Effects of changes in weather conditions upon grasslands in the northern great plains
Publ American Assoc Advance Science 53: 291-306
Recent extensive studies in 48 stands of grassland throughout a 250-mile-long section of mixed prairie in the brown soil zone of Saskatchewan and Alberta show that since 1944 the weather has not interfered with continued increases of the species formerly occupying only the more sheltered locations. From 1950 to 1954 the growing seasons favored this increase with precipitation about 20% above average, mean temperatures 3 to 5% F below average, and 16% lower evaporation. The mixed (mid and short) grass cover of ungrazed to moderately grazed ranges has responded remarkably to these changes in environment. Since 1944 the percentage of the soil surface occupied by grasses has doubled. The proportion of the grass cover contributed by the two principal species of dry situations (Stipa comata and Bouteloua gracilis) has declined from 62% in 1944 to 40% in 1955-56, while the portion of the forage yield furnished by these species has declined from 54 to 28%. The two principal species of more favored locations (Stipa spartea var. curtiseta and Agropyron dasystachyum) have increased from 15% to 31% in percentage composition and from 29% to 50% in proportion of the forage contributed. Calculated yielding capacity of these ranges has increased on the average by 137%. Changes in botanical composition have been greatest in areas intermediate in moisture relations, while yield increases have been greatest in areas where the basal cover in 1944 was relatively sparse (in the moistest and driest areas). These observations suggest that the extent of fluctuations in species composition occurring within vegetation in equilibrium with the climate is much greater in grassland than in forest and is probably greatest in parts of the grassland adjacent to the forest zone, but where droughts are of sufficient frequency to prevent invasion of tree species.