HOW EMERGENT PLANTS EXPERIENCE WATER REGIME IN A MEDITERRANEAN-TYPE WETLAND

In Southern Australia where water levels fluctuate and are naturally v ariable, the importance of water regime, as distinct from water depth, was shown and the difference between depth and elevation gradients wa s verified. The water regimes at sites across an elevation gradient we re described and quantified. The Sum Water Regime, an index of the hyd rological past that plants had experienced, was a summary of depth and duration of inundation. It was calculated over monthly intervals (1, 3, 6, 12 months) prior to plant harvests and on a daily scale (for eac h record of water level: 1-30 days). It identified the response times of two emergent species at the level of the population (live abovegrou nd mass, g m(-2), and shoot density, number per m(2)). Triglochin proc erum R.Br. had a rapid response time with population changes matching changes in depth. In contrast, Baumea arthrophylla (Nees) Boeckeler ha d a slower response time with its characteristics correlated to the pa st water regime integrated over 6-12 months. At the level of photosynt hetic organs, the change in the mean heights of T. placer um leaves ac ross a spring depth gradient, paralleled the change in the maximum hei ghts. Height and diameter also changed in proportion. For B. arthrophy lla, the maximum height of its stems increased across the sampled grad ient, but the mean heights fell at the deeper sites. Coupled with the finding that increases in diameter did not keep pace with increases in height, these results showed that T. procerum, unlike B. arthrophylla , was well within its depth limits. Populations were sampled across th e range of water regimes they experienced. This highlighted the signif icant effect small changes in water regime have on the structure and p erformance of aquatic plants. Measuring the range of characteristics, not only identifies the upper and lower limits, but may be used to pre dict the condition of other populations.