The distribution, growth, reproduction and population genetics of a mangrove species, Rhizophora stylosa Griff, near its southern limits in New South Wales, Australia
Creators: name: Wilson, Nicholas
Publisher: name: Australian Catholic University
Year: 2009

description: Rhizophora stylosa is a common and widespread mangrove species on tropical coasts in the Indo-West Pacific and very common and widespread in northern Australia. This aim of this study was to investigate R. stylosa's distribution, growth, reproduction and population genetics over the last 300 kms of its range at its southern limits in northern New South Wales on the Australian east coast. Rhizophora stylosa was found to be more widespread between and within estuaries in northern New South Wales than previously recorded, with evident spread over recent decades. A new southern limit of South West Rocks Creek was determined, with only two mature trees are present. Rhizophora stylosa is in small numbers in most the 16 estuaries now known to contain R. stylosa in NSW. Its distribution is illustrated from intra-estuarine scales in small 'incipient' stands developing within estuaries to the colonisation of new estuaries. Despite the evidence of recent population spread, several restricted old stands were recorded, indicating a limited presence of R. stylosa at high latitudes for much longer periods, somewhat complicating a simple model of expanding range with recently warming climate. Mangrove species are generally understood to be limited by cold, but significant provisos on ascribing R. stylosa's recent spread to the warming trends of the last several decades exist. A shoot tracking methodology was applied for 2.5 years in three estuaries, including the most southerly, and detected little if any reduction of growth over a range of 260 km. Rhizophora stylosa has a reiterative mode of growth and an average of three leaf emergences (6 leaves) per annum was found. The growth results overall are comparable to some tropical studies, particularly if herbivory is accounted for, and R. stylosa appears not to be at its thermal tolerance at its known southern limits.;Leaf longevity was greater than comparable tropical studies, suggesting the trees were compensating for climatic factors in this manner. A reproductive study conducted at the same time on the same shoots found trees reproduce successfully even at the southern limits, with little or no evidence of a decline with latitude across the study area. Only a small proportion of buds finally form propagules (overall 2.2%), but again fecundity is comparable to tropical studies, although the full reproductive cycle from flower bud primordia to viviparous propagule may be slightly longer. A generally low level of genetic diversity measured as allelic richness and heterozygosity and evidence of inbreeding was detected during the genetic study on samples from eight New South Wales estuaries, three localities in Moreton Bay and eight localities in north Queensland. Spatially, the New South Wales and Moreton Bay localities are from one pool, but there is differentiation between localities and little geographic coherence along the coast. Different origins and histories are likely for the estuaries, supporting a hypothesis arising from the distribution of R. stylosa in New South Wales that colonisation from the ocean has been and continues to be sporadic on the northern New South Wales coast. The major variation in the genetic data is between North Queensland and the southern localities, as expected, although relatedness remains, raising interesting biogeographic questions on current or recently historic gene flow between distant populations. Many questions remain about R. stylosa at the ends of its range, but there are indications that it is well within its tolerance ranges in northern New South Wales, perhaps contrary to some expectations of a mangrove species at its southern limits.