Comparison of three vegetation monitoring methods: Their relative utility for ecological assessment and monitoring
Introduction
A primary objective of many ecological monitoring programs is to detect changes in ecosystem functions and processes (National Research Council, 1994, Heinz Center, 2002, Niemi and McDonald, 2004). Vegetation cover and composition are two of the most commonly used groups of indicators in many terrestrial ecosystems. These indicators have been correlated with a large number of ecosystem services including biodiversity and soil and water conservation, habitat for wildlife, food and fiber production (National Research Council, 2000, Millenium Ecosystem Assessment, 2003). They are commonly used to evaluate land degradation and recovery, and the success of restoration projects.
A large number of methods are currently used to quantify various forms of these indicators (Bonham, 1989, Elzinga et al., 2001) and a large number of datasets already exist that include them for tens of thousands of sites in the US (Spaeth et al., 2003; http://fia.fs.fed.us/library/field-guides-methods-proc/version 4.0). A number of new proposals would require a significant expansion in the spatial and temporal extent of this type of data (e.g., National Research Council, 1994, Heinz Center, 2002). While many of these initiatives will rely on new remote sensing technologies and analyses, including high resolution aerial photography (Laliberte et al., 2007), ground-based measurements will continue to be used for both calibration and local monitoring.
Despite the widespread interest in vegetation cover and composition indicators, there have been few successful attempts to standardize them so that they can be compared across space and time. These differences persist within and among agencies in the United States and throughout the world. For example the term ‘vegetation cover’ is commonly used to refer to both ‘canopy cover’ or the proportion of the soil surface included within the (variably defined) perimeter of any plant canopy, and ‘foliar cover’, which includes only those parts of the soil surface that are covered by a plant part (Bonham, 1989). While a diversity of measurement and reporting standards is often required by the scientific community in order to address specific research objectives at the local level, this same diversity has limited attempts to synthesize data to address regional, national, and international policy and management issues.
The lack of consensus on vegetation cover and composition monitoring protocols can be attributed to a number of factors, including personal and institutional traditions, and the fact that the optimal method varies with the relative importance of different monitoring objectives. It is also due to the relative paucity of studies that have systematically compared different methods in order to determine (1) which methods generate data that are statistically identical, or that can be systematically converted (interchangeability), (2) the level of precision that can be achieved at a particular cost, and (3) the number and value of different indicators that can be generated with each method. With a few exceptions (see reviews in Elzinga et al., 2001), most of the published comparisons have focused on the ability of these methods to measure indicators related to biodiversity, such as diversity of native plant species, detection of exotic species, and monitoring of rare species (Stohlgren et al., 1995, Stohlgren et al., 1998, Campbell et al., 2002, Leis et al., 2003, Prosser et al., 2003). The detection of plant species through accumulation curves is another issue that has been discussed in a number of papers (Stohlgren et al., 1995, Stohlgren et al., 1998). These discussions provide relevant information about the utility of sampling methods for preservation of biological diversity. With a few exceptions, however (e.g., Sykes et al., 1983, Stohlgren et al., 1995, Stohlgren et al., 1998), most of the studies have focused on just one or two plant communities and there are few studies from arid environments.
The objective of this study was to compare three commonly used vegetation monitoring methods (line-point intercept, grid-point intercept, and ocular estimates) in five different plant communities, with respect to (1) interchangeability of the data, (2) precision, (3) cost, and (4) value of each method relative to its potential to generate multiple indicators. In order to effectively address criteria 1, 2 and 4, we attempted to keep cost (3) approximately constant across all methods. We included point-based methods and ocular estimates because, in one form or another, at least one of them is applied by virtually every organization in the world today that is collecting ground-based monitoring data. Both line and quadrat-based point methods were included to specifically test the hypothesis that data collected in the same plot using these two methods are interchangeable. For simplicity, we have focused our analysis on just two key indicators, species richness and foliar cover. These indicators are frequently used to monitor biodiversity and ecosystem functioning, respectively.
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Study sites
This study was conducted in the Jornada Basin, which is located approximately 37 km north-east of Las Cruces, New Mexico, USA. The climate of the basin is semiarid with a mean annual precipitation over 80 years (1916–1995) of 248 mm. The mean monthly temperature ranges from 3.8 °C in January to 26.1 °C in July (Hochstrasser et al., 2002). Plant communities are dominated by grasses such as Bouteloua eriopoda Torrey (Torrey) or Pleuraphis mutica Buckley, shrubs such as Larrea tridentata (Sess. &
Results
Ocular estimates generally detected more species than either of the point-based methods, although the significance of the results depended on the analysis method. The comparison of species richness for each plant community showed that there were no significant differences among methods, except for tarbush shrublands. In these shrublands ocular estimates detected more species than either of the point-based methods. The comparison across all plant communities showed that significantly more
Discussion
The results showed that line- and grid-point intercept methods provide similar estimates of species richness across all plant communities, and that these were generally lower than those obtained with ocular estimates. This same tendency was also observed within each plant community, although there were no significant differences among monitoring methods, except for tarbush shrublands. The non-significant differences among methods could be due to the limited number of replicates conducted within
Acknowledgments
We thank Debra Peters and John Anderson for access to the LTER NPP sites. We also thank Ericha Courtright for her logistical support. This paper was written during H.G.A.’s sabbatical at USDA-ARS Jornada Experimental Range, NMSU. Fellowship was provided by Dirección General de Asuntos del Personal Académico, UNAM. This work was supported by a National Science Foundation award to the LTER programs at the Jornada Basin at New Mexico State University (DEB 0080412) and by a Conservation Effects
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