City Trees

thick mulch that adds to the effect. The interiors are an open structure of radiating branches (Photo 6) and from clear signs of use make wonderful dens and climbing structures for young children. What the air photography also revealed was another sense in which these oaks could be characterised as space invaders. Around each is a very distinc- tive "halo" where the shingle is devoid of any other vegetation excepting where other holm oaks grow closely together. While the effect is visible on the ground, it was the photographs that drew our atten- tion to the phenomenon. Beyond the crowns, the roots of the oaks appear to be sucking all the avail- able moisture and nutrients from this impoverished habitat. Even the highly adapted valerian, which happily grows on the exposed walls of the nearby Canterbury UNESCO World Heritage Site (the loca- tion of our university) and on the arid ash cone of volcanic Mount Vesuvius, is unable to grow within the oaks' sphere of influence. This has obvious consequences for the native flora of the shingle, especially as the oaks spread further along the coast. Our observation of the halo effect prompted a survey to be undertaken to assess just how important the impact could be. Casual observation of the air photography suggested that the larger the oak, the larger the halo, with whole areas of the beach being devoid of other vegetation where mature holms dominated. A ground survey was undertaken in the summer of 2016. Several measurements provided an average radius for each crown and for the zone of influence from which the areas could be calculated. The results did confirm that larger oaks with crowns tended to have a greater influence in terms of area of suppression, but that other factors were involved. One of the largest crowns (average radius of 9.4 m/30.8 feet) had a relatively small halo extending slightly less than 5 metres (16.4 feet), while one of the largest halos extending to nearly ten metres (33 feet) was associated with a crown of radius less than 5 metres. Both examples, however, impact on over 600 square metres (718 square yards) in total. Differences in effect seem to be partly related to the impact of the urbanised sections of the beach. A problem identified by the White Cliffs partnership (2010) is the dumping of garden waste on the beach which encourages invasion by exotic species, but also nutrient enrichment of the shingle environment to the detriment of native species. The air photography shows this issue clearly, with lush ground cover on the shingle immediately to the front of the housing (Photo 7), compared with much more exposed shingle to the north, where no housing exists. The large holm oaks close to the housing seem to have much less impact due to the nutrient enrichment and the fact that the species surrounding them, including grasses, are adapted to making use of this. In conclusion, the holm oaks of Walmer provide fascinating insight into the ability of some tree species to survive and thrive in a harsh environ- ment. The wonderful genetic plasticity of trees is evidenced by this case study, with the oaks hun- kering down in their modified giant "cushion" form. Good for them, if not great for the native species of this important and rare habitat type. Beyond the crowns, the roots of the oaks appear to be sucking all the available moisture and nutrients from this impoverished habitat. 36 CityTREES

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