Riparian vegetation varies according to hydrogeomorphic processes operating across different scales over two didmensions: transversely (across-stream) and longitudinally (parallel to stream). We tested the hypothesis that vegetation patterns reveal the scale and direction of underlying processes. We correlated patterns of dominant woody vegetation with environmental variables at 28 sites located within four geomorphologically distinct regions along the length of the South Alligator River catchment of Kakadu National Park, northern Australia. Across the catchment there existed a strong transverse boundary between upland savanna vegetation and two zones of riparian vegetation: Melaleuca-spp.-dominated closed-forest vegetation along stream channels and mixed open-woodland vegetation adjacent to closed forest. We surmise that there is hierarchic constraint on smaller-scale catchment processes due to fire incursion into the riparian zone and access to water during the dry season. Within the closed-forest zone, vegetation did not vary transversely, but did longitudinally. Riparian woodlands also varied longitudinally, but in the upper reaches varied independently of stream variables. By contrast, in the lower reaches woodland was strongly correlated with stream variables. The observed pattern of weak transverse linkages in headwaters but strong linkages in lower reaches is analogous to models developed for in-stream patterns and processes, particularly the river continuum and flood-pulse concepts.

The reproductive biology of female pig-nosed turtles Carettochelys insculpta was studied for 4 years in the wet–dry tropics of northern Australia. Females matured at around 6 kg body mass (38.0 cm curved carapace length, 30.5 cm plastron length). Turtles produced egg sizes and clutch sizes similar to that of other turtle species similar in size. Turtles reproduced every second year, but produced two clutches within years, about 41 days apart. Thus, it appeared that females were energy limited, possibly due to the low available energy content of the dry season diet (aquatic vegetation). Life-history theory predicts that some costly behaviour associated with reproduction exists, such that by skipping years turtles could reduce that cost and put the savings into future reproduction. Previous work revealed no behaviour associated with reproduction in the population. Within years, clutch mass did not differ between early (first) and late (second) clutches. However, early clutches tended to have more eggs per clutch but smaller eggs than late clutches, a new finding for turtles that has been demonstrated in lizards and other animals. Because the study spanned both years with ‘big’ and ‘small’ wet seasons, we were able to examine how the magnitude of the wet season influenced reproductive characteristics. Following big wet seasons, turtles produced larger, heavier, and more eggs per clutch than they did after small wet seasons. Relationships among body size, egg size and clutch size were evident after two big wet seasons but not apparent after two small wet seasons. Collectively, annual variation in reproductive characteristics and current life-history theory suggest that a big wet season is a time of high energy accumulation for the turtles.