Mountain Meadows and Snowglades - Mountains and Plains: The Ecology of Wyoming Landscapes - page 228

Forest Expansion into Meadows Response to Climate Change

Although some meadows appear to be stable features
of the landscape, others are being invaded by trees.
As noted, the origin of such meadows is often difficult
to explain, but the presence of young trees suggests a
change of one or more environmental factors over time.
the factors mentioned most often are less frequent fires,
which enables trees to grow where they could not sur-
vive when fires were more frequent; grazing by domestic
livestock, which can favor trees by reducing competi-
tion from herbaceous plants; and climate variation.
Meadows that are otherwise apparently stable could
also be undergoing very slow reforestation following a
severe disturbance that occurred long ago. An example
of slow invasion is at high elevations on parts of Libby
Flats in the Medicine Bow Mountains (see chapter 14). 17
in considering the effects of climate variation, sev-
eral scenarios are plausible. First, consider what might
happen if the climate were to become cooler. evidence
suggests that tree establishment after such disturbances
as fire would be slowed. For example, pollen data from
the Wind River Mountains indicate that forests gave
way to meadows about 3,000 years ago during a period
of cooling . 18 nocturnal summer frosts at this time
probably became frequent enough to restrict tree seed-
ling establishment as the older conifers died, thereby
enabling herbaceous plants to dominate instead after
fires or windstorms. the long-term persistence of pine,
spruce, and fir depends on seedling establishment, as
these trees lack the ability to sprout from stumps or
roots.
in contrast, if warming occurs, then summer frosts
at night become less frequent, and tree seedlings may
be able to survive more frequently (see chapter 14). A
contributing factor would be that warmer air holds
more water vapor, which itself is a greenhouse gas that
enables warmer temperatures at night. expansion of
lodgepole pine into dry meadows since about 1870 in
Yellowstone national Park apparently is driven by the
regional warming trend that began at the end of the
Little ice Ag e. 19 Similarly, trees became established in
some meadows in the Medicine Bow Mountains during
this time, apparently for the same reason. 20 Such obser-
vations suggest that tree expansion is likely to continue
as the global climate warms over the coming century.

Predicting the effects of warming (or cooling) on plant
growth in meadows is currently hampered by un-
certainties about how water availability will change
under future climates. if the annual precipitation
increases or remains about the same, then concomitant
higher temperatures will create drier conditions, because
the water will evaporate more rapidly—often before it
can enhance plant growth. the annual growth of many
plants would be slower if summer drought stress were
to begin earlier. if summer precipitation increases to a
level that more than compensates for increased evapo-
transpiration, then a warmer climate could result in a
longer growing season, depending on whether the addi-
tional water comes as snow or summer rain. if summer
rainfall were to increase sufficiently, then the growing
season for many mountain plants could be lengthened,
and the annual production of new plant biomass would
increase. Alternatively, more snow may not have much
effect on plant growth, because even low snowfall years
most likely would produce enough meltwater to saturate
the soils. the additional water could augment stream-
flow and reservoir levels but often would not increase
plant growth in the meadows.
of course, not all meadows and plant growth forms
would respond in the same way. ecologist Diane
Debinski and her colleagues studied dry, moist, and
wet meadows in the Yellowstone area during a transi-
tion from very wet conditions (1997-1999) to very dry
ones (2000-2007). 21 the amount of plant cover gener-
ally declined, especially in dry meadows, but changes
in forb and shrub cover varied considerably. Shrub
cover actually increased in several of their dry, moist,
and wet study areas. in addition to the direct effects
of drought, various studies have documented variable
plant responses to changes in the duration of snow
cover. 22
to learn more about the mechanisms whereby sub-
alpine meadows might respond to climate warming,
small-scale experiments have been conducted using elec-
tric heaters. one study in the colorado Rockies found
warming increased the water-use efficiency of mountain
big sagebrush, enabling higher rates of photosynthesis
early and late in the growing season. 23 the results suggest
that mountain big sagebrush could become more abun-