Growth, flowering, and population dynamics of temperate Western Australian seagrasses

Abstract

Quantification of module size, leaf, rhizome and clonal growth, flowering intensity and shoot population dynamics of 7 temperate Western Australian seagrasses [Amphiboljs antarctica, A. griffithii, Posidonia australis, P, sinuosa, P. angustifolia, Heterozostera tasmanica, Thalassodendron pachyrhizum) developing 8 n~onospecific stands reveals that these plants have different plant morphologies, display a wide repertoire of growth patterns, and exhibit substantial variabhty in their capacity to flower. Leaf production rate ranged between 2.6 (P. sinuosa) and 26 leaves yr.' (A. antarctica), and leaf life-span varied between 85 (A. antarctica) and 245 d (P. sinuosa). Most of the species extended their horizontal rhizomes at rates slower than 10 cm yr-' The highest rate of new short shoot formation by vertical rhizomes was observed in A. antarctica (on average each vertical rhizome annually produced 1.5 new short shoots), and the lowest in T pachyrhizum (only l new short shoot would be recruited from 65 vertical rhizomes). Horizontal rhizomes produced 1 new short shoot every l? (H. tasrnanica) or 582 d (P. sinuosa). Flowering intensity varied from 0 (P. sinuosa) to 7 % flowering short shoots yr-' (P. australis). The median age of the short shoots in these populations ranged from 367 (H. tasmanica] to 1000 d (P. sinuosa). Clonal growth was the main mechanism providing short shoots in these temperate Western Australian meadows. Short shoot recruitment balanced short shoot mortality rate in most populations, indicating that they were in steady-state with the colonisation process. These species maintain their meadows through different plant strategies as a result of their different growth programmes and the variability in sexual reproduction success. The large differences in flowering intensity found across temperate Western Australian seagrasses suggest that sexual reproduction may play an important role for meadow maintenance and recovery.

Recovery from the impact of light reduction on the seagrass Amphibolis griffithii, insights for dredging management

“…Generally, recovery in these species takes years, decades or has been predicted to take centuries (Boese et al, 2009;Bryars and Neverauskas, 2004;Collier et al, 2009;Gonzalez-Correa et al, 2005;Hammerstrom et al, 2007;Neckles et al, 2005). Interestingly, this relatively fast recovery occurred despite up to 72% loss of leaf biomass, highlighting the fast leaf production rates of Amphibolis compared to other large seagrasses (Marba and Walker, 1999), and high recovery potential if actively growing clusters (up to 42%) remain on the stem from which new leaves can form. However, it should be noted that belowground material, a significant component of these rhizomatous plants (~ 50% relative to the above-ground biomass (Lavery et al, 2009)) was not taken into account in assessing recovery.…”

“…Aspects of A. griffithii's morphology and growth characteristics are important to understand in the context of predicting recovery from impact. Amphibolis is placed towards the centre of the seagrass functional form model (Walker et al, 1999). This clonal plant is composed of underground roots and rhizomes with a vertical, branching stem that holds terminal leaf clusters (Cambridge, 1999).…”

“…There are generally 2-5 leaves per cluster and 6-20 clusters per vertical stem (Cambridge, 1999;Ducker et al, 1977). Stems are long lived, generally 2-3 years (den Hartog, 1970) whilst leaves are much shorter lived, generally 90 days (Marba and Walker, 1999).…”

“…The plastochrone interval of vertical stems (short shoots or branches of a stem) is 277 days, horizontal rhizome 509 days and leaves 32 days (Marba and Walker, 1999). Upright stems are produced every 4-6 horizontal rhizome internodes and branches are produced every 3-17 vertical stem internodes (Coupland, 1997).…”

“…Recovery following severe impact will be slower than that following moderate impact as it depends on stem recruitment, which is slower than leaf production (Marba and Walker, 1999) (i.e. the plastochrone interval for stems is 259 days vs. 32 for leaves (Marba and Walker, 1999)) and seedlings are only produced for a few months during a year (Walker et al, 2001).…”