AbstractsBiology & Animal Science

A five-year clipping experiment was conducted to determine maximum sustained utilization levels for three big game browse plant species in central Arizona. The three species were mountain-mahogany (Cercocarpus breviflorus) and desert ceanothus (Ceanothus Greggii) in the chaparral type, and cliff-rose (Cowania mexicana var. Stensburiana) in the pinyon-juniper type. Experimental treatments were removal of 0, 25, 50, 75, and 100 percent of current annual twig growth plus a more severe treatment involving removal of all twigs larger than four millimeters in diameter. Treatments were applied each October to ten replications. Data recorded were green and oven-dry weight per plot, total number of current twigs per plot, and mean twig length. Other data obtained were monthly precipitation, weekly maximum-minimum temperatures, monthly twig elongation, and phenological observations. Soil moisture trends were traced by electrical resistance methods during 1961 and 1962. Soil samples were subjected to mechanical analyses and bulk density determinations. Soil moisture content at permanent wilting point and field capacity were estimated from field trend curves, from sample evaporation curves, and from pressure membrane tests one-third and 15 atmospheres pressure. The five-year duration of the experiment was not sufficient to establish a definitive response to clipping. All species were stimulated initially by heavy clipping. This growth stimulation persisted throughout the study in cliff-rose and, to a lesser extent, in mountain-mahogany. Ceanothus four-millimeter plots were rapidly dying off at the close of the study and 100 percent plots were showing loss of vigor. It was tentatively concluded that 75 percent use was permissible for cliff-rose and probably also for mountain-mahogany. Use on ceanothus probably should not exceed 50 percent. Throughout the study, production under moderate use was higher than under no use. The chaparral site was located on deep granitic soils and was characterized by separate and independent spring and summer growth seasons. Annual, browse production was correlated best with annual or summer rainfall. The pinyon-juniper site was located on a limestone outcrop, and growth was strongly dependent upon winter precipitation. After a good winter, browse growth continued uninterrupted throughout the summer. The difference in growth regimes at the two sites appeared to be the result of highly efficient retention of moisture in the limestone substrate. Soil moisture depletion in the spring proceeded from the surface downward. The depletion curves were very steep, as would be expected in sandy loam sells. The range of available moisture was about ten percent. Spring browse growth began in late March when soil temperatures reached 45 to 50° F. Growth cessation occurred about October 1 each year and appeared to be the result of early frost. The maximum annual growth season ran about 190 days, but high soil moisture stress reduced this season drastically when either…