Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 343

Forest Succession and Disturbance
The dominance of a particular plant type or species depends on its relationship with
the physical environment and other organisms. The prevalence of either conifers or
broadleaf deciduous trees in a given forest depends on which are better suited to the
local conditions, including their relationship with each other. In marine and higher-lat-
itude climates, deciduous tree species are typically less shade tolerant than conifers,
and reproduce poorly under closed forest canopies. These conditions hasten the rate of
species turnover and relegate broadleaf deciduous trees to early successional stages. In
midlatitude deciduous forests, these successional trends are reversed, with pines and
hemlock as the early successional shade-intolerant species. The presence of these suc-
cessional patterns are, however, highly variable and may be locally dependent on envir-
onmental conditions and human activities.
FIGURE 7.5 Cross section of major plant communities in (A) the equatorial Andes and (B) subtropical
Andes at 16°S. Symbols represent: (1) semideciduous lowland forest; (2) tropical lowland forest;
(3) tropical lower-montane forest; (4) tropical upper-montane forest (cloud forest); (5) humid
alpine communities (páramos); (6) evergreen shrub and Polylepis woodland; (7) thorn and succu-
lent woodlands; (8) thorn and succulent shrubs; (9) desert; (10) moist grass puna; (11) dry thorn
puna; (12) moist lowland savanna; (13) snowline; (14) cloud belt. (From Troll 1968.)
Many western North American forests have experienced dramatic changes in their
composition and structure over the past 100 years as a result of fire exclusion (e.g.,
Covington and Moore 1994). The outcome is an acceleration of succession toward more
shade-tolerant species and a corresponding decrease in community and landscape di-