Tree mortality events on the increase worldwide, is Nardoo Hills next?

Published 07 Mar 2013 
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There has been a sharp increase of tree mortality reports from many of the world’s forests over the last few years. Now maybe such an event is under way at Nardoo Hills. Of late a sudden loss of green colour in Nardoo’s canopy has surprised Victorian Field Officer Jeroen van Veen. Not because it is happening, but because of the suddenness of this phenomenon and the scale. All of a sudden a large number of trees seem to be dead or dying, while there was no sign of this two months ago. It is also surprising these deaths are mainly amongst the younger trees, anything younger than 20 years, while the larger trees seem so far unaffected.

At the height of the 1996-2008 drought, right in its final year, there were many large trees that were severely stressed and quite a few did not make it through. The ecological experts that studied this episode concluded this was mainly because these larger trees were losing contact with the dropping groundwater table while being prevented from receiving moisture from rains by the small roots networks of their direct young offspring surrounding them. Bush Heritage seriously considered intervening and taking out many of these smaller regrowth trees to give the larger ones access to water, but then the really wet years of 2010 and 2011 came and this intervention was no longer necessary.

Now, the tree deaths are mainly amongst the smaller trees that grow away from the larger ones, by themselves or in small groups dispersed over the hills. The species that are affected are mainly Greybox (Euc macrocarpa) and Yellowbox (Euc meliodora). They grow on top of the ridges at Nardoo because the geology there provides these species with better water holding soils. This is quite unique, as especially the Yellowbox is normally found in gullies, low in the landscape. It is generally considered a more thirsty tree compared to other species in Central Victoria. At Nardoo Hills, the Yellowbox makes up between 30 and 40 percent of the canopy overall. If this species would retreat from the hills it would have a major impact on bird, insect and reptile populations.

It has been extremely dry in Central Victoria this spring and summer. In Wedderburn, the town closest to Nardoo Hills, rainfall has been limited to 35 mm since August (6 months ago), and most of that fell last week. On top of that, it has also been very hot over long periods, declining moisture levels much faster than normal. Could it be that a tree species particularly vulnerable to drought conditions, is retreating because of the climate? It is unclear if the trees seen dying and dead already are just the precursor to a larger event or they represent the final bodycount. There is no rain forecast for the next two weeks and temperatures are set to rise to the high thirties once again this week.

Large episodes of tree deaths have been reported from the cedar forests of Algeria, the Juniper woodlands of Saudi Arabia, the nothofagus forests of Chile, the pine forests of Switzerland,  Spain and China and the Aspen forests of Canada of late. Could we soon add the Nardoo Hills woodlands to that list?