Sensitivity of limber pine (Pinus flexilis) seedling physiology to elevation, warming, and water availability
across a timberline ecotone
Andrew B. Moyes1, Cristina Castanha2, Scott M. Ferrenberg3, Matthew J. Germino4, and Lara M. Kueppers1
1
University of California, Merced; 2 Lawrence Berkeley National Laboratory; 3 University of Colorado, Boulder; 4 Idaho State University
2. Rationale:
1. Objective:
- Characterize the sensitivity of limber pine seedling physiology to manipulations of
climate (heating and watering) across an elevation gradient extending above and
below the current treeline.
- Climate change is likely to cause warmer temperatures in the Rocky Mountains via increased incoming long-wave radiation associated with accumulated greenhouse gases
such as CO2. Heating may lead to soil drying and reduced water availability for plants, or this effect may be alleviated by increases in precipitation.
- Alpine treelines are defined where climatic gradients limit tree growth and survival. Physiological sensitivity to environmental variables determines spatial patterns of
seedling establishment success, leading to maintenance or shifts in tree ranges with climate change. To predict and model these potential changes we need to understand the
physiological mechanisms underlying the elevational limits of Rocky Mountain forest trees.
3. Experimental Treatments:
SAMPLING PERIODS
-Study sites at three elevations from above treeline to forest (ALP, USA, and LSA) at Niwot Ridge,
Colorado USA
ALP
- Heating and watering treatments and controls (C, W, H, and HW) replicated (n = 5) within sites
ALP - W
ALP - H
ALP - HW
- Limber pine locally collected and seeded into plots, protected from herbivory
USA
ALP: Alpine – Above treeline
4. Measurements:
USA - W
- “Whole canopy” seedling gas exchange measured using Li-6400
- Net assimilation under saturating light
- Dark respiration
- Net assimilation with inlet air stream supplied from N2 cylinder (index of
photorespiration)
USA - H
USA - HW
LSA
LSA - W
-Afternoon stem water potential measured with a pressure bomb
USA: Upper Subalpine – Just below treeline
LSA - H
LSA - HW
HEATING:
WATERING
Heater output constant to simulate increased IR flux with
greenhouse gases
Summer soil moisture lowest in heated plots
- Chlorophyll efficiency (Fv/Fm) at night
Heaters raised soil temps (7 cm depth) by 1.2 – 3.2 °C
Heater effects were variable:
- stronger with decreasing elevation
- reduced by wind (sensible heat loss)
- not strongly impacted by observed soil moisture variation
LSA: Lower Subalpine – Within forest
5. Temperature explained a limited amount of physiological variation
In general, photosynthesis decreased
and respiration increased with
increasing leaf temperature among all
elevations and treatments.
These data suggest a cool optimum
leaf temperature, and a potential for
seedling carbon balance to be
negatively impacted by warming.
6. Water availability was critical for seedling carbon balance
8. Temperature, moisture, and seedling age interacted for strong treatment effects
Early snow melt led to
earlier germination in
heated plots and at lower
elevations. As seedlings
survived into summer, soil
moisture depletion and
seedling age were
associated with reduced
photosynthetic capacity. By
the end of summer, those
seedlings which had
germinated early (reaching
ages >80 days) in heated
plots and at low elevation
were exhibiting very low
rates of gas exchange.
Decreasing soil moisture was
associated with limited rates of
photosynthesis (Anet) and
respiration (R).
However, most of the variation in
photosynthesis and respiration rates
was not explained by temperature
Assimilation in the absence of O2 (AN2,
without photorespiration) was ~27%
higher than assimilation in air (Aair) for
all elevations and treatments.
Although photorespiration increases
with rising temperature at the tissue
level, our results at the population
level showed that seedlings acclimated
to cooler conditions were losing more
carbon via photorespration following
oxidation of RuBP than plants in
warmer conditions.
Elevation and heating
treatments led to differences
in growing season (e.g. snowfree period) length and timing
Drying soils led to negative stem
water potentials (Ψ, inset), which
limited stomatal conductance (gs)
and photosynthesis (Anet).
7. Chlorophyll efficiency (Fv/Fm)
showed no indication of
photoinhibition. Measurements
from each of the four treatments at all
three elevations were similarly near 0.8
(not shown).
9. Conclusions: Increasing temperature led to decreasing carbon balance (reduced photosynthesis
and enhanced respiration). Soil moisture limitation is likely a more important limitation to seedling
establishment than previously thought. Lengthening of the growing season by early snow melt is
detrimental to seedling carbon balance by extending the exposure of seedlings to high summer
temperatures and moisture limitation. Range shifts of limber pine with climate change will be driven
by responses to both temperature and moisture, as well as their interactive effects.
Acknowledgements: Funding was provided by a DOE PER grant (DE-FG02-07ER-64457). Thanks to the
Mountain Research Station (UC, Boulder) and the many field technicians who helped set up and conduct this study.
Correspondence:
email: amoyes@ucmerced.edu