Trees in forests withstand dry summer better than those in cities

Long, rainy, and warm autumn could hurt trees most

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Dry birch leaves and pine needles are a common sight in Finland this summer. Trees on high ground, on cliffs, and in urban areas are suffering from a lack of rainfall. Leaves have been dropping prematurely, and if the drought continues, root systems might also be threatened.
      Risto Jalkanen, special researcher at the Finnish Forest Research Institute, says that the needs of teres are not sufficiently taken into consideration in urban areas.
      He says that drainage systems prevent rain water from accumulating in the ground. Roots are damaged when district heating pipes or broadband cables are installed. Furthermore, nearly all areas in cities are paved, and the blasting away of rock interferes with the flow of water inside.
     
Commercial forests growing in moraine soil are not especially threatened by dry conditions, even though many forest owners have been worried. Although the trees are surviving, the dryness does slow down the growth.
      "If a forest owner has been active, cutting and planting trees, forests will do well, and sapling stands look good. A warm and dry summer does affect the populations of certain pests", Jalkanen says.
      Tuula Vanhatalo, a forest owner from Hyvinkää, says that the dry conditions do not seem to affect large trees very much. She adds that the conditions are actually good for saplings, because there was sufficient rain in the early summer. However, forest undergrowth has suffered.
      "This year there are very few berries, and hardly any mushrooms at all. Last summer people were carrying away boletus mushrooms in large plastic shopping bags."
     
The true survival test for forests will be the winter, which they will have to struggle through after being weakened by the dry summer.
      Normally, trees in Finland begin biological preparations for the winter soon after midsummer, when the days start getting shorter.
      The worst possible scenario for the health of Finnish trees would be if the hot and dry summer were followed by a long, rainy, and warm autumn. If this were to occur, trees would continue to grow, even though they should be dropping their leaves.
      Ideally, there would be just enough rain for the needs of the roots, but not so much that the tree would keep growing.
      The full impact of the drought will not be seen in Finnish trees until next June when the growth period has begun. In addition to the death of trees, growth and leaf production can be affected.


Previously in HS International Edition:
  Drought prompts mysterious messages urging residents to water trees in Helsinki (3.8.2006)

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