How Trees are Killing Our Rivers

In the past decade, acid raid has caused a drop in water quality in almost 60 miles of Welsh Rivers. A much grater length of small tributary streams has been equally degraded. Most of the streams flow thorough plantations of conifer tress.

Now, it seems many of the same streams are facing another problem. The shading effect of the dark, densely planted conifer crops is altering water temperatures, causing a considerable decline in the development of many aquatic invertebrates and fish.

Anything that affects stream invertebrates and fish has a knock-on effect for other groups of animal such as otters and the characteristic birds of upland waters-dippers, grey wagtails and common sandpipers, in particular.

An assessment of the impact of conifer plantation, which now cover a quarter of the hills and uplands of Whales, on stream water temperatures has been made by Neil Weatherly and Stephen Ormerod, of the University of Wales College, Cardiff.

They studied year-round temperature variation in six streams, all of which run into the River Tywi in central Wales. Three of the rivers drained open moorland; the other tree drained large plantation of 30-year-old Sitka spruce, the most commonly planted conifer in these hills. Two of the spruce plantation had their trees cut down in a band 45 feet wide either side of the stream so that shading effect on the water was at least slightly reduced. In the other plantation-draining stream, the conifer grew right to its banks. All tree conifer-counterparts in the Welsh Hills were without fish and supported only a few varieties of aquatic invertebrates. Acidification – made worse by conifer plantation whose needle-shape leaves effectively filter pollutants out of the air – is the likely cause. Most of the streams formerly supported good populations of brown trout and Atlantic salmon.

Doctors Weatherly and Ormerod found that average daily water temperatures between April and August were between 0.6 ^oC and 2.8 ^oC higher in moorland streams compared with the totally forested stream. In the winter, the pattern was reversed. Then the forested stream water was up to 0.9 ^oC warmer. Water temperatures in the streams in the conifer plantations with trees growing well back from the banks were between those of the open moorland streams which was completely forested. Their water became up 1 ^oC warmer between May and June compared with the forested stream.

Temperature differences were mainly the result of trees shading out both direct and indirect sunlight. Because their trees are so closely spaced, conifer plantation are highly effective at blocking sunlight while small streams are much more susceptible to shading effects than larger rivers.

Based in the substantial changes in water temperature caused by the panting of conifer, Doctors Weatherly and Ormerod then used published biological models – developed from laboratory studies – to stimulate the impact on invertebrate and fish development.

According to these simulations, the eggs of mayfly species developed faster in a moorland stream after having been fertilized in the spring. This, combined with predicted faster growth of the hatched nymph, means adults would emerge during November in a forested stream but before the end of October in a moorland stream.

Similar effects to choose forecast for mayflies were predicted for brown trout by the models. Trout eggs took up to nine days longer to hatch in forest streams. Subsequent fish growth was faster in the moorland stream; a little under a year from egg fertilization the previous November, moorland stream trout were predicted to be between 22 % and 39 % heavier than fish in forest stream.

By the end of the second growing season the effects still held, even though weight loss over winter was less in the forested stream (because its winter water temperature was slightly higher). Similar fish are uniquely to be as fertile. Trout in streams where the conifers had been failed from the banks did virtually as well as those in moorland streams.

Doctors Weatherly and Ormerod are quick to point out the possible pitfalls of this sort of biological modeling. Numerous assumptions had to be made along the way. For instance, water temperature was measured using thermostat probes in the water flow. This works well for organism with life stages in the surface water. On the stream bed and among the bed gravels, however, temperatures wary less than at the stream surface. Deeper down in the gravels and mud, temperatures can be more stable still and even more different from those near the surface.

It was also assumed that no other environmental variables apart from temperature was limiting organism development.

The modeling suggests that temperature changes brought about by putting dense conifer plantations close to streams may have important consequences for stream life, similar to “moving” the stream to a colder climate, either by shifting it north or its altitude upwards.