Mature trees as keystone structures in Holarctic ecosystems - a quantitative species comparison in a northern English park

Background: Mature trees often provide ecological niches of value to specialised flora and fauna, signalled by such attributes as epiphytes, trunk rot and dead branches. In Britain, they are often found in parklands and wood pastures, which are rare habitats in Europe. Aims: As species differences in veteran attributes of such trees have not been studied, we surveyed eight Holarctic tree species in Chillingham Park, in north-east England, where the stems are of broadly similar age (200-250 years). Methods: The following variables were scored for 779 trees: presence or absence of veteran attributes, community status (alone, in a group, or in a linear feature), stem diameter, altitude at which growing, and the ground vegetation. Results: Trees were generally of only moderate mean diameter. Alder (Alnus glutinosa) and ash (Fraxinus excelsior) had the most veteran attributes (4.30 and 4.16, respectively), followed by oak (Quercus sp.) (3.65), then by birch (Betula agg.) (3.49), beech (Fagus sylvatica) (3.12), sycamore (Acer pseudoplatanus) (2.77), larch (Larix sp.) (2.47) and Scots pine (Pinus sylvestris) (1.92). Trees growing at middle altitudes and alone, or in linear features (rather than in groups), had most veteran attributes; 32% of trees exhibited three or more. Conclusions: To capture the veteran tree interest of a site, a survey protocol must consider the history of a site as well as the numbers of veteran attributes exhibited by individual trees, which may differ among species. Finally, alder has not attracted particular attention in these habitats, and we suggest that its fast-growing and rot-prone nature may make it of particular interest for conservation of saproxylic biodiversity.