Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 178

the density of trees and understory plants in forests
varies considerably from place to place and also fluctu-
ates through time . 15 Where tree density is high, there
may be hardly any understory herbs or shrubs because
of too much competition from the trees for light, water,
and nutrients; in more open forests, the abundance
and diversity of smaller plants are often much greater . 16
Lower tree density is found on sites that are too dry or
infertile to support a dense tree canopy, or in forests that
have been thinned by disturbances, as discussed below.
Snowdrifts persisting until early July restrict the growth
of some understory species, probably because the grow-
ing season is too short after the snow disappears. 17
Plant parasites reduce tree growth, produce deformed
trees, or even kill some trees—all problems in a setting
where wood production is a primary objective. other-
wise, native parasites add diversity to the forest com-
munity. two that are especially important on Rocky
Mountain trees are dwarf mistletoe and comandra blis-
ter rust, both of which have coexisted with their hosts
for millennia.
Dwarf mistletoe ( Arceuthobium americanum and other
species) is a flowering plant that is an obligate parasite
on pines (fig. 11.3). the pale green leaves and stems
are capable of some photosynthesis, but the plants
extend their roots into the host's inner bark (phloem)
and sapwood, where they obtain energy in the form
of carbohydrates, as well as water and nutrients. 18 tree
growth is slowed, and sometimes the trees die . 19 How-
ever, bird species richness can be higher with mistletoe
infection, suggesting that mistletoe eradication may be
undesirable where wood production is a less important
objective.
Mistletoe seed dispersal is accomplished by the ejec-
tion of seeds from the fruits by hydrostatic pressure,
with the seeds often traveling 20-40 feet before landing
on the leaves of another tree. the seeds have a slippery
coating and slide to the leaf base, where they germinate
in the spring. infection occurs when the root success-
fully penetrates the bark of 1- to 3-year-old twigs. the
mistletoe plant is hardly visible for several years, and
new seeds are not produced for 5-6 years. Both healthy
and stressed pine trees are susceptible to mistletoe
infection.
Forest managers sometimes work to reduce the abun-
dance of mistletoe to maximize tree growth. Herbi-

Fig. 11.3. Dwarf mistletoe is a parasitic flowering plant com-
mon on lodgepole and limber pine. the seed is spread to other
trees by explosive fruits, propelled by hydrostatic pressure.

cides are avoided, because they generally have not been
effective, are expensive to apply over large areas, and
have adverse secondary effects. Harvesting obviously
infected trees is the most common approach, but often
young mistletoe plants are difficult to see. Where foster-
ing the growth of a new generation of trees is a prior-
ity, clearcutting is the preferred option, whereby nearly
all trees are cut, whether they are infected or not. this
helps ensure that no infected trees are left behind. Most
likely, the prehistoric control of dwarf mistletoe was
periodic forest fires. Mistletoe now is probably more
widespread than ever before because of fire exclusion,
and also because some early logging practices removed
only the healthy trees.
comandra blister rust ( Cronartium comandrae ),
another common native plant parasite, is a fascinat-
ing fungus that requires three very different plant spe-
cies in close proximity to accomplish its life cycle—in
this case, lodgepole pine, big sagebrush (or a few other