At Kestrel Bay, marram grass (Ammophila arenaria) grows on exposed foredunes above the high-tide line, where the sand is dry and shifting and winds are strong. Grey mangrove trees (Avicennia marina) grow in the lower intertidal estuary, in a mangrove swamp with waterlogged sediment. Marram leaves roll inward during dry, windy periods; the stomata are on the inner surface of the rolled leaves. Grey mangroves have aerial roots that project above the sediment and have openings exposed to the air.
Table 1. Conditions measured in the two habitats
| Habitat | Soil water content (%) | Pore-water salinity (g kg⁻¹) | Soil oxygen (%) |
|---|
| Foredune | 6 | 2 | 18 |
| Mangrove swamp | 68 | 25 | 3 |
To estimate the number of grey mangrove trees in a 0.80 ha area of swamp, six non-overlapping 100 m² quadrats were selected at random. The numbers of trees counted were:
| Quadrat | 1 | 2 | 3 | 4 | 5 | 6 |
|---|
| Trees counted | 8 | 11 | 9 | 10 | 7 | 9 |
Seedlings of both species were also grown for eight weeks under two substrate conditions. The “dune-like” and “swamp-like” treatments reproduced the corresponding moisture, salinity and soil oxygen conditions in Table 1. Light, temperature and nutrient availability were kept constant. Each value is the mean dry-mass increase per seedling for 10 seedlings.
Table 2. Mean dry-mass increase after eight weeks
| Species | Dune-like substrate (g) | Swamp-like substrate (g) |
|---|
| Marram grass | 2.4 | 0.5 |
| Grey mangrove | 0.7 | 2.2 |