60
at this time and will probably be determined by future conditions, especially amount
and intensity of grazing, fi re, and certainly the amount of rainfall or drought.
Apparently these C 4 warm season grasslands and woody legume communities
will not remain the same in the future unless management is modifi ed. Grazing intensity by domestic animals and fi re or burning frequencies should be adjusted to at least
moderate levels. Grazing and fi re frequency as well as intensity are factors of paramount importance to understanding woody legume encroachment and the functioning of these communities. These factors are all interrelated and interdependent. The
intensity and frequency of fi re depends on many factors including grazing intensity,
precipitation, temperature, soil depth, soil nutrient levels, and probably others that
are not known for many woody legume communities. If grasslands are not burned,
ecological succession will change. The grasslands will not remain the same in time,
and woody species such as the various woody legumes and a few other woody species will encroach into the grasslands, and the grasslands will become woody legume
savannas, and savannas will become woodlands that will continue to change.
Fire and its effects on most of these legume communities have not been carefully
examined. This is especially true for the Vachellia and Senegalia communities in the
Americas. Fire has been used in attempts to control Prosopis encroachment, but
usually late in the encroachment process, and required conditions and results are not
yet well defi ned. What happens if the legume communities are not burned? Results
seem varied. In more mesic areas, succession continues and the woodlands are
replaced in time with species more tolerant of shaded understory conditions, and
probably requiring higher levels of soil nitrogen or possibly other undefi ned conditions. This woodland successional sequence has been demonstrated for two woody
legume species in south central North America, and a similar sequence has been
demonstrated in southern Africa. However, in the grasslands, legume savannas, and
woodlands of the southern Great Plains of central North America including north
Texas, the conclusions are not as clear. The same is true for some of the arid and
semiarid grassland biomes and former grasslands of southwestern North America.
It is not clear if other drought-tolerant and shade-tolerant species will replace the
various woody legumes in these habitats. The answer to this question remains unrequited at this time. Have the potential replacement species been identifi ed? In a few
cases they have been identifi ed, but in most cases and for most conditions, they have
not been identifi ed.
The encroachment or invasion process really seems to be a modifi cation of ecological succession known and studied for many years. However, in grasslands, the
fi nal stage in the succession has long been considered a mixture of various grasses
and herbaceous plants, the grassland stage. The grassland stage has been considered
limited and the “climax” community because of the amount of annual rainfall in the
area of the community. This does not appear to be correct. In the past, the grassland
communities seem to have been limited or controlled by the frequency and intensity
of fi re in what was called a “pyric climax.” Today, grasslands are modifi ed by the
amount of herbivory and thus the amount of fuel for a fi re. Could the succession in
these grasslands or legume shrublands or woodlands be cyclic? Fire seems to reset
the successional process. If the woodlands are burned, would or could these
9 The Future
at this time and will probably be determined by future conditions, especially amount
and intensity of grazing, fi re, and certainly the amount of rainfall or drought.
Apparently these C 4 warm season grasslands and woody legume communities
will not remain the same in the future unless management is modifi ed. Grazing intensity by domestic animals and fi re or burning frequencies should be adjusted to at least
moderate levels. Grazing and fi re frequency as well as intensity are factors of paramount importance to understanding woody legume encroachment and the functioning of these communities. These factors are all interrelated and interdependent. The
intensity and frequency of fi re depends on many factors including grazing intensity,
precipitation, temperature, soil depth, soil nutrient levels, and probably others that
are not known for many woody legume communities. If grasslands are not burned,
ecological succession will change. The grasslands will not remain the same in time,
and woody species such as the various woody legumes and a few other woody species will encroach into the grasslands, and the grasslands will become woody legume
savannas, and savannas will become woodlands that will continue to change.
Fire and its effects on most of these legume communities have not been carefully
examined. This is especially true for the Vachellia and Senegalia communities in the
Americas. Fire has been used in attempts to control Prosopis encroachment, but
usually late in the encroachment process, and required conditions and results are not
yet well defi ned. What happens if the legume communities are not burned? Results
seem varied. In more mesic areas, succession continues and the woodlands are
replaced in time with species more tolerant of shaded understory conditions, and
probably requiring higher levels of soil nitrogen or possibly other undefi ned conditions. This woodland successional sequence has been demonstrated for two woody
legume species in south central North America, and a similar sequence has been
demonstrated in southern Africa. However, in the grasslands, legume savannas, and
woodlands of the southern Great Plains of central North America including north
Texas, the conclusions are not as clear. The same is true for some of the arid and
semiarid grassland biomes and former grasslands of southwestern North America.
It is not clear if other drought-tolerant and shade-tolerant species will replace the
various woody legumes in these habitats. The answer to this question remains unrequited at this time. Have the potential replacement species been identifi ed? In a few
cases they have been identifi ed, but in most cases and for most conditions, they have
not been identifi ed.
The encroachment or invasion process really seems to be a modifi cation of ecological succession known and studied for many years. However, in grasslands, the
fi nal stage in the succession has long been considered a mixture of various grasses
and herbaceous plants, the grassland stage. The grassland stage has been considered
limited and the “climax” community because of the amount of annual rainfall in the
area of the community. This does not appear to be correct. In the past, the grassland
communities seem to have been limited or controlled by the frequency and intensity
of fi re in what was called a “pyric climax.” Today, grasslands are modifi ed by the
amount of herbivory and thus the amount of fuel for a fi re. Could the succession in
these grasslands or legume shrublands or woodlands be cyclic? Fire seems to reset
the successional process. If the woodlands are burned, would or could these
9 The Future