ENVIRONMENTAL CONSEQUENCES in rocky mountain coniferous ...

canopy. Light then indirectly causes dessication and lethal effects through in-
creasing transpiration and evaporation.
The intensity of light and radiation outside of the visible spectrum increases
the temperature of plants and other objects. Plants and animals have many mechanisms
for controlling or reducing the effects, some of which will be discussed later.
Light is known to have potentially lethal effects on plants and to be effective
in control ling or a1 tering plant distribution. Reports indicate that high 1 ight
intensities injure the photosynthetic process of some sensitive species (Ronco
1 970), but this damage may be related more to 1 ight qua1 i ty than intensity. Ultra-
violet radiation is increased considerably at high elevations and apparent1 y damages
nucleic acids in the cells of some species (Caldwell 1971). Ultraviolet light appears
to affect tree development and possibly influences species distribution (Klein 1978).
Other wavelengths of light, such as far red, seem to reduce germination and affect
other physiological processes. Many studies have shown shade to be beneficial to
growth and survival. However, it is not stated whether 1 ight, temperature, or some
other operational factors were a1 tered,
Heat
Heat and energy exchange that changes temperature regimes are biologically
significant. Temperature describes a body's potential for exchanging heat with its
surroundings. Our study's results indicate that different kinds of microsites in the
same locality at the same time do not have the same temperatures (fig. 19). Too
often we have unwittingly assumed that measurement of air temperature will tell us
what is going on at a particular location. The differences between air temperature
and that of a plant or plant part can be significant. Small differences about thresh -
old values can mean 1 ife or death, or produce significant differences in growth.
The lethalohigh temperature for most plant tissue is about 54'~; exposure of
seedlings to 54 C for about 30 minutes will cause death. Research has documented the
occurrence of high temperatures kill ing tree seed1 ings (Shearer 1967; Silen 1960).
Our data indicate lethal temperatures do occur on residue and litter surfaces in
clearcuts in the Northern Rocky Mountain area. Factors such as the age of plants and
the insulation properties of their bark, determine the potential for exchange between
the soil surface and the organisms. This, in turn, determines the temperature of
tissues. We must recognize again that there are species differences and these factors
need to be considered. For only a few species do we have complete records of maximum
temperature tolerance and information on specific hardening behavior. Data indicate
that temperatures several centimeters below the surface seldom, if ever, reach lethal
levels, except during fires (Shearer 1975). Roots, therefore, are not ldkely to be
killed by heat. Root growth is generally limited or stopped at about 35 C (Hermann
1977).
Low temperatures are often lethal to seedlings and other organisms, although
plant tissue properly hardgned or conditioned to low temperatures can withstand
temperatures as low as -55 C. Species differences exist, but 1 imits for native
species are seldom exceeded. Planting of exotic species can lead to disasterous
results if to1 erance is not considered. Young seed1 ings, are particularly suscept-
ible, and unseasonable frosts frequently are lethal or cause significant injury.
Insect larvae and fungi can also be harmed by overly low temperatures. Data for our
study sites indicates that frequent frosts do occur as a result of harvesting. These