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AS Geography

       

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          The effects of a woodland on microclimate

On a hot summer’s day it is noticeable that temperatures in a forest are much lower than those outside. You are no longer walking under a hot, baking sun. It feels sheltered and it may feel slightly more humid.

Radiation exchanges in woodland
  • Examples of tree albedos: Scots pine 9%; Oak (summer) 15%, Oak (spring) 12%; Sitka spruce 12%; Norway spruce 12%; Orange trees 32%; Tropical forest 13%.

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The effects of woodland type

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Temperature in woodlands
  • Woodlands are normally cooler in summer and slightly warmer in winter.

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Moisture in woodlands
  • Increased output of moisture - In a forest , leaves are continually transpirating moisture into the atmosphere. As winds inside a woodland area are usually light, this moisture is not easily dispersed.
  • Interception of moisture input - On the other hand, vegetation is continually intercepting moisture, so less of it reaches the forest floor.
  • The net effect on humidity levels within a woodland is small.
  • Daytime temperatures within a woodland are cooler than those outside - this makes the relative humidity of the air greater within a forest (even if the forest atmosphere contains the same absolute amount of water vapour as outside it).
  • Experiments suggest a 5% difference, although much depends on the time of year and weather conditions.

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Difference of relative humidity (per cent) between the inside and outside of a forest

Forest type

January

April

July

October

Year
Deciduous broad leaf

3.4

3.2

-0.8

1.1

2.2
Needle tree conifer

4.8

4.8

6.5

9.5

6.8
Japanese cedar

1.6

-1.1

1.5

0.5

0.8
Positive values indicate that the inside of the forest was more humid. The research was undertaken by the University of Tokyo, Japan. (Briggs & Smithson p140)

References: David Briggs and Peter Smithson (1985) Fundamentals of Physical Geography pub Hutchison, chp 9.

  • Chapter 8 of Lenon & Cleves Fieldwork Techniques includes suggestions for studying local climate. Towns, woods and lakes all create their own climate.

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