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.



A. Data set identity:

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

B. Data set identification code:


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 We used an extensive review of the literature, including both published and unpublished data, to summarize available data on the effects of a change from open woodland or grassland to shrubland or closed woodland, on richness of plant and animal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.


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.

B. Specific project description

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]).


Variable information:






Unique alphanumeric identifier for each unencroached (U) and encroached (E) pair in the data set



Reference for the study


Country code

Country descriptor: 1 = Argentina, 2 = Australia, 3 = Botswana, 4 = Canada, 5 = China, 6 = Ethiopia, 7 = Greece, 8 = Kenya, 9 = Mexico, 10 = Namibia, 11 = Portugal, 12 = South Africa, 13 = Spain, 14 = Turkey, 15 = USA, 16 = Venezuela, 17 = Zimbabwe


Site name

The geographic locality of the study in terms of the general area, study site or locality name



Degrees and minutes of latitude; ‘regional’ indicates that the study was conducted at a regional scale



Degrees and minutes of longitude; ‘regional’ indicates that the study was conducted at a regional scale



Average annual rainfall (mm) data extracted from the original data source or obtained from literature searches or world wide web searches.



Average annual temperature (degrees C) data extracted from the original data source or obtained from literature searches or world wide web searches.


Woody species

The dominant woody plant species involved in the encroachment. In some cases more than one species are noted if they are co-dominant


Encroachment status

0 = unencroached, 1 = encroached



The number of independent sites from which the data were derived


Soil depth

The depth of the soil profile, where relevant, in cm. ‘-‘ indicates that soil depth data is not applicable



Soil texture descriptor: S = sand, LS = loamy sand, SL = sandy loam, FSL = fine sandy loam, L = loam, Si = silt, SiL = silty loam, CL = clay loam, SCL = sandy clay loam, SiCL = silty clay loam, SiC = silty clay, C = clay, na = no data available



The response variables being assessed. They are described in detail below



The value of each variable averaged over all replicates.



Standard error of the mean as reported in the study. nd indicates that the data are either not available or there is only one replicate.



The units of measurement


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 ( 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:

Response variable


Number of records

Above-grnd C

Aboveground plant carbon


Above-grnd N

Aboveground plant nitrogen


Aggregate stability

Stability of soil aggregates based on the ASWAT scale (Field et al., 1997)



Annual net primary productivity


Ant abundance

Abundance of ants


Ant richness

Richness of ant taxa


Arthropod abundance

Abundance of arthropods


Arthropod richness

Number of arthropods Recognizable Taxonomic Units (RTUs)


Bare soil

Cover of bare soil


Beetle richness

Richness of beetle taxa



Aboveground plant biomass


Bird richness

Richness of bird taxa



Soil cation exchange capacity


Cryptogam cover

Surface cover of cryptogamic soil crust


Darkling beetle abundance

Abundance of darkling beetles



Litter decomposition


Dung beetle abundance

Abundance of dung beetles



Soil electrical conductivity


Erosion potential

The Reynolds number; a dimensionless number that quantifies the relative importance of smooth and turbulent flows of a liquid


Erosion score

Scale from 1 (nil erosion) to 5 (abundant) based on the amount of soil pedestalling, and in some severe cases, the presence of pavements (terraces of flat soil) lacking plant cover (Angassa and Baars 2000)



Exchangeable sodium potential



Potential evapotranspiration


Fetch length

The average inter-patch distance between permanent structures such as logs, perennial grass tussocks and perennial plants (sensu Tongway 1995)


Grass cover

Cover of perennial grasses


Grass richness

Richness of grass taxa


Grass root biomass

Below-ground perennial grass biomass


Grass root diameter

Average diameter of grass roots


Grass root length

Specific grass root length


Grass rooting depth

Maximum grass rooting depth


Grasshopper richness

Number of grasshopper OTUs


Gravimetric moisture

Gravimetric soil moisture content


Herbaceous biomass

Aboveground herbaceous biomass


Herbage cover

Cover of herbaceous plants



Water infiltration


Infiltration index

A derived index related to the capacity of the soil to accept rainfall (sensu Tongway 1995)


Infiltration ponding

Steady-state infiltration under +10 mm ponding


Infiltration tension

Steady-state infiltration under -40 mm tension


Litter biomass

Biomass of surface-resident litter


Litter cover

Cover of surface-resident litter


Lizard richness

Richness of lizard taxa


Mammal richness

Richness of mammal taxa


Microbial biomass

Microbial biomass


Microbial biomass C

Carbon component of microbial biomass


Microbial biomass N

Nitrogen component of microbial biomass


Nutrient index

A derived index related to the capacity of the soil to cycle nutrients (sensu Tongway 1995)



The density of permanent structures such as logs and perennial grasses on the soil surface expressed per unit length of transect (sensu Tongway 1995)


Plant cover

Cover of perennial plants


Plant richness

Richness of all plant taxa


Potential N mineralization

Potential nitrogen mineralization



Near ground radiation


Rodent density

Density of rodent taxa


Rodent richness

Richness of rodent taxa


Root biomass

Biomass of roots


Root density

Density of plant roots


Runoff coefficient

The percentage of rainfall shed as runoff


Runoff rate

Rate of water runoff


Sediment concentration

Concentration of sediment in runoff water


Shoot biomass

Biomass of plant shoots


Shrub biomass

Aboveground shrub biomass


Shrub cover

Cover of shrubs


Shrub density

Density of shrubs


Shrub richness

Richness shrub taxa


Shrub root biomass

Biomass of shrub roots


Shrub root depth

Maximum shrub rooting depth


Shrub root diameter

Average shrub root diameter


Shrub root length

Length of shrub roots


Soil aluminium

Soil aluminium content


Soil ammonification

Soil ammonification


Soil available K

Available soil potassium


Soil available N

Available soil nitrogen


Soil available P

Available soil phosphorus


Soil bulk density

Soil bulk density


Soil calcium

Soil calcium


Soil CEC

Soil cation exchange capacity


Soil extractable P

Soil extractable phosphorus


Soil inorganic C

Soil inorganic carbon


Soil inorganic N

Soil inorganic nitrogen


Soil labile C

Soil labile carbon


Soil magnesium

Soil magnesium


Soil NH4-N

Soil ammonium


Soil nitrate

Soil nitrate


Soil nitrification

Soil nitrification


Soil organic C

Soil organic carbon


Soil organic matter

Soil organic matter


Soil P mineralization

Soil P mineralization


Soil pH

Soil pH


Soil potassium

Soil potassium


Soil respiration

Soil respiration


Soil sodium

Soil sodium


Soil sulphur

Soil sulphur


Soil temperature

Soil temperature


Soil total C

Soil total carbon


Soil total N

Soil total nitrogen


Soil total P

Soil total phosphorus


Soil total S

Soil total sulphur


Sorptivity ponding

Sorptivity under +10 mm ponding


Sorptivity tension

Sorptivity under -40 mm tension


Spider richness

Richness of spider OTUs


Stability index

A derived index related to the capacity of the soil to resist erosion (sensu Tongway 1995)


Tissue N

Plant tissue nitrogen


Tree cover

Cover of trees


Tree richness

Richness of all tree taxa


Tree root depth

Maximum tree rooting depth


Tree root diameter

Average diameter of tree roots


Tree root length

Length of tree roots


Vertebrate richness

Richness of all vertebrate taxa


Volumetric moisture

Soil volumetric moisture content


Water isotope

Isotopic ratio of water


Wetting front

Rate of movement of wetting front in the soil


Woody density

Density of all shrubs and trees


Woody richness

Richness of all shrubs and trees



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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