Subalpine Conifer Seedling Demographics: Species Responses to Climate Manipulations
Across an Elevational Gradient at Niwot Ridge, Colorado
Cristina Castanha1,4, Matthew J. Germino2, Margaret S. Torn 1,4, Scott M. Ferrenberg 3, John Harte 4, and Lara M. Kueppers 5
GC51D-0778
(1) Lawrence Berkeley National Lab, (2) Idaho State University, (3) University of Colorado Boulder, (4) University of California Berkeley, (5) University of California Merced
INTRODUCTION
Historical observations and models suggest that with climate
warming, treeline tree species ranges will shift upward in elevation.
For the migration of tree line, seedling establishment and survival is
essential, yet little is known about seedling survival in the initial life
history stages. At Niwot Ridge, Colorado, we compare germination
and survival of seedlings from two local species (limber pine vs.
Englemann spruce) sown along an elevation gradient spanning
subalpine forest to alpine tundra, and subjected to heating and
watering treatments.
Questions
Do limber pine and Englemann spruce differ in their ability to recruit
within and above their current range?
How will warming affect recruitment? e.g. will it lead to a contraction
of the warm edge and an expansion at the cool edge of the species’
ranges?
METHODS
Location: Niwot Ridge, Rocky Mountain Front Range, Co.
Design
• Three sites along an elevation gradient,
subalpine forest (3060m), treeline
(3430m), alpine tundra (3540m).
•Twenty 3m diameter plots per site.
• Seed from 2 local treeline species:
Limber pine and Engelmann spruce.
• Climate manipulations: Heat,
Heat+Water, Water, and Control.
Infrared overhead heaters mimic extra radiation from greenhouse
effect (target +4 ° C). Manual water additions of 3mm per week to
offset drying due to heating.
Seedling emergence and death surveyed weekly.
Limber pine
•Large seed (100mg)
•Bird-dispersed
•Drought-tolerant
•Pioneer species
Factorial ANOVA results
Englemann spruce
•Small seed (3 mg)
•Wind-dispersed
•Shade-tolerant
•Climax species
Survival
Cumulative (fraction of
germination
seed)
*
**
***
***
RESULTS
Timing of snowmelt
Heating advanced snowmelt by 3 months at the forest site, by
1 month at the alpine site, and by 4 months at the treeline site.
Germination
Heating advanced timing of germination.
Germination rates of Pine > Spruce and this difference was
greatest in alpine and lowest in treeline site.
Overall germination rates in forest ≈ treeline > alpine.
Species responses to treatment varied by site: In the forest
heat+water enhanced pine germination. In the treeline heat
enhanced pine germination but depressed spruce germination.
No effect of treatment was observed in the alpine.
Germination:
Survival
In addition to generally poor germination, spruce seedlings
suffer total mortality with heating -- except in the alpine site,
where watering enhanced survival.
Treatment effects on pine survival -- as a fraction of
germinants or as fraction of seed -- were similar in the alpine
and the forest sites, where water enhanced seedling survival.
In the treeline heat depressed survival as fraction of
germinants but promoted germination, and the net effect was
higher survival of pine under heating.
***
**
**
*
*** P<0.0001, **P<0.05, * P<0.1
CONCLUSIONS
Survival:
Fraction of germinants
Treeline
Site
Species
Treatment
Site x Species
***
Site x Treatment
Species x Treatment
Site x Species x Treatment **
Survival
(fraction of
germinants)
Fraction of seeds
Treeline
Species effect? Yes. The ability to germinate and survive -within and above-- their existing ranges is higher for limber pine
than Englemann spruce.
Elevation effect? Yes. Site had a complex effect on seedling
success: For both species germination was greatest and
survival was lowest in the treeline site.
Warming effects on geographic ranges?
•Expansion above high edge? Spruce: Unlikely. Pine: If
seeds are dispersed pine may invade alpine tundra.
Watering beneficial.
• Infilling at treeline? Spruce: Unlikely. Pine: Yes, with
warming.
• Contraction at low edge of species ranges? Spruce &
pine likely: Potentially big effect for spruce, a wide-ranging
dominant climax species.
∴Potential for big changes in composition of forest and tundra!
ACKNOWLEDGMENTS
This research was supported by the Office of Science (BER), U.S. Department of
Energy, Grant No. DE-FG02-07ER64457. Many thanks to S. Sawyer, A.Faist,
M.Redmond, S.Taylor Smith, S.Love Stowell, A. Allen, M. Anantharaman, W.Baird,
M.Barlerin, S.Barlerin, J.Curtis, M.Daly, E.Flemming, A.Farnham, H.Finkel,
R.Gaffney, A.Howell, M.McLaughlin (USFS CDA Nursery), A.Peterson, A.Qubain,
K.Riddel, L.Senkyr, X.Zhai, and F. Zuest, for their valuable assistance constructing
exclosures, monitoring cone maturity, collecting, processing, and sowing seed,
transplanting and surveying seedlings, and entering survey data.
For more information contact ccastanha@lbl.gov