5.2: Dispersal mechanisms of willows in South Eastern Australia

Project overview

Sub-Project progress

Challenges

Willows are widespread invasive riverine exotic weeds through southeastern Victoria. Willow invasion effects water flow, chemistry and local native vegetation dynamics.

Current control efforts are extensive, but could be improved with a thorough understanding of the dispersal dynamics of willows.

Seeking solutions

The fundamental scientific and environmental management question that needs to be addressed is: "On what geographical scale must willow eradication and post eradication monitoring be conducted to ensure that these efforts will be effective over the long term?" To answer this question it is necessary to know the extent and pattern of seed and pollen movement across catchments as this will allow removal efforts to be targeted at spatial scales that will minimise the risk of reinfestation – which is currently quite common. Knowledge of the distances that willow seeds and pollen move across landscapes will also allow more realistic risk assessment and management with regard to predicting which native plant communities that are currently willow free have a significant chance of being colonized by willows species in the future.

Approaches

This project will use high-throughput DNA analysis of adult and seedling willow genotypes to:

1. examine historical patterns of willow movement across Victorian catchments through the analysis of the geographical genetic structure of established populations of these species and;
2. determine current patterns of seed dispersal and pollen movement by using parentage and paternity analysis to ascertain the origin of invading seedlings and thus directly measure dispersal distances.

Collaboration

Melbourne Water, Goulburn Broken Catchment Management Authority, Corangamite CMA, West Gippsland CMA, North East CMA, North Central CMA, Department of Sustainability and Environment (Victoria).

Tools for Decision Makers

Our results will be published in scientific journal articles.

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Sub-Project progress

January 2010

Australian patterns

It has been found that Salix cinerea maintains high levels of genetic variation and there is sufficient genetic variation within and among populations for successful assignment of seed and seedlings to source populations across the study catchment.

From pollination experiments it was found wind pollination accounts for an average of 23% of observed seed set events, with insect pollination required to maintain typical levels of open pollinated seed set. The level of wind pollination is highly variable between trees with some having no wind pollination at all and another having 38% wind pollination.

From paternity analysis it was found that the number of seedlings per mother whose father could not be accounted for by males within the population varies from 20 to 80% per mother. However every mother has some foreign pollen contributing to her seedlings. Overall significant levels, 50%, of inter-population pollination events suggest inter-population dynamics are critical to local willow reproductive dynamics.

Of the 50% of local pollinations that are occurring, 66% occur at less than 50 metres, though pollinations were observed occurring at over 200 metres. These local pollinations on a single mother were generally sired by multiple males. However, male fitness is highly skewed; of the 61 males just 5 were responsible for 41% of pollinations.

Overall we know that a single population is not functioning in isolation, an average of 50% of seedlings in the population are sired by males outside the population. Preliminary results on locating the source populations of seedlings have indicated that seed tends to be restricted in its movement to along rivers and only a limited amount of seed immigration is due to between river movements. More data and analysis is needed to confirm these findings.

Implications

Results of genetic and pollination studies are contributing significantly to our knowledge of willow reproductive ecology and dispersal dynamics in Victoria and will enable the future optimisation of conventional control efforts.

The variation in male fitness could potentially lead to focused removal of trees to significantly reduce seed set. Unfortunately it was found that these trees are not necessarily bigger than other male trees so it would not be effective to just clear the larger individuals and no other correlation between these males and other measured characteristics could be found.

The high proportion of wind pollination has the implication for long distance dispersal and hence long distance pollination of isolated individuals. This along with the identified high levels of foreign pollen coming into a population tell us that control strategies must focus on areas larger than a single population to be effective. Our initial examination of possible source population has suggested that management strategies may best be targeted at clearing along rivers to maximise effectiveness.

 

August 2009

Poster: Dispersal dynamics of willows (Salix cinerea) in southeastern Australia – implications for catchment-scale control [pdf]

 

 

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