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Assessment and prediction of tree encroachment into a serpentine Jeffrey pine savanna
For over a century, increases in the abundance of woody plant species in savannas have been occurring worldwide, and this woody plant encroachment directly threatens the existence of savanna ecosystems. Savannas are known for their striking composition of herbaceous grassland interspersed with scattered woody vegetation, and the development of contiguous shrub or tree canopies threatens both large-scale landscape diversity and smaller-scale taxonomic diversity. Rates of afforestation vary in encroached savannas and can be mediated by topographical gradients such as slope, aspect, and elevation. The major cause of encroachment in many systems has been attributed to a change in disturbance regimes, most notably declines in fire frequency. Encroachment has potentially irreversible ecological consequences that include changes in carbon storage, soil chemistry, landscape heterogeneity, and taxonomic richness. One unique type of encroached savanna is restricted to serpentine soils of the Klamath Mountains in the northwest United States. Composed primarily of Idaho fescue (Festuca idahoensis) grassland in the understory and scattered Jeffrey pine (Pinus jeffreyi) in the overstory, these savannas contain high frequencies of rare and endangered taxa. My research focused on a Jeffrey pine savanna in northwest California called Little Bald Hills, where I utilized two distinct approaches to studying encroachment, each providing unique but complementary information about the encroachment process. First, I used dendroecology to determine encroachment rates, establish historical site reference conditions, and project tree-growth trends in Little Bald Hills. Additionally, through the use of historical aerial photographs, I constructed a spatial model of tree encroachment that allows for prediction of encroachment decades into the future. I found that encroachment began ~1850 and was not related to differences in topography across the landscape. Encroachment has driven a large shift in the size and age structure of the woody plant community, especially since the 1940s when a sharp spike in establishment occurred after the last known fire burned in Little Bald Hills. Concomitant with the increase in tree density and size is the contraction of the grassland component of the savanna. Grassland represented ~36% of Little Bald Hills in 1942, but if current encroachment rates continue less than 5% of Little Bald Hills will be grassland in 50 years. Ecological restoration is needed to preserve this uncommon habitat type, and the results of my study will allow land managers to set appropriate restoration goals, prioritize restoration areas, and evaluate restoration efforts.