Conceptual Model


In mesic environments, the hydrologic cycle couples biological processes (e.g., C and N dynamics, NPP) in ecosystems. Water transports nutrients, sustains biotic activity, and drives microbial processes, thereby linking production, decomposition and storage of organic matter (OM), and controlling the rates of reactions that drive community structure and ecosystem processes. Organic nutrient pools accumulate in mesic environments over time and most of the N required for primary production and trophic interactions comes from OM decomposition. Where moisture is predictable and sufficient, C and N cycles are more or less closed and tightly coupled through production and decomposition of OM (Asner et al. 1997).


Like other regions, the arid Southwest is experiencing climatic change and increasing rates of atmospheric N deposition (Fenn et al. 2003, Báez et al. 2007). The consequences of increased N loading and climate change for community structure and ecosystem processes in arid lands are poorly understood, and likely related to seasonal timing, microbial and plant functional types, and precipitation (Zeglin et al. 2007, Porras-Alfaro et al. 2008, Hall et al. 2011, Ladwig et al.

2011). Our overarching conceptual framework is designed to understand the mechanistic interactions of climate change and chronic resource alterations (Smith et al. 2009), particularly N availability, on community and ecosystem processes in aridland ecosystems, with an emphasis on the degree to which interactions and processes are coupled in time. Coupled-decoupled interactions range across scales from the timing of microbial decomposition and plant uptake of nutrients to periodic hydrological linkages between landscape components (Van Horn 2010). Thus, our research on pulse dynamics and coupled-decoupled processes integrates the principal structural and functional elements of aridland ecosystems across multiple spatial and temporal scales, and determines how those elements will respond to global change drivers.

 

The pulse-reserve paradigm (Noy-Meir 1973) was developed to describe aridland responses to individual precipitation events. Each event triggers a pulse of growth that yields reserves (energy, seeds, biomass) until moisture from the rain event is depleted. Focusing only on individual rainfall events, however, may limit our understanding of aridland ecosystem processes. Reynolds et al. (2004) modified the pulse reserve model such that pulses of precipitation generate and/or add to existing soil moisture leading to production responses that may vary among plant functional types depending on prior conditions, seasonality, etc. The direct linkage between antecedent soil moisture and plant functional types allows for variable rates of production in response to variation in rainfall event sizes, intervals and seasonality. The Reynolds et al. (2004) model is a significant improvement over the original model, but it lacked explicit inclusion of pulse-driven microbial processes (fixation and transformation of C and N, decomposition, nutrient storage and translocation) whose thresholds and response intervals differ substantially from plants. We modified the Reynolds et al. model (Collins et al. 2008) to include contributions and feedbacks between soil moisture dynamics, microbial functional types, soil nutrient pools and plant production. This Threshold-Delay-Nutrient Dynamics (TDND) model is the conceptual paradigm for the Sevilleta LTER research program. Current and proposed research explicitly addresses all components of this paradigm.



Although the original pulse reserve model is primarily terrestrial, pulse dynamics applies to riparian and riverine processes as well (Ludwig et al. 2005). For example, storm events are relatively discrete pulses that affect microbial activity, nutrient availability, and producer and consumer dynamics in riparian systems (Molles et al. 1998, Valett et al. 2005, Martinet et al. 2009, Van Horn 2010). Thus, our general conceptual model can be applied in all of the core landscape units at SEV. The pulse reserve model as developed here is spatially static in that it describes local-scale processes in time. A related concept, the Trigger-Transfer-Reserve-Pulse (TTRP) model (Ludwig et al. 2005), was developed to link trigger events, such as rainfall, with reserve processes through spatial transfer of nutrients and water (runoff–runon, flood spates) in areas with complex topography (Belnap et al. 2005, Turnbull et al. 2010a,b). It is these large-scale but rare events that couple terrestrial and aquatic landscapes at SEV.


In addition to spatial connectivity, temporal changes in the frequency, intensity, and seasonality of abiotic drivers will have a significant impact on community structure and ecosystem processes (Gerten et al. 2008, Luo et al. 2008, Knapp et al. 2008, Diffenbaugh et al. 2008). Our existing long-term measurements and experiments along with our proposed new research are designed to understand how aridland ecosystems will change in response to alterations in the spatial and temporal dynamics of key abiotic and biotic drivers. For example, regional climate is predicted to become even more variable in the future (Diffenbaugh et al. 2008, Schoof et al. 2010, Min et al. 2011). Surface and subsurface hydrological processes (evapotranspiration [ET], surface flow rates, ground water recharge) are likely to change in upland areas as shrubs encroach on former grasslands (Turnbull et al. 2010a, b, c), and drought mortality, bark beetles and N deposition combine to reduce tree abundance in piñon-juniper woodlands (Breshears et al. 2005, McDowell et al. 2009, Allen et al. 2010, Shim et al. 2011). River regulation and invasive species have altered groundwater availability, hydrological flow regimes, nutrient dynamics and groundwater-riparian coupling in the Middle Rio Grande basin (Tibbetts & Molles 2005, Follstad Shah & Dahm 2008, Harner et al. 2009, Follstad Shah et al. 2010). Atmospheric N deposition and runoff events are increasing the amount of seasonally available soil N in historically N-poor terrestrial and aquatic ecosystems (Fenn et al. 2003, Báez et al. 2007, Van Horn 2010). Climate change has the potential to alter long-term precipitation regimes contributing to the replacement of C4 grassland by C3 shrubs (Van Auken 2000, 2009, Peters 2003, Geist & Lambin 2004, Báez & Collins 2008, Báez et al. in review). Such changes in plant species composition and functional types will alter habitat structure, resource availability, trophic interactions, and C storage and fluxes. Thus, our goal of understanding how changes in key abiotic drivers and constraints (climate, water, soils) interact with ecosystem processes (NPP, NEP, biogeochemistry) to affect the dynamics, stability, and interaction of key producers and consumers integrates SEV research.