The aquatic downside of forestry

Posted on December 15, 2011

In response to growing global demands for biofuel there are moves to intensify forestry practices and introduce new extraction techniques such as whole-tree harvesting.  These changes have potential implications for soil productivity and downstream water quality through their effects on cation dynamics.  Trees acidify soils by taking up base cations (calcium, magnesium, sodium and potassium) and releasing hydrogen ions.  In the short-term, the removal of trees through harvesting reduces cation uptake, which can increase leaching of cations to surface streams.  On the other hand, in the longer-term, tree extraction will reduce the pool of cations in tree biomass and the soil, leading to lower cation flows.  How do  long term changes in cation availability affect the ecology of receiving streams, particularly in regions with acid-sensitive soils, such as Scandinavia and North America?  By applying a charge-balance model for pH to streams in northern Sweden, it’s been possible to examine the response of stream acidity to different levels of tree harvesting.  Using previous estimates of the effects of whole-tree cropping on cation loss, the model simulated the removal of up to 50 µequiv / litre of base cations from streams.  While the spring snowmelt normally leads to a drop in stream pH of about 0.9 pH units, the addition of tree harvesting could further reduce pH by up to 0.6 units.  This means that over much of the stream system pH would enter a biologically sensitive range (below pH 5.0-5.5), which would eliminate a number of acid-sensitive fish and invertebrates.  The removal of 30-50 µequiv / litre of base cations would lead to 58-100% of the stream length being classified as artificially acidified.  The pH sensitivity of different stream reaches to cation reduction is likely to be highest in small headwaters and densely forested areas.

Reference: Agren, A., Buffam, I., Bishop, K. & Laudon, H.  2010.  Sensitivity of pH in a boreal stream network to a potential decrease in base cations caused by forest harvest..  Canadian Journal of Fisheries and Aquatic Sciences, 67, 1116–1125.

Posted in: plants, water quality