Ecological Archives --D1

David J. Eldridge, Fernando T. Maestre, Sara Maltez-Mouro, and Matthew A. Bowker. 2012. A global database of shrub encroachment effects on ecosystem structure and functioning. Ecology 93:2499. http://dx.doi.org/10.1890/12-0749.1

METADATA

CLASS I. DATA SET DESCRIPTORS

A. Data set identity:

A global database of shrub encroachment effects on ecosystem structure and functioning.

B. Data set identification code:

WE_summary_V2_May2012.csv

C. Data set description

Principal Investigators:

David J. Eldridge, Evolution and Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia,

Fernando T. Maestre and Sara Maltez-Mouro, Área de Biodiversidad y Conservación, Departamento de Biología y Geología, ESCET, Universidad Rey Juan Carlos, 28933 Móstoles, Spain.

Matthew A. Bowker, US Geological Survey, Southwest Biological Science Center, Northern Arizona University, 2255 N. Gemini Drive, Flagstaff, AZ 86001, USA.

Abstract: The encroachment of woody plants into grasslands, and the conversion of savannas and open woodlands into shrublands, has been widely reported during the past decade. Encroachment has generated considerable interest among ecologists worldwide, but there have been few quantitative syntheses of the effects of encroachment on ecosystem processes across large areas of the globe. Here we summarize the results of observations of the effects of encroachment by woody plants on 111 ecosystem response variables using data obtained from 1722 encroached–unencroached pairs, reported in 160 studies from North and South America, Africa, Europe, Australia, and Asia. We used an extensive review of the literature, including both published and unpublished data, to summarize available data on the effects of a change from open woodland or grassland to shrubland or closed woodland, on richness of plant and animal taxa, soil chemistry, and the status of the soil surface. Our database is restricted to arid, semi-arid, and dry sub-humid environments (drylands) receiving average annual rainfall between 850 and 200 mm. An analysis of the impacts of shrub encroachment on ecosystem structure and function has already been reported using a large subset of these data. This updated data set can provide an opportunity to test further hypotheses about the effects of encroachment on plant and animal communities and on soil processes related to ecosystem functioning.

D. Key words: degradation; desertification; ecosystem processes; encroachment; invasion; shrub; shrubland; thickening; woody.

CLASS II. RESEARCH ORIGIN DESCRIPTORS

A. Overall project description

Identity: A global database of shrub encroachment effects on ecosystem structure and functioning: WE_summary_V2_May2012.csv

Size: 3444 records (1722 pairs), 474 kB.

Format and storage mode: ASCII text, comma separated.

Missing or unavailable data:  marked as '-'

Variation n data: For records where there is only one site, reported values of the standard error of the mean (SE) refer to the variability among sampling units (sub-plots, quadrats), as reported in the original source.

Header information: The first row of the file contains the variable names listed below.

Alphanumeric attributes: Mixed

Special characters/fields: None.

Authentication procedures: None.

An analysis of a large part of this data set has been reported in Ecology Letters (Eldridge et al. 2011). Since publication, the data set has been updated and extended to include new material published since the Ecology Letters paper, further unpublished material, data from additional soil layers not considered in the original publication, and new data that was drawn to our attention by colleagues. This data set contains functional and structural attributes that have been shown to change with encroachment of grasslands or open woodlands/forests. Clarification on any parts of the data set should be directed to David Eldridge ([email protected]).

CLASS III. DATA STRUCTURAL DESCRIPTORS

Variable information:

Methods and selection of studies:

We performed a systematic search of the scientific literature to identify quantitative evidence of the impacts of shrub encroachment on ecosystem structure and/or functioning. We searched for relevant studies, using the ISI Web of Knowledge (www.isiwebofknowledge.com) database (1945–2011 period) using the keywords “encroachment”, “competition”, “shrub”, “bush”, “thickening”, “grassland”, “desertification”,  “arid”, “semi-arid”, “semiarid”, “dryland” and “woody”, and searched for published and unpublished material as well as recent reviews (e.g., Hibbard et al. 2001, Huxman et al. 2005, van Auken 2009). We restricted this review to arid, semi-arid, and dry sub-humid environments (“drylands”; rainfall ≤ 850 mm, range: 200–850 mm; median: 400 mm) because the focus of our work was land degradation, which is a key environmental issue in these ecosystems (Reynolds et al. 2007), and because the encroachment–degradation paradigm was initially developed for the boundary between arid and semi-arid systems around which our data set is centered (Schlesinger et al. 1990). Those papers that seemed suitable for our quantitative review were carefully examined by at least one of us. To be included in our database, a study had to meet certain criteria.

1. The study must have been conducted under natural field conditions. We did not include data from studies that used greenhouse or growth chambers, or studies using cultivated plants.


2. The study had to contain quantitative and therefore analyzable data.


3. The study had to contain data on variables that were measured in plots with (encroached) and without (unencroached) woody vegetation and located in the same geographical area. This allowed us to be certain that any effects observed at each study could be attributed to the effects of encroachment, and not to variations in climatic or soil type between encroached and unencroached plots.


4. Studies were included only if the reported data were representative of whole grassland (unencroached) and encroached plots. In some cases this required that we extrapolate values of soil variables obtained from different microsites (e.g., under plant canopies and in bare ground areas devoid of vascular vegetation) by the cover of each microsite, in other cases the sampling was conducted over all the plot, so that data were representative of either the grassland or the encroached plot. Studies that evaluated the effects of woody vegetation on the response variables considered at the microsite level (e.g., comparing the effects of shrub vs. grass canopies on vegetation or soil attributes within the same site) were not considered. When suitable studies lacked information, or we could not retrieve it, we contacted the authors and requested the original data.


5. Most, but not all, of these data represented encroachment scenarios. A small number represented naturally-occurring areas of dense shrubland (e.g., the Succulent Karoo in Southern Africa; Todd and Hoffman 1999) and these were compared with shrub-free, generally grass-dominant sites. While not all of these grassland–shrubland plots were strictly cross-fence comparisons, i.e., they did not always occur immediately adjacent to each other, they were always within a few kilometers away, and therefore represented two different ends of the woody encroachment–grassland continuum. Many of the studies reported results from only a single encroached-grassland plot at the same site and therefore lacked measurement errors.


6. Most of the studies present the results of field surveys carried out at one point in time. In some cases, the authors present results from several years or from several months and/or seasons within a given year for a given set of paired plots. In these cases, we considered the average value of the response variable.


7. Some of the studies differed in their degree of encroachment sampled at the same area or site (e.g., the extensive reported studies from La Copita in Texas, Jornada Experimental Range in New Mexico, and Flint Hills in Kansas). While strictly speaking, the results of these plots are not independent, we retained each combination of unencroached and encroached plot as a separate case study to ensure that the database was as general as possible.

We reviewed data on a total of 111 response variables from 1722 plot-level case studies, each with an unencroached and encroached pair, all except for one reported in the same publication. Of all the case studies, 653 were from North America (USA, Canada, Mexico), 141 from Africa (Botswana, Ethiopia, Kenya, Namibia, South Africa, Zimbabwe), 282 from Europe (Greece, Portugal, Spain), 528 from Australia, 96 from Asia (China, Turkey), and 22 from South America (Argentina, Venezuela).

Response variables:

Data-use policy

Copyright or Proprietary Restrictions: This data set is freely available for non-commercial scientific use, given the appropriate scholarly citation. The authors have spent a year compiling data from a range of literature sources and checking these data for use in meta-analyses. Those using these data to undertake further meta-analyses may wish to consult the original publications. The data set should be cited as:

Eldridge, D. J., F. T. Maestre, S. Maltez-Mouro, and M. A. Bowker. 2012. A global database of shrub encroachment effects on ecosystem structure and functioning. Ecology 93:2499.

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