Many of the world's grassland and savanna communities have experienced significant changes in the abundance of shrubs and trees over the last century. Woody plants have dramatically increased in many different types of grassland and declined in others. In light of these shifts and of the high proportion of threatened plants and animals that occur in grasslands, there is a need to better understand the influences of trees on the surrounding plant communities, how these impacts vary along regional climatic gradients, and the resistance of these ecosystems to further change. In particular, the potential for trees to exert long-term impacts on neighboring communities following their death has never been evaluated. This project will explore the structure of the California oak savanna at local and regional scales through time, focusing on the relationship between the presence of oak trees and the diversity of other native and non-native species. Research will focus on two main questions: (1) How long do the effects of oaks on species diversity and soil properties last? and (2) How do the resource patches created by savanna trees contribute to species richness? Researchers will survey the surroundings of living and dead oak trees, determine the time of death for each tree with radiocarbon dating, and use mapping software and spatial analyses. At this time there is no overarching theory predicting when canopy trees will have positive or negative impacts on understory species diversity. This work will provide a unified theory for understanding impacts of trees in grasslands.
Savannas and grasslands are economically important grazing lands in North America and throughout the world. This project will help understand how changes taking place now in these habitats may affect them in the future, helping society adapt management to keep pace with change. In addition, the project will provide opportunities to mentor high school students in science and will work with outreach programs at the University of California to introduce students from groups under-represented in science to habitat conservation and restoration.
Over the last century many of the world's grassland and savanna communities have experienced significant changes in the abundance of shrubs and trees. Woody plants have dramatically increased in many different types of grassland while declining in others. In light of these shifts and the high proportion of threatened plants and animals that occur in grasslands, there is a need to better understand the influences of trees on the surrounding plant communities, how these impacts vary across regional climate gradients, and the resistance of these ecosystems to further change. In particular, the potential for trees to exert long-term impacts on neighboring communities following their death and the role of spatial factors such as isolation from other trees have never been evaluated. In this project we explored the structure of the California oak savanna at multiple scales, focusing on the relationship between the diversity of native and non-native species and the presence of oak trees at local versus regional scales and over time. We focused on two main questions: (1) How long does the oak impact on understory species diversity and soil properties last? and (2) How does the spatial heterogeneity in resources created by savanna trees contribute to local versus landscape patterns of species richness? To answer these questions we conducted surveys of vegetation and soil attributes surrounding living and dead trees over several years, determined the date of death for trees using historic aerial photos, and used mapping software to explore how the distance between trees influenced the presence of rare species. We also collected wood samples and used the anatomy of the cells within the dead wood to determine the species of dead trees that we surveyed. We found similar, strong effects of the living trees at all locations we surveyed, across a broad gradient in climate from northern to southern California. Oaks generally enhanced the presence of non-native annual grasses, and this effect lasted many decades following tree death. There were very few differences between the types of plants surrounding a dead oak and those around a living oak. Soil attributes also changed very slowly following death. These results suggest that changes to plant communities where trees are dying or declining will not be immediate and that the past presence of trees can have strong legacies on the plants currently present at a site. There were differences between the species, however, which suggest that trees with certain attributes (such as evergreen leaves and shallow roots) may have stronger impacts on surrounding plants. Although trees locally decreased the abundance of native species, and appeared to be "islands of invasion" within the grassland, they were also associated with the presence of species that otherwise were absent from open grassland. Trees that had large canopies were much more likely to contain these species in the understory. At the southernmost site, trees that were less isolated from other trees were also more likely to add new species to the list of those found in the open. This pattern was not true at the other three locations, which suggests that this pattern is not generalizable to all locations. The specific history of how tree abundance and density has changed from historic conditions, for example, could differ between our surveyed sites. In addition to furthering our understanding of how trees can exert long-term impacts on their environment and change the distribution of various species, this project has supported the scientific training of numerous high school and undergraduate students. Many of these students have gone on to continue their interest in the life sciences at higher levels. The principal investigators have disseminated their findings via many public outreach activities through seminar series at the field sites and their university which are open to the general public. They have attended national and regional scientific meetings to present their findings to colleagues and shared their knowledge with local land managers. Undergraduates who contributed significantly to these results have also shared their work with the public and their peers through poster presentations at several conferences. Lastly, this project has contributed images and slides of oak anatomy to the collections at UC Santa Barbara museum, housed at the Cheadle Center for Biodiversity and Ecological Restoration.