Cultivars of lime (Tilia spp.) have a long history of use as ornamental trees across northern Europe, particularly in avenues, and remain popular due to their tolerance of a range of growing conditions, biodiversity support and low susceptibililty to pests and diseases.
"In contrast to a diversity in tree species planted in parks, trees that line historic avenues are usually of one type, as was popular at the time of planting in the 17th and 18th century," Newcastle University reader in evolutionary genetics Dr Kirsten Wolff and colleagues argue.
"Maintaining the historic integrity of these avenues means maintaining the phenotype of the trees in the avenue and that ultimately means sourcing trees of the same genotype."
Historically, selections were less based on "a tree's suitability to habitat conditions and restrictions" than on "fashion and availability", they conclude.
The common or "Dutch" lime, (Tilia x europaea) is a hybrid of T. cordata, the small-leaved lime, and T. platyphyllos, the large-leaved lime, and genetic markers indicate most of those planted in northern Europe are of two cultivars, 'Pallida' and 'Zwarte Linde', each with distinct forms.
Those in the UK would have originally been imported from the Netherlands and later also propagated at the estates themselves or in nurseries, as both are amenable to layering, the researchers suggest.
A third type, 'Hatfield', "is found in most UK stately parks, and should therefore be available from a collection in case historically correct replacements are needed" - but "this seems currently not to be the case", they found.
While simply replicating these ultimately ad-hoc historical choices may not seem ideal, "selecting ‘better’ or specific cultivars for certain situations is inevitably a very long and difficult process" given the time they take to mature, they point out - though they suggest that such a task could be based on criteria of resistance to pests, diseases, drought and pollution, ultimate height, form, leaf colour, density and ease of management.
The research is published in Urban Forestry & Urban Greening.
Take care to match new limes with old, researchers say
Most of the "heritage" lime trees planted in the UK are of three particular varieties, and managers should consider genotyping existing trees when replacing or adding to them to maintain uniformity, a new research paper argues.
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