Regular commentator at this blog Ian Mott sent me the following email:
Hello Jen,
We have all grown accustomed to the notion that it is the removal of trees from the landscape that causes salinity. But recent research from the Argentine Pampas indicates that the addition of trees to a natural grassland can also increase the salinity of groundwater flow systems (GFS).
This could have major implications for the management of salinity in the Murray Darling Basin, particularly in rangeland areas where major thickening events have taken place or where existing small clusters of forest have expanded onto grassland ecosystems.
The study, by Esteban G. Jobbagy and Robert B. Jackson, published in Global Change Biology compared 20 paired plots of forest and grassland and found a significant increase in groundwater salinity under the forested plots. “Afforested plots (10-100 ha in size) showed 4-19-fold increases in groundwater salinity on silty upland soils but less than twofold increases on clay loess soils and sand dunes.”
While this study has been limited to planted forest plots on previously grassland ecosystems, the same causal factors are at play whenever forest vegetation expands on grassland. And it logically follows that the same causal factors will be at play when, for example, a 10% canopy woodland thickens to become a 60% canopy forest.
Jobbagy & Jackson have concluded that “Soil cores and vertical electrical soundings indicated that …salts accumulated close to the water table and suggested that salinization resulted from the exclusion of fresh groundwater solutes by tree roots.”
To which the average farmer would say, “Well, they would do that, wouldn’t they”.
The extensive, 1400 plus, rangeland sample plots done by Bill Burrows confirm that more than 60 million hectares of rangeland in Queensland is subject to thickening at an average rate of circa 0.25m2 increase in basal area per hectare. There is a further estimated 30 million hectares in NSW. And there are also numerous landholder reports of properties that had only 3,000 ha of Gidgee in the early 1900’s but have in the order of 50,000 ha today as a result of major encroachment onto grassland.
And this poses an interesting question for the publicly funded anti-salinity industry and the policy arms that have focussed so much public attention on the removal of trees as salinity causal agent. If the lowering of a water table by excess bore irrigation can be widely recognised as a causal factor in increasingly brackish ground water resources, why has it taken so long to recognise that a similar lowering of a water table by the addition of trees can produce the same result?
It certainly invites the question, is there any similar research conducted here in Australia?
Clearly, the political exploitation of salinity appears to be sinking deeper and deeper into murkier water.
Regards,
Ian Mott
Jennifer Marohasy BSc PhD has worked in industry and government. She is currently researching a novel technique for long-range weather forecasting funded by the B. Macfie Family Foundation.