Demography

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To date, demographic analyses of the genets (an individual originated from a seed, sensu Harper, 1977) of Thalassia have not been attempted because following a genet in the field is virtually impossible. Tracing the shoots of an individual in the field is difficult because their interconnecting horizontal rhizomes occur belowground and are entangled. Also, the connections between the shoots break with time, either through mechanical damage or decay of the oldest end of the horizontal rhizomes, thus a genet consists of many separate rhizomes sections (i.e. clones), and it is impossible to tell one genet apart from another. In contrast, the shoots arising from the horizontal rhizomes, which can considered being the ramets (sensu Harper, 1977), are easily identified, and techniques of age determination of the shoots (Patriquin, 1973; Duarte et al., 1994) make it possible to study their demography.

The longevity of the shoots can be determined by 'translating' their size (expressed as the number of scars of abscised leaves) into age using information based on the number of leaves produced yearly. For example, a shoot with 30 leaf scars plus standing leaves, which produces ten leaves per year, has an age of 3 years. There exist various techniques for the determination of annual leaf production, or its derivative, annual mean Plastochron Interval, which was defined by Patriquin (1973) as the difference in age between two successive leaves, measured when they have reached the same stage of development. The techniques applied to Thalassia spp. are: (1) Leaf marking, which determines the rate of leaf initiation per foliar shoot (Durako, 1994; van Tussenbroek, 1994a; Kaldy et al., 1999; Peterson and Fourqurean, 2001), (2) differences in internode's length of the vertical rhizomes (Gallegos et al., 1993; Marba et al., 1994), (3) number of leaves formed between successive flower scars (Gallegos et al., 1992; van Tussenbroek, 1994a), and (4) annual cohort analysis based on size frequency histograms of the vertical rhizomes (Durako, 1994; Duarte et al., 1994). All of these techniques, except for leaf marking, utilize cyclical variations caused by seasonal differences in vertical rhizome morphology to indicate the passage of a year, and the number of leaf scars between these seasonal imprints is considered to be the number of leaves produced in that year. The important underlying assumption of all techniques of age-determination is that the number of leaves produced annually does not differ significantly between years, but only a few authors have tested this, and results vary from place to place. T. testudinum showed large inter-annual variability in annual leaf formation rates in the Laguna Madre, Texas (Kaldy et al.,

1999), whereas this species showed similar annual rates of leaf production over a period of 13 years in a tropical Mexican reef lagoon (van Tussenbroek, 2002), and neither Durako (1994) nor Peterson and Fourqurean (2001) found significant differences in annual mean leaf emergence rates between two consecutive sampling years at a large number of sites in Florida Bay.

The ideal study of shoot demography would be to mark a large number of shoots and follow their development through time, allowing for production of a dynamic life-table, but this requires excavation of the rhizomes, which is virtually impossible without considerable damage. Therefore, all demographic studies have involved examination of the shoot population sampled at one particular time, resulting in a static age-distribution. Applications of such static age-distributions have been many-fold; and amongst others they have been used to retrospectively assess flowering events and the effects of a hurricane on T. testudinum in the Mexican Caribbean (Gallegos et al., 1992; Marba et al., 1994; van Tussenbroek, 1994a,b). The principal application of the static age-distributions has been for the derivation of population growth rate and related parameters (such as mortality, recruitment, shoot turnover rates, and half life), according to the model proposed by Duarte et al. (1994), which has been used for populations of both T. testudinum (Gallegos et al., 1993; Durako, 1994; Peterson and Fourqurean, 2001) and T. hemprichii (Vermaat et al., 1995; Rollon et al., 2001). There are questions, however, over whether these static models can be applied to determine population growth (see Durako and Duarte, 1997; Jensen et al., 1997; Kaldy et al., 1999; van Tussenbroek,

2000), or whether the prerequisites of constant annual shoot mortality and recruitment rates and age-independent mortality, required by this model are tenable (Jensen et al., 1996; Kaldy et al., 1999; van Tussenbroek, 2000; Peterson and Fourqurean, 2001). Another application of shoot demography has been that of van Tussenbroek (2002), who applied a classic static life-table analysis to various stable shoot-populations of T. testudinum, and found that mortality of the shoots was age-dependent, with lower mortality for the younger shoots. This is not uncommon for clonal plants, and is an indication of support of the younger ramets by the older ones (Sarukhan and Harper, 1973; Hartnett and Baz-zaz, 1983). Also, elasticity analysis of a derived Leslie matrix showed that ramification of 1-year-old shoots was by far more important to net population growth than survival of the foliar shoots of all other age classes. The latter indicates that proliferation through branching is not only important for expansion of populations to nearby virgin areas (see above), but also for maintenance of established Thalassia populations.

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