Life stage filters on conifer encroachment into subalpine meadows in the central Sierra Nevada, California
Mountain meadows provide a number of important ecosystem services, including maintenance of biodiversity, carbon sequestration, flood mitigation, and water storage, as well as aesthetic value. Encroachment of woody species into mountain meadows would bring a shift from a graminoid/herbaceous community to one dominated by woody species, potentially diminishing a meadow’s ability to provide critical ecosystem services. Meadows in the Sierra Nevada mountain range in California have a long history of conifer encroachment. In order for conifers to successfully encroach a meadow, they must pass through several life stages: seed production, germination, establishment, and survival of established individuals. This study aims to explore these four life stages and the environmental factors influencing each. We conducted field surveys of 30 subalpine meadows in wilderness areas of Yosemite National Park; five meadows were instrumented with seed traps and soil moisture and temperature sensors. We documented new germinants and survival of encroaching conifers of various ages over four years of repeat surveys. We then correlated temporal patterns of encroachment with historical climate and correlated spatial patterns with field measurements of soil moisture, melt date, and seed availability.
Seed abundance differed substantially between two years, but was abundant up to 40 meters from the nearest cone-bearing tree. Abundance of new germinants was also highly variable, with 2009 germinant abundance an order of magnitude higher than the following three years. Presence and abundance of new germinants was somewhat spatially consistent across years (r=0.35 ± 0.04). High germinant abundance was in locations with late melt dates and slow soil dry down.
96% of conifer mortality within meadows occurred in the first 10 years following germination, leading us to define establishment as reaching 10 years. Unlike germination, establishment showed no spatial consistency (r=0.08 ± 0.01). This could indicate that microsite has little effect on seedling establishment, or that there are strong interactions between microsite and climate. While most mortality occurred within a conifer’s first 10 years, the 2011-2012 season was an exception to this, with trees up to 30 years old dying over a winter characterized by low snowpack. It appears that while background mortality is low among established conifers, periodic mortality events may play an important role. Our results give insight into factors influencing life stages of conifers encroaching into an ecologically important system.