276
Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
within a species can be as large as that across the
seagrass flora, and there can be considerable variability between years and sites in the rate of recruitment of new shoots into populations (Durako, 1994;
Marb` a et al., 1996b). Hence, for the relatively large
species Thalassia testudinum, characterized by moderate rates of vegetative shoot formation, annual recruitment can vary from 0.01–1.30 year
−1 among
populations (Peterson and Fourqurean, 2001). Despite the very low rates of vegetative shoot production in the large seagrass species, however, the much
longer shoot life-span of these species ensure a close
balance between shoot recruitment and losses in stable populations.
Vegetative rather than sexual recruitment is generally considered the primary mechanism to the
maintenance of shoot density within closed seagrass
vegetation. Firstly, the sexual reproductive effort is
low in many seagrass species, the proportion of
shoots that flower being generally less than 10%,
and seed set occur irregularly in many populations
(Duarte et al., 1997b; Durako and Moffler, 1985;
Marb` a and Walker, 1999; Campey et al., 2002). Secondly, large plants suppress the growth of small ones,
such that the entry of new sexual recruits inside areas
occupied by adult genets can be expected to occur
only when established individuals are lost and vacate
space. Most information of seedling recruitment and
establishment come from studies performed outside
established vegetation where it is less problematic
to discern sexual recruits from shoots derived from
already established clones. However, these studies
suggest low survival rates of seeds and newly established seedlings (Hootsmans et al., 1987; Duarte
and Sand-Jensen, 1990a; Harrison, 1993; Kirkman,
1998; Kaldy and Dunton, 1999; Balestri and Cinelli,
2003) supporting the contention that successful sexual recruitment events must be rare within closed
vegetation.
Even though vegetative shoot formation is the
dominant reproductive mode in seagrass meadows,
large differences in recruitment strategies among
species (Inglis, 2000) and considerable variation in
spatial and temporal extent of seed production suggest that sexual recruitment can play a potential role
in meadow maintenance, particularly in populations
where the risk of adult mortality is high, leaving
open space available for seedling establishment and
growth (see Orth et al., Chapter 5). In the extensively studied seagrass Zostera marina, the reproductive effort is highly plastic and populations adopting an annual growth strategy, typically in physically
harsh environments, produce significant number of
seeds (>20,000 seeds m
−2 ) and regenerate completely from seeds each year (Harlin et al., 1982;
Phillips et al., 1983; Phillips and Backman, 1983;
van Lent and Verschuure, 1994). Also, the ability to
accumulate stocks of persistent seeds inside the parent meadow of some of the small, shorter-lived seagrass species producing poorly-dispersed seeds (e.g.
Cymodocea nodosa; Terrados, 1993 and Halophila
spp; McMillan, 1988; Kuo et al., 1993; Preen et al.,
1995; Kenworthy, 2000; also see Ackerman, Chapter
4 and Orth et al., Chapter 5) may promote meadow
persistence following natural senescence of plants
or disturbances by recruiting new sexual propagules. Hence, the relative importance of sexual and
asexual shoot recruitment to meadow maintenance
may vary considerable among species and environments. While sexual recruitment can be critical for
meadow maintenance in highly disturbed and extreme environments inhabited by small shorter-lived
seagrass species, the quantitative importance of sexual recruitment in meadows of larger and longerlived species is low relative to asexual recruitment
and seeds primarily contribute to the establishment
of new patches.
B. Shoot Mortality
Specific shoot mortality rates range greatly both
across seagrass species (Hemminga and Duarte,
2000) and across meadows for any one species
(Marb` a et al., 1996b; Peterson and Fourqurean,
2001), from lowest values of 0.06 year
−1 (i.e. 6%
of shoots dying in a year) for a stand of the longlived Mediterranean seagrass Posidonia oceanica
to a maximum estimated mortality rate of 4.47
year
−1 for Cymodocea nodosa (Fig. 4). These shoot
mortality rates incorporate two additive components, a baseline mortality corresponding to an
internally-controlled mortality rate necessary to
maintain shoot turnover, and a component derived
from stresses and disturbances to the meadows.
Shoot mortality is not only a prominent component of the dynamics of seagrass meadows, but is
indeed a necessary one. In an established, steady
meadow, the continuous recruitment of seagrass
shoots resulting from branching processes cannot be sustained without a parallel mortality of
shoots, as crowding would otherwise impare recruitment. Shoot mortality is, however, insufficiently
Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
within a species can be as large as that across the
seagrass flora, and there can be considerable variability between years and sites in the rate of recruitment of new shoots into populations (Durako, 1994;
Marb` a et al., 1996b). Hence, for the relatively large
species Thalassia testudinum, characterized by moderate rates of vegetative shoot formation, annual recruitment can vary from 0.01–1.30 year
−1 among
populations (Peterson and Fourqurean, 2001). Despite the very low rates of vegetative shoot production in the large seagrass species, however, the much
longer shoot life-span of these species ensure a close
balance between shoot recruitment and losses in stable populations.
Vegetative rather than sexual recruitment is generally considered the primary mechanism to the
maintenance of shoot density within closed seagrass
vegetation. Firstly, the sexual reproductive effort is
low in many seagrass species, the proportion of
shoots that flower being generally less than 10%,
and seed set occur irregularly in many populations
(Duarte et al., 1997b; Durako and Moffler, 1985;
Marb` a and Walker, 1999; Campey et al., 2002). Secondly, large plants suppress the growth of small ones,
such that the entry of new sexual recruits inside areas
occupied by adult genets can be expected to occur
only when established individuals are lost and vacate
space. Most information of seedling recruitment and
establishment come from studies performed outside
established vegetation where it is less problematic
to discern sexual recruits from shoots derived from
already established clones. However, these studies
suggest low survival rates of seeds and newly established seedlings (Hootsmans et al., 1987; Duarte
and Sand-Jensen, 1990a; Harrison, 1993; Kirkman,
1998; Kaldy and Dunton, 1999; Balestri and Cinelli,
2003) supporting the contention that successful sexual recruitment events must be rare within closed
vegetation.
Even though vegetative shoot formation is the
dominant reproductive mode in seagrass meadows,
large differences in recruitment strategies among
species (Inglis, 2000) and considerable variation in
spatial and temporal extent of seed production suggest that sexual recruitment can play a potential role
in meadow maintenance, particularly in populations
where the risk of adult mortality is high, leaving
open space available for seedling establishment and
growth (see Orth et al., Chapter 5). In the extensively studied seagrass Zostera marina, the reproductive effort is highly plastic and populations adopting an annual growth strategy, typically in physically
harsh environments, produce significant number of
seeds (>20,000 seeds m
−2 ) and regenerate completely from seeds each year (Harlin et al., 1982;
Phillips et al., 1983; Phillips and Backman, 1983;
van Lent and Verschuure, 1994). Also, the ability to
accumulate stocks of persistent seeds inside the parent meadow of some of the small, shorter-lived seagrass species producing poorly-dispersed seeds (e.g.
Cymodocea nodosa; Terrados, 1993 and Halophila
spp; McMillan, 1988; Kuo et al., 1993; Preen et al.,
1995; Kenworthy, 2000; also see Ackerman, Chapter
4 and Orth et al., Chapter 5) may promote meadow
persistence following natural senescence of plants
or disturbances by recruiting new sexual propagules. Hence, the relative importance of sexual and
asexual shoot recruitment to meadow maintenance
may vary considerable among species and environments. While sexual recruitment can be critical for
meadow maintenance in highly disturbed and extreme environments inhabited by small shorter-lived
seagrass species, the quantitative importance of sexual recruitment in meadows of larger and longerlived species is low relative to asexual recruitment
and seeds primarily contribute to the establishment
of new patches.
B. Shoot Mortality
Specific shoot mortality rates range greatly both
across seagrass species (Hemminga and Duarte,
2000) and across meadows for any one species
(Marb` a et al., 1996b; Peterson and Fourqurean,
2001), from lowest values of 0.06 year
−1 (i.e. 6%
of shoots dying in a year) for a stand of the longlived Mediterranean seagrass Posidonia oceanica
to a maximum estimated mortality rate of 4.47
year
−1 for Cymodocea nodosa (Fig. 4). These shoot
mortality rates incorporate two additive components, a baseline mortality corresponding to an
internally-controlled mortality rate necessary to
maintain shoot turnover, and a component derived
from stresses and disturbances to the meadows.
Shoot mortality is not only a prominent component of the dynamics of seagrass meadows, but is
indeed a necessary one. In an established, steady
meadow, the continuous recruitment of seagrass
shoots resulting from branching processes cannot be sustained without a parallel mortality of
shoots, as crowding would otherwise impare recruitment. Shoot mortality is, however, insufficiently