Fluvial seed dispersal processes and the sustainability of riparian vegetation

Abstract

Hydrochory, the aquatic dispersal of diaspores, is one method that many species use to extend their range. Fluvial dispersal can transport large quantities of seed hundreds of kilometres through a catchment. Depending on the composition of the seed rain such movement can potentially have detrimental or beneficial effects on the structure of the riparian vegetation. To gain the core knowledge of the dynamics of hydrochory and its potential impact in shaping the riparian vegetation structure, the object of the thesis was to determine the spatio-temporal patterns of hydrochorous seed dispersal and their sources at the catchment scale. Incorporated into this theme is that most fluvial seed dispersal occurs during high flow or flood pulses. This study has focused on floating fluvial seed transport in the Wingecarribee catchment. The aim of this thesis was achieved by sampling the instream species pool with floating aquatic traps and aerial seed rain with bucket seed traps over 17 months; also by surveying the standing riparian vegetation and sampling the soil seed bank upstream from each of the six sites throughout the Wingecarribee catchment, in south eastern Australia. The abundance and richness of the seed rain and soil seed bank was determined by germination trials. To quantify the distance that fluvial seed disperse the dispersal kernel of Helianthus annuus (sunflower) achenes along the Wingecarribee River and two of its tributaries was quantified over 24 hrs. Concurrently, the stranding pattern of the achenes were related to abiotic and biotic factors that may influence their aggregation and vegetative cover along the channel banks, while also taking into account seed loss from waterlogging and predation. From the data collected a semi-empirical model of seed dispersal by water was developed, which predicts the parameters of the dispersal kernel. The instream seed composition varied in relation to the local species, season and water velocity. Over the sampling period,11 577 viable seeds of 79 species were caught in the instream species pool, with >60% of them being non-native to Australia. Spatial groupings divided the catchment between up and down stream, and secondly between the main channel and the tributary, indicating that at low flow only locally derived species were found in the water column. Similarly there were distinct seasonal changes in the instream seed density and species richness within the catchment, with 30% of it being explained by seed release phenology of riparian vegetation. Increase in water velocity correlated with greater instream seed density (R2 = 0.62) and species richness (R2 = 0.25); but seed release phenology was the main control of the instream species pool, making the river flow regime a secondary influence, showing that the catchment was seed limited. During higher flows seed may be transported long distances from upstream riparian vegetation or be derived from other sources such as the soil seed bank. The sources of hydrochorous seed and their respective spatial divisions were measured by surveying the standing riparian vegetation, and sampling the soil seed banks and aerial seed rain. A total of 208 species were found from all survey and sampling methods, with 90% of the instream species pool accounted for by the local vegetation and seed banks, indicating that most species were locally derived. Cluster analysis of the species composition found distinct separation between the survey/sampling methods, with secondary spatial separation delineating land use patterns. Furthermore, along the Wingecarribee River there were seasonal shifts in the instream species pool seed source, controlled by the relative seed availability from the standing vegetation and soil seed bank. Large amounts of non-native species found throughout all survey/sampling methods, highlights the need for integrated alien species eradication programs that target not only the riparian zone but also upland vegetation and soil seed banks. To investigate the dispersal kernel of hydrochorous seed and what factors influence seed aggregation along the banks Helianthus annuus (sunflower) achenes were released in three different river reaches. Over 50% of the achenes were deposited within 1000 m and 100 m of the point of release on the Wingecarribee River and its two tributaries respectively, with the furthest being transported 4500 m. At the reach scale (>100 m),water velocity was found to be a significant predictor of the dispersal kernel. 

Date of Award	2010
Original language	English
Supervisor	Richard Norris (Supervisor), David Williams (Supervisor) & David Caitcheon (Supervisor)