Savannas and woodlands are open forest phases that occur along a gradient between grasslands and closed canopy forests. These ecosystems are characterized by open to nearly closed canopies of overstorey trees, relatively sparse midstorey and understorey woody vegetation, and dense, species-rich ground flora. In contrast to closed forests, the dominant and codominant trees in the canopy of open forests often have large, spreading crowns. Relatively open structure allows light to the floor, which is critical for grasses and forbs; the greatest plant species diversity and turnover rates occur in the herbaceous layer of open forest ecosystems in temperate zones (Gilliam 2007). Open structure, old trees, and variation of light and microclimate are important for lichens (Paltto et al. 2011), insects (e.g. carabids; Taboada et al. 2011), birds (Hunter et al. 2001), mammals and other taxa.