Tukituki Riparian Bitter Willow (Salix elaeagnos) Canopy Opening Indigenous Revegetation Experiment

 Even though in some cases they may be invasive, introduced trees can modify ecological conditions, helping the establishment and growth of indigenous forest plants beneath them.  Over time, and with appropriate management intervention, sites dominated by introduced trees could feasibly become sites dominated by native forest.  This project focuses on an invasive willow species and investigates how succession to indigenous forest can be accelerated.

EXPERIMENT OBJECTIVE

The experiment investigates the use of poisoning of the invasive tree, bitter willow, as a management intervention to facilitate the establishment and growth of indigenous tree species, within the bitter willow understorey – in riparian zone of the middle Tukituki River, central Hawke’s Bay.  

BACKGROUND

The deciduous introduced tree – Salix elaeagnos – is also referred to as hoary willow or sage willow.  Of European origin, the species was first recorded in New Zealand in 1968.  Today it has discrete distributions in the North and South Islands of New Zealand (N, Poverty Bay, Hawke’s Bay; S, Westland, Canterbury).  In New Zealand the species typically colonises riverbanks and is generally uncommon or rare.  However, an apparent invasive stronghold for this species is on the middle reaches of the Tukituki River in central Hawke’s Bay.  Here bitter willow forms contiguous stands colonising both stabilised river gravels and the shallow soils of recent river terraces.

Bitter willow gets its name from its unpalatable bitter tasting leaf characteristics.  Here on the middle reaches of the Tukituki River, central Hawke's Bay, bitter willow invades the river gravels and low riparian floodplains.  Photograph shows the study area in August 2012.

Young bitter willow colonising the riparian zone of a gravel river bed. 

Bitter willow develops a stature of tree or large shrub of up to 6 m high, its habit being dense and bushy.  Within the study area, average annual rainfall is 1,000 – 1,200 mm and indigenous forest seed sources are close.  As a consequence, indigenous woody plants and ground ferns colonise the seasonally shaded bitter willow understorey.  It is this successional development which is at the centre of the experiment.

It is hypothesised that through poisoning of mature bitter willow, increased light transmission through the open canopy will promote the establishment (germination) and growth of indigenous forest tree species.

Common broadleaved indigenous shrub species invading the bitter willow understorey.  As well as common shrub species, seedlings of indigenous canopy trees are present, including titoki, totara, and kahikatea—suggesting that successional processes could lead toward tall indigenous forest from these bitter willow sites.

METHODS

Experimental Design. At treatment sites the willow canopy was opened by basal drilling and poisoning using the broad spectrum systemic herbicide Glyphosate, applied neat to drilled holes.  Poisoning occurred over the late spring – early summer period 2012.  The vegetation composition at all control and treatment plots was assessed within 10 × 10 m permanently established plots using the RECCE vegetation survey method.  Plots were established during November and December 2012, at the same time as the willows were poisoned.  Invasive climbing weeds, Clematis vitalba (old man’s beard) and Rubus fruticosus (blackberry), growing within or near plots were controlled at the time of plot establishment and one year later in early Summer 2013. 

 

Poisoned bitter willow trees as part of this experiment.  Fresh Spring growth of mahoe (indigenous tree) apparent within the experimental plot.  Eleven months from willow poisoning.  Photograph November 2013.

Experimental Parameters. Parameters being assessed across treatment and control plots are:

Explanatory variables

   % transmission of photosynthetic active radiation. 

     Response variables

• ·        Seedling establishment.
• ·        Seedling, sapling and tree composition.
• ·        Species-cover across height tiers.

Next update will include hemispherical photography assessment of canopy structure and light transmission.