Resprouting and multi-stemming and the role of the persistence niche in the structure and dynamics of subtropical coastal dune forest in KwaZulu-Natal province, South Africa
Loading...
Date
2008
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of KwaZulu-Natal
Abstract
Resprouting is an important means of plant regeneration especially under conditions that do
not favour regeneration through seeding such as frequent disturbances, low productivity,
unfavourable soil conditions, extreme cold and limited understorey light availability. Sprouts
may be advantageous over seedlings because they have higher survival and growth rates than
seedlings, since they use resources from parent plants unlike seedlings that have to acquire
their own resources. Resprouting is well documented for ecosystems that experience severe
disturbances that damage aboveground biomass. For example, resprouting is important for
plant persistence against fire in fire-prone savannas and Mediterranean shrub-lands, and
hurricanes and cyclones in tropical forests. In these ecosystems, resprouting often results in
multi-stemming, because this dilutes the risk of damage among many stems, improving the
chances of individual survival.
This study was conducted at coastal dune forest at Cape Vidal in north-eastern South
Africa, where there is a high incidence of multi-stemmed trees due to resprouting in response
to chronic disturbances of low severity. This study examines (1) the importance of resprouting
to tree survival and dynamics in an environment where disturbance severity is low but
pervasive, and (2) how this resprouting strategy differs from the more familiar sprouting
response to severe disturbances such as fire and hurricanes.
Analysis of the relationship between multi-stemming and a number of disturbances
potentially causing multi-stemming revealed that stem leaning and substrate erosion were the
most important disturbances associated with multi-stemming. There were fewer multi
stemmed trees on dune slacks that had a stable substrate and were protected from sea winds
than on dune crests and slopes that had unstable substrate and were exposed to sea winds.Trees resprouted and became multi-stemmed from an early stage to increase their chances of
survival against leaning caused by strong sea winds and erosion, and occasional slumping of
the unstable dune sand substrate. These low severity disturbances are persistent and are
referred to as chronic disturbances in this thesis. As a result of these chronic disturbances, both
single and multi-stemmed trees had short stature because taller individuals that emerged above
the tree canopy would be exposed to wind damage.
Under chronic disturbances plants may manifest a phylogenetically determined
sprouting response. However, in this study resprouting and multi-stemming were the results of
the tree-disturbance interaction and not a property of a plant or species and were not
phylogenetically constrained.
Because the disturbances are predominantly of low severity, leaning trees were able to
regain the vertical orientation of the growing section by turning upward (a process referred to
as ‘turning up’ in this study) and hence survive without resprouting. Species that were prone
to turning upward had a low incidence and degree of leaning of their individuals, low
frequency of loss of primary stems and high abundance of individuals. Although turning up is
less costly to the individual than resprouting, it could only be used by leaning trees that had
small angles of inclination and were not eroded. High intensities of the latter require that
individuals resprout to survive.
The form and function of resprouting varied between seedlings and juvenile and
mature trees. Resprouting in seedlings resulted in a single replacement shoot, unlike sprouting
in juvenile and mature trees that resulted in multi-stemmed trees. Like sprouting in juvenile
and mature trees, sprouting in seedlings was not phylogenetically constrained. Resprouting in
seedlings increased seedling persistence; hence species with more sprout seedlings had larger
individual seedlings and seedling banks. Resprouting in seedlings increased the chances of Trees resprouted and became multi-stemmed from an early stage to increase their chances of
survival against leaning caused by strong sea winds and erosion, and occasional slumping of
the unstable dune sand substrate. These low severity disturbances are persistent and are
referred to as chronic disturbances in this thesis. As a result of these chronic disturbances, both
single and multi-stemmed trees had short stature because taller individuals that emerged above
the tree canopy would be exposed to wind damage.
Under chronic disturbances plants may manifest a phylogenetically determined
sprouting response. However, in this study resprouting and multi-stemming were the results of
the tree-disturbance interaction and not a property of a plant or species and were not
phylogenetically constrained.
Because the disturbances are predominantly of low severity, leaning trees were able to
regain the vertical orientation of the growing section by turning upward (a process referred to
as ‘turning up’ in this study) and hence survive without resprouting. Species that were prone
to turning upward had a low incidence and degree of leaning of their individuals, low
frequency of loss of primary stems and high abundance of individuals. Although turning up is
less costly to the individual than resprouting, it could only be used by leaning trees that had
small angles of inclination and were not eroded. High intensities of the latter require that
individuals resprout to survive.
The form and function of resprouting varied between seedlings and juvenile and
mature trees. Resprouting in seedlings resulted in a single replacement shoot, unlike sprouting
in juvenile and mature trees that resulted in multi-stemmed trees. Like sprouting in juvenile
and mature trees, sprouting in seedlings was not phylogenetically constrained. Resprouting in
seedlings increased seedling persistence; hence species with more sprout seedlings had larger
individual seedlings and seedling banks. Resprouting in seedlings increased the chances of Trees resprouted and became multi-stemmed from an early stage to increase their chances of
survival against leaning caused by strong sea winds and erosion, and occasional slumping of
the unstable dune sand substrate. These low severity disturbances are persistent and are
referred to as chronic disturbances in this thesis. As a result of these chronic disturbances, both
single and multi-stemmed trees had short stature because taller individuals that emerged above
the tree canopy would be exposed to wind damage.
Under chronic disturbances plants may manifest a phylogenetically determined
sprouting response. However, in this study resprouting and multi-stemming were the results of
the tree-disturbance interaction and not a property of a plant or species and were not
phylogenetically constrained.
Because the disturbances are predominantly of low severity, leaning trees were able to
regain the vertical orientation of the growing section by turning upward (a process referred to
as ‘turning up’ in this study) and hence survive without resprouting. Species that were prone
to turning upward had a low incidence and degree of leaning of their individuals, low
frequency of loss of primary stems and high abundance of individuals. Although turning up is
less costly to the individual than resprouting, it could only be used by leaning trees that had
small angles of inclination and were not eroded. High intensities of the latter require that
individuals resprout to survive.
The form and function of resprouting varied between seedlings and juvenile and
mature trees. Resprouting in seedlings resulted in a single replacement shoot, unlike sprouting
in juvenile and mature trees that resulted in multi-stemmed trees. Like sprouting in juvenile
and mature trees, sprouting in seedlings was not phylogenetically constrained. Resprouting in
seedlings increased seedling persistence; hence species with more sprout seedlings had larger
individual seedlings and seedling banks. Resprouting in seedlings increased the chances of seedling recruitment, whereas resprouting in juvenile and mature trees increased the chances
of an established plant maintaining its position in the habitat.
After disturbances of high severity, which destroy the photosynthesizing parts, plants
resprout using carbohydrates stored below- or aboveground. In this study, good resprouters
stored more carbohydrates both below- and aboveground than poor resprouters. The
carbohydrates were mobilized for resprouting after disturbance. More carbohydrates were
stored in stems than in roots because the prevailing disturbances were mostly of low severity
and hence aboveground resources were readily available. Similar to storage by plants in
severely disturbed habitats, carbohydrates were stored by reserve formation, which competes
for carbohydrates with growth and maintenance and forms permanent storage, rather than
accumulation, which temporarily stores carbohydrates in excess of demands for growth and
maintenance. Stored carbohydrates are not necessary for resprouting of plants after
disturbances of low severity because they can resprout using resources remobilized directly
from the disturbed photosynthesizing parts. However, in this study, stored carbohydrates
served as a bet-hedge against occasional severe disturbances that occurred in addition to
chronic disturbances. Allocation of carbohydrates to permanent storage diverts them from
growth and reproduction and hence good resprouters had lower growth rates, seed output, seed
size and seedling recruitment than poor resprouters. However, the costs of these traits that
resulted in low recruitment from seed by good resprouters, were compensated for by high
persistence of established individuals of good resprouters through recruitment of sprout stems.
This study demonstrates that resprouting is not only advantageous in severely
disturbed environments, but also in environments where disturbances are of low severity but
nevertheless confer an advantage on individuals that persist. Thus in forest environments where aboveground biomass is seldom destroyed and individuals are relatively long-lived.
resprouting can confer significant fitness and selective advantage on individuals.
Description
Thesis
Keywords
Coastal dune, South Africa, Forest, Resprouting, Multi-stemming