Plant diversity and ecosystem multifunctionality peak at intermediate levels of woody cover in global drylands.
OBJECTIVE
The global spread of woody plants into grasslands is predicted to increase over the coming century. While there is general agreement regarding the anthropogenic causes of this phenomenon, its ecological consequences are less certain. We analyzed how woody vegetation of differing cover affects plant diversity (richness and evenness) and multiple ecosystem functions (multifunctionality) in global drylands, and how this changes with aridity.


METHODS
224 dryland sites from all continents except Antarctica widely differing in their environmental conditions (from arid to dry-subhumid sites) and woody covers (from 0 to 100%).


METHODS
Using a standardized field survey, we measured the cover, richness and evenness of perennial vegetation. At each site, we measured 14 ecosystem functions related to soil fertility and the build-up of nutrient pools. These functions are critical for maintaining ecosystem function in drylands.


RESULTS
Species richness and ecosystem multifunctionality were strongly influenced by woody vegetation, with both variables peaking at relative woody covers (RWC) of 41-60%. This relationship shifted with aridity. We observed linear positive effects of RWC in dry-subhumid sites. These positive trends shifted to hump-shaped RWC-diversity and multifunctionality relationships under semiarid environments. Finally, hump-shaped (richness, evenness) or linear negative (multifunctionality) effects of RWC were found under the most arid conditions.


CONCLUSIONS
Plant diversity and multifunctionality peaked at intermediate levels of woody cover, although this relationship became increasingly positive under wetter environments. This comprehensive study accounts for multiple ecosystem attributes across a range of woody covers and environmental conditions. Our results help us to reconcile contrasting views of woody encroachment found in current literature and can be used to improve predictions of the likely effects of encroachment on biodiversity and ecosystem services.
Área de Biodiversidad y Conservación. Departamento de Biología y Geología, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, 28933 Móstoles, Spain. ; Institute of Plant Sciences, University of Bern, 3013 Bern, Switzerland.
Santiago
S
Soliveres
Área de Biodiversidad y Conservación. Departamento de Biología y Geología, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, 28933 Móstoles, Spain.
Fernando T
FT
Maestre
School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales 2052, Australia. DJE: d.eldridge@unsw.edu.au.
David J
DJ
Eldridge
Área de Biodiversidad y Conservación. Departamento de Biología y Geología, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, 28933 Móstoles, Spain. ; Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, 2751, New South Wales, Australia. MDB: mdelbaq@upo.es.
Manuel
M
Delgado-Baquerizo
Departamento de Ingeniería Forestal, Escuela Técnica Superior de Ingeniería Agronómica y de Montes. Universidad de Córdoba. Edificio Leonardo da Vinci, 1 planta. Campus de Rabanales. Ctra N-IV km 396. C.P. 14071, Córdoba, Spain. JLQ: jose.quero@uco.es.
José Luis
JL
Quero
School of Forestry, Northern Arizona University, 200 East Pine Knoll Drive, AZ 86011, Flagstaff, USA. MAB: matthew.bowker@nau.edu.
Matthew A
MA
Bowker
Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Carretera de Utrera kilómetro 1, 41013 Sevilla, Spain. AG: agallardo@upo.es.
Antonio
A
Gallardo
aridity; semiarid; shrub encroachment; soil; species evenness; species richness; thicketization