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(mgermino@usgs.gov)
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• An understanding of how alpine climates affect trees is important for predicting treeline and forest responses to climate change.
• Most studies on the physiological basis for treeline responses to climate have focused on growth limitations, whereas survival of extreme stress that could quickly result in mortality (acute event) has received less attention.
• Variability in survival of chilling or desiccation events should be most evident in very young seedlings, an important demographic stage for treeline change.
R ESEARCH QUESTION : Do threshold physiological responses to low temperatures or water status correspond to seedling establishment patterns at treeline ?
Specifically, do the thresholds correspond with:
• Temporal variation in temperature/moisture and mortality?
• Interspecific differences in establishment?
• Age-based differences in survival ?
• Seedling survivorship patterns were compared to lab-based measurements of physiological/threshold responses to temperature or water, on freshly collected field samples, including:
Thresholds in freezing:
• temperature causing leaf freezing (=freezing point)
• physiological response to freezing
Thresholds in water status:
• turgor loss point, TLP; and related physiological adjustments
• Subject seedlings were from 80 plots at treeline and in the alpine, at the
Alpine Treeline Warming Experiment in Colorado
(alpine.ucmerced.edu/pub/htdocs).
• Seedlings were in their first or second years since germination, and emerged from seed sown into 20 replicate plots per zone that were part of a fullfactorial of warming and rain-on treatments.
• Foliar temperatures were modeled from measurements of shallow-soil temperature; parameterized from several weeks of leaf-temperature data.
• Species :
Limber pine, stress-adapted and has ~70% seedling survival;
Pinus flexilis, PICO
Engelmann Spruce, common at timberline and treeline;
Engelmann Spruce, PIEN, 90% seedling mortality
Lodgepole pine, low-elevation forest species; rare at timberline; Pinus contorta, PICO, 30-40% survival
(photo to left, of 1 st yr seedling)
TABLE 1: ~35% of mortality occurs in cold months, in limber pine (2011/12 winter). 66% of plots cooled to -6
C,
29% cooled to <-18
C
Time period Proportion mortality (s.e.)
First winter
Second spring
Second fall
0.207 (0.016)
0.130 (0.013)
0.054 (0.009)
Second winter 0.044 (0.006)
FIG 1: Appreciable mortality in field occurred as limber pine leaf temperatures dropped from -6 to -9 ° C
(2011/12 winter)
0.15
0.10
winter winter+spring
0.05
0.00
-16 -14 -12 -10 -8 -6 -4
Minimum temperature ( ° C)
FIG 3: Physiological response
(photosynthetic, chlorophyll fluoresence) to deep cooling in lab freezer differs among species. See lower left of poster for species code. No recovery is observed for Fv/Fm <0.5. 50% loss of function (death) occurred at:
• -37 ° C in limber pine
• -23 ° C in spruce
• -15 ° C in lodgepole pine
FIG 4: Turgor loss points for first year seedlings in 2013 were very high
(wet), but decreased considerably with age.
In 2012, TLP was -2.3 MPa for year-old limber pine
FIG 2: Freezing points ranged from 6 to -9 ° C (in 2012 and ’13); no increase with seedling age.
FIG 5: Freezing and desiccation tolerance are positively related, through osmotic potential (not shown, r 2
= 0.53). Multiple regression
showed that fall/cold survival (Table 1 and Fig 1) is greater in plots where plant osmotic potential along with minimum temperature were greater.
1) …when seedlings are culled from treeline or alpine sites?
For limber pine:
• 2/3 of mortality in warm periods, 1/3 occurs when minimum temperatures are <0 ° C (Table 1)
• Turgor-loss point for limber pine in 2012 matches water stress when summer mortality occurred in Moyes et al 2012 (Fig 4), but mortality in 2013 probably occurred at much lower MPa than TLP
• Mortality increased when leaves cooled below cell freezing points
( >-9 ° C, Figs 1, 2)
• Low-temperature tolerances are much lower than temperatures experienced in field, except for in lodgeople (Fig 3).
Freezing avoidance and turgor-loss tolerance thresholds may help explain/predict some but not all mortality events .
2) …interspecific differences in establishment above forest?
(greatest for limber, lodgepole is a low-elevation species).
Interspecific variation in:
• …freezing point and establishment do not match well (Fig 2)
• …freezing tolerance matches lack of lodgepole at treeline (Fig 3)
• …turgor-loss tolerances match dominance at treeline (Fig 4)
Thresholds in stress tolerance but not avoidance match species’ differences in establishment.
3) …age-based differences in survival? (older=greater survival).
• Freezing points overall do not decrease with age (Fig 2)
• Freezing tolerance increases with age (not shown)
• Turgor-loss tolerance increases with age (Fig 4)
Thresholds in stress tolerances but not avoidance can help explain high initial mortality of seedlings at treeline.
Additional finding: cold- and desiccation tolerance are highly interrelated (Fig 5), suggesting that multiple stress interactions limit tree establishment at treeline.
Overview : Previous literature points to the importance of slowacting growth and carbon balance limitations on seedling survival and establishment. Here, we show that physiological stress thresholds can explain some but not all seedling establishment patterns. Chronic limitations in carbon balance, and slow-acting climate responses therefore likely play an important role in establishment patterns.
Funding and assistance: DOE Terrestrial Ecosystems Program
IAA#DE-SC0007071; USGS ; and the technicians in the ATWE team.