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Water Harvesting for Afforestation: II. Survival and Growth of Trees¹

ABSTRACT

Alternative methods of afforestation which do not require irrigation must be developed for arid regions. One such method is to concentrate water received by a watershed without vegetation on a smaller area where trees are planted. This can be done by making portions of the surface of a watershed area impervious to water through application of asphalt. This principle was tested near Tehran, Iran, by constructing 2-m wide terraces on contour lines at 5-m intervals resulting in a watershed to spreading area ratio of 1.5. Asphalt was sprayed at the rate of 1 liter/m² in December 1969. Seedlings of Robinia pseudacacia L., Cupressus arizonica G., and Fraxinus rotundifolia Mill., tree species commonly used in irrigated afforestation projects in Iran, were planted in March 1970 on the terraces. Runoff from asphalt-treated areas did not significantly increase the survival of the tree seedlings. The 23.4 mm of rain in July 1970, completely overwhelmed treatment effects. This rain was a rare event for the Tehran area. The increases in growth of height, crown cover, and stem cross section due to the asphalt treatment at the end of a 5-year period were 61.5, 60.9, and 53.0%, respectively, for Robinia pseudacacia L.; 14.6, 15.4, and 31.6%, respectively, for Cupressus arizonica G.; and 29.4, 79.6, and 23.9%, respectively, for Fraxinus rotundifolia Mill.

Regression equations were developed correlating growth with amount of water and age of trees. Amount of water received in April and May in any given year was found to determine growth of height and crown during the following year. Amount of water received in April through October was found to determine the growth of stem cross section during the following year.

¹ Contribution from the Research Institute of Forests and Rangelands, 569 Pahlavi Avenue, Tehran 15, Iran and the Department of Soil Science, Oregon State Univ., Corvallis, OR 97331.

² Research Scientist, President, and Research Scientist, Research Institute of Forests and Rangelands and Professor, Dept. of Soil Science, Oregon State Univ.

Received for publication October 3, 1977. Accepted for publication April 4, 1978.