Brush Management & Ecosystem Services - Study Summary | SNRE

Study | SUMMARY

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Background & Motivation

Grasslands, shrublands, and savannas, collectively "rangelands", constitute about 50% of the Earth’s land surface. These geographically extensive arid and semi-arid landscapes represent 30-35% of terrestrial net primary productivity, contain greater than 30% of the world’s human population, and support the majority of the world’s livestock production. As such, rangelands play an important role in global carbon, water, and nitrogen cycles and human health. Their extensive air- and watersheds provide habitat for wildlife and a myriad of ecosystem goods and services important to rapidly growing settlements and cities, some of which may be geographically distant.

Management over the decades has focused on reducing the abundance of woody vegetation using a variety of technologies, with the intent of reversing real or perceived declines in forage production, streamflow, or groundwater recharge. Recent research suggests that woody plant encroachment (WPE) and actions taken to reduce their abundance also affect a variety of other ecosystem services (ES). However, our understanding of the impacts of brush management (BM) effects on ES have lagged behind that of WPE. As a result, we are poorly positioned to predict or evaluate trade-offs or benefits in management scenarios including or excluding BM options (e.g. among forage production, wildlife, carbon storage, biodiversity, water and sediment retention, soil productivity). Furthermore, economic analyses of BM suggest that returns based solely on improvements in livestock carrying capacity are often not economically justified, especially when external subsidies are not available. Full consideration of other ES could, however, change these cost-benefit assessments, as their inclusion would underscore the true value of rangelands and their functions. The proposed research will position us to better quantify the effects of BM on key provisioning, supporting and regulating ES, to evaluate trade-offs between them that may occur, and to conduct a more rigorous analysis of the cost-effectiveness of providing these services.

Goals

Our overall goal is to quantify and assess trade-offs between woody plant encroachment (WPE) and BM so that policymakers and rangeland and forest managers can better differentiate among the consequences and cost-benefit of competing land use and land management scenarios at relevant scales. Our overall objective is to compare and contrast the provision of a portfolio of ES on instrumented watersheds with intact and cleared woody vegetation. ES to be quantified include: provisioning services (forage production, water yield), supporting services (ecosystem [woody + herbaceous] primary production, soil moisture), and regulating services (carbon sequestration, peak flows, sediment yield, land surface-atmosphere interactions). All ES will be assessed for the duration of the project. All field data will be collected in a spatially explicit fashion and used in conjunction with existing dynamic simulation environments (Automated Geospatial Watershed Assessment tool with the RHEM-KINEROS2-OPUS models; tRIBS, TIN-based Real-time Integrated Basin Simulator; CENTURY biogeochemical process model) to evaluate how contrasting land cover, climate, and land management scenarios will impact the provision of ESs at watershed scales.

The proposed research will address these limitations for a portfolio of ES on instrumented watersheds with intact and cleared woody vegetation using an integrated combination of field surveys, flux towers and high resolution remote sensing. The provisioning and support services we are addressing will be quantified at multiple scales, creating an opportunity to examine their spatial variability and determine:

(i) how well our models predict those patterns, and
(ii) how well those patterns aggregate to watershed scales.

A recent review of BM noted the lack of watershed-scale research and the lack of long-term data on ecosystem responses. The proposed research will address the former and lay a strong foundation for the latter. Our study site, the Santa Rita Experimental Range (SRER) has a storied history of supporting long-term research and has recently been designated a long-term (35 year) NEON core site. As such, mechanisms are already in-place for site/treatment protection and maintenance, as are protocols for sample and data archiving. Furthermore, the watersheds we will be using are managed by the ARS Southwest Watershed Research Center and are part of the recently initiated Long-Term Agro-Ecosystem Research (LTAR) Network. This network currently consists of 18 nodes comprised of USDA-ARS and university research sites across the country. The network has four priority areas of concern: agro-ecosystem productivity; climate variability and change; conservation and environmental quality; and socio-economic viability and opportunities. The proposed research is designed with these priorities in mind. Our four year project will quantify important pre-treatment, antecedent conditions and initial ecosystem responses to BM. As such, we will lay a solid foundation for subsequent research and monitoring by the scientific community, a necessary first step toward elucidating long-term ES responses to BM.

Study Site

The 210 square-kilometer Santa Rita Experimental Range (SRER; elevation range 900 - 1400 m), created in 1903, is located on the western alluvial fans of the Santa Rita Mountains, 45 kilometers south of Tucson. The climate is subtropical semi-arid with precipitation dominated by North American Monsoon. Precipitation is roughly split between summer (July-September) and winter (December-March). Mean annual precipitation ranges from 282 mm to 492 mm at low and high elevations, respectively. Average daily temperature at 1340 m is 26.4 °C in July and 8.6 °C in January. Vegetation on the long-term watersheds is dominated by velvet mesquite (Prosopis velutina) and a ground layer of black grama (Bouteloua eriopoda), lovegrass (Eragrostis lehmanniana and E. curvula), Arizona cottontop (Digitaria californica), and threeawn (Aristida spp.). The primary land use has been cattle grazing.

One advantage of this study is that we have the opportunity to monitor BM effects on ES in the context of instrumented watersheds that provide long-term data. The USDA-ARS instrumented eight unit-source watersheds in 1975 throughout SRER to record rainfall, runoff, and sediment, as well as to investigate the effects of manipulative treatments on hydrological processes. A subset of these watersheds (WS 5, 6, 7, and 8) are being used for this study.

The treatment area is centered on an eddy covariance tower managed by Arizona State University and constructed in 2011. We have the capacity to compare the changes in ecosystem flux measured by this tower post-treatment with our "control" tower managed by USDA-ARS about 1.5 kilometers to the west. Both towers are equipped with identical instrumentation to measure ET and NEE using the eddy covariance technique.