61
communities revert to open grasslands or savannas? This question is only partially
answered and seems to depend on the stage of succession and fi re frequency and
intensity. What is the timeline of these changes? Also largely undefi ned at present
and is dependent on local conditions that modify the rate of change.
Are domestic grazing animals important in this encroachment and succession of
woody legumes? Yes, they are. Would the removal of domestic grazing animals
prevent the encroachment of various woody legumes and other species into the arid
and semiarid C 4 warm season grassland communities? Apparently the answer is no;
the grasslands should be allowed to burn and then the woody species could be controlled or reduced. The removal of the domestic herbivores would modify the rate of
change, but it would not prevent the woody legume plant encroachment. Grazing by
domestic cattle would speed up the encroachment process, if the grazing was heavy
and continuous. If grazing were at a lower level and frequency, the rate of the
encroachment would be slower. The presence and competition of various C 3 and C 4
grasses with the woody legumes would slow down the encroachment process but
not prevent it from occurring. Dead grass biomass is fuel for fi res that can move
through these biomes. If there was no grass biomass or it was reduced because of
heavy grazing, there would be little or no fuel for the grassland fi res. With enough
fuel, the fi res could be at a relatively high temperature and frequency, and the fi res
would top kill the woody legumes and reverse to some extent woody legume and
other woody plant growth and succession. But, 200 years of heavy and almost continuous grazing would not be reversed with one fi re.
Control of encroachment of woody legumes into these grassland communities
must occur early in the encroachment or woody legume establishment process. It
would probably be best if it occurred in the seedling stage or early growth stage of
the encroaching plants. Control or management of woody legumes and other woody
species in grasslands usually is attempted well after, usually years after, the woody
plants germinate and establish. Thus, only top killing occurs and multiple fi res are
necessary to attempt density reduction or control. Seasonality of the fi res is also an
important potential effect on woody legume encroachment, but effects of seasonality of fi res need to be examined more carefully, especially summer fi res.
Large domestic grazing animals reduce the amount of aboveground grass biomass, and if grazing is continuous and intense, belowground grass biomass will be
reduced as well. This would reduce the ability of the grass to regrow leaves and
stems and belowground parts as well and therefore reduce the grasses ability to
obtain belowground resources including water, nitrogen, phosphate, and other soil
components. It would also reduce the grasses’ ability to grow and compete in these
ecosystems. Another thing that continuous and heavy grazing does is reduce the
fuel available for grassland fi res and the ability of a grassland community to carry a
fi re. So, any normal fi re-caused woody plant mortality would cease.
What about the invertebrate herbivores in these grassland communities?
Unfortunately, this topic has mostly been ignored. The upper layer of soil litter is
where most of these invertebrates live, and this is where the woody legume seed
germination, seedling emergence, and seedling establishing take place. If there is
little or no litter or biomass from grass growth and decomposition, the soil surface
9 The Future
communities revert to open grasslands or savannas? This question is only partially
answered and seems to depend on the stage of succession and fi re frequency and
intensity. What is the timeline of these changes? Also largely undefi ned at present
and is dependent on local conditions that modify the rate of change.
Are domestic grazing animals important in this encroachment and succession of
woody legumes? Yes, they are. Would the removal of domestic grazing animals
prevent the encroachment of various woody legumes and other species into the arid
and semiarid C 4 warm season grassland communities? Apparently the answer is no;
the grasslands should be allowed to burn and then the woody species could be controlled or reduced. The removal of the domestic herbivores would modify the rate of
change, but it would not prevent the woody legume plant encroachment. Grazing by
domestic cattle would speed up the encroachment process, if the grazing was heavy
and continuous. If grazing were at a lower level and frequency, the rate of the
encroachment would be slower. The presence and competition of various C 3 and C 4
grasses with the woody legumes would slow down the encroachment process but
not prevent it from occurring. Dead grass biomass is fuel for fi res that can move
through these biomes. If there was no grass biomass or it was reduced because of
heavy grazing, there would be little or no fuel for the grassland fi res. With enough
fuel, the fi res could be at a relatively high temperature and frequency, and the fi res
would top kill the woody legumes and reverse to some extent woody legume and
other woody plant growth and succession. But, 200 years of heavy and almost continuous grazing would not be reversed with one fi re.
Control of encroachment of woody legumes into these grassland communities
must occur early in the encroachment or woody legume establishment process. It
would probably be best if it occurred in the seedling stage or early growth stage of
the encroaching plants. Control or management of woody legumes and other woody
species in grasslands usually is attempted well after, usually years after, the woody
plants germinate and establish. Thus, only top killing occurs and multiple fi res are
necessary to attempt density reduction or control. Seasonality of the fi res is also an
important potential effect on woody legume encroachment, but effects of seasonality of fi res need to be examined more carefully, especially summer fi res.
Large domestic grazing animals reduce the amount of aboveground grass biomass, and if grazing is continuous and intense, belowground grass biomass will be
reduced as well. This would reduce the ability of the grass to regrow leaves and
stems and belowground parts as well and therefore reduce the grasses ability to
obtain belowground resources including water, nitrogen, phosphate, and other soil
components. It would also reduce the grasses’ ability to grow and compete in these
ecosystems. Another thing that continuous and heavy grazing does is reduce the
fuel available for grassland fi res and the ability of a grassland community to carry a
fi re. So, any normal fi re-caused woody plant mortality would cease.
What about the invertebrate herbivores in these grassland communities?
Unfortunately, this topic has mostly been ignored. The upper layer of soil litter is
where most of these invertebrates live, and this is where the woody legume seed
germination, seedling emergence, and seedling establishing take place. If there is
little or no litter or biomass from grass growth and decomposition, the soil surface
9 The Future