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A tree, by its very nature, is a closed hydraulic system. Water is absorbed from the soil via the roots during a process called osmosis and it moves through the plant to the leaves where a small quantity is combined with carbon dioxide and made into sugars by a procedure known as photosynthesis. The bulk, possibly as much as 99% of the water absorbed from the soil, evaporates from the leaves into the atmosphere by transpiration. Water rises up through the tree in the outer edge of the woody material - the xylem. Its movement is assisted by capillary action in the very fine xylem vessels which connect the roots to the trunk, branches and eventually to the leaves. The water, which is called sap, is a solution of mineral salts that have been absorbed from the soil. In late winter through early spring the sap is enriched with sugars being transported from storage in the roots to the shoots where they are used to provide energy necessary for shoot growth and leaf expansion.
On warm days, when the air is dry and a gentle breeze is blowing, evaporation from leaves can be very rapid with considerable quantities of water being drawn through the tree. Figures of 1000 litres per day have been quoted for mature oak trees Quercus spp. Conversely, on cool damp days with little or no air movement, loss of water to the atmosphere will be slow; it may even stop.
Once there is positive water pressure in the tree, damage to the tree, by various means, may release the pressure and allow sap, usually a colourless watery liquid, to escape and the tree will 'bleed'. The result is often a white frothy ooze that has a sweet fermenting odour similar to beer. Anyone who has ever sugared for moths will know how effective a mix of molasses and beer can be in attracting a host of insect species; the odour of fermenting sap having a similar appeal. The accompanying images show the magnificent Purple Emperor Apatura iris feeding on sap runs high in the woodland canopy.
References
Patch, D. (2004). Trees Bleeding. Arboricultural Practice Notes (APN 8). Farnham: Arboricultural Advisory and Information Service.
The Purple Empire. http://apaturairis.blogspot.com [Accessed, 9 July 2018].
UK Butterflies. www.ukbutterflies.co.uk/species.php?species=iris [Accessed, 9 July, 2018].