TheImprovement ofTropical and Subtropical Rangelands

SIT8 EVALUATION 109
the better is the estimate of site quality and, conaequently, the
understanding of the potential of a site for improvement practices.
Comprehensive reviews and comparisons ofsite evaluation, including
its history, methode, and applications, have been prepared by Jones
(1969) and Carmean (1975).
Prod.etivit, Letle" Knowledge of the productivity levels (that is,
amounts of plant material present) of plants growing on a site can
provide insight into what might be expected from any range improvement
practice. Information regarding the total production of
all herbaceous plants, taking into account the loss of plant material
to utilization, is often used as a "threshold" productivity value. In
other words, improvement should be expected to exceed the existing
productivity levels. Ifexceuive utilization of the plants has occurred,
the measures of existing production may be biased downward.
Volumetric measurements ofplants are seldom made to quantify
productivity levels. Instead, weights are used to measure the biomus
ofthe plant material present. The weights ofplants are most precisely
obtained by the clipping of sample plots. But, since clipping is timeconsuming
and coetly, a double sampling procedure is frequently
employed to measure productivity on an extensive basis; weights of
plants are estimated on all plots, with only a few plots clipped to
derive a factor to correct the estimates, ifnecessary.
Whenever feasible, productivity levels should be obtained on the
basis of individual plant species to allow subsequent groupings into
plant-form categories or grazing value classes for decision-making
purpoees.
Plaftt Cotler In addition to the productivity or biomus available
for utilization, the ability of the plant community to stabilize the
site and arrest the soil erosion process should also be determined.
Productivity information alone does not provide the manager with
this knowledge. The percentage of the soil surface that is covered
by plants, either only by the base of the plant (basal cover) or by
all above-ground plant parts when viewed from above the canopy
(canopy cover), indicates both the susceptibility of the site to erosion
and the established dominance of one plant species over another.
PltJftt Num6er A plant community might be dominated, in terms of
productivity and cover, by one or two plant species, the individuals
of which are old and decadent. As these individuals die, they will
be replaced by the same or new species. Data on number of plants
of each species may give the land manager an indication of the