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other.bioregions.are.drier,.so.increasing.moisture.would.improve.conditions.for.tree.
establishment. and. growth.. Improvement. would. be. greatest. in. the. southern. Rocky.
Mountains.and.least.in.the.Sierra.Nevada,.because.of.differences.in.substrate.devel-
opment. and. water-holding. capacity.. For. example,. tree. growth. increased. in. Sierra.
Nevada. tree. lines. for. warmer,. wetter. periods. in. the. twentieth. century. (Millar. et.
al..2004)..Millar.et.al..(2004).also.found.increased.tree.establishment.in.subalpine.
meadows.during.drier.periods.when.the.limitation.of.deep.snow.was.reduced..At.all.
three.sites,.the.effect.is.likely.to.be.largest.in.the.lower.AFTE.where.deeper.soils.and.
root.zones.can.utilize.increased.water.
In.warmer,.drier.conditions,.the.Paciic.Northwest.will.differ.from.the.other.sites:.
less.moisture.will.not.reduce.tree.species.establishment.and.growth.in.the.AFTE,.and.
reduced.snow.suppression.could.allow.expansion.in.the.upper.ecotone.and.inilling.
of.meadows.in.the.lower.ecotone..At.the.other.tree-line.sites,.warmer.and.drier.cli-
mate.could.reduce.tree.establishment.in.the.ecotone..While.much.of.the.evidence.for.
AFTE.response.suggests.upslope.advance.during.warmer.conditions,.in.many.places.
advance. may. be. moisture. limited.. For. example,. Glacier. NP. experienced. upward.
movement. of. the. ecotone. in. the. nineteenth. century. (Bekker. 2005),. while. during.
the.twentieth.century,.advance.was.limited.but.density.of.existing.patches.increased.
(Butler. et. al.. 1994;. Klasner. and. Fagre. 2002).. Tree-line. advances. in. the. latter. half.
of.the.twentieth.century.coincided.with.the.cool.(wet).phase.of.the.Paciic.Decadal.
Oscillation,.and.advance.stopped.coincident.with.the.warm.(dry).phase.of.the.1980s.
and.1990s.(Alftine.et.al..2003)..The.effects.of.general.warming.on.the.AFTE.must.
be.evaluated.in.the.context.of.multidecadal.modes.of.climatic.variability.
Advance.of.trees.into.tundra.can.alter.ecosystem.services,.such.as.carbon,.water,.
and.nutrient.cycles;.feedbacks.to.the.climate.system;.and.maintenance.of.biodiversity..
Tree.establishment.at.some.locations.has.increased.carbon.storage.through.accretion.
of.living.vegetation,.and.transfer.of.carbon.to.dead.biomass.and.soil.organic.matter.
results.in.a.potential.long-term.carbon.sink..Advance.of.trees,.or.change.in.pattern.
in.general,.also.has.the.potential.to.alter.snow.retention.and.hydrology,.with.implica-
tions.for.local.soil.moisture.and.nutrient.transport.
e CologiCal d isTurbanCe and i nTeraCTion of s Tressors
WMI.disturbance.research.addresses.spatial.scales.from.landscapes.to.regions..We.
have.used.both.process-based.and.empirical.models.to.estimate.the.extent.and.mag-
nitude. of. future. disturbances. across. the. West.. Data. sources. for. modeling. include.
observational.records.from.recent.decades,.and.in.the.case.of.ire,.historical.tree-ring.
reconstructions.dating.back.to.the.sixteenth.century.and.charcoal.deposits.in.sedi-
ments.extending.back.as.much.as.15,000.years.before.present.
Changing. disturbance. regimes. in. a. warmer. climate. are. likely. to. precipitate.
more.rapid.changes.in.ecosystems.than.the.direct.effects.of.warmer.temperatures..
Particularly.in.forest.ecosystems.dominated.by.long-lived.conifer.species.and.sub-
ject. to. stand-replacing. disturbances,. accelerated. species. turnover. will. occur. after.
severe. disturbance,. because. seedlings. are. less. resistant. to. changing. climate. than.
mature.individuals.(FigureĀ 8.3)..In.light.of.this.central.role.of.disturbance.in.Western.
forests,.WMI.research.has.recently.emphasized.quantifying.the.effects.of.climatic.
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