PARENT SESSION
Contributed Oral Session 40: Forest Understories
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 514 A, Level 5, Palais des congrès de Montréal

Dispersal vs. recruitment limitation of fern populations in forests of different history.

Flinn, Kathryn^*,1, ¹ Cornell University, Ithaca, NY, kmf27@cornell.edu

ABSTRACT- Plant population dynamics and species distributions may be limited either by the availability of propagules or by the availability of suitable sites, and the relative importance of these mechanisms remains a fundamental question in plant ecology. Landscapes with a history of agriculture provide an ideal opportunity to assess the influence of dispersal vs. recruitment limitation on current vegetation, as species vary in their associations with forests that were never cleared (primary) and forests on abandoned agricultural lands (secondary). To examine processes underlying this pattern, I focused on several fern species of northern hardwood forests in central New York, Polystichum acrostichoides, Dryopteris carthusiana and D. intermedia, that show contrasting distributions despite similar capacities for long-distance dispersal. First, I measured numbers of fern spores released in the wind and stored in the soil. Though about twice as many spores were deposited in primary forests as in secondary, and primary forest soils contained slightly more spores, even secondary forests without adult plants received 88,000 P. acrostichoides spores/m² over one growing season. To compare germination, establishment and juvenile survival among species, between forest types and among microsites, I conducted a three-year field experiment, apparently the first to address the critical juvenile stages of the fern life cycle. I sowed spores and monitored plants on different forest-floor substrates and with or without elevated humidity in paired primary and secondary forests. All species had the highest rates of germination, establishment and survival on mineral soil at high humidity, regardless of forest type. Finally, I quantified the availability of different microsites in the two forest types and surveyed the natural distribution of juvenile plants across these habitats. Some sites that juvenile plants clearly preferred, such as rotten logs and exposed tree roots, were more common in primary forests than in secondary. Together these results suggest that microsite availability overrides spore dispersal in determining the distributions of these species, and that small-scale habitat heterogeneity facilitates the recovery and maintenance of diversity in these forests.

Key words: land-use history