Tree and scrub species of the Treeline Ecotone in the Cairngorms National Park, Scotland

Abstract

Expansion of native woodland, in places up to a natural treeline with montane scrub above, is a priority within the Cairngorms National Park (CNP). Thirty native and six non-native tree and scrub species (phanerophytes and nano-phanerophytes) have records within the CNP treeline ecotone, defined as between 500 and 1000 m altitude. The altitudinal distribution (using a 25 m altitude interval) of the native species is analysed at monad (1 km grid square) precision. Results are displayed as the raw monad counts, monad counts calibrated to take account of altitudinal recording bias, and calibrated monad counts as a percentage of monads within the CNP. Summing the calibrated species monad counts in each altitude (25 m) class provides a combined measure of spatial and altitudinal frequency.

The same ten species occupy the top ten rank frequency positions in each of the altitudinal zones (forest line, treeline woodland and montane scrub). Therefore, across the whole treeline ecotone, we can expect woodland and scrub to comprise the same mix of species, though their proportions will vary, both altitudinally and geographically. Juniperus communis and Sorbus aucuparia are the two most widespread species in all three zones, Pinus sylvestris is in 4th or 5th place, while Betula pubescens is in 7th or 8th place.

In Norway there is typically a 'birch belt', in which Betula pubescens var. pumila (L.) Govaerts forms the highest treeline, above the altitudinal limit of Pinus sylvestris. It has recently been argued that an equivalent birch belt used to be present in Scotland, and should be a target for conservation interventions to restore it. The present analysis does not provide support for this view. While the low rank position of B. pubescens may reflect lack of seed sources and browsing pressure, that P. sylvestris occurs to very high altitudes, and comprises a higher percentage of the combined CNP calibrated taxon monad totals above 700 m than it does from 500-700 m strongly suggests that B. pubescens will not be able to form an exclusive 'birch belt' at elevations above the P. sylvestris altitudinal limit.

While B. pubescens var. pumila has not been confirmed as occurring in Scotland, introgression from B. nana into B. pubescens is widespread here, though only at a low level. The most parsimonious scenario that explains this is that during the post glacial period any highly introgressed B. pubescens, derived from tetraploid hybrids crossing with B. pubescens, rapidly crossed with 'ordinary' B. pubescens, hence diluting and dispersing the genetic contribution of B. nana. Within the CNP, Mar Lodge NNR is the most likely location where future hybridisation and introgression of B. nana into B. pubescens will occur.