Geography in America at the Dawn of the 21st Century

spatial scale, and disturbance type (Veblen 1992). For
instance, D. A. Brown (1993) found that disturbance was
a major factor affecting the local and landscape patterns
of Great Plains grasses. For forests, which have received
the major share of recent research efforts, studies of
within-stand dynamics focus mainly on endogenous
disturbance in the form of fine-scale tree-fall gaps.
However, forest dynamics are also studied in relation
to coarse-scale exogenous disturbances, environmental
fluctuations, and demographic changes of the tree
species related to whole stand (cohort) replacement patterns.
Data on tree size and age structures and tree spatial
distributions have facilitated interpretations of ecological
processes and stand history in a range of forest
types across North America (Taylor and Halpern 1991;
Frelich and Graumlich 1994; Daniels et al. 1995;
K. C. Parker et al. 1997b; Arabas 2000; Ziegler 2000),
and elsewhere (Grau et al. 1997; Enright and Goldblum
1998; Rigg et al. 1998). Occasionally, such approaches
have been combined with monitored permanent plots
(Kupfer and Runkle 1996). Kellman and Tackaberry
(1993) evaluated differential mortality due to fire and
tree-falls in tropical riparian forest fragments in Belize.
An important research theme in the study of gap
dynamics has been species partitioning of gap environments
that might be explained by differences in life history
traits. For example, tree-ring studies in Argentina
and New Zealand show species-specific differences in
responses to gap creation (Rebertus and Veblen 1993;
Runkle et al. 1995). Demographic parameters such as
mortality and recruitment rates have been measured for
tree species in tropical rain forests of Australia (Herwitz
and Young 1994), temperate forests of western North
America (Daniels and Klinka 1996), subalpine forests in
China (Taylor et al. 1996), and timberline forests in Peru
(K. R. Young 1991). Malanson (1996) used models to
assess the effects of demographic parameters on standlevel
dynamics. The composition of recovering fragmented
and human-altered forests can also be affected
by edge effects and by the available species pools (Beatty
1991; Goldblum 1997; Kupfer et al. 1997).
Research on the effect of coarse-scale disturbances
on vegetation dynamics includes detailed analyses of
how stands develop following fires, windstorms, forest
dieback, insect outbreaks, and geomorphic events, as
well as studies of the spatial and temporal variations in
the disturbance per se. An important theme has been the
heterogeneity of burns and of post-fire patterns across a
wide range of ecosystem types from tropical ecosystems
(Horn 1997; Keating 1998) to eucalypt woodlands
(Enright et al. 1997), the prairie-forest ecotone (Leitner
et al. 1991), chaparral and sage scrub (O’Leary 1990b;
Biogeography ·19
Minnich and Bahre 1995), oak woodlands (Callaway and
Davis 1993), and pine forests (Parker and Parker 1994).
A common landscape pattern in the western US is
a shift from park-like stands to denser forests of ponderosa
pine, which coincides with changes in livestock
grazing, decreased ignitions by Native Americans, fire
exclusion, and climate changes (Savage 1991; Mast et al.
1998). Changes in fire regimes related to settlement have
been reported throughout North America (Goldblum
and Veblen 1994; Grissino-Mayer et al. 1995; Taylor and
Skinner 1998; Wolf and Mast 1998), for temperate latitudes
of South America (Szeicz et al. 1998; Veblen et al.
1999), and for the neotropics (Horn 1998). Wind disturbance
may preferentially damage larger, faster-growing
early successional species and accelerate succession
towards later stages (Veblen et al. 1989a; Dyer and Baird
1997b). Similarly, Hadley and Savage (1996) suggest that
wind disturbance can hasten the development of forestinterior
characteristics by creating gaps and heterogeneity.
Repeated wind-caused damage was the main
determinant of forest structure on Tierra del Fuego
(Rebertus et al. 1997), as were the hurricanes that affect
Puerto Rico (Scatena and Lugo 1995).
In a southern California montane forest, Savage
(1994) attributed widespread tree mortality to fire suppression,
air pollution, drought, competition, and insect
infestation. Tree-ring reconstructions have proven useful
for understanding interactions among fire regimes,
forest health (disease and insect outbreaks), and topographic
controls (Hadley and Veblen 1993; Hadley 1994;
Veblen et al. 1994).
Reviews by Malanson (1993) and K. C. Parker and
Bendix (1996) evaluated interactions between landform
and vegetation processes, identifying riparian environments
as a major focus of research. This approach
addresses the influences both of the physical environment
on vegetation (Bendix 1994), and of vegetation
on flooding and sedimentation (Malanson and Kupfer
1993). Vegetation composition, structure, and succession
have been attributed to characteristics of gravel bars
and flood plains in Montana (Malanson and Butler
1991), Iowa (Craig and Malanson 1993), Wyoming
(Miller et al. 1995), Colorado (Baker 1990), and Kenya
(Medley 1992). Floods have a dominant influence on
many types of riparian vegetation (Kupfer and Malanson
1993; Birkeland 1996), including that of abandoned
channels (Shankman and Drake 1990). Flooding due
to beavers also has important impacts on vegetation
(Butler 1995).
Mass movements can have major influences on
vegetation (Hunter and Parker 1993; K. C. Parker and
Bendix 1996). Impacts of avalanches have been modeled