Woody encroachment decreases diversity across North American grasslands and savannas

• Published in: Ecology (Durham) 2012-04, Vol.93 (4), p.697-703
• Main Authors: Ratajczak, Zakary, Nippert, Jesse B, Collins, Scott L
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biodiversity; Biological and medical sciences; Biological Evolution; bush encroachment; Conservation of Natural Resources; Correlation analysis; Demecology; Demography; ecosystem structure; Encroachment; Fundamental and applied biological sciences. Psychology; General aspects; global change; Grasses; Grasslands; Herbivores; Meta-analysis; Net Primary Productivity; North America; Plant ecology; Plant species; Plants; Plants - classification; Plants and fungi; Population decline; Precipitation; primary productivity; Productivity; productivity-richness relationship; Rain; Savannahs; Savannas; shrub encroachment; Shrubs; Species; species diversity; Species richness; Terrestrial ecosystems; tree encroachment; tree-grass coexistence; Woody plants; woody release
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/11-1199.1

Woody encroachment is a widespread and acute phenomenon affecting grasslands and savannas worldwide. We performed a meta-analysis of 29 studies from 13 different grassland/savanna communities in North America to determine the consequences of woody encroachment on plant species richness. In all 13 communities, species richness declined with woody plant encroachment (average decline = 45%). Species richness declined more in communities with higher precipitation ( r 2 = 0.81) and where encroachment was associated with a greater change in annual net primary productivity (ANPP; r 2 = 0.69). Based on the strong positive correlation between precipitation and ANPP following encroachment ( r 2 = 0.87), we hypothesize that these relationships occur because water-limited woody plants experience a greater physiological and demographic release as precipitation increases. The observed relationship between species richness and ANPP provides support for the theoretical expectation that a trade-off occurs between richness and productivity in herbaceous communities. We conclude that woody plant encroachment leads to significant declines in species richness in North American grassland/savanna communities.