PHYSIOLOGICAL STRESSES OF LIMBER PINE SEEDLINGS AT AND ABOVE TREELINE IMMEDIATELY FOLLOWING NATURAL AND EXPERIMENTALLY ADVANCED SNOWMELT Andrew B. Moyes1, Matthew J. Germino2, Lara M. Kueppers1, 3
(1) University of California, Merced, CA 95343, (2) US Geological Survey, Boise, ID 83706, (3) Lawrence Berkeley National Laboratory, Berkeley, CA 94720
Abstract: Treeline positions are anticipated to shift uphill in response to climate warming, a prediction which depends on future seedling recruitment at and above the current distribution limits of subalpine trees. To examine the role of cold temperature in seedling establishment at treeline, we measured physiological performance of limber pine seedlings after surviving their first winter in heated and ambient temperature plots within the Alpine Treeline Warming Experiment. Fluorometric measurements of maximum photosystem II efficiency (Fv/Fm) indicated severe photoinhibition 10 days following melt (Fv/Fm near 0), but photoinhibition diminished over the subsequent 10 days. Over the same period, seedlings showed highly variable degrees of moisture stress in midday stem water potentials (Ψ) associated with frost drought and freeze‐thaw embolism, despite consistently abundant afternoon soil moisture. Many seedlings would not exude water under maximum measurement pressure (Ψ < ‐5 MPa) across all sampling dates (up to 60 days following melt), indicating high spatial variability in cold‐associated moisture stress, and possibly limited xylem conduit refilling. Net CO2 assimilation appeared to be independently limited by photoinhibition and moisture stress. Although these stresses were not significantly impacted by the timing of snowmelt or whether seeds were sourced from high or low elevation provenances, the observation of severe cold stress indicates that seedling establishment above treeline will be sensitive to future patterns of snowmelt and air temperature. Ambient
Heated
Objective: to characterize photoinhibitory and moisture stress experienced by overwintering seedlings at and beyond the upper elevation limits of trees following natural and experimentally advanced snowmelt. Two seed source provenances were compared, collected from local low and high elevation sources of limber pine to investigate genetic differentiation in susceptibility to spring cold stress. Ambient Heated
High Elev. source
Low Elev. source
Methods: ‐ Infrared heaters on 10 of 20 study plots at treeline and in the alpine tundra
‐ Fluorescence measurement of maximum photosystem II efficiency (Fv/Fm) at night
‐ Midday stem water potential using a pressure bomb
‐ Net assimilation and dark respiration with a Licor 6400
Comparison of the temperature time series of the two study sites. Average day of melt for 10 replicate plots is shown with a large circle. Small circles show dates of first and last plots to melt. The treeline site (USA) melted a few weeks later than the alpine site (ALP), and with a larger effect of the heating treatment on melt day. Alpine plots experienced freezing temperature due to low insulating snow cover caused by high wind over all plots.
Upcoming additions to this data set:
‐ Complimentary autumn shoulder season data (collected last week)
‐ Temperature response curves during summer
‐ Pressure‐Volume curves
‐ Needle freezing point analysis
Ambient Heated
High Elev. source
Low Elev. source
Impact of photoinhibition (as reduced maximum photosystem II efficiency (Fv/Fm)) and water stress (midday stem water potential (Ψ)) on net assimilation (Anet) and dark respiration (R) for individual seedlings. Low values of Fv/Fm were associated with low values of assimilation and respiration, while low water potential may have limited assimilation to a greater extent than respiration. A multiple regression approach will be used to identify relative limitations to carbon balance by photoinhibition and water stress.
Photoinhibition (as reduced maximum photosystem II efficiency (Fv/Fm)) and water stress (midday stem water potential (Ψ) were not correlated among seedlings, indicating separate predisposing conditions. Maximum photosystem II efficiency (Fv/Fm) was low following snowmelt, indicating strong photoinhibition of seedlings upon exposure to cold night air and bright daytime sun. Fv/Fm rose to healthy levels (near 0.8) over the following few weeks. Midday stem water potential (Ψ) was variable across sites, treaments, and provenances, without a clear association with minimum temperature or time since melt. Values of Ψ on the dotted line represent seedlings that did not exude water under maximum chamber pressure, and may have lost hydraulic conductivity.
Midday stem water potential was not strongly associated with soil water content, and was likely affected by frost drought and formation of freeze/thaw emboli as soils froze at night. Only some individuals within 1 m2 study plots were affected, possibly indicating the importance of soil water availability at smaller spatial scales.