Geoscience Reference
In-Depth Information
T
ree
-l
ine
d
ynamiCs
The.transition.from.subalpine.forest.to.alpine.tundra.(alpine.forest-tundra.ecotone,.
AFTE).is.one.of.the.most.distinctive.features.of.mountain.environments..Although.
the.ecological.dynamics.of.the.AFTE.are.inluenced.by.climate,.it.is.an.imperfect.
indicator. of. climatic. change.. Mechanistic. processes. that. shape. the. ecotone—seed.
rain,.seed.germination,.seedling.establishment,.and.subsequent.tree.growth.form—
depend. on. microsite. patterns.. Although. mechanistic. processes. are. similar. for. all.
AFTEs.in.western.North.America,.prior.climate,.geology.and.geomorphology,.and.
genetic.constraints.of.tree.species.create.geographic.differences.in.responses.of.eco-
tones. to. climatic. change.. Thus,. signiicant. variation. in. tree-line. spatial. composi-
tion.and.history.can.occur.within.and.among.mountain.ecosystems..We.examined.
tree-line. phenomena. and. limiting. factors. (Malanson. et. al.. 2007):. in. the. northern.
Rockies.centered.on.Glacier.NP,.the.southern.Rockies.centered.on.Niwot.Ridge.and.
Rocky.Mountain.NP.(with.insights.from.the.Medicine.Bow.Mountains,.Wyoming),.
the.Sierra.Nevada.centered.on.Sequoia-Kings.Canyon.NP,.and.the.Paciic.Northwest.
centered.on.Olympic.NP.
Biological. mechanisms. controlling. tree. establishment. in. the. AFTE. are. similar.
across.western.North.America..Climate,.energy,.and.water.directly.affect.these.pro-
cesses,.with.differing.outcomes.due.to.abiotic.differences.and.biogeographic.history,.
although.thresholds.for.effects.on.carbon.balance.of.seedlings.is.unknown.(Cairns.
and. Malanson. 1998).. Climatic. change. may. affect. successful. dispersal,. germina-
tion,.and.survival.by.modifying.soil.availability,.quality,.and.moisture;.precipitation.
fraction.arriving.as.snow;.snow.redistribution.and.melt;.extent.of.glacial.foreields;.
extent.of.permanent.snowields;.and.disturbance.regimes.
The.three-dimensional.pattern.at.tree.line.is.often.patchy,.including.krummholz.
and.dwarf.trees.(Allen.and.Walsh.1996),.with.expansion.often.facilitated.by.other.
plants.(Resler.2006)..The.formation.of.vegetation.structures.that.add.wind.protec-
tion,.snow.collection,.and.soil.development.allows.subalpine.forest.species.to.initiate.
patches.and.then.to.have.patches.expand.in.the.upper.tree.line.(Smith.et.al..2003)..
These.structures.can.affect.the.inilling.of.openings.in.tree.cover.in.the.lower.AFTE,.
sometimes.with.a.linear.coniguration.of.trees.(Holtmeier.1982).
Climate. and. variation. in. geomorphology. or. geology. control. the. dynamics. of. tree.
lines.(e.g.,.Butler.and.Walsh.1994)..For.example,.the.freeze-thaw.cycle.in.the.periglacial.
climate.of.tree.line.leads.to.soliluction,.soil.creep,.landslides,.and.erosion..Snow.ava-
lanches.and.debris.lows.cause.tree.mortality.and.limit.tree-line.elevation..Insects.and.
pathogens.are.spatially.diffuse.and.alter.tree.line.at.large.spatial.scales..Recent.examples.
include.outbreaks.of.mountain.pine.beetle.(MPB,.
Dendroctonus ponderosae
).in.high-
elevation. pine. species. (Logan. and. Powell. 2001). and. ongoing. infection. by. white. pine.
blister.rust.(
Cronartium ribicola
),.in.ive-needle.pines.(Tomback.and.Resler.2007).
At. regional. to. continental. scales,. control. of. AFTE. by. temperature. is. locally.
modiied. by. moisture. (Malanson. and. Butler. 2002).. In. warmer,. wetter. conditions.
signiicant.changes.in.the.structure.of.tree.line.in.all.WMI.bioregions.are.likely..The.
Paciic. Northwest. has. ample. moisture,. and. increased. snow. will. inhibit. the. estab-
lishment. and. growth. of. trees,. so. longer-lasting. snowpack. could. limit. the. expan-
sion. of. krummholz. higher. in. the. ecotone. and. could. expand. lower. meadows.. The.
