UC Merced

Mountain meadows provide a number of ecosystem services, including sequestering carbon, storing and regulating the release of water, and maintaining biodiversity. However, as with other grassland systems, many meadows are currently experiencing woody plant encroachment. In my first chapter, I used a landscape scale survey of mountain meadows in the central Sierra Nevada to quantify the extent of conifer encroachment, the density of encroaching conifers, and determine if synchronous temporal patterns of encroachment existed. By examining overall encroachment density among 310 meadows in the central Sierra Nevada, I found that essentially all high elevation meadows, up to the krummholz vegetation zone near treeline, have some degree of encroachment. Using natural variation in physical parameters among meadows, as well as variation in climatic conditions since the early 1900s, I was able to identify conditions conducive to conifer recruitment and/or survival. When I correlated annual recruitment with climatic conditions since 1930, conditions during the year of germination were unimportant with success instead depending on conditions during a 3-year seed production period and a 6-year seedling establishment period. Applying a temporal model based on climate during both seed production and seedling establishment periods to downscaled output from four global climate models indicates that the average meadow will convert to forest by the end of the 21st century. Given the ubiquitous nature of conifer encroachment in the central Sierra Nevada, in my second chapter I explored important life stages driving encroachment and the environmental factors influencing each. Using a 4-year study of 30 meadows in Yosemite National Park, I examined seed availability, germination, establishment, and adult survival. I found that seed was abundant throughout most of the interior of the meadows. Germination and seedling emergence showed high interannual variability, and locations with new germinants tended to be those that melted later and experienced more gradual summer dry-down than locations without new germinants. The mortality curve showed that mortality levels off around age 10, leading me to define establishment as survival to 10 years of age. During this establishment phase, most trees occurred in locations where soils reached relatively dry levels by the end of the summer, as opposed to locations that did not dry down as strongly, which had no establishing trees. Adult trees (>10 years) were also found in locations that experienced strong dry down. However, matrix population models built for Pinus contorta in these meadows showed that the asymptotic growth rate of tree numbers in meadows was more sensitive to adult survival than to germination or establishment. Thus, it appears that adult survival drives overall population growth, while microsite constraints on germinants and young seedlings drives the spatial distribution of encroaching trees. Having determined that adult survival has a strong influence on population growth rate of conifers in meadows, and assuming that years of poor adult tree growth indicate years of increased mortality, my third chapter focused on adult growth rates and the influence of climate. Using tree cores from 80 trees in 12 different meadows and from 144 trees in the forest immediately surrounding those 12 meadows, I was able to show that growth patterns of trees in meadows are more similar to each other than they are to growth patterns of trees in the adjacent forest. Thus, there appears to be a distinct topoclimate shared by geographically separate meadows. Meadow tree growth was sensitive to interannual climate variability both during the winter and during the growing season, with growing season effects dependent on the phase of the Pacific Decadal Oscillation. My work shows that conifer encroachment into subalpine meadows is common throughout the central Sierra Nevada. Both recruitment and adult growth are likely to increase with anthropogenic climate change, assuming that responses to climate are stationary. Even if these assumptions are violated, meadows are likely to change toward forest habitat in the short term. This reduction in meadows has management implications, and may result in the loss of important ecosystem services that meadows provide.