All seagrasses are rhizomatous plants that grow by reiteration of a limited set of modules. Their past growth history can therefore be reconstructed from the scars left by abscised leaves and flowers on the long-lived rhizomes or the seasonal slgnals Imprinted in the frequency and size of their modules. We provide here the basic foundations and assumptions of these reconstruction techniques and the calculations involved in their application. We then show their reliability and potential to quantlfy an array of ecological processes, such as plant demography, leaf and rhlzome production, flowering ~ntensity, and seagrass responses to anthropogenic perturbations, based on our recent studies of Mediterranean, Caribbean and Indo-Pacific seagrass species. Reconstruction techniques have also proven useful in demonstrating the role of seagrasses as tracers of sedlment movement over seagrass beds and the rates of colonisation and expansion of seagrass patches. These reconstruction techniques should provide a powerful tool to improve our knowledge of seagrass species and populations from remote areas based on a single or just a few visits This should, therefore, allow us to sample many seagrass meadows using limited resources, thus generating a strong foundation for the study of comparatlve seagrass ecology and testing of theories previously applied to terrestrial plant populations.
ABSTRACT. We report here data on biomass and dynamics of shoots and rhizomes of Thalassia testudinum in the Mexican Caribbean based, primarily, on aging of plant material. We found 7 testudinum to be able to reach high biomass (> 1000 g dry wt m-') and develop highly productive populations (1500 to 4500 g dry wt m-' annually, of which 10% were allocated to the rhizomes) in the Mexican Caribbean. The plants appeared to grow slowly, as reflected in longer time intervals in between production of leaves and shoots, and slower rhizome elongation rates and leaf production rates than found in the past, possibly as a result of their low nutrient, particularly phosphorus, content. Calculated maximum shoot life spans ranged between 6 and 9 yr, shoot recruitment rates ranged from 0.02 to 0.07 In units per plastochrone interval (PI) and shoot mortality rate was substantial and averaged about 0.045 In units PI-' Shoots turned over at ca 0.6 yr-', which also represents the rhizome turnover rate.
ABSTRACT. We examined the leaf age dependence, magnitude and variability of herbivore consumption of Posidonia oceanica (L.) Delile leaves in the Spanish Mediterranean Two patterns of herbivore consumption along the life-span of P. oceanica leaves were found, namely a linear and a parabolic curve of cumulative leaf consumption versus leaf age, which are indicative of leaf-age-independent consumption rates and preference for mid-aged leaves, respectively. The flsh Boops salpa contributed about 75% to the total herbivore consumption, and seemed to be responsible for the parabolic pattern of consumption with leaf age. Herbivory appeared to be a minor factor in the control of P. oceanica production since it only accounted, on the average, for about 2 % of its leaf production. This percentage tends to increase in the northern populations of the Spanish Mediterranean as a result of the longer leaf life-span and associated higher cumulative consumption values, and the lower leaf production of these populations. It is concluded that, in spite of the low levels of herbivory on P oceanica in the Mediterranean, this seagrass supports a high herbivore production due to its large primary production.
The shoot demography and rhizome growth of Syringodium filiforme Kutz. and Halodule wrightii Aschers. were studied, based on plant dating techniques, to account for their role as pioneer in the succession sequence of Canbbean seagrasses. Results demonstrated that these species are able to develop dense meadows, supporting biomasses in excess of 500 g DW m" They produced more than 2000 g DW m -2 yr-' due to their high leaf (5.0 to 8 5 yr.') and rhizome (2 0 to 3.3 yr-') turnover. Rhizome growth and branch~ng rates were very high, allowing these seagrasses to rapidly occupy the space they colonise. The rapid rhizome turnover involved, however, a high shoot mortality rate and low 11fe expectancy (average shoot life expectancy 100 to 180 d). This implies that, while these pioneer species are able to rapidly occupy the space they colonise, their established shoots cannot occupy that space for a s long a s the more longlived species Thalassia testudinum. We suggest, therefore. that the role of seagrass species a s pioneer or climax species is independent of their capacity to support dense, productive populations, and is closely related to shoot longevity and rhizome turnover.
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