12
Ecological succession has been studied for many years. In 1863, Henry David
Thoreau recognized that upland pine communities were replaced after logging by
hardwood communities (see Spurr and Barnes 1973 ). Clements ( 1916 ) suggested
that succession would proceed to a climax community that would dominate a particular climatic region. This appears to be too simplistic an approach for a complex
theory. The climate in a local area may control the climax plant community type, but
usually there is a combination of factors that control the type of climax community
in an area in addition to climate, such as soil conditions, topography, fi re, and possibly others. All communities appear to continue to change in time but late in succession as the climax community is approached, the rate of change slows.
Disturbances, large and small, would result in patches or gaps in a community and
seem to reset a part of the community to an earlier stage of succession, which could
slow the successional process or the approach to the climax community (Begon
et al. 2006 ; Smith and Smith 2012 ).
Many of the woody legume communities have seemed diffi cult to study because
conditions necessary to understand the start of the succession have not been well
recognized or described. This may be in part because of the relatively long time
interval required to start the process in relatively dry or arid conditions and areas.
Or, it may be the relatively long time it takes for the woody legumes to overtop the
grasses and be recognized as encroaching. Or, it could be the level or degree of
disturbance, which may be continuous or discontinuous. The disturbances could
also interact in various ways with a highly variable climatic condition such as
amount of rainfall. Curiously, for the most part, connections have not apparently
been made between the encroachment process and secondary succession. This
incursion or encroachment of woody legumes into grasslands has been going on for
a long time, but has not been recognized by many as secondary succession.
There are many different ways to look at and explain succession (Begon et al.
2006 ; Smith and Smith 2012 ). But the driving force or forces of succession are much
harder and more diffi cult to fi nd, describe, and explain. These are the biological
mechanisms that underlie succession. There are lots of suggestions concerning
mechanisms, and there are many mechanisms that fi t certain community types and
conditions. One theory that seems to make considerable sense concerning woody
legume encroachment and the successional sequences that follow is the resource
ratio hypothesis (Tilman 1985 ). This theory suggests that the dominance of a species
in time and space during terrestrial succession is dependent on the relative amount
or availability of two potentially limiting resources. The two resources that he
focused on were light levels and a limiting soil resource such as nitrogen (Fig. 3.1 ).
Surface light levels are high early in terrestrial succession (Fig. 3.1 ) and then
decrease as shade-tolerant woody species other than the woody legumes establish,
grow, and become a dominant part of the overstory or canopy. The second resource,
soil nitrogen level, is low early in succession and then increases late in succession.
In the case of woody legumes in encroachment or secondary succession, the
sequence is slightly different. Soil nitrogen levels are low early in succession, but
the woody legumes or more particularly their symbiotic nodule-forming bacteria fi x
atmospheric nitrogen. This nitrogen is used by the legume, and then the nitrogen is
3 Encroachment and Secondary Succession
Ecological succession has been studied for many years. In 1863, Henry David
Thoreau recognized that upland pine communities were replaced after logging by
hardwood communities (see Spurr and Barnes 1973 ). Clements ( 1916 ) suggested
that succession would proceed to a climax community that would dominate a particular climatic region. This appears to be too simplistic an approach for a complex
theory. The climate in a local area may control the climax plant community type, but
usually there is a combination of factors that control the type of climax community
in an area in addition to climate, such as soil conditions, topography, fi re, and possibly others. All communities appear to continue to change in time but late in succession as the climax community is approached, the rate of change slows.
Disturbances, large and small, would result in patches or gaps in a community and
seem to reset a part of the community to an earlier stage of succession, which could
slow the successional process or the approach to the climax community (Begon
et al. 2006 ; Smith and Smith 2012 ).
Many of the woody legume communities have seemed diffi cult to study because
conditions necessary to understand the start of the succession have not been well
recognized or described. This may be in part because of the relatively long time
interval required to start the process in relatively dry or arid conditions and areas.
Or, it may be the relatively long time it takes for the woody legumes to overtop the
grasses and be recognized as encroaching. Or, it could be the level or degree of
disturbance, which may be continuous or discontinuous. The disturbances could
also interact in various ways with a highly variable climatic condition such as
amount of rainfall. Curiously, for the most part, connections have not apparently
been made between the encroachment process and secondary succession. This
incursion or encroachment of woody legumes into grasslands has been going on for
a long time, but has not been recognized by many as secondary succession.
There are many different ways to look at and explain succession (Begon et al.
2006 ; Smith and Smith 2012 ). But the driving force or forces of succession are much
harder and more diffi cult to fi nd, describe, and explain. These are the biological
mechanisms that underlie succession. There are lots of suggestions concerning
mechanisms, and there are many mechanisms that fi t certain community types and
conditions. One theory that seems to make considerable sense concerning woody
legume encroachment and the successional sequences that follow is the resource
ratio hypothesis (Tilman 1985 ). This theory suggests that the dominance of a species
in time and space during terrestrial succession is dependent on the relative amount
or availability of two potentially limiting resources. The two resources that he
focused on were light levels and a limiting soil resource such as nitrogen (Fig. 3.1 ).
Surface light levels are high early in terrestrial succession (Fig. 3.1 ) and then
decrease as shade-tolerant woody species other than the woody legumes establish,
grow, and become a dominant part of the overstory or canopy. The second resource,
soil nitrogen level, is low early in succession and then increases late in succession.
In the case of woody legumes in encroachment or secondary succession, the
sequence is slightly different. Soil nitrogen levels are low early in succession, but
the woody legumes or more particularly their symbiotic nodule-forming bacteria fi x
atmospheric nitrogen. This nitrogen is used by the legume, and then the nitrogen is
3 Encroachment and Secondary Succession