Grasslands, woodlands and savannas occupy large portions of the Earth´s surface, but are particularly prevalent in dryland, temperate and tropical/subtropical regions. These vegetation types play a key role in the functioning of the biosphere and in supporting the needs of human population through the provision of multiple ecosystem services. An important part of the world’s grasslands and savannas is undergoing a rapid shift from herbaceous to woody-plant dominance, which is promoting large changes in their structure and functioning, and thus on the services they provide.
In the introduction to the Special feature, Sala & Maestre (2014) discuss the importance of having a mechanistic understanding of the factors driving grassland/woodland transitions and of their ecological consequences, and illustrate how the different articles included in the special feature contribute to fill important gaps in our knowledge of this topic.
Anadon, Sala & Maestre (2014) used models of current distribution of treeless vegetation, savanna and forest coupled with an ensemble of global circulation models, to forecast how climate change will affect the distribution of these vegetation types, and of the transition zones between them, in tropical and subtropical Americas. Using data from The Konza Prairie Research Natural Area spanning multiple decades, Ratajczak et al. (2014) explore the role of fire as an important driver of grass-woodland dynamics in mesic grasslands. Abades, Gaxiola & Marquet (2014) describe the theory behind the spatial structure of woodlands and grasslands, and point out the importance of critical transitions associated with percolation phenomena as a potential mechanism that may underlie grass-woodland transitions. Archer & Predick (2014) evaluate the known and potential consequences of management actions on a wide range of ecosystem services provided by grasslands and woodlands, the scientific challenges to quantifying these services and the trade-offs existing among them. They also provide a roadmap of priority areas for research that can reduce uncertainty and improve predictions of the outcomes of brush management activities. Using an extensive database with over 900 study sites coupled to an ecosystem water balance model, Bradford et al. (2014) evaluate in this issue the hydrological impacts of the removal of big sagebrush (Artemisia tridentata) in shrub steppe ecosystems across western North America. In the last article from the special feature, Gaitán et al. (2014) used 311 sites located across a broad natural gradient in Patagonian rangelands and structural equation modelling to evaluate the relative importance of climate (temperature and precipitation) and vegetation structure (grass/shrub cover and species richness) as drivers of aboveground net primary productivity (ANPP), precipitation-use efficiency (the ratio of annual ANPP to annual precipitation) and precipitation marginal response (the slope of the annual precipitation-ANPP relationship).
Understanding the causes and consequences of grass-woodland transitions requires the joint consideration of the multiple drivers affecting this phenomenon and their consequences for ecosystem functioning and services. The collection of reviews, empirical and modelling studies included in this Special Feature contribute towards this goal, and will motivate further research in these and other important issues on grass-woodland transitions.
Osvaldo Sala & Fernando Maestre