A land grant from the king of Spain initiated what is now the Sevilleta Wildlife Refuge in central New Mexico. The Nature Conservancy purchased the 100,000 ha property in 1970 and turned over its management to the U.S. Fish and Wildlife Service.
The uniqueness of the Sevilleta LTER (see photo, top of page 4) is its location at the junction of at least 4 major blomes. This allows quantification of
- Gradient relationships with distance
- The scale-dependent or independent nature of spatial variability
- How steep environmental gradients influence system properties
- Integrated responses across the region
Although blomes are delineated with a boundary line, environmental factors occur as gradients across regions. Quantifying those gradients, including changes in steepness at boundaries, is critical for extrapolating to regional scales. Biological responses to gradients are likely to be nonlinear and magnify the effect of the gradient (e.g. an ecotonal region between species, communities, and life forms).
Our approach is to measure ecological parameters over gradients ranging from small scale (microtopographic redistribution of water) to medium (watershed-level topographic redistribution of water and species) to large (between LTER sites). By studying a range of scales, we hope to understand how processes operating over gradients can be used to predict long-term change. Studying the biotic responses to climate change is enhanced in semiarid systems where abiotic signals are strong. Abiotic constraints, inverse threshold effects, nonlinear hydrological scaling effects, and climatic signals originating outside of North America motivate our proposed investigations of biological responses to climatic fluctuations on gradients within the Sevilleta and along gradients connecting regional LTER sites.
A number of counter-intuitive aspects of semiarid regions add to our investigations. Widely scattered convective storms yield a variance in precipitation that is highest for low mean annual precipitation. Annual runoff volume from watersheds decreases linearly with the log of basin size. Because of the influence of winter precipitation (El Nino) on basin characteristics, annual runoff can vary inversely with antecedent moisture conditions.
Small watersheds exhibit high frequency, low volume flows and large watersheds exhibit low frequency, high volume flows allowing the use of watersheds for measuring responses to qualitatively different climatic “signals”. Above a precipitation threshold (e.g. 370 mm) productivity increases with soil moisture holding capacity and is highest on fine textured soils. Below the threshold, productivity decreases with moisture holding capacity and is highest on coarse textured soils. This threshold effect allows study of nonlinear dynamics, multiple stable states, and hysteresis at various levels of biological organization.
Climatic factors combined with high species diversity (e.g. 1140 plant taxa), heterogeneous topography and soils provide a complex template for ecological studies of species response, population/community dynamics and functional and structural properties of ecosystems.
A major theme is that biological response to climate change will occur in this “tension” zone before it occurs within the core of a biome.
A combination of major transects (several km wide), and independent study sites captures the variability of the Sevilleta LTER. Multiple scale studies, from vegetation plots to remote sensing from space, will be performed. A network of different sized watersheds (from 20 to 350,000 ha) can be studied for the influence of short and long-term climatic variation on the hydrology and ecology of ephemeral streams.
Watershed size is a surrogate for scale and enables study of the consequences of scale- dependent biotic responses. Replicated irrigation/exclosure experiments will assess the effects of supplemental spring moisture, kangaroo rat mound construction, and large mammal grazing and trampling on a wide range of ecological processes. Common garden studies will be used to identify the role of genetic variation and plasticity in species response to environmental constraints and change. Reciprocal transplant studies will be performed on the CPEIR and Jornada LTER sites. A number of innovative technologies are being tested from small to large (1 km) scales. Remote sensing and Geographic Information Systems are standard tools.
To obtain additional information contact James R. Gosz, Department of Biology, University of New Mexico, Albuquerque, NM 87131.
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