Chapter 11 Dynamics of Seagrasses
281
IV. Clones and Patch Dynamics
A. Processes of Patch Formation
The spatial structure of seagrass populations is
highly variable among sites ranging from extant,
nearly continuous meadows to meadows that are
highly fragmented and arranged into a mosaic of
discrete patches. Patchy seagrass vegetation often
reflects processes of recovery from disturbances,
whether natural or human-induced, that occurred at
different times in the past, as well as the particular hydrodynamic conditions of the seagrass habitats (cf. Bell et al., Chapter 26). Seagrass meadows
have, therefore, not only spatial but also temporal
dynamics involving the continuous recruitment, expansion, and mortality of patches. Hence, knowledge
of these dynamic properties is essential to gain insight into the dynamics and persistence of seagrass
populations.
Patches may result from fragmentation or colonization processes. Loss of seagrass cover may
lead to fragmented beds resulting in a patchy,
rather than continuous meadow distribution. Alternatively, patches may result from a colonization process, where propagules, whether established
seeds or vegetative fragments initiate clonal growth,
thereby producing a patch. Patch formation through
seedling establishment has been well documented
(e.g. Duarte and Sand-Jensen, 1990a; Olesen and
Sand-Jensen, 1994b; Vidondo et al., 1997), although
estimates of patch formation rates are still few.
In contrast, patch formation through the anchoring of detached vegetative fragments has received
limited attention (e.g. Campbell, 2003), although
it may be an important process for seagrass patch
formation.
Seedling establishment is a precondition but not
a sufficient condition for patch formation, as available evidence suggests that many seedlings may die
without ever producing patches (e.g. Duarte and
Sand-Jensen, 1990a; Olesen and Sand-Jensen, 1994;
Olesen et al., 2004). For instance, a study of a Cymodocea nodosa population growing in a patchy lagoon showed that only small fractions of established
seedlings initiated patch formation through clonal
growth (Duarte and Sand-Jensen, 1990a). Failure to
initiate clonal growth was attributed, in this particular population, to nutrient limitation (Duarte and
Sand-Jensen, 1996).
B. Patch Growth and Loss
Seagrass patch growth proceeds by the horizontal
extension of rhizomes at the patch edge and the
subsequent branching and vegetative production of
new shoots at the rhizome apex to fill out the open
space between expanding rhizomes. The branching
frequency and the angle between the horizontal rhizome and the rhizome branches that are formed on it
are, therefore, important determinants of the capacity to spread in two dimensions (Marb` a and Duarte,
1998). However, the main controlling factor on the
patch growth rate is the elongation of horizontal rhizomes, extending the patch through its periphery.
Realized patch growth rates may be lower than the
potential rates set by rhizome extension rate whenever sediment dynamics and hydrodynamics interfere with plant growth or create disturbance (cf. Bell
et al., Chapter 26).
The elongation rate of horizontal rhizomes is
species specific (Table 2) and range from about
2 cm year
−1 in the large slow-growing species as
Enhalus acoroides and Posidonia oceanica to more
than 300 cm year
−1 in small fast-growing species as
Halophila ovalis (Duarte, 1991; Marb` a and Duarte,
1998). The close, negative scaling between rhizome
elongation rates and seagrass module size, suggests
that shoot size is a strong predictor of patch extension through clonal growth for the different seagrass
species.
The maximal rate of rhizome growth sets the
upper rate of patch extension possible although
this capacity is not necessarily realized in natural patches. Seagrasses display considerable plasticity in formation rates and size of modules
(Duarte, 1991). Variability in rhizome growth often has a distinct seasonal pattern, particularly
in temperate and subtropical climates, where rhizome growth is minimized during winter as a result of low light and temperature conditions. Rhizome growth can also be expected to respond
to resource availability, e.g. through enhanced
elongation rates in deep growing stands, thereby
reducing internal self-shading by increased distance between neighboring shoots (Olesen et al.,
2002). This response pattern does not apply to
all species, however, and experimental evidence
is needed to evaluate the adaptive significance of
seagrass rhizome growth to various environmental
conditions.
281
IV. Clones and Patch Dynamics
A. Processes of Patch Formation
The spatial structure of seagrass populations is
highly variable among sites ranging from extant,
nearly continuous meadows to meadows that are
highly fragmented and arranged into a mosaic of
discrete patches. Patchy seagrass vegetation often
reflects processes of recovery from disturbances,
whether natural or human-induced, that occurred at
different times in the past, as well as the particular hydrodynamic conditions of the seagrass habitats (cf. Bell et al., Chapter 26). Seagrass meadows
have, therefore, not only spatial but also temporal
dynamics involving the continuous recruitment, expansion, and mortality of patches. Hence, knowledge
of these dynamic properties is essential to gain insight into the dynamics and persistence of seagrass
populations.
Patches may result from fragmentation or colonization processes. Loss of seagrass cover may
lead to fragmented beds resulting in a patchy,
rather than continuous meadow distribution. Alternatively, patches may result from a colonization process, where propagules, whether established
seeds or vegetative fragments initiate clonal growth,
thereby producing a patch. Patch formation through
seedling establishment has been well documented
(e.g. Duarte and Sand-Jensen, 1990a; Olesen and
Sand-Jensen, 1994b; Vidondo et al., 1997), although
estimates of patch formation rates are still few.
In contrast, patch formation through the anchoring of detached vegetative fragments has received
limited attention (e.g. Campbell, 2003), although
it may be an important process for seagrass patch
formation.
Seedling establishment is a precondition but not
a sufficient condition for patch formation, as available evidence suggests that many seedlings may die
without ever producing patches (e.g. Duarte and
Sand-Jensen, 1990a; Olesen and Sand-Jensen, 1994;
Olesen et al., 2004). For instance, a study of a Cymodocea nodosa population growing in a patchy lagoon showed that only small fractions of established
seedlings initiated patch formation through clonal
growth (Duarte and Sand-Jensen, 1990a). Failure to
initiate clonal growth was attributed, in this particular population, to nutrient limitation (Duarte and
Sand-Jensen, 1996).
B. Patch Growth and Loss
Seagrass patch growth proceeds by the horizontal
extension of rhizomes at the patch edge and the
subsequent branching and vegetative production of
new shoots at the rhizome apex to fill out the open
space between expanding rhizomes. The branching
frequency and the angle between the horizontal rhizome and the rhizome branches that are formed on it
are, therefore, important determinants of the capacity to spread in two dimensions (Marb` a and Duarte,
1998). However, the main controlling factor on the
patch growth rate is the elongation of horizontal rhizomes, extending the patch through its periphery.
Realized patch growth rates may be lower than the
potential rates set by rhizome extension rate whenever sediment dynamics and hydrodynamics interfere with plant growth or create disturbance (cf. Bell
et al., Chapter 26).
The elongation rate of horizontal rhizomes is
species specific (Table 2) and range from about
2 cm year
−1 in the large slow-growing species as
Enhalus acoroides and Posidonia oceanica to more
than 300 cm year
−1 in small fast-growing species as
Halophila ovalis (Duarte, 1991; Marb` a and Duarte,
1998). The close, negative scaling between rhizome
elongation rates and seagrass module size, suggests
that shoot size is a strong predictor of patch extension through clonal growth for the different seagrass
species.
The maximal rate of rhizome growth sets the
upper rate of patch extension possible although
this capacity is not necessarily realized in natural patches. Seagrasses display considerable plasticity in formation rates and size of modules
(Duarte, 1991). Variability in rhizome growth often has a distinct seasonal pattern, particularly
in temperate and subtropical climates, where rhizome growth is minimized during winter as a result of low light and temperature conditions. Rhizome growth can also be expected to respond
to resource availability, e.g. through enhanced
elongation rates in deep growing stands, thereby
reducing internal self-shading by increased distance between neighboring shoots (Olesen et al.,
2002). This response pattern does not apply to
all species, however, and experimental evidence
is needed to evaluate the adaptive significance of
seagrass rhizome growth to various environmental
conditions.