Forestry alters the circulation of nutrients

Forestry operations, such as felling, drainage, transport, soil cultivation, road construction and seedling maintenance have an impact on the circulation of nutrients in forest environment. The removal of trees, for instance, decreases evaporation, which will be seen in that runoff will increase for up to 30 years. Drainage will increase runoff for 15–20 years. Fertilisation increases the biomass of trees rapidly, increasing evaporation and decreasing runoff.

The load caused to waterways is non-point in character, and therefore it cannot be diminished except by decreasing it at source. It is also difficult to measure, which is why all estimates are only indicative. The load consists of solids, dissolved organic matter, i.e., humus, and dissolved nutrients. The specific load caused by forestry on Finnish waterways is about one tenth of the load caused by agriculture, but since forestry encompasses about ten times the area that agriculture does, the total load may come up to similar figures. On the other hand, it is very difficult of estimate the share of natural leaching, for this has only been studied after forestry spread to practically all forests in the country.

The nutrient load caused by forestry mainly affects surface waters, while the soil and groundwater are only affected to some extent. As regards surface waters, the species most affected are the benthic (bottom-dwelling) species and micro-organisms in the headwaters and the fish stocks. An important factor here is the improved passage of light through the water. The effect will last for over ten years after the forestry operations.

The leaching of nutrients to waterways causes eutrophication, leading to algae bloom and oxygen depletion. These are harmful to the entire water ecosystem, but also to the recreational and commercial uses of waterways. Among fish species, those most vulnerable to eutrophication are trout and grayling, but crayfish also suffer from it. Perch, pike and carp-like fishes, however, manage better, which may even accelerate eutrophication: they are in the habit of disturbing the bottom, thus mixing the nutrients stored there with the water and perhaps contributing to the growth of algae. On the other hand, forestry operations do not cause the mercury content of fish to rise, although this is a frequent assumption.

Forests also have an indirect impact on the circulation of nutrients. As an example, forests will alleviate flood peaks by slowing down the runoff of surface waters into waterways. The drainage of mires decreases this effect, which is thought to be one factor behind massive spring floods. The removal of trees and soil cultivation will also accelerate the runoff and increase the leaching of nutrients, which will impoverish the forest environment and increase the nutrient load to waterways.

18. Nitrogen and phosphorus emissions from forestry.

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