Deciduous trees slied their foliage in the autumn, 

 are bare over winter, antl obtain new foliage in the 

 spring. Coniferous trees, on tlie otlier hand, retain 

 their foliage throughout the year, although old dried 

 leaves fall a few at a time at all seasons. Diflferences 

 in the size, shape, and structure of the leaves are ini- 

 |K)rtant to many animals. The lack of foliage in de- 

 ciduous forests during the winter permits a greater 

 light penetration to the forest floor, more wind circu- 

 lation, and relatively lower temperatures than in 

 coniferous forests. During the summer, deciduous 

 forests generally have higher hut more variable tem- 

 peratures and lower relative humidities than do co- 

 niferous spruce and fir forests (Blake 1926, Dirks- 

 Edmunds 1947). Pine forests, however, commonly 

 develop in habitats that are warm and dry. 



As shade producers, the deciduous and coniferous 

 trees do not vary as groups, but only as individual 

 species (Weaver and Clements 1938) : 



Deciduous trees Coniferous trees 



Hcaz'y shade producers 

 Sugar maple Yew 



Beech Spruce 



Basswood Hemlock 



Firs 

 Thujas 



Medium shade producers 

 Elms Eastern white pine 



White oak Douglas-fir 



Northern red oak 

 Ash 

 Black oak 



Silver maple 

 Bur oak 

 Birches 

 Poplars 

 Willows 



Light shade producers 



Ponderosa pine 

 Tamarack 

 Lodgepole pine 



It is interesting that light shade producers are spe- 

 cies found in the early stages of succession while the 

 heavy shade producers are mostly climax species. 



There is an important difference between decidu- 

 ous and coniferous forests in the nature of the de- 

 composing dead leaves that fall from the trees. De- 

 composition of broad leaves is rapid and relatively 

 complete to form a rich humus that mixes gradually 

 with the mineral soil beneath. Needle leaves decom- 

 jxjse slowly and form a somewhat acid humus sharply 

 defined from the underlying mineral soil. Humus 

 formed in humid grasslands is similar to but richer 

 than that of deciduous forest : in arid grasslands it is 

 poorly developed. The nature of the humus and litter 

 affects the number and kinds of animals that occur 

 in the soil. 



In grassland tiiere are three strata of vegetation : 

 subterranean, composed of roots and other under- 

 ground plant parts as well as bacteria, fungi, and 

 algae, i/routid. including the surface litter, and herb. 

 tlie stems and leaves of the grasses and forbs. The 

 forest not only has these strata, but also one of shrubs 

 and one or more of trees. Animals characteristically 

 limit their major activity to one or more of these 

 strata. 



HABITAT 



Grassland, forest-edge, 



and forest-interior compared 



.\t the L'niversity of Illinois, no significant dif- 

 ference in mean monthly temperatures, calculated bi- 

 hourly day and night, has been found between the 

 interior of a virgin oak-maple forest and an adjacent 

 open grassland. In the forest, however, the daily 

 extremes are not so great; i.e., the maximum mid- 

 afternoon temperature is not as high, nor the mini- 

 mum night temperature so low, as in the grassland. 



Relative humidity during a summer day in Iowa 

 was found (Aikman and Smelser 1938) to average 

 20 per cent lower in grassland than in a shrubby 

 forest-edge, and .^ to 8 per cent lower in the forest- 

 edge than in the forest-interior. There is less differ- 

 ence between the three habitats, however, at night. 

 Rate of evaporation, as measured with Livingston 

 atmometers, is inversely correlated with humidity, 

 being greatest in grassland and least in the forest- 

 interior. Daily changes in relative humidity between 

 day and night tend to vary inversely with the tem- 

 perature, except when there is rain. 



During four years at the University of Illinois 

 woods, rain gauges recorded 88.8 cm (35.5 in.) per 

 year in the adjacent grassland, upon which full pre- 

 cipitation fell, and 70.1 cm (28.0 in.) throughfall 

 (the amount reaching the ground) under the tree 

 canopy of the forest. There was variation of through- 

 fall from spot to spot in the forest, depending on the 

 location of openings in the canopy and drip-points 

 from the leaves and stems. Stem-fiorv of water down 

 the tree trunks was not measured. Throughfall and 

 stem-flow together make up the net rainfall. In a 

 shortleaf pine plantation in southern Illinois (Bog- 

 gess 1956), the net rainfall over three years averaged 

 91.2 per cent of the total rainfall. Interception, the 

 amount of rainfall presumably evaporated back into 

 the air, was 100 per cent in very light rainfalls but 

 less than 5 per cent of rainfalls exceeding 5 cm 

 (2 in.). 



In a beech-maple forest in northern Ohio, which 

 bordered on an open field, wind velocity at a distance 

 245 meters (about 800 ft) inside the west margin 



Grassland, forests, and forest-edges 



21 



