I recently met Jon Banks, the plant diagnostician from the Bartlett Tree Research Laboratory at Shinfield near Reading. He was visiting the Ely, Cambridgeshire, nursery of Barcham Trees as part of the work he is compiling for his PhD thesis on drought tolerance in trees. Jon told me that at present we do not fully know how drought-tolerant a species is, and especially cultivars within genotypes. He is studying Acers, and comparing species and cultivar drought tolerance.

There are various ways of measuring drought tolerance, and first we looked at the pressure bomb, which can make pressure volume curves. Leaves are squeezed with increasing amounts of air pressure, and the amounts of sap expressed are measured. From this, a characteristic curve is formed, and a point on the curve tells when the plant will wilt. There is a correlation between wilting point and drought tolerance. This is, however, a very slow method, as it can only process four leaves a day.

Another device, the chlorophyll fluorimeter, measures fluorescence. Jon also measures dehydrating leaves with the aim of giving each species a number which relates to its drought tolerance. Trees such as London plane and common oak could be numerically ranked 10, and could be super-drought-tolerant on his scale.

Jon is spending three weeks on Barcham’s nursery in an attempt to solve experimentally some of the uncertainty surrounding drought tolerance of trees. He identifies characteristics of one species, such as small leaves or hairs on the undersides of leaves, and sees if they have a similar tolerance to another species or cultivar.

As part of his work at Barcham, Jon is deliberately subjecting some trees to drought and then looking at them with a porometer, which measures stomatal apertures. The pores on the undersides of leaves are one of the first parts of the tree to respond to drought – but we do not yet know whether rapidly closing pores signify tolerance or intolerance of drought. It will, however, be used to compare when a species responds to drought stress as a measure of the species tolerance.

Jon’s work is continuing, and he told me how rewarding it is that two companies such as Bartlett and Barcham are collaborating, encouraging his work and hoping to promote the nursery trade’s understanding of tree physiology, as well as selecting trees for tough and dry sites.

By Colin Hambidge