Connectivity between savanna and woodland habitats benefits plant species richness at landscape scales

Background/Question/Methods

Trees within savannas affect patterns of species occurrence and diversity. Oaks in Californian savannas are associated with increased dominance of exotic species beneath their canopy, reducing native and non-native species richness. At a landscape scale, however, oak trees may increase the number of available niches relative to open grassland and thereby increase overall species richness. This effect may depend on tree distribution; more isolated trees may contribute fewer species to the landscape due to fewer opportunities for colonization. Alternatively, a species-rich tree may provide more unique resource space compared to the open grassland. To examine these hypotheses we surveyed vegetation, soil and light conditions around savanna trees at four sites. We counted the number of new species the plots associated with an individual tree added to the landscape pool relative to those in open grassland and measured distances from the nearest oak woodland and neighboring oak individual using recent aerial photographs. Using Poisson generalized linear regression, we evaluated the relationship between the number of unique species added by a given tree and its canopy size and distance from nearby trees. To test the alternative hypothesis we also compared models with and without parameters accounting for the degree of difference in soil and light.

Results/Conclusions

Our data support the importance of connectivity: trees closer to oak woodland added significantly more species to the landscape pool than those that were more isolated. The degree of proximity to neighboring savanna oak trees, however, did not influence this pattern. Canopy size and local environmental variables were also unrelated to the presence of new species and did not add significantly to the fit of the models. This analysis suggests that savanna trees may be important in overall biodiversity in a region but this positive effect is highly sensitive to isolation and landscape fragmentation. In order to maintain high species richness in oak savanna it will be important to minimize future fragmentation and also preserve existing remnant oak woodland patches neighboring savanna.