Geoscience Reference
In-Depth Information
in. temperature. of. this. magnitude. will. further. fragment. available. suitable. habitat,.
increase.the.risk.of.severe.ire,.change.the.timing.and.quantity.of.water.from.snow-
pack,. increase. winter. looding,. and. create. habitat. that. favors. introduced. species.
(Williams.et.al..2007a).
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WMI.places.a.strong.emphasis.on.understanding.the.direct.and.indirect.inluences.
of.climatic.variability.and.change.on.Western.forests,.and.has.contributed.to.recent.
efforts.linking.regional.warming.to.chronic.and.acute.increases.in.tree.mortality.in.
the.West..Even.small.changes.in.background.tree.mortality.rates.(rates.considered.
“normal”.or.not.steeply.declining),.when.compounded.over.time,.could.substantially.
alter.forest.structure,.composition,.and.function..For.example,.a.persistent.doubling.
of.mortality.rate.(such.as.from.1%.to.2%.per.year).ultimately.would.cause.a.greater.
than.50%.reduction.in.average.tree.age,.and.hence.a.potential.reduction.in.carbon.
storage.. Increasing. background. mortality. rates. could. indicate. forests. approaching.
thresholds.for.abrupt.die-back.
We.compiled.and.analyzed.long-term.data.from.unmanaged.old-forest.plots.across.
the. western. United. States,. inding. that. background. tree. mortality. rates. have. more.
than. doubled. in. recent. decades. (van. Mantgem. et. al.. 2009). (Figure 8.2).. Mortality.
rates.not.only.increased.in.each.of.three.subregions.(Paciic.Northwest,.California,.
and.interior),.but.also.within.dominant.taxonomic.groups.(
Abies
,.
Pinus
,.
Tsuga
,.and.
all.other.genera.combined).and.across.elevation.zones.and.tree.size.classes..In.sharp.
contrast,.tree.recruitment.rates.remained.unchanged..Consistent.with.what.would.be.
expected.when.mortality.increases.without.compensating.increases.in.recruitment,.
in.recent.decades.the.density.and.basal.area.of.old-forests.in.the.West.have.declined.
slightly.(van.Mantgem.et.al..2009).
We.examined.two.broad.classes.of.possible.causes.of.the.observed.increases.in.
tree.mortality.rates:.endogenous.processes.(those.inherent.to.forest.dynamics).and.
exogenous.drivers.(such.as.environmental.changes)..Several.endogenous.processes—
such.as.increasing.competition.among.trees,.successional.dynamics,.aging.cohorts.
of.trees,.and.effects.of.ire.exclusion—were.not.considered.to.be.causes.of.elevated.
tree. mortality.. Available. evidence. was. also. inconsistent. with. a. signiicant. role. for.
two.possible.exogenous.causes:.forest.fragmentation.and.air.pollution..For.a.number.
of. reasons,. regional. warming. appears. to. be. a. likely. contributor. to. the. widespread.
increases.in.tree.mortality.
Speciically,. from. the. 1970s. to. 2006. mean. annual. temperature. of. the. West.
increased.at.a.rate.of.0.3-0.4°C.per.decade,.even.approaching.0.5°C.per.decade.at.
the.higher.elevations.typically.occupied.by.forests.(Diaz.and.Eischeid.2007)..This.
regional.warming.has.contributed.to.widespread.hydrologic.changes.such.as.declining.
fraction.of.precipitation.falling.as.snow.(Knowles.et.al..2006),.declining.snowpack.
water.content.(Mote.et.al..2005),.earlier.spring.snowmelt.and.runoff.(Stewart.et.al..
2005),.and.a.consequent.lengthening.of.the.summer.drought.(Westerling.et.al..2006)..
Thus,. both. temperature. and. climatic. water. deicit. (evaporative. demand. that. is. not.
met. by. available. water). increased. over. the. study. period,. and. both. were. positively.
correlated.with.tree.mortality.rates.(van.Mantgem.et.al..2009).
