Individual clumps of the perennial grass Bouteloua gracilis from which 25% or 75% of neighbors had been removed to a radius of 50 cm displayed significantly higher biomass production over one summer than did control plants. Neighbor removal also resulted in increased tiller production and flowering as compared to control plants. These results clearly demonstrate the importance of competition to B. gracilis in this system. In contrast, significant differences in predawn leaf water potential were found on only two of four sampling dates during the growing season. On the driest sampling date, increased variability in leaf water potential within treatments obscured the effect of treatment, even though the difference in mean leaf water potential between 75% removal and control treatments (0.35 MPa) was quite high. On the wettest sampling date, all plants appeared well-hydrated, and there was only negligible difference among treatments. In a second study, I examined the effect of near neighbors on the fecundity of an annual herb, Machaeranthera tanacetifolia (Asteraceae). Fecundity was negatively related to the number of neighbors within a biologically meaningful neighborhood radius for both conspecific neighbors and grass neighbors in 1984, and for grass neighbors in 1985. However, in no case did neighbors explain more than 6% of the variance in fecundity of M. tanacetifolia, and most plants produced few or no seeds regardless of the number of neighbors. I present a simple graphical model in which competition from neighbors serves as a necessary but not sufficient condition for high fecundity; other factors must also be favorable if a plant is to be successful. Application of this model to my data revealed that the amount of area occupied by neighbors had an important effect on fecundity for the subset of the population for which other conditions were inferred to be most favorable. As other conditions became less favorable, neighbors became less relevant to fecundity. I then discuss these results in terms of a general model of the interacting effects of competition and other factors, and distinguish four possible classes of outcome.

Germination and establishment was evaluated in Arizona cottontop (Digitaria californica (Benth.) Chase), bush muhly (Muhlenbergia porteri Scribn.) and plains lovegrass (Eragrostis intermedia Hitchc.) from southern Arizona. Germination was determined in different aged seed harvested from native and irrigated plots of various populations. Results indicated variability in germination characteristics of study species that appeared unrelated to afterripening or dormancy. Germination ranged from 62 to 96% for Arizona cottontop, 34 to 99% for plains lovegrass, and 75 to 99% for bush muhly. Bush muhly had highest average velocity of germination (58.0 ± 4.1), followed by plains lovegrass (35.9 ± 1.91), and Arizona cottontop (31.4 ± 1.13). Effects of 3 surface treatments (furrows, gravel, litter) and 2 cultivation treatments on establishment of study species and yellow bluestem (Bothriochloa ischaemum (L.) Keng var. ischaemum) was investigated in the field. Bush muhly did not emerge and plains lovegrass had minimal emergence. Arizona cottontop had similar emergence in cultivated and uncultivated plots whereas yellow bluestem had greater emergence in cultivated plots. Litter promoted greater survival of Arizona cottontop. Lower survival in furrows may have resulted from fewer days with available soil moisture and interference from greater density of non-seeded species. Soil covers appear to positively influence survival rather than emergence. Effect of rainfall distribution on germination was investigated using 2 seed retrieval methods. Arizona cottontop and bush muhly germinated after at least 3 days of available soil moisture. Plains lovegrass seeds did not germinate in the field but retrieved seeds had 83% germination, possibly due to light exposure under laboratory conditions. Another study determined whether species have different density and cover under overstory species compared to open areas. Arizona cottontop had either greater, less, or no differences in density under canopied versus open areas. Cover of Arizona cottontop was greater or similar among canopied and open areas. Plains lovegrass had lower or similar density and cover under canopies than in open areas. Bush muhly had greater density and cover under canopies. Overall, studies suggested that species are easily germinated, but accept relatively narrow ranges of environmental conditions for establishment. Establishment may be enhanced by sowing species when rainfall is consistent, extending available moisture through use of soil covers, or mimicking microsites where a species occurs naturally.

Levels of nitrogen, phosphorus, potassium, calcium, magnesium, zinc, manganese, copper and selenium in grasses grown under rainfed conditions on different soils in the Santa Rita Experimental Range and other areas in southern Arizona were evaluated to determine whether these nutrients are deficient, sufficient or toxic to cattle. In the greenhouse, two grasses grown on five different soils representing four soil orders were fertilized with nitrogen and phosphorus and the uptake of a number of nutrients was determined. For the grasses collected from the field, the nutrient contents generally decreased with time to maturity and the highest levels were attained during August when the grasses were young, and March, the start of the spring growth. The nitrogen and phosphorus contents were lower than the requirement for grazing cattle indicating the need for supplementing protein and phosphorus. The contents of potassium, calcium, zinc, manganese, copper and selenium were adequate for grazing animals. Although the magnesium level in some cases was lower than the requirement, it was considered adequate because its deficiency is now known in warm season grasses. The nutrient contents in the grasses grown on Comoro soils were generally higher than when grown on an adjacent Sonoita soil. Soil analysis revealed no consistent pattern of the levels in these grasses and in Comoro and Sonita soils on which they were grown, except for nitrogen. The nutrient contents in the heads and leaves of two species of grasses were higher than in their stems. The greenhouse experiments showed that the vegetation on the N + P treatment was more vigorous than on the nitrogen treatment and the control, respectively. For the two grasses grown on the five different soils, the nitrogen and magnesium contents mostly were not significantly different between the three treatments, probably due to the dilution effect resulting from the vigorous vegetative growth on the N + P treatment. The phosphorus content in the grasses grown on the N + P treatment was significantly higher than the other two treatments. The nitrogen and phosphorus levels in the soils before and after planting were similar indicating that nitrogen and phosphorus fertilization helped maintain their levels in the soils. Other nutrients in the soils decreased after planting. Most of the nutrients were higher in the surface soils than in the subsurface soils, probably due to the ability of grasses to recycle nutrients from the subsoil.

The ability of different seedbed preparation treatments to enhance seedling establishment of 'Vaughn' sideoats graina (Bouteloua curtipendula (Michx.) Torr.), 'A-130' blue panic (Panicum antidotale Retz.), 'A-68' Lehmann lovegrass (Eragrostis lehmanniana Nees) and 'Cochise' Atherstone lovegrass (Eragrostis lehmanniana Nees X E. tricophera Coss and Dur.) was determined in field and greenhouse experiments. Field experiments compared seed burial and seedling emergence on undisturbed plots with that of plots lightly or heavily trampled by cattle, furrowed with a land imprinter or plowed. Seed burial was nearly always greatest in plots disturbed by these 3 latter treatments compared to undisturbed or light-trampled plots. Summer thunderstorms increased burial on treated and undisturbed plots. These seedbed treatments likewise increased seedling emergence of all species during a moderately-wet summer but not during a dry summer when emergence was limited. Sideoats graina emergence was low all 3 years. Seed burial and emergence of the small-seeded lovegrasses was high in a wet year even on undisturbed plots. Greenhouse experiments were designed to determine effects of seedbed microsites, sowing depth and soil water on emergence, survival and morphology of sideoats graina, blue panic and Cochise lovegrass. Emergence of all species was highest from seeds under gravel, followed by seeds under litter, seeds in cracks and finally seeds on the bare soil surface. Cochise lovegrass had high emergence under gravel for both continuously-wet and drying seedbeds. Cochise lovegrass had greater survival, followed by blue panic and then sideoats graina. All 3 species initiated permanent roots from nodes near the soil surface regardless of sowing depth. Seedlings from deeper-sown seeds had shallower primary roots and less survival than seedlings from shallow-sown seeds. Although seedbed treatments may increase the number of seeds buried and thereby increase seedling emergence when summer storms are frequent, treatments which bury seeds too deep may decrease seedling emergence. Seedbed treatments do not ensure successful emergence in a dry year.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

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No description available.

No description available.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

No description available.