Threshold responses to cold and drought in tree seedlings vary among species at and above alpine treeline, but can they help explain establishment patterns?

Background/Question/Methods

Species’ distributional ranges may be limited by physiological response to acute stress events (drought, freezing) or chronic, sublethal limitations on photosynthesis and growth, or a combination of the two.  Few studies have evaluated the role of acute stress events for treeline.  We investigated differences in cold and drought threshold responses for three widespread subalpine conifer species and asked whether these responses varied with changing temperature and soil moisture.  We harvested first year seedlings of Pinus flexilis, a species able to colonize relatively cold and bright sites at treeline and a wide range of habitats; Picea engelmannii, common at treeline but considered less drought-hardy and limited to a narrower range of habitats; and Pinus contorta, which is dominates lower elevation forests.  Seedlings were grown from seed in a factorially warmed and watered common garden experiment at treeline and in the alpine at Niwot Ridge, Colorado.  Harvests occurred at approximately three weeks and eight weeks after germination.  We measured the threshold temperatures for freezing (freezing point) and physiological response to freezing (chlorophyll fluorescence, F_v/F_m) and threshold water status (turgor loss point) and osmotic and elastic properties that contribute to maintain tissue water content or turgor in spite of water loss (pressure-volume curves).

Results/Conclusions

Preliminary findings suggest Pinus flexilis had the greatest resistance to freezing (greater Fv/Fm after freezing), Picea engelmannii had the greatest freezing avoidance (indicated by freezing-point depression), and Pinus contorta had the least freezing resistance.  Overall drought avoidance (osmotic pressure) was greatest for Picea engelmanii while drought resistance (elasticity) was greatest for Pinus contorta.  Related studies found seedling survival at and above treeline was greatest in Pinus flexilis and least in Picea engelmannii.  The ranking of low-temperature and desiccation resistances or avoidances only partly corresponds to these interspecific differences in survival, indicating chronic growth responses may be relatively better predictors of seedling establishment patterns and treeline change. However, a degree of tradeoff between stress avoidance and tolerance exists, and this consideration may help in relating threshold physiological responses to seedling establishment.  Furthermore, freezing responses as well as pressure-volume traits acclimated to temperature variation among sites and treatments, and there was also some influence of plot water availability on freezing responses.  Interactions among low-temperature and desiccation responses indicate the importance of multiple stress effects for understanding treeline change.