Temporal and spatial variability of the importance of regeneration microsite for Abies magnifica seedlings on arid range margins

Abstract

Microsites created by vegetation can influence the emergence and establishment of seedlings by altering local temperature, soil moisture, and ameliorating stressful environmental conditions. Forest structure and composition can be influenced by conifer regeneration in favorable patches, and in arid and semi-arid environments, microsites that ameliorate drought stress are important regeneration sites. <italic>Abies magnifica</italic> (California red fir) trees have a wide distribution across the western slope of the Sierra Nevada, but grow in a narrow band on the eastern slope of the Carson Range in the rain shadow of the Sierra Nevada crest. <italic>A.magnifica</italic> seedlings can be sensitive to summer drought, extreme soil temperatures, and intense solar radiation. Microsite amelioration may aid <italic>A.magnifica</italic> establishment and survival, and my goal was to understand the role of vegetative microsites in successful <italic>A.magnifica</italic> regeneration under some of the most arid conditions the species encounters. I compared <italic>A.magnifica</italic> natural seedling establishment in open and shaded microsites across an altitudinal gradient of environmental stress on the eastern slope of the Carson Range. Seedling distribution, growth, soil moisture and soil temperatures were sampled in the summer of 2011 and 2012. In the fall of 2011, <italic>A.magnifica</italic> seeds were planted in open and shrub-shaded microsites across the altitudinal range and monitored for emergence and survival the following summer. 2011 and 2012 differed in winter weather conditions, and temperature stress and soil moisture limitations were more extreme following a winter of low snowpack. In 2011, (a year with higher than average snowpack), seedling observations and soil moisture were uniform among microsites and elevations. However, in 2012, (a year with lower than average snowpack), soil moisture was non-uniform across microsites and elevations and seedling observations were higher in microsites with greater soil moisture. Specifically, more seedlings were observed in microsites with greater subsurface moisture, which were open microsites at high elevations and shrub-shaded microsites at low elevations. A very low proportion of seedlings emerged in experimental plots and nearly complete mortality signified significant issues with environmental stress in 2012. Poor success could also be related to seed genetic variability, as <italic>A.magnifica</italic> seeds were harvested from trees on the western slope of the Sierra Nevada and may not have been as genetically tolerant of drier conditions experienced on the eastern Carson Range. Fewer first-year seedlings were observed in 2012 than 2011, suggesting pulses of seedling emergence in wet years and declines in dry years. Facilitative effects of shrubs varied spatially in the low snowpack year, with shrubs tending to have greater positive effects on soil moisture and temperature at low elevations, where soil moisture was chronically low. The importance of shrub facilitation changed over the altitudinal gradient of stress, consistent with the stress gradient hypothesis. Long-term declines of winter snowpacks could result in declines of <italic>A.magnifica</italic> seedling emergence, which could alter the overall structure of <italic>A.magnifica</italic> forests. Facilitative effects were stronger in years with greater environmental stress and weaker in years with less environmental stress, showing evidence of temporal variability of the stress gradient hypothesis. Thus, shrub cover may be vital for maintaining forest structure and composition at low elevations if snowpacks decline in a changing climate. These results highlight the temporal and spatial shifts of shrub facilitation on the emergence and establishment of <italic>A.magnifica</italic> seedlings, and can help direct future forest management.