C.1 Biology and Ecology relevant to threC.1 Biology and Ecology relevant to threatening processes

C.1 Biology and Ecology relevant to threatening processes

Asplenium listeri is a small terrestrial fern with shortly creeping rhizomes and fronds up to 9cm long held in a crown. The species is described (DuPuy 1993b) as an occasional (rare) component of the sparse vegetation community characteristic of inland cliffs which rise above marginal rainforest on the terraces. It is not recorded from the other vegetation zones (being primary rainforest; marginal terrace rainforest; open, scrubby and vine forests; coastal fringe; shorecliffs/spray zone; and mined areas).

Asplenium listeri appears to be quite specialised in its habitat of limestone rock crevices in otherwise dry and exposed sites, where few other species are found. These sites tend to be rocky and narrow cliff-top strips up to 15 metres wide, between a very open aspect on the seaward side and a forest structure increasing up to 40 metres high on the inland side (Reddell pers. comm.). Such situations are well placed to interrupt and capture moist flow from south-easterly trade winds (Tranter pers. comm.). Some sites may be partly shaded, beneath or near Ficus microcarpa (Holmes & Holmes 2002).

It is highly likely that distribution and propagation of the species are related to moisture retention. While the island experiences generally high rainfall, the limestone outcrops are highly porous due to joints and solution. As a result, surface water is rare, being restricted to perennial springs. These are effluxes associated with the junction of limestone with underlying basalt, and surface flow continues where they flow over volcanic rock. Most effluxes lie at elevations below the occurrences of Asplenium listeri (which are >100 metres elevation).

It is interesting to note that the first collection of the species by Lister took place at the peak of the dry season and probably on the northern side of the island (DuPuy 1993b). The specimen that Lister collected may have been in an ideal microhabitat, perhaps associated with a seep, or it may have been collected after unseasonal rain. In 2001 specimens kept at a nursery on Christmas Island would apparently die off when deprived of water for a short period and quickly recover when watered again. This suggests that the vegetative parts would more normally die off during the dry season, and re‑sprout during the next wet (Sewell pers. comm.). It was observed that the specimens did not appear to benefit from regular abundant irrigation, as befits the habitat preferences of the species (Claussen pers. comm.).

The habit of growth in rock crevices is likely to afford the rhizome a degree of protection from drying out in the dry season, a protective effect heightened by accumulation of organic debris as mulch in the crevices (Hart pers. comm.). In other situations, this habit may also protect the rhizome from occasional fire, which is not currently seen as an issue on Christmas Island (Claussen pers. comm; Reddell pers. comm.)