Warming reduces cold stress but increases moisture stress for establishing limber pine within and above alpine treeline

Background/Question/Methods

Climate warming is anticipated to promote an upward redistribution of subalpine tree species, but this depends on seedling establishment within current alpine ecosystems.  To evaluate this potential, we examined warming affects on physiological function of establishing limber pine (Pinus flexilis) seedlings at treeline and in the alpine at Niwot Ridge CO, USA in 2012.  We measured thresholds and responses to environmental stresses in ambient and experimentally warmed plots at the two sites, including photoinhibition and hydraulic status related to cold and bright sun exposure, tissue freezing point, responses of photosynthesis and respiration to leaf temperature, and tissue vulnerability to low midsummer soil moisture.  We then used continuous soil microclimate data to estimate the effect of warming on occurrence of abiotic conditions associated with each kind of stress. 

Results/Conclusions

Preliminary results indicated that photoinhibition occurred when nighttime soil temperature was below 5 °C, and corresponded with reduced photosynthesis, but lasted only a few days.  Prolonged low stem water potentials (<-3 MPa) occurred when soil underwent repeated freezing.  Soil moisture became limiting below 0.12 m³ m^-3, corresponding with daytime water potential at average turgor loss point of -2.3 MPa.  Tissues froze at -7.8 °C, which occurred only in the alpine site.  Net photosynthesis was consistently optimal near leaf temperature of 21 °C, with broad response curves indicating little effect of warming on carbon supply.  Photosynthetic temperature acclimation was not observed.  Dark respiration at 10 °C decreased with higher growing temperature, but respiratory sensitivity to T (Q­₁₀) was unresponsive to growth temperature. 

Continuous microclimate data revealed that the alpine environment exposed seedlings to greater cold stress and early summer moisture stress than treeline due to lower snow accumulation and earlier melt.  Warming enhanced exposure to soil moisture stress, but decreased likelihood of photoinhibition and hydraulic problems associated with nighttime cold exposure.  Thus, while warming may alleviate stresses associated with low temperature, it may exacerbate exposure to low soil moisture, limiting the rate and extent of upward range shifts by limber pine.