as 1% to 2% of the total solar radiation reaches the forest floor (Vezina 

 1961). Only highly tolerant plant species survive under these low light 

 conditions (such as mosses and lichens). The conical shape of spruce and fir 

 allows sunlight to penetrate deeper into the canopy but on the ground light 

 levels may still be only about 3% of those in the open (Vezina 1961; and 

 Vezina and Pech 1964). Red and white pines have less dense canopies and as 

 much as 10% to 15% of incident solar radiation may reach the ground (Spurr 

 1964) . The sunlight that many species of understory plants receive in early 

 spring, before the trees develop leaves, gives them the energy to survive for 

 the remainder of the year. In addition, small patches of direct sunlight 

 reaching the forest floor through holes in the canopy (called sunflecks) are 

 important to understory plants. Although sunflecks are small, changes in the 

 angle of the sun causes flecks to move across the forest floor and provide 

 direct sunlight to most of the ground vegetation for at least part of the day. 



Temperature 



Air temperatures in a forest are somewhat stratified, because of the amount of 

 solar radiation reaching each stratification level and the degree of air 

 circulation permitted by the vegetation. In deciduous stands in summer, 

 maximum temperatures are reached just below the top of the canopy, where solar 

 radiation is most intense and air circulation is reduced. Air temperatures in 

 the canopy of coniferous stands are usually less than those in hardwood 

 stands, because the open canopy of coniferous allows more air circulation. 

 Minimum temperatures in forests are found near the ground (Geiger 1965). 



Temperature extremes are much greater in open ground than in forests. During 

 the day the canopy intercepts solar radiation, which reduces maximum 

 temperatures beneath the canopy. At night the canopy traps thermal radiation 

 from the ground and reduces wind velocity, thus increasing nighttime 

 temperatures. Forests usually reduce daytime maximum temperatures more than 

 they increase nighttime temperatures (Kittredge 1948 and Spurr 1957). In 

 deciduous forests in winter, these differences in temperatures are much 

 reduced (Spurr 1964) . 



Wind 



Wind velocity is reduced by friction with forest vegetation and is deflected 

 up and over the canopy. The degree of interference is dependent on the 

 density of vegetation. In tall dense stands wind velocity may be only 20% of 

 that in the open (Fons 1940). In stands with dense understory, wind velocity 

 decreases near the ground, whereas stands with more open understories have 

 higher wind velocities. 



Water 



Precipitation falling on a forest may be either intercepted by the vegetation 

 (interception) or fall directly to the ground (throughfall) . Precipitation 

 that is intercepted is either absorbed by the leaves, evaporated into the 

 atmosphere, or drained down the stem to the ground (stemflow; Zinke 1967; 

 Geiger 1965; and Kittredge 1948). During light rains nearly all of the 

 precipitation may be intercepted by the canopy, whereas in heavy, prolonged 

 rains interception may be as little as 10% (Voight 1960). Dense canopies 



9-25 



10-80 



