TAKING THE PULSE OF MOUNTAINS: ECOSYSTEM RESPONSES
TO CLIMATIC VARIABILITY
DANIEL B. FAGRE
, DAVID L. PETERSON
and AMY E. HESSL
U.S. Geological Survey, Northern Rocky Mountain Science Center, West Glacier, MT 59936, U.S.A.
E-mail: dan_ email@example.com
USDA Forest Service, Paciﬁc Northwest Research Station, 400 N. 34th Street, Suite 201,
Seattle, WA 98103, U.S.A.
Department of Geology and Geography, West Virginia University, Morgantown, WV 26505, U.S.A.
Abstract. An integrated program of ecosystem modeling and ﬁeld studies in the mountains of the
Paciﬁc Northwest (U.S.A.) has quantiﬁed many of the ecological processes affected by climatic
variability. Paleoecological and contemporary ecological data in forest ecosystems provided model
parameterization and validation at broad spatial and temporal scales for tree growth, tree regener-
ation and treeline movement. For subalpine tree species, winter precipitation has a strong negative
correlation with growth; this relationship is stronger at higher elevations and west-side sites (which
have more precipitation). Temperature affects tree growth at some locations with respect to length
of growing season (spring) and severity of drought at drier sites (summer). Furthermore, variable
but predictable climate-growth relationships across elevation gradients suggest that tree species re-
spond differently to climate at different locations, making a uniform response of these species to
future climatic change unlikely. Multi-decadal variability in climate also affects ecosystem processes.
Mountain hemlock growth at high-elevation sites is negatively correlated with winter snow depth and
positively correlated with the winter Paciﬁc Decadal Oscillation (PDO) index. At low elevations, the
reverse is true. Glacier mass balance and ﬁre severity are also linked to PDO. Rapid establishment
of trees in subalpine ecosystems during this century is increasing forest cover and reducing meadow
cover at many subalpine locations in the western U.S.A. and precipitation (snow depth) is a critical
variable regulating conifer expansion. Lastly, modeling potential future ecosystem conditions sug-
gests that increased climatic variability will result in increasing forest ﬁre size and frequency, and
reduced net primary productivity in drier, east-side forest ecosystems. As additional empirical data
and modeling output become available, we will improve our ability to predict the effects of climatic
change across a broad range of climates and mountain ecosystems in the northwestern U.S.A.
During the past two decades, many different approaches have been used to predict
the potential response of ecosystems to climatic variability and change. Typically,
various general circulation models (GCM) have been used to establish scenarios
that reﬂect the effect of increased greenhouse gases on temperature and precip-
itation. Modeling at various spatial scales (initially global to continental) (e.g.,
VEMAP members, 1995; Cramer et al., 2001) has been used to estimate changes
in vegetation cover, species distribution, and carbon balance. Recently ecosystem
and vegetation models have been linked explicitly with GCM scenarios to provide
Climatic Change 59: 263–282, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.