Snow depth effects on seedling recruitment at the Great Basin Desert, Sierra Nevada ecotone

Background/Question/Methods Snowfall is an important input of water in high-elevation and high-latitude arid and semi-arid ecosystems of western North America.  Climate change may alter snowfall amounts and the timing of snowmelt, thereby altering seedling recruitment success. Our objective was to experimentally test hypotheses linking snow climate (depth, melt timing), recruitment, and species composition driven by annual, interannual, and long-term snow climate forcing. Experiments were conducted at the ecotone between Great Basin Desert sagebrush steppe and Sierra Nevada conifer forest, near Mammoth Lakes, Mono County, CA.  The experiments addressed a series of questions, including:  How do snow depth and microhabitat affect water relations, survival, and recruitment for dominant shrubs and trees of a widespread ecotone type? And, what recruitment stages/processes are most sensitive to snow climate and microhabitat?  Experiments utilized large-scale (> km), long-term (>50 yr) snow fences. We used a combination of in situ  instrumentation and monitoring of natural recruits, and experimental plantings of seedlings of the two co-dominant shrub species at this site (Artemisia tridentata (Asteraceae) and  Purshia tridentata (Rosaceae)), to link variation in snow depth with soil water, recruitment niches (under adult shrubs and in open bare soil habitats), and community patterns.

Results/Conclusions Snowfall varied 2-fold between 2007 – 2011, and soil moisture following snowmelt mirrored snowfall trends.  Reproductive effort did not differ across snow depth treatments. There were over 8000 seedlings found in 6300 m² total research plot area in summer 2009, following about 1400 mm of winter snow and spring rain. This is much higher than in the drier years of 2007 and 2008. The frequency of seedlings of A. tridentata and P. tridentata were much lower on increased-depth (“+snow”) plots compared to ambient-depth, and reduced-depth (“-snow”) plots. Survival of the first year was lowest on ambient-depth plots for A. tridentata. Survival was highest for A. tridentata in open bare soil sites, and highest for P. tridentata under adults of A. tridentata. Survival of seedlings planted in various microhabitats was about 30% for A. tridentata and about 15% for P. tridentata; there were no microhabitat or snow depth effects for A. tridentata. Survival of planted seedlings of P. tridentata was somewhat higher on –snow plots compared to ambient and +snow plots. We conclude that spring rain contributes to higher seedling survival following a high-snowfall winter, microhabitat affects survival, and that survival of planted seedlings is not similar to survival of natural recruits.