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Arizona
Range Grasses GRASS MORPHOLOGY,
GROWTH Plants on rangelands are commonly grouped into the categories of grasses, forbs, shrubs and trees. Grasses are monocotyledonous plants, or monocots, meaning that they have just one seed leaf. Monocots can be recognized by the parallel venation of their leaves. Dicotyledonous plants or dicots are broad-leaved plants with net or branched venation in their leaves. Forbs are herbaceous or non-woody dicots whereas shrubs are usually broad-leaved plants with woody bases and/or stems. Trees are woody plants over some predetermined height, usually 10 or 20 feet. Even within the grass family (Poaceae) there is much life form diversity. There are annual grasses, which grow from seed and complete their life cycle in one growing season. There are perennial grasses that live for many years. There are cool season grasses that begin their growth early in the spring and complete their reproductive cycle (produce seeds) before hot summer weather sets in. And there are warm season grasses that begin their surge of annual growth later in the summer, after the soil warms and summer rains begin. Grasses can be further classified according to their growth stature and form. There are tall, mid and short grasses in addition to bunch grasses and sod-forming grasses.
Knowledge of how grasses grow and respond to defoliation is fundamental to most aspects of range management. Typically, grass plants are made up of roots, root crown and tillers (figure 1a). Some grasses also have specialized organs called stolons or runners (trailing above ground stems), and some have rhizomes which are underground stems. A grass plant is really a group of individual tillers that originate from the root crown at the base of the plant. The grass tiller consists of a series of units each made up of a leaf blade and sheath, collar, ligule and axillary bud (figure 1b). Grass flowers form at the top of each reproductive tiller. These flowers are made up of spikelets, each of which contain one or more florets. The fertile florets develop into seeds upon maturity.
During the growing season, new tillers of a grass plant are produced.
New tillers arise in the axillary buds of older tillers. The new tillers
originate at the base of the plant but once they start to grow, each
new leaf unit develops from the apical meristem. Apical meristems may
remain at the bottom of the tiller, near the ground, or become elevated
when the internodes elongate. Thus, the youngest leaf is at the top
of the tiller and the oldest is at the bottom. The location of the apical
meristem is important to how the grass responds to defoliation. Vegetative growth will continue to be produced as long as growing conditions
are favorable, but not after the apical meristem becomes reproductive
(starts sending up a flowering stalk) or is removed through grazing
or by other means. Once a tiller starts a seed stalk or if the apical
meristem is removed no new leaves will grow on that shoot. The effects of grazing or any other form of defoliation on a grass
plant are directly related to how grasses grow. These effects depend
upon the following:
Grazing resistance or tolerance are general terms used to describe
the relative ability of plants to survive defoliation by grazing animals.
Strategies to cope with grazing vary. For example, plants with thorns
or certain chemical compounds are less likely to be grazed than plants
that don't produce these. Most grass plants can be heavily or frequently
grazed but not both. Grasses are usually considered to be the plant
group with the highest grazing tolerance. Plant stress and even death can result from frequent and intense defoliation.
This is due to a combination of reduced leaf area for photosynthesis,
the need to initiate new tillers from axillary meristems in order to
produce new leaves, and reduced plant root growth following defoliation.
Reduced root mass decreases the ability of the plant to obtain water
and nutrients for growth and survival. Restricted root systems can be
especially harmful during times of drought. Plants can recover from defoliation if allowed periodic deferments
or occasionally complete rest from grazing. The amount of plant material
remaining and the ability to quickly produce new growth after defoliation
are the most important factors related to grazing tolerance. While grazing
during the dormant season has little effect on the physiology of the
plant, excessive grazing during this time may affect grasses indirectly.
If grazing is too heavy, all litter may be removed, soil erosion may
occur and much of the water that should go into the soil may be lost
as runoff. The following characteristics help determine grazing tolerance.
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Cooperative
Extension,College of Agriculture & Life Sciences, The University of
Arizona
