13
added to the soil via leaf and litter fall, followed by decomposition by soil bacteria
and fungi. The nitrogen is released from the litter, and soil nitrogen levels increase.
Next, this nitrogen is available to nonlegumes for their growth and metabolism. As
a result, other woody species requiring higher levels of soil nitrogen but are tolerant
of shade establish in the low light environment below the legume canopy. These
nonlegumes that are tolerant of low light conditions then overgrow and shade the
high light requiring early successional, woody legumes that previously encroached
the area (Van Auken 2000 ).
This is the encroachment process and successional sequence that has occurred
and seems to be occurring in many southwestern North American warm season
grasslands and probably other grasslands around the world. We will present evidence and results of experiments to show that this sequence is in fact true.
References
Begon M, Townsend CR, Harper JL (2006) Ecology: from individuals to ecosystems, 4th edn.
Blackwell, Malden, MA
Clements FE (1916) Plant succession: analysis of the development of vegetation. Carnegie
Institute of Washington Publication No. 242, Washington, DC
Grover HD, Musick HB (1990) Shrubland encroachment in southern New Mexico, USA: an analysis of desertifi cation processes in the American southwest. Climate Change 17:305–330
Humphrey RR (1958) The desert grassland: a history of vegetation change and an analysis of
causes. Bot Rev 24:193–252
Knapp AK, Briggs JM, Collins SL, Archer S (2008) Shrub encroachment in North American
Grasslands: shifts in growth form dominance rapidly alters control of ecosystem carbon inputs.
Glob Change Biol 14:615–623
Laurenroth WK (1979) Grassland primary production. In: French NR (ed) Perspectives in grassland ecology: results and applications of the US/IBP grassland biome study, 3–24. Springer,
New York
Smith TM, Smith RL (2012) Elements of ecology, 8th edn. Benjamin Cummings, New York
0
20
40
60
80
100
0
50
100 150 200 250 300
SUCCESSIONAL TIME-YEARS
RELATIVE VALUES
LIGHT
SOIL N
N -FIX
Fig. 3.1 Theoretical fi gure showing relative changes in surface light levels, nitrogen fi xation, and
soil nitrogen levels during secondary succession in a warm season C 4 arid zone grassland. Early
secondary succession or encroachment would be on the left , development of the legume savanna
or woodland would be toward the center , and the late successional nonlegume woodland or forest
would be on the right
References
added to the soil via leaf and litter fall, followed by decomposition by soil bacteria
and fungi. The nitrogen is released from the litter, and soil nitrogen levels increase.
Next, this nitrogen is available to nonlegumes for their growth and metabolism. As
a result, other woody species requiring higher levels of soil nitrogen but are tolerant
of shade establish in the low light environment below the legume canopy. These
nonlegumes that are tolerant of low light conditions then overgrow and shade the
high light requiring early successional, woody legumes that previously encroached
the area (Van Auken 2000 ).
This is the encroachment process and successional sequence that has occurred
and seems to be occurring in many southwestern North American warm season
grasslands and probably other grasslands around the world. We will present evidence and results of experiments to show that this sequence is in fact true.
References
Begon M, Townsend CR, Harper JL (2006) Ecology: from individuals to ecosystems, 4th edn.
Blackwell, Malden, MA
Clements FE (1916) Plant succession: analysis of the development of vegetation. Carnegie
Institute of Washington Publication No. 242, Washington, DC
Grover HD, Musick HB (1990) Shrubland encroachment in southern New Mexico, USA: an analysis of desertifi cation processes in the American southwest. Climate Change 17:305–330
Humphrey RR (1958) The desert grassland: a history of vegetation change and an analysis of
causes. Bot Rev 24:193–252
Knapp AK, Briggs JM, Collins SL, Archer S (2008) Shrub encroachment in North American
Grasslands: shifts in growth form dominance rapidly alters control of ecosystem carbon inputs.
Glob Change Biol 14:615–623
Laurenroth WK (1979) Grassland primary production. In: French NR (ed) Perspectives in grassland ecology: results and applications of the US/IBP grassland biome study, 3–24. Springer,
New York
Smith TM, Smith RL (2012) Elements of ecology, 8th edn. Benjamin Cummings, New York
0
20
40
60
80
100
0
50
100 150 200 250 300
SUCCESSIONAL TIME-YEARS
RELATIVE VALUES
LIGHT
SOIL N
N -FIX
Fig. 3.1 Theoretical fi gure showing relative changes in surface light levels, nitrogen fi xation, and
soil nitrogen levels during secondary succession in a warm season C 4 arid zone grassland. Early
secondary succession or encroachment would be on the left , development of the legume savanna
or woodland would be toward the center , and the late successional nonlegume woodland or forest
would be on the right
References