Interacting effects of climate on life stages of conifers encroaching into subalpine meadows
High elevation meadows provide important ecosystem services, such as maintenance of biodiversity, carbon sequestration, regulation of water storage and release, and aesthetic value. However, many meadows are experiencing woody plant encroachment, potentially diminishing the ability of meadows to provide these services. To predict the persistence of mountain meadows into the future, we need to understand the drivers of encroachment. Of particular importance are the responses of woody plants to climate variability and change. Here, we evaluate the responses to climate of conifer seedlings and adults encroaching into subalpine meadows in the central Sierra Nevada, California. We conducted field surveys of 30 subalpine meadows in wilderness areas of Yosemite National Park to examine seedling survival and establishment. We then correlated temporal patterns of recruitment with historical climate to further determine the relationship between climate and successful recruitment. We combined these methods to evaluate seedling dynamics with data from 244 tree cores to examine adult growth patterns in response to climate. This combination of seedling dynamics and adult growth patterns allowed us to assess the interacting effects of climate across conifer life history.
We found high interannual variability in new germinant abundance, with 2009 abundance an order of magnitude higher than the following three years. Locations with new germinants melted on average 3.8 days later and maintained near saturated soil moisture levels one month later following snowmelt than locations without new germinants. Unlike germination, establishment showed no microsite consistency. Over the 20th century, recruitment in meadows depended on climate conditions during both the seed production period (3 years prior to germination) and the seedling establishment period (6 years following germination), while not influenced by conditions during the year of germination. High snowpack during reproduction and establishment periods increased recruitment, as did high summer precipitation and high summer temperatures during establishment.
Adult trees in meadows were sensitive to climate conditions. Unlike germination and recruitment, which responded positively to high April 1st snow water equivalent (SWE), adult meadow tree growth was limited by high SWE. Adult tree growth was positively correlated with previous fall precipitation, negatively correlated with previous winter and spring precipitation, and positively correlated with spring temperatures. Reduced snowpack with climate change may slow the pace of recruitment in meadows but increase tree growth rates. Higher temperatures may counteract the effects of lower snowpack.