SUMMARY The concept of woodland being the climax community in temperate ecosystems has a long history. However this can be difficult to reconcile with the fact that large herbivores which can browse trees are characteristic of the vast majority of ecosystems (or were before humans made them extinct). This paper indicates eight scenarios which allow woodland to be the climax vegetation in the presence of browsing. Where herbivores are naturally present, regeneration is only possible if young trees are protected from grazing by thorny shrubs, winter snow cover, unpalatable leaves, rough topography, or regeneration conditions are optimal for young trees so the probability of some surviving browsing is high. If these conditions are not met then an open landscape is to be expected at this, the oligocratic phase of postglacial succession. The Scottish Highlands with its open landscape is a case in point: evidence suggests a long period of natural woodland regression from a postglacial maximum extent, with minimal anthropogenic input. The moorland vegetation characteristic of the area is more resilient than woodland over long time scales because, to persist in the landscape, woodland always has a sensitive period when young trees have to out-compete the other vegetation without being browsed.
WOODLAND OR OPEN GROUND? Scenarios for the persistence of woodland in the presence of grazing The Highlands of Scotland as an example
Introduction In this paper the Scottish Highlands (here referred to as the Highlands) are used as an example of temperate ecosystems which have retained a population of large indigenous herbivores, the red deer (Cervus elephas), throughout the postglacial period. The landscape is largely unwooded (Figure 1) and Fenton (2008) has explained how its openness can be explained through natural causes. In summary, the ecological carrying capacity of the vegetation for the main indigenous large herbivore is an order of magnitude greater than that which would allow for persistence of woodland in the landscape in this, the oligocratic phase of an interglacial. However in Scotland the dominant view amongst conservation organisations is that the open nature of the Highlands is anthropogenic: human activity has resulted in hills that are overgrazed – there are now too many deer and, in the past, too many sheep (see for example John Muir Trust 2019). The main justification for this view is the absence of trees in the upland landscape: the argument is along the lines ‘we know there are too many deer because there is too little woodland and grazing prevents regeneration.’ In practice this leads to a circular, self-fulfilling argument: ‘the reason we know there is too little woodland is because there are too many deer.’ This paper takes forward the arguments pursued in Fenton (2008) by addressing what should be the two more pertinent questions. A. How many deer would you expect in a natural system? – addressed through trophic levels. B. What is the expected natural distribution of woodland at this stage of the postglacial cycle, assuming herbivores are present at the natural level? The paper identifies eight different scenarios which would allow woodland to persist in the presence of herbivores. In the analysis which follows the following three statements are taken to be self-evidently true: 1. Trees are successful because their height means they can outcompete lower-growing competitors to form woods. 2. Regenerating trees are vulnerable until they are tall enough to be safe from browsing. 3. Large herbivores which can browse trees are characteristic of the vast majority of European ecosystems (or were before humans made them extinct). The aim of this paper is to answer the two questions above and to analyse the implications of these three statements.
Figure 1. Distribution of unwooded moorland (red), woodland (green) and agricultural land (white) in mainland northern Scotland, the area here described as ‘the Highlands’. This is the area used as an example in this paper. Most of the mapped woodland consists of post-1900 plantations. Contains OS Data Crown Copyright 2018. Licensed under Open Government Licence v3.0. Produced on QGIS by James Fenton.
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References Fenton JHC. 2008. A postulated natural origin for the open landscape of upland Scotland. Plant Ecology & Diversity 1:1, 115-127. John Muir Trust 2019. Management standards: biodiversity and woodland. [accessed 3 October 2019]. standards/biodiversity-woodland
Note: This paper represents the personal views of the author, James Fenton, and is based on a lifetime of observation, reading and cogitation. In many cases I have gone back to first principles rather than starting from the established position or majority view. This paper is the copyright of James Fenton but all or part may be copied for non-commercial use as long as due acknowledgement is given to the author. Date of first publication on 9 October 2019.
WOODLAND OR OPEN GROUND? Scenarios for the persistence of woodland in the presence of grazing
CONTENTS Introduction       Page 1 1 2 3 3 3 3 4 4 4 5 5 5 5 5 5 5 7 7 8