Responses of subalpine tree recruitment to warming within and above current altitudinal ranges

Background/Question/Methods

Niche models and paleoecological observations indicate that climate change will alter the geographic distributions of plant species. An upward shift in the ranges of subalpine forest trees would displace alpine species and alter high elevation hydrology and carbon cycling. We established the Alpine Treeline Warming Experiment at Niwot Ridge, CO, to examine effects of climate warming on tree seedling establishment near the lower limit of subalpine forest, at upper treeline, and in the alpine. We are using infrared heaters to increase growing season surface soil temperatures, and to lengthen the growing season. The warming treatment is crossed with a summer soil moisture manipulation to distinguish effects due to higher temperatures from those due to drier soil. Each plot is a common garden sown with two populations each of limber pine (Pinus flexilis) and Engelmann spruce (Picea engelmannii).

With multiple cohorts of seedlings (2010-2012), we are asking, (1) How does warming alter seed germination and seedling recruitment rates within and above the altitudinal limits of subalpine forest? And, (2) Can differences in first year recruitment and survival between years and treatments be explained by variation in soil moisture and temperature during the growing season?

Results/Conclusions

Preliminary analyses of 2010-2011 data show that heating reduced germination and initial seedling survival for both species and populations near the low elevation limit of subalpine forest, consistent with expectations of reduced recruitment at lower elevations with warming. At upper treeline, heating effects on seed germination and survival were contingent on year and species, with increased limber pine but decreased Engelmann spruce germination with warming in 2010. In the alpine, contrary to expectations of increased recruitment with warming, experimental heating did not increase germination or survival for any species or population. Soil moisture at 5-10cm depth is a key covariate of seedling survival in the first growing season, with limber pine seedling photosynthesis declining on days with soil moisture below ~0.16 m3/m3, and stopping completely with soil moisture below 0.8 m3/m3; 2010 survival is correlated with number of days below 0.8 m3/m3 moisture across all sites. Further, Engelmann spruce survival to two years in treeline and alpine sites occurred only in watered plots. Taken together, these preliminary analyses suggest that warming may result in lower elevation range contraction but not upper elevation range expansion of subalpine trees in the absence of additional growing season soil moisture.