Secondary seed dispersal of Erodiophyllum elderi, a patchily distributed short‐lived perennial in the arid lands of Australia

Abstract

We investigated secondary dispersal of propagules of Erodiophyllum elderi (Asteraceae), a short-lived perennial plant growing in small patches in the arid lands of southern Australia. In spite of its importance for population dynamics, secondary dispersal is a little understood process. We monitored the dispersal of 2280 large woody capitula (seed heads) released in six E. elderi patches for 9 months. Colour-coded seed heads were located at night using UV light and their distance and direction from the release point were measured. Over the 9-month period, more seed heads moved, and those that did, moved further in areas with high herbivore activity. Overall dispersal distance across the ground was limited to less than 30 m. Dispersal patterns were related to the topographical slope at the release site: seed heads moved further, and more dispersed on steeper slopes unless the steep slopes had sandy soil in which case seed heads were buried, caught or there was reduced sheet water flow limiting their dispersal potential. After several months, seed head dispersal virtually ceased as seed heads became stuck in the debris and soil after heavy rains or further dispersal became unlikely when seed heads reached locally low-lying areas. Secondary dispersal patterns suggest two distinctly different influences associated with the presence of herbivores: the direct movement of seed heads by trampling from sheep (an introduced herbivore) and the indirect effect of a reduced standing biomass from grazing. Reduced vegetation cover allows seed head redistribution via sheet water flow during large rainfall events.

The fate of seeds in the soil: a review of the influence of overland flow on seed removal and its consequences for the vegetation of arid and semiarid patchy ecosystems

“…These studies highlight the absence of seed movement from the bare interpatch areas to the vegetation patches in combination with a low seed trapping capacity by the vegetation (Aerts et al, 2006) or with short dispersal distances relative to the pattern of spatial heterogeneity (Venable et al, 2008;Emmerson et al, 2010Emmerson et al, , 2012. The authors concluded that successful recruitment of the species used in these experiments could not rely on seed transport by runoff but depended on other mechanisms such as primary dispersal (Aerts et al, 2006) or the temporal delay of germination (Venable et al, 2008;Siewert and Tielborger, 2010).…”

Changes in seed dispersal processes and the potential for between‐patch connectivity for an arid land daisy

“…This species is ideal for such studies because of its production of hard, roughly spherical capitula (seed heads ;Black 1986) that can be monitored for long periods (Emmerson et al 2010). The lack of obvious dispersal-promoting structures makes it typical of other xeric species.…”

“…The paint does not affect seed head dispersal (Emmerson et al 2010). Seed head dispersal was monitored on and off animal tracks in eight E. elderi patches located within 10 km of each other.…”

“…Does secondary seed head dispersal along animal tracks enhance connectivity between patches? Finally, we incorporate results from this and a previous experiment (Emmerson et al 2010) to explain patch contraction and expansion by linking dispersal outcomes to the fluctuating environment, the state of the vegetation, and soil condition.…”

“…However, because of the intrinsic difficulty in studying dispersal, there has not been much direct experimental research of dispersal itself (e.g., Venable et al 2008). Comparing dispersal in nearby sites with original and modified herbivore communities, e.g., where rangelands adjacent to protected areas can be studied (Emmerson et al 2010), can provide this information.…”