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Weeping Willow

“I believe that the presence of willows along streams in agricultural zones can be shown to be almost universally preferable to cleared streams in those zones. I would also suggest that even relatively low-disturbance eucalypt-Acacia dominated riparian vegetation may not have compelling benefits over willows under many circumstances.” (Wilson, 2007)

It would be fair to presume the comment above had been made by Peter Andrews, Natural Sequence Farming originator and outspoken champion for the much maligned willow. In fact, this statement came from Dr Michael Wilson, a stream ecologist who supervised numerous PhD and Masters research projects in Ballarat, Victoria, during the early 2000s, comparing streams flanked by willows; 100 year old, multi-strata, native regrowth, and cleared land with introduced pasture.

The full paper which is linked to at the bottom of this article goes into more detail, but here’s a summary from Wilson (2007) to give you the gist:

– On average, willow-lined streams had a higher retention of sediment (187t more/km) and organic matter (30t more/km) than the native forest.

– “Willow-mediated aggradation in these channels is converting them from incised channels to in-fill channels that are more characteristic of pre-European conditions”.

Litterfall of willow and native-reveg reaches had a similar annual distribution pattern due to the not-so-well-known summer dominant leaf drop habit of many Eucalypts.

– The annual weight of leaves, twigs, bark and flowers was very similar at the willow and native sites.

– With similar annual litterfall amount and distribution, coupled with dense shade patterns in the seasons of maximum productivity, the overall metabolism (and resulting biological oxygen demand) was also very similar.

– Root mats of willows were found to provide beneficial habitat to native fish in the absence of large woody debris.

– There was a disproportionately large association between pool-riffle sequences and willows, formed by the root mats of the willows.

“Pool-riffle sequences are extremely valuable habitat and for that reason alone it is worthwhile (maintaining willows). But it becomes even more valuable when it can contribute to ideas focused on restoring the whole of the floodplain complex in agricultural landscapes.”

“In all the streams we have studied, clearing willows will mobilise sediment, nutrients and organic matter, will make heterotrophic streams more autotrophic, will threaten habitat values for invertebrates and fish and will threaten pool-riffle sequences. Native vegetation planted where willows are cleared will take many decades if not hundreds of years to mature, for the canopy to close over and for significant limb fall to occur.”

View the full article:

Click here to view the full article, Willows: Weeds of Retention 

Wilson, M., 2007. Willows: Weeds of Retention. Proceedings of the 1st Natural Sequence Farming Workshop. ‘Natural Sequence Farming: Defining the Science and the Practice’, Hazell, Peter and Norris, Duane, Bungendore, NSW,  2007. http://www.nsfarming.com/workshop/

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Increasing atmospheric CO2 levels during the industrial age have gained a lot of exposure in the link with a changing climate. An area which has received far less attention are the significant changes to both local and global water cycles and vegetation patterns during the same period of time.

Water For the Recovery of the Climate – A New Water Paradigm (Kravcik et al, 2007) explores the science behind the very direct links between vegetation and both temperature and precipitation.

It’s a read I highly recommend and it can be downloaded by clicking on the title in the paragraph above. So that the time-poor can get the gist, I have included a number of the diagrams from the paper in a slideshow at the bottom of this article.

Here’s a very simple summary as well:

1) At any given time or place on the planet, there’s a certain amount of solar energy hitting the surface of the earth. That energy can be put to two very different uses.

The distribution of solar energy on drained land and on a landscape saturated with water (Kravcik et al, 2007)

At one extreme, a bare soil (or concrete, steel, etc), the majority of that energy is absorbed and re-radiated as (sensible) heat, warming the local environment.

At the other extreme, a saturated wetland, the majority of that energy is utilised by plants for photosynthesis, with the resulting transpiration drawing (latent) heat from the surrounding area.

So the sun’s energy is used to either power a landscape-scale radiant heater (above left) or a landscape-scale evaporative air conditioner (above right).

2) Of the average 720mm of precipitation that falls on land, the input from the sea is about 310 mm (the large water cycle). Hence, the land provides the larger part of its own precipitation (410 mm) from its own land-based evaporation (the small water cycle).

Small water cycle

Kravcik et al (2007)

Therefore, a drained and dehydrated landscape, coinciding with relatively shallow rooted plants (ie conventional western agriculture) ultimately means less rain over the land, in a (not so) positive feedback loop.

A landscape which is primed to accept whatever rainfall or overland flow that arrives and to send that moisture through actively growing plants, means more rain over the land in a (more) positive feedback loop.

More soil moisture = more evapotranspiration = more precipitation = more soil moisture etc

The radiant heater mentioned previously means less rain; the evaporative air conditioner, more.

In closing, a couple of short paragraphs from the paper:

“The renewal of the domination of the small water cycle, which is advantageous for humanity, vegetation and the land, depends on the renewal of the functional plant cover of a territory and water surfaces in a country.”

“With sensible management of water and vegetation we can curb climatic change on the local level; if we can act in the same way across larger areas, perhaps we can expect a tempering of global climate change.”

We’re in the business of landscape-scale air conditioners, so if you’re interested, please get in touch. Or, visit and ‘Like’ our Facebook page to hear about future posts.

 

Reference:

Kravcik M., Pokorny J., Kohutiar J., Kovac M., Toth, E, 2007. Water For the Recovery of the Climate – A New Water Paradigm

Click here to download a pdf of the paper

Click on the images for a slideshow of the diagrams from the paper.