PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



217 



than over the ungrazed treatment during all 3 

 weeks reported. 



Average vapor pressure deficits at the 15 cm. 

 height were slightly greater on the grazed area 

 than on the ungrazed area during each week 

 (table 16). Prasad (10) demonstrated a strong- 

 correlation between vapor pressure deficits at this 

 level and the status of water in grass leaves. At 

 1 m. above the canopy, vapor pressure deficit 

 values were slightly higher on the ungrazed site 

 than on the grazed site in June and July, and 

 slightly lower in August. The differences in aver- 

 age relative humidity and vapor pressure values 

 between the treatments are not great, taken as a 

 whole, but the results illustrate that evaporative 

 conditions near the ground on the grazed site were 

 somewhat more severe than on the ungrazed site. 



Table 16. — Average weekly vapor pressure defic- 

 its (mm. of mercury) at two heights above the 

 ground surface on the grazed and, ungrazed 

 sites for selected, weeks in the, 1,970 season 



Week 



Height 



Vapor pressure 



deficit on 



of 

 season 



above 

 ground 







Grazed 



Ungrazed 







Mm. Hg. 



Mm. Hg. 



•Tune 14-20 



15 cm. 



3.63 



3.60 



July 5-11 



do. 



12.15 



10.58 



Aug. 23-29 



do. 

 Canopy 



11.98 



11.67 



June 14-20 



+ 1H1 1 



3.75 



4.25 



July 5-11 



do. 



12.17 



12.25 



Aug. 23-29 



do. 



12.87 



12.18 



1 On grazed site canopy + 1 m. = 130 cm. 

 site canopy |lm.z 145 cm. 



on ungrazed 



Summary And Conclusions 



Microenvironmental conditions in heavily 

 grazed Northern Plains mixed grass prairie were 

 found to be appreciably different than conditions 

 in similar ungrazed grassland when compared in 

 the 1970 season. The observed differences ap- 

 peared to be primarily the result of differences in 

 vegetation bulk on the two sites. Total vegetation 

 mass, including standing crop at maximum sea- 

 sonal development, standing dead, and litter was 

 441.3 g./m. 2 on the grazed area and 1,185.2 g./m. 2 



on the ungrazed area. Underground biomass was 

 greater under the grazed than under the ungrazed 

 treatment, averaging 2519 g./m. 2 and 1643 g./m. 2 . 

 respectively. 



Air temperatures over the grazed treatment 

 averaged 1.5° C. higher than temperatures over 

 ungrazed vegetation, although at a height of 

 canopy plus 1 m. the temperatures averaged about 

 the same on the two treatments. Soil temperatures 

 to a depth of 120 cm. were also higher under the 

 grazed treatment, averaging 21.4° C. for the sea- 

 son, while the average on the ungrazed treatment 

 was 17.9° C. Wind movement at 15 cm. averaged 

 2.1 km./hr. on the grazed area and 0.6 hm./hr. on 

 the ungrazed. At canopy height plus 1 m. wind 

 movement was about the same on the grazed and 

 ungrazed areas averaging 8.7 and 8.4 km./hr.. 

 respectively. 



Soil moisture was slightly greater under the 

 ungrazed than under the grazed treatment until 

 after midsummer, but available moisture was ex- 

 hausted about the same time on both areas. Rela- 

 tive humidity and vapor pressure deficit differ- 

 ences were relatively small between the two treat- 

 ments, but the results indicate that evaporative 

 conditions near the ground were somewhat more 

 severe on the grazed than on the ungrazed area. 



It would be highly desirable to obtain infor- 

 mation about several other microenvironmental 

 factors to increase the validity of comparisons be- 

 tween grazed and ungrazed vegetation. A refine- 

 ment of techniques involving soil- moisture ten- 

 sion measurements, to make possible a more pre- 

 cise evaluation of water balance under the two 

 treatments, seems indicated. An evaluation of the 

 moisture stress status of the plant tissues in rela- 

 tion to microenvironmental stresses on the grazed 

 and ungrazed areas, accompanied by precise 

 growth and production measurements of the ma- 

 jor species, would contribute substantially to 

 understanding just how the microenvironmental 

 influences directly affect grassland yield. Prelim- 

 inary measurements of net radiation and soil heat 

 flux indicate that there are differences in the en- 

 ergy budgets under the two treatments. More in- 

 formation on plant responses could be developed 

 by additional studies in this area. A comparison 

 of evaporation potentials throughout the growing 

 season on the two areas could provide usefuj data 

 on relative severity of plant stress conditions. 



