62
will be essentially devoid of these invertebrate herbivores and the emerging woody
legume seedlings would not be threatened by them. In the past, these soil or litter
invertebrate herbivores could have been a major barrier to woody legume seedling
establishment, but they have never been studied in this context.
Because of natural and anthropogenic factors, the story of woody legume
encroachment into grasslands is more complicated. Natural atmospheric carbon
dioxide levels of the Earth have increased over the past 12–15,000 years, and they
are continuing to increase at rates modifi ed by human activities. Increased atmospheric levels of CO 2 have aggravated the greenhouse effect, which has caused the
mean annual temperature of the Earth to increase. Higher levels of atmospheric CO 2
will also stimulate plant carbon uptake or photosynthetic activity in many species.
This could result in an increase in annual net primary production, as long as another
required nutrient or factor such as soil nitrogen, phosphate, or water is not limiting
to the plants. Higher temperatures will also increase photosynthetic rates and should
promote the upward (elevation) spread as well as the more northward and southward (from the tropics) extension of temperate and tropical species and communities with a simultaneous narrowing of the ranges of cold-tolerant species.
What does this portend for the distribution of the woody legume savannas and
woodlands of western and southwestern North America and throughout the world?
The potential effects of these anthropogenic changes are diffi cult to envision over
the relatively short term because of the age or potential age that the woody legumes
and their communities can attain. Besides, experimental controls are lacking, and
the last time high levels of atmospheric CO 2 and temperature occurred, there were
no humans on the Earth. High atmospheric temperatures at the surface of the Earth
imply that temperate communities and species will migrate farther north or south
and to higher elevations. Those species that are cold adapted and cold tolerant,
already present in northern and southern regions and higher elevations, will relocate, moving further north or south and up in elevation until they cannot move further and then they will be extirpated.
Prior suggestions involve relatively long-term changes. What is expected regarding the short term? There are expected changes in water, nitrogen, and other nutrients. Dramatic shifts in weather patterns are expected, including more severe
droughts and shifts in rainfall distribution, but the effects of these changes on woody
legume establishment, encroachment, growth, and community structure are all
unknown. Longer-term effects of these potential changes are also speculative at this
time. Supplementary studies are needed to clarify the longer-term effects of climate
modifi cations, including rainfall amounts and distribution in addition to water use
and storage. These climatic and atmospheric changes will no doubt cause modifi cations in the overall water cycle, as well as the carbon and nitrogen cycle and their
storage, use, and metabolism. As the use and cycling of these resources change, the
plant and animal species in the current grasslands, woody legume savannas, and
woodland communities will also change. As physical and chemical changes occur,
changes in the vertebrate, invertebrate, plant, bacteria, and fungi populations and
diversity will follow. These community modifi cations will be benefi cial to some
species and detrimental to others. But, what happens to which ones is unclear.
9 The Future
will be essentially devoid of these invertebrate herbivores and the emerging woody
legume seedlings would not be threatened by them. In the past, these soil or litter
invertebrate herbivores could have been a major barrier to woody legume seedling
establishment, but they have never been studied in this context.
Because of natural and anthropogenic factors, the story of woody legume
encroachment into grasslands is more complicated. Natural atmospheric carbon
dioxide levels of the Earth have increased over the past 12–15,000 years, and they
are continuing to increase at rates modifi ed by human activities. Increased atmospheric levels of CO 2 have aggravated the greenhouse effect, which has caused the
mean annual temperature of the Earth to increase. Higher levels of atmospheric CO 2
will also stimulate plant carbon uptake or photosynthetic activity in many species.
This could result in an increase in annual net primary production, as long as another
required nutrient or factor such as soil nitrogen, phosphate, or water is not limiting
to the plants. Higher temperatures will also increase photosynthetic rates and should
promote the upward (elevation) spread as well as the more northward and southward (from the tropics) extension of temperate and tropical species and communities with a simultaneous narrowing of the ranges of cold-tolerant species.
What does this portend for the distribution of the woody legume savannas and
woodlands of western and southwestern North America and throughout the world?
The potential effects of these anthropogenic changes are diffi cult to envision over
the relatively short term because of the age or potential age that the woody legumes
and their communities can attain. Besides, experimental controls are lacking, and
the last time high levels of atmospheric CO 2 and temperature occurred, there were
no humans on the Earth. High atmospheric temperatures at the surface of the Earth
imply that temperate communities and species will migrate farther north or south
and to higher elevations. Those species that are cold adapted and cold tolerant,
already present in northern and southern regions and higher elevations, will relocate, moving further north or south and up in elevation until they cannot move further and then they will be extirpated.
Prior suggestions involve relatively long-term changes. What is expected regarding the short term? There are expected changes in water, nitrogen, and other nutrients. Dramatic shifts in weather patterns are expected, including more severe
droughts and shifts in rainfall distribution, but the effects of these changes on woody
legume establishment, encroachment, growth, and community structure are all
unknown. Longer-term effects of these potential changes are also speculative at this
time. Supplementary studies are needed to clarify the longer-term effects of climate
modifi cations, including rainfall amounts and distribution in addition to water use
and storage. These climatic and atmospheric changes will no doubt cause modifi cations in the overall water cycle, as well as the carbon and nitrogen cycle and their
storage, use, and metabolism. As the use and cycling of these resources change, the
plant and animal species in the current grasslands, woody legume savannas, and
woodland communities will also change. As physical and chemical changes occur,
changes in the vertebrate, invertebrate, plant, bacteria, and fungi populations and
diversity will follow. These community modifi cations will be benefi cial to some
species and detrimental to others. But, what happens to which ones is unclear.
9 The Future