The Willow

The willow (Salix taxa) has recently been declared a weed of national significance (WONS) by the National Weed Strategy Executive Committee (DPIWE, 02). The pretense for this classification was that willows pose ten possible threats; 

( Threats to forestry management ( threats to water quality and supplies ( threat of damage to infrastructure ( threat to endangered species ( threat to plant management (threat to cultural values ( threat to tourism ( threat to the community and, ( threat to recreation and amenities. For the fishermen (and anybody else that frequents Australia's waterways) the crack willow (S. fragilis) is the most commonly encountered willow, typically found as impenetrable infestations along riverbanks and other riparian areas. One of the most common willows in Australia, the crack willow was introduced as a male clone and consequently can only reproduce by vegetative means (DPIWE, 02). In laymen terms this means that the willow does not use seeds or flowers to reproduce but instead relies on attached branches, broken branches or root suckers to develop into new trees (usually downstream of the parent). This is an important fact and for any bait fishers this should be good enough reason not to use a forked willow branch as a rod rest.

 

Impacts on Rivers

Obviously willow trees and their typical infestations are very different from our typical riparian vegetation of tea-trees, wattles, eucalyptus etc. The crack willow differs from traditional riparian vegetation in that the willow is deciduous and that it often grows into riverbeds some distance beyond the riverbank. These differences between the willow and native vegetation leads to three potential effects to riparian and river functions (from the draft willow management strategy (DWMS));

  • River geomorphology (including surface water flows)
  • Stream ecology
  • Riparian ecology

A primary reason that willows were first planted in Australia, apart from aesthetic reasons, was to reduce riverbank erosion. Unfortunately the success of the willow in spreading out great mats of roots and multiplying like rabbits has actually proved its downfall. (The Meander Valley Weed Strategy for the crack willow estimates that 19% of the riverbanks in the catchment are covered in willows - typically near settlements and areas of disturbed native vegetation).

The DWMS identifies five impacts of willows on river geomorphology. These impacts are changes in the dynamics of rivers, pool-rifle sequences, frequency (and period) of flooding, increased rates of bank erosion and the differing depositing of large woody debris (LWD).

River features such as gravel bars and silt beds are normally mobile, typically disturbed during periods such as floods and deposited in other parts of the stream. Due to the dense matting of willow root systems and the increased friction along the river bed floor these otherwise motile sand, gravel and silt beds become trapped resulting in "sediment starvation" to downstream sections of the river channel.

Alterations of typical pool-riffle sequences in rivers is another feature of willow impacts. As the dense mats of willow roots and even the trees themselves invade shallow riffles, the altered flow may result in the surface flow between pools to decrease, even become disconnected. Needless to say such invasions in to rivers may limit access and cause danger to boaters on the water.

Typical native riparian vegetation is evergreen, shedding old or dead foliage throughout the year. In contrast, the willow is deciduous loosing all of its leaves in autumn, gaining them again in spring. The implications are higher autumn and spring water temperatures for rivers impacted by willows due to the lack of shading (DWMS) which can alter algae / lichen (epilithon) biomass on the riverbed (Read, 99).

High nutrient loads on the riverbed caused by leaf litter can lead to an increased oxygen demand as the leaves breakdown leading to decreased water quality. Reduced surface water flows across riffles can also reduce the amount of re-oxygenation occurring. As willows impinge into the river channels and trap sediments, sand and gravel beds, the depth (capacity) of the river channel decreases, increasing its propensity to flooding. As the river capacity decreases, normal flows as well as any additional flows (e.g. floods) are diverted to the banks and cause erosion in an attempt to widen the river and compensate for decreased capacity (previously in the form of depth).

As any fisherman will testify, where there is one willow there is likely to be a hundred. This feature is referred to as a "monoculture" where a single species spreads and dominates, and obviously for ecological biodiversity this is extremely bad. The willow tree monoculture is detrimental to surrounding flora and fauna biodiversity eventually creating an area where nothing but willows grow (Meander Valley Weed Strategy).

Impacts on River and Riparian Ecology

Large woody debris is important in our rivers for shelter, spawning sites and as a source of food items etc. An example of the importance of woody debris is to the native Tasmanian blackfish (Gadopsis marmoratus) that relies of LWD for shelter and as a spawning site. (The blackfish lays its eggs in the hollows of logs (IFS pamphlet)). Whilst there is no doubt that the willow contributes woody debris to river ecosystems, the quality of willow LWD is inferior ( different in form, texture and longevity (DWMS)) to the quality of the LWD of native vegetation. Numbers of native species such as the blackfish that relies on appropriate LWD for spawning and the Giant Freshwater Lobster (Astacopsis gouldi) that feeds on decaying wood would therefore likely be detrimentally affected by willow tree invasion. The monoculture of willow trees along riverbanks and riparian areas leads to dense canopy's and sparse under-stories due to shading. This leads to the suppression of most native species which through the food chain may effect the whole food web.

A small unpublished Rivercare study in the North Esk Catchment noted a lower diversity of birdlife compared with native sites demonstrating the effect of willow monoculture upon the foodchain. As noted by the DWMS, willow trees do not develop hollows suitable for native bird or mammal shelter.

Eradication / Control measures

The Meander Valley Weed Strategy for the crack willow identifies foliar sprays and the "cut stump" method for control / eradication of the willow. The foliar spray is suitable for small trees, suggested for trees / regrowth of up to 2 metres high. This technique involves applying a chemical or herbicide to the foliage of the tree during summer and once dead removing the tree. The suggested applications are for young trees, new infestations and regrowth from previous control programs.

The cut stump method is for larger trees and involves cutting the tree at or above ground level and treating the stump with a herbacide. All debris obviously has to be carefully removed and disposed of, e.g. burnt. The advantage of this method is that the riparian area still benefits from erosion protection by the dead root systems whilst being re-vegetated with native vegetation. The root systems of the willows will decay with time. It is important to note that consultation with DPIWE and other appropriate bodies is recommended (and compulsory depending on situation) before the commencement of willow tree control programs.

The Future

Threats in the future lie in many quarters. One possible threat is the hybridization of the male crack willow with the female of a different species. This would enable the hybrid crack willow to spread sexually by seed! What the fishers can do is physically minimise the spread of the willow (no willow rod rests etc). Secondly, if time permits volunteer with local applicable organisations (e.g. Rivercare, Landcare, Trout Unlimited in Northern Tasmania etc) and get directly involved in protecting the environment. For all those parents out there encourage your children to get involved so that they can also have fish for the future.

As a side note, the complete removal of willows from any riparian area all at once is not the answer. Although willows are no substitute for native vegetation they do provide excellent shelter, feeding areas and structure for fauna and fishers alike. Wholesale clearing that has occurred in parts (eg: the Northern Midlands) often leaves minimal riparian vegetation, no shade for the watercourses (increased temperatures), increased rates of erosion and deposits into streambeds and leaves all fauna without cover. Progressive control of the willow may be a better alternative, gradually substituting the introduced species with appropriate native substitutes. As the older (often wiser) fishers will tell you, willows have been in Tasmania on our famous rivers for over a hundred years. During these years the number and spread of willows was maintained and everyone may have actually benefited from the willows" presence. Unfortunately due to a number of factors that have ultimately led to neglect, the willows have been allowed to take over many of our prime recreational areas.

Glossary of term

Riparian land? "Simply put riparian land is any land which adjoins or directly influences a body of water". The definition of riparian land includes land on or along any watercourse or catchment area as well as floodplains (LWRRDC, 1996). - Large woody debris? Woody debris is the branches, limbs, trunks or even trees that fall in to the waterways. - Ecology? "The interrelationships between organisms, their environment and eachother" (Lawrence, 1997). - Vegetative means? Attached branches, broken branches or root suckers can all grow in to new plants when in contact with the ground. (LWRRDC, 1996). - River geomorphology? The geological forms and changes in forms of rivers, in this case riverbeds (and their formation changes), banks and riparian areas (surface water flows).
Acknowledgements / References Thanks to Mike Noble and the Rivercare team who compiled a package of information for me pertaining to Willow trees and riparian land management. The TFN editor supplied the draft Tasmanian Weed Management Strategy that formed the basis of the article and other information was collected from DPIWE.
* Cale, Peter. Unpublished data on bird assemblages in the North Esk Catchment. Rivercare. * DPIWE, 2002. "Draft Tasmanian Weed Management Strategy- Willow Salix Taxa". Weeds of National Significance Program, National Weeds Strategy. * Inland Fisheries Service. "Importance of Woody Debris". Information booklet. IFS PO Box 288, Moonah, TAS. 7009. * Reed, Martin (Report 1999). Abstracts from "Comparison of the in-stream fauna and resources of willow-lined river reaches with those of other riparian types in Tasmania". Land and Water Resource Research and Development Corporation, Canberra ACT. Project code UTA2. * "Riparian Management" information booklets 1-7. Published by the Land and Water Resources R&D Corporation (LWRRDC) 1996. * "The Meander Valley Weed Strategy - Crack Willow". www.microtech.com.au/mvws/willow.htm