRID obviates many of the limitations of both gap models and ecosystem process models. The growth equations of gap models are replaced with functionally realistic equations and processes for carbon fixation and partitioning, resulting in a dynamic model in which competition and physiology play important roles. The model is used to predict ecosystem processes and dynamics in oak forests in Knoxville, Tennessee (USA), and pine forests in Missoula, Montana (USA) between the years 1910 and 1986. The simulated growth of individual trees and the overall ecosystem- level processes are very similar to observations. A sensitivity analysis performed for these sites showed that predictions of net primary productivity by HYBRID are most sensitive to the ratio of CO2 partial pressure between inside the leaf and the air, relative humidity, ambient CO2 partial pressure, precipitation, air temperature, tree allometry, respiration parameters, site soil water capacity, and a carbon storage parameter.

Viewpoint: Plant community thresholds, multiple steady states, and multiple successional pathways: legacy of the Quaternary?

Abstract: The climate cycles of the 2 million years of the Quaternary were a major force in the evolution of plant response to change. Quaternary climate has been primarily glacial with interglacials such as the current Holocene a minor component. Plant species responded individually to climate changes and, consequently, species composition has continually changed. The legacy of Quaternary climate change is that plant communities are far less stable than they appear to be from our perspective. They are unique at each location, difficult to define, and communities that are relics from a previous environment can be sensitive to small or transient environmental changes. Plant communities are variable both in space and time. Many ecological principles and concepts, and ecosystem paradigms derived from them, require revision to incorporate this variation. The concepts of habitat type and condition and trend, for example, do not reflect dynamic vegetation response to changes in climate. Our knowledge is presently insufficient to adequately describe interactions between ecosystems changing climate, but the patterns of vegetation response to environmental changes of the past may provide important information on vegetation response to present and future climate change. The concepts of thresholds, multiple steady states, and multiple successional pathways are helpful in understanding the dynamic interrelationships between vegetation and environmental changes.

Modeling the effects of climatic and co2 changes on grassland storage of soil C

Abstract: We present results from analyses of the sensitivity of global grassland ecosystems to modified climate and atmospheric CO2 levels. We assess 31 grassland sites from around the world under two different General Circulation Models (GCM) double CO2 climates. These grasslands are representative of mostly naturally occurring ecosystems, however, in many regions of the world, grasslands have been greatly modified by recent land use changes. In this paper we focus on the ecosystem dynamics of natural grasslands. The climate change results indicate that simulated soil C losses occur in all but one grassland ecoregion, ranging from 0 to 14% of current soil C levels for the surface 20 cm. The Eurasian grasslands lost the greatest amount of soil C (~1200 g C m-2) and the other temperate grasslands losses ranged from 0 to 1000 g C m-2, averaging approximately 350 g C m-2. The tropical grasslands and savannas lost the least amount of soil C per unit area ranging from no change to 300 g C m-2 losses, averaging approximately 70 g C m-2. Plant production varies according to modifications in rainfall under the altered climate and to altered nitrogen mineralization rates. The two GCM’s differed in predictions of rainfall with a doubling of CO2, and these differences are reflected in plant production. Soil decomposition rates responded most predictably to changes in temperature. Direct CO2 enhancement effects on decomposition and plant production tended to reduce the net impact of climate alterations alone.

Mutualism and competition between plants and decomposers: implications for nutrient

Abstract: By examining the consequences of simultaneous mutualism and competition be- tween plants and decomposers, we show that testable predictions about nutrient allocation in ecosystems follow from the assumption that decomposers allocate for their own growth the fraction of mineralized nutrient that maximizes their population biomass, leaving the remainder available for plant uptake. Available data for a wide variety of ecosystems are nearly all consis- tent with the predicted quantitative relationships among nitrogen flow rates and nitrogen frac- tions in plants, decomposers, and nonliving organic matter. Our predictions are robust against changes in the detailed structure of the nutrient-cycle models we use for our derivations. In the past two decades, much progress has been made in understanding the role of decomposers (primarily bacteria and fungi) in nutrient cycles and in eluci- dating biological, chemical, and physical factors that regulate the activity of these organisms. The mechanisms of nutrient cycling, and, in particular, the functions of microorganisms, have been shown to be essentially the same in all ecosystems (Pomeroy 1970; Anderson and Macfadyen 1976; Clark and Rosswall 1981; Ross- wall 1981; Sprent 1987; Coleman et al. 1989). But there is little understanding of whether universality of mechanism implies universality of allocation-in particu- lar, whether quantitative regularities exist in the distribution of stocks and flows of nutrients within a wide variety of ecosystems. Here we demonstrate that such ecosystem regularities can be deduced from a maximization constraint on decom- poser populations and that available data from a wide variety of ecosystems are generally consistent with our predicted patterns of nutrient allocation. The simplified nutrient cycle shown in figure 1 includes four flow streams: death, mineralization, which refers to the final process of decomposition in which dead organic matter is converted to inorganic nutrient (e.g., ammonium or phos- phate), immobilization, which refers to net uptake of inorganic nutrient by the mineralizers, and assimiliation, which refers to net uptake of inorganic nutrient by photosynthesizers. Curiosity over what determines the ratio of nutrient assimi-

Stable carbon isotope analysis of soil organic matter illustrates vegetation change at the grassland/woodland boundary in southeastern Arizona, USA

Abstract: In southeastern Arizona, Prosopis juliflora (Swartz) DC. and Quercus emoryi Torr. are the dominant woody species at grassland/woodland boundaries. The stability of the grassland/woodland boundary in this region has been questioned, although there is no direct evidence to confirm that woodland is encroaching into grassland or vice versa. We used stable carbon isotope analysis of soil organic matter to investigate the direction and magnitude of vegetation change along this ecotone. δ13C values of soil organic matter and roots along the ecotone indicated that both dominant woody species (C3) are recent components of former grasslands (C4), consistent with other reports of recent increases in woody plant abundance in grasslands and savannas throughout the world. Data on root biomass and soil organic matter suggest that this increase in woody plant abundance in grasslands and savannas may increase carbon storage in these ecosystems, with implications for the global carbon cycle.