33
O.W. Van Auken and J.K. Bush, Invasion of Woody Legumes, SpringerBriefs in Ecology 4,
DOI 10.1007/978-1-4614-7199-8_6, © Springer Science+Business Media New York 2013
The interactions between individuals within species populations or competition
have been considered one of the major factors controlling or determining community characteristics (Harper 1977 ; Grime 1979 ; Grace and Tilman 1990 ). The interactions occur because of shared requirements for the same resource and can lead to
negative, positive, or no effect on one or both individuals, species, or populations
(Begon et al. 2006 ). However, the topic has been continually debated (Grace 1995 ;
Twolan-Strutt and Keddy 1996 ; Van Auken 2009 ). We have done a number of
greenhouse-controlled environment competition studies and fi eld studies between
V. farnesiana, P. glandulosa, and various grasses from former C 4 grasslands to help
to understand reasons for the changes that have occurred with encroachment of
woody plants and long-term, constant, and high levels of herbivory in these grasslands (Cohn et al. 1989 ; Van Auken 1994 ).
In one experiment, V. farnesiana was grown with a C 4 grass ( Cynodon dactylon
Bermuda grass) which is an introduced Eurasian grass that is now ubiquitous in
Texas and many C 4 grasslands (Gould 1969 ; Correll and Johnston 1979 ). The experiment was carried out in pots, in a low nutrient soil (clay-loam Mollisol of the Frio
series) (Taylor et al. 1966 ). Little growth per plant occurred for either species in
mixtures of the two species or monoculture (by themselves) in low nutrient soil, but
there was more grass (Fig. 6.1 ). In addition, there were no signifi cant differences in
dry mass between proportional treatments between mixture and monoculture for
either species (Cohn et al. 1989 ). Relative yields were higher for C. dactylon in
mixture (30–50 %) and were lower for V. farnesiana in mixtures (10–20 %), but
neither was signifi cantly different. When nutrients were added, growth of C. dactylon doubled in monoculture and tripled in 2:4 mixtures ( Cynodon/Vachellia ), and
the growth of V. farnesiana decreased in competition with the grass, but not in
monoculture where it increased slightly (Fig. 6.1 ) (Cohn et al. 1989 ). Cynodon dry
mass was higher than the V. farnesiana dry mass in all cases. Both species appear to
be early successional colonizers and probably germinate and start growth in gaps
caused by disturbances. Cynodon was the better competitor in the soils with added
nutrient, but the species were approximately equal in low nutrient soil in spite of
greater grass dry mass. In addition, there were no mortalities of either species.
Chapter 6
Competition
O.W. Van Auken and J.K. Bush, Invasion of Woody Legumes, SpringerBriefs in Ecology 4,
DOI 10.1007/978-1-4614-7199-8_6, © Springer Science+Business Media New York 2013
The interactions between individuals within species populations or competition
have been considered one of the major factors controlling or determining community characteristics (Harper 1977 ; Grime 1979 ; Grace and Tilman 1990 ). The interactions occur because of shared requirements for the same resource and can lead to
negative, positive, or no effect on one or both individuals, species, or populations
(Begon et al. 2006 ). However, the topic has been continually debated (Grace 1995 ;
Twolan-Strutt and Keddy 1996 ; Van Auken 2009 ). We have done a number of
greenhouse-controlled environment competition studies and fi eld studies between
V. farnesiana, P. glandulosa, and various grasses from former C 4 grasslands to help
to understand reasons for the changes that have occurred with encroachment of
woody plants and long-term, constant, and high levels of herbivory in these grasslands (Cohn et al. 1989 ; Van Auken 1994 ).
In one experiment, V. farnesiana was grown with a C 4 grass ( Cynodon dactylon
Bermuda grass) which is an introduced Eurasian grass that is now ubiquitous in
Texas and many C 4 grasslands (Gould 1969 ; Correll and Johnston 1979 ). The experiment was carried out in pots, in a low nutrient soil (clay-loam Mollisol of the Frio
series) (Taylor et al. 1966 ). Little growth per plant occurred for either species in
mixtures of the two species or monoculture (by themselves) in low nutrient soil, but
there was more grass (Fig. 6.1 ). In addition, there were no signifi cant differences in
dry mass between proportional treatments between mixture and monoculture for
either species (Cohn et al. 1989 ). Relative yields were higher for C. dactylon in
mixture (30–50 %) and were lower for V. farnesiana in mixtures (10–20 %), but
neither was signifi cantly different. When nutrients were added, growth of C. dactylon doubled in monoculture and tripled in 2:4 mixtures ( Cynodon/Vachellia ), and
the growth of V. farnesiana decreased in competition with the grass, but not in
monoculture where it increased slightly (Fig. 6.1 ) (Cohn et al. 1989 ). Cynodon dry
mass was higher than the V. farnesiana dry mass in all cases. Both species appear to
be early successional colonizers and probably germinate and start growth in gaps
caused by disturbances. Cynodon was the better competitor in the soils with added
nutrient, but the species were approximately equal in low nutrient soil in spite of
greater grass dry mass. In addition, there were no mortalities of either species.
Chapter 6
Competition