The flooding tolerance of two critical habitat-forming wetland shrubs, Leptospermum lanigerum and Melaleuca squarrosa, at different life history stages

Abstract

Understanding the effect of water regime on the different life history stages of woody wetland plants is essential to managing their persistence. The common and widespread myrtaceous shrub species, Melaleuca squarrosa Donn. ex Sm. and Leptospermum lanigerum (Aiton) Sm., provide habitat for two critically endangered fauna within the Yellingbo Nature Conservation Reserve (south-eastern Australia), but are in decline putatively because of the altered flooding regimes. We, thus, tested the effects of flooding depth and duration on their seed germination and seedling establishment, and seedling growth and survival in two separate glasshouse experiments. We also compared the condition of mature plants of both species at an intermittently flooded (reference) site, and two near permanently flooded (impact) sites. Seeds of both species were able to germinate underwater, but early flooding reduced seedling establishment. Seedling growth of both species was greater in waterlogged than in well drained or inundated conditions, whereas no seedlings of either species survived >8 weeks of submergence. Leptospermum lanigerum seedlings were generally more flood tolerant than were M. squarrosa seedlings. Correspondingly, crown condition of mature M. squarrosa, but not L. lanigerum, was poorer at impact than reference sites. Prolonged flooding in swamp forests is likely to (1) limit woody plant recruitment, because flooding reduces seedling establishment, growth and survival, and (2) be deleterious to the maintenance of less flood-tolerant species (e.g. M. squarrosa). Moist exposed substrate is likely to be best for promoting the recruitment of both study species, and intermittent flooding for maintaining adult M. squarrosa plants.