Can ecological land classification increase the utility of vegetation monitoring data?
Introduction
Vegetation monitoring is one of the principal methods used to assess the ecological consequences of management actions and climate change at local to landscape scales (Herrick et al., 2005). Vegetation dynamics at these scales can vary strongly in response to topoedaphic heterogeneity (Bestelmeyer et al., 2011, Pringle et al., 2006, Wu and Archer, 2005). For example, even relatively subtle variations in soil profile properties, such as the depth to clay- or carbonate-rich horizons in otherwise similar soils, can cause variations in rates of shrub encroachment or grass mortality (Bestelmeyer et al., 2006, Browning et al., 2012). Vegetation monitoring programs, however, often do not consider the impact of topoedaphic heterogeneity on the temporal patterns observed, which can lead to misinterpretation of early warning indicators or the importance of anthropogenic or climatic variables being studied (Pringle et al., 2006).
To address the effects of topoedaphic properties, some authors have recommended that monitoring sites be linked to soil- and climate-based land classification systems (Herrick et al., 2006, Karl and Herrick, 2010) such as the ecological site (ES) classifications used widely in the United States (Brown, 2010, USDA-NRCS, 2013 and similar classifications used worldwide (Blanco et al., 2014, Green and Klinka, 1994, Ray, 2001, van Gool and Moore, 1999). ES classes are subdivisions of a landscape based on soil, topographic, and/or climate properties known to influence vegetation composition and change (Duniway et al., 2010). Each ES class is associated with a state-and-transition model describing the vegetation changes that are likely to occur following specific management actions or natural events (Bestelmeyer et al., 2009, López et al., 2013). Land areas belonging to the same ES class are expected to provide the same general environment for plant establishment and growth. This expectation can give land managers increased confidence that the knowledge they have acquired from a particular vegetation monitoring effort can be effectively applied to other areas belonging to the same ES class (and only cautiously applied to other areas). In addition, the criteria used to differentiate ES classes are in most cases explicitly defined, which enables land managers to assess the degree of similarity between two classes and determine the suitability of applying ecological knowledge across class boundaries. In the United States separate ES classifications are created on a per-region basis, and individual ES classes are typically only utilized in that region they were developed for.
Given the important role of topoedaphic properties in controlling vegetation composition and dynamics, best practices commonly call for the incorporation of topoedaphic strata into vegetation monitoring designs. Use of ES classifications for landscape stratification is likely to increase with official commitment by three prominent US land management agencies − the Natural Resources Conservation Service, Forest Service, and Bureau of Land Management − to utilize ES classifications as a basis for monitoring, assessment, and planning in rangelands (BLM, 2010). ES classifications are already applied to a number of conservation activities and therefore represent a sensible tool for linking monitoring programs to other aspects of land management such as restoration projects and grazing plans. Nevertheless, there has been little empirical study aimed at supporting or refuting the utility of ES classifications with regard to ecosystem monitoring, despite recommendations to further incorporate ES classifications or similar frameworks into vegetation monitoring programs (Bestelmeyer et al., 2009, Herrick et al., 2006, Karl and Herrick, 2010).
We used an uncommonly long (>40 years), well-studied, and spatially extensive monitoring dataset available from the Santa Rita Experimental Range (SRER) to test for differences in vegetation trajectories among ES classes reflecting differences in subsoil properties in sandy soils of piedmont slope landforms. Long-term monitoring of ecological indicators is essential for resolving critical uncertainties in the detection of ecosystem trends, such as whether or not environmental degradation or improvement is taking place in ecosystems, like deserts, that respond slowly or episodically to management or climatic drivers. Increasing the effectiveness of ecological indicators may require addressing topoedaphic variation in a more systematic and detailed way than typically occurred in the past, and ES classification has been identified as one tool that could be used to address topoedaphic variation in such a manner (Bestelmeyer et al., 2009, Herrick et al., 2006). Our study provides a rare, empirical assessment of ES classification utility using an existing long-term monitoring dataset. By associating each SRER monitoring site with an ES class, we sought to determine if the detection of changes in plant species recognized as having heightened ecological or management importance in our study area would be improved. We also sought to determine whether previously unrecognized edaphic effects on vegetation trajectories had the potential to produce erroneous interpretations of vegetation monitoring data and associated indicators of ecosystem change. The ES classes studied here reflect differences in subsoil clay content that would likely go unnoticed by many observers without explicit consideration of ES classes, and earlier published analyses of the SRER long-term monitoring data did not address such soil variations. Finally, our study offered an opportunity to refine interpretations of a high-value long-term dataset and evaluate the need to modify the current ES classification system.
Section snippets
Focal species
We limited our analysis to two plant species having great management significance in the southeastern Arizona region: velvet mesquite (Prosopis velutina Woot.) and Lehmann lovegrass (Eragrostis lehmanniana Nees). Velvet mesquite is a small tree native to portions of Arizona, California, and New Mexico. Historically abundant on the SRER primarily along ephemeral drainages, the species has since colonized most upland areas of the research property (McClaran, 2003, McClaran et al., 2010).
Time series summary statistics
Compared to SLU plots (n = 18), plots assigned to the SLD (n = 24) class exhibited a broader distribution of mean and maximum velvet mesquite canopy cover values (Fig. 2). Median values of these two summary statistics were also higher for the SLD class than for the SLU class. Kruskal-Wallis tests indicated a significant difference in mean velvet mesquite canopy cover between SLD and SLU plots and a nearly significant difference (P = 0.0534) in maximum canopy cover values between these two ES classes (
Discussion
This study utilized a long-term vegetation record from southeastern Arizona to address a simple question: Can a readily-accessed ecological land classification, representing subtle topoedaphic variations that might otherwise go unnoticed by researchers, improve interpretations of vegetation monitoring data? We found that incorporating an ES classification into our analysis of a long-term monitoring dataset helped to explain spatial variations in the magnitude of historical vegetation change,
Summary
ES classifications have been adopted by land management agencies in the United States and other countries as frameworks for developing land unit-specific management recommendations, models of vegetation dynamics (e.g., state-and-transition models), and protocols for assessing ecosystem health. Monitoring activities, in turn, are recognized as an important tool for collecting information that can be used to support the development, testing, and refinement of vegetation change models, management
Acknowledgements
This research was supported in part by USDA CSREES grants 2007-38415-18637 and 2008-51130-19567.
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