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 We used an extensive review of the literature, including both published and unpublished data, to summarize available data on the effects of a change from open woodland or grassland to shrubland or closed woodland, on richness of plant and animal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

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 (d.eldridge@unsw.edu.au).

CLASS III. DATA STRUCTURAL DESCRIPTORS

Variable information:

Column

Header

Description

1

Code

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

2

Source

Reference for the study

3

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

4

Site name

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

5

Latitude

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

6

Longitude

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

7

Rainfall

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

8

Temperature

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

9

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

10

Encroachment status

0 = unencroached, 1 = encroached

11

Replicates

The number of independent sites from which the data were derived

12

Soil depth

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

13

Texture

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

14

Attribute

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

15

Mean

The value of each variable averaged over all replicates.

16

SEM

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.

17

Units

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 (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:

Response variable

Description

Number of records

Above-grnd C

Aboveground plant carbon

18

Above-grnd N

Aboveground plant nitrogen

12

Aggregate stability

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

2

ANPP

Annual net primary productivity

96

Ant abundance

Abundance of ants

2

Ant richness

Richness of ant taxa

26

Arthropod abundance

Abundance of arthropods

2

Arthropod richness

Number of arthropods Recognizable Taxonomic Units (RTUs)

2

Bare soil

Cover of bare soil

58

Beetle richness

Richness of beetle taxa

 

Biomass

Aboveground plant biomass

30

Bird richness

Richness of bird taxa

18

CEC

Soil cation exchange capacity

2

Cryptogam cover

Surface cover of cryptogamic soil crust

44

Darkling beetle abundance

Abundance of darkling beetles

2

Decomposition

Litter decomposition

2

Dung beetle abundance

Abundance of dung beetles

2

EC

Soil electrical conductivity

44

Erosion potential

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

6

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)

2

ESP

Exchangeable sodium potential

2

Evapotranspiration

Potential evapotranspiration

6

Fetch length

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

66

Grass cover

Cover of perennial grasses

80

Grass richness

Richness of grass taxa

2

Grass root biomass

Below-ground perennial grass biomass

2

Grass root diameter

Average diameter of grass roots

4

Grass root length

Specific grass root length

4

Grass rooting depth

Maximum grass rooting depth

4

Grasshopper richness

Number of grasshopper OTUs

2

Gravimetric moisture

Gravimetric soil moisture content

10

Herbaceous biomass

Aboveground herbaceous biomass

2

Herbage cover

Cover of herbaceous plants

2

Infiltration

Water infiltration

18

Infiltration index

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

80

Infiltration ponding

Steady-state infiltration under +10 mm ponding

2

Infiltration tension

Steady-state infiltration under -40 mm tension

2

Litter biomass

Biomass of surface-resident litter

8

Litter cover

Cover of surface-resident litter

26

Lizard richness

Richness of lizard taxa

2

Mammal richness

Richness of mammal taxa

2

Microbial biomass

Microbial biomass

2

Microbial biomass C

Carbon component of microbial biomass

10

Microbial biomass N

Nitrogen component of microbial biomass

2

Nutrient index

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

80

Obstructions

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

66

Plant cover

Cover of perennial plants

60

Plant richness

Richness of all plant taxa

170

Potential N mineralization

Potential nitrogen mineralization

62

Radiation

Near ground radiation

2

Rodent density

Density of rodent taxa

2

Rodent richness

Richness of rodent taxa

2

Root biomass

Biomass of roots

90

Root density

Density of plant roots

2

Runoff coefficient

The percentage of rainfall shed as runoff

2

Runoff rate

Rate of water runoff

4

Sediment concentration

Concentration of sediment in runoff water

6

Shoot biomass

Biomass of plant shoots

2

Shrub biomass

Aboveground shrub biomass

2

Shrub cover

Cover of shrubs

120

Shrub density

Density of shrubs

4

Shrub richness

Richness shrub taxa

34

Shrub root biomass

Biomass of shrub roots

2

Shrub root depth

Maximum shrub rooting depth

4

Shrub root diameter

Average shrub root diameter

4

Shrub root length

Length of shrub roots

4

Soil aluminium

Soil aluminium content

6

Soil ammonification

Soil ammonification

2

Soil available K

Available soil potassium

2

Soil available N

Available soil nitrogen

22

Soil available P

Available soil phosphorus

42

Soil bulk density

Soil bulk density

146

Soil calcium

Soil calcium

60

Soil CEC

Soil cation exchange capacity

18

Soil extractable P

Soil extractable phosphorus

14

Soil inorganic C

Soil inorganic carbon

2

Soil inorganic N

Soil inorganic nitrogen

18

Soil labile C

Soil labile carbon

16

Soil magnesium

Soil magnesium

56

Soil NH4-N

Soil ammonium

10

Soil nitrate

Soil nitrate

8

Soil nitrification

Soil nitrification

10

Soil organic C

Soil organic carbon

264

Soil organic matter

Soil organic matter

6

Soil P mineralization

Soil P mineralization

4

Soil pH

Soil pH

182

Soil potassium

Soil potassium

94

Soil respiration

Soil respiration

76

Soil sodium

Soil sodium

34

Soil sulphur

Soil sulphur

10

Soil temperature

Soil temperature

2

Soil total C

Soil total carbon

204

Soil total N

Soil total nitrogen

344

Soil total P

Soil total phosphorus

64

Soil total S

Soil total sulphur

24

Sorptivity ponding

Sorptivity under +10 mm ponding

2

Sorptivity tension

Sorptivity under -40 mm tension

2

Spider richness

Richness of spider OTUs

2

Stability index

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

80

Tissue N

Plant tissue nitrogen

4

Tree cover

Cover of trees

12

Tree richness

Richness of all tree taxa

8

Tree root depth

Maximum tree rooting depth

4

Tree root diameter

Average diameter of tree roots

4

Tree root length

Length of tree roots

4

Vertebrate richness

Richness of all vertebrate taxa

50

Volumetric moisture

Soil volumetric moisture content

82

Water isotope

Isotopic ratio of water

2

Wetting front

Rate of movement of wetting front in the soil

2

Woody density

Density of all shrubs and trees

2

Woody richness

Richness of all shrubs and trees

2

 

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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