12 Jun 2023

How high can a tree grow?

Written by Dan Watson, Senior Nature Conservation Officer

An old Scots pine stands alone in a mountainous landscape. The hills behind are covered in snow and the sky is blue.

Scots pine at Mar Lodge Estate NNR

Within the botanical community there is a small group of people who scour the mountains for the highest altitudinal record of each species of plant. The Botanical Society of Britain and Ireland (BSBI) maintains a spreadsheet with all the ‘altitudinal records’ for each species.

Ben Nevis, of course, holds the record – for starry saxifrage growing close to the summit cairn. But what about trees? In 2011, I found a rowan growing at 940m in the shelter of a boulder at the summit of Am Bodach at the eastern end of Aonach Eagach in Glencoe – this held the record for over a decade. In 2019, I spotted downy birch at 795m at the bealach (mountain pass) between Meall Corranaich and Meall a’ Choire Lèith in the Ben Lawers range – this held the record for a shorter amount of time. In 2022, eight rowans were recorded higher than 940m and eight downy birches higher than 795m.

Why this sudden influx of records? In May 2022 Sarah Watts, former seasonal ecologist at Ben Lawers NNR and now Corrour Estate’s Conservation Manager, set up the High-altitude Trees of Britain and Ireland Facebook group. Over on Twitter, the hashtag #HighMountainTrees was also encouraged as a means of sharing records of trees found growing at over 900m in altitude. This resulted in an upsurge in the number of people, not just botanists, who started photographing and recording trees they saw on their mountain visits. Incredibly, the altitudinal records for nine species were broken over one summer, showing the power of social media. In ascending order, the tree species recorded above 900m are:

Goat willow at 984m on Beinn Eibhinn in the Grampians in 2022
Downy birch at 1,026m on Ben Nevis in 2022
Rowan at 1,150m on Sgurr nan Ceathreamhnan in West Affric in 2022
Scots pine at 1,160m on Cairn Lochan in the Cairngorms in 2003
Sitka spruce at 1,210m on Braeriach in Mar Lodge Estate NNR in 2023 – this is the highest recorded tree in Scotland!

Unfortunately, there is no precise location for the Scots pine record, so it is unknown if it is still there. However, in 2022 a record was made of a tiny Scots pine at 1,100m on Sgoran Dubh Mor in the Cairngorms. Several other willow species occur above 900m but these aren’t included here, as they are deemed shrubs rather than trees.

A close-up of a very small birch tree, growing very close to the ground. It is surrounded by moss and montane grasses.

A tiny rowan tree grows out of some rocks on top of a very tall mountain.

If you’re tempted to get out this summer and beat these records, it’s worth bearing in mind that it takes a bit of searching. Anyone who spends much time on the high mountains of Scotland will know that winds can be brutal in a way that makes storms at lower altitudes seem tame in comparison. Any trees at high altitude are going to be small, nearly always growing no higher than whatever is providing them with a bit of shelter from the prevailing wind.

Size is not necessarily a good indicator of age in such situations. A photo of a downy birch taken in 2022 at 986m on Stob Coire Sgriodain (in the Grampians) has been compared to one taken in 2016 when it was first found (but not formally recorded) – it shows no discernible difference in size. The only practical way to age such a tree would be to cut the main stem and count the rings, which would probably need a microscope as annual growth is so low. It would take a cold-hearted botanist to carry out that deed on a tree which has survived thus far against all odds.

Some botanists might find this an easier task to commit on the one other high-altitude tree that can be found in the Scottish mountains: the Sitka spruce. There were 40 records of this tree above 900m in the last year, the highest being at 1,210m on Braeriach. This non-native spruce is becoming a species of increasing concern to many conservationists, because it seeds out from plantations into many different habitats and is protected from grazing by its sharp, needle-like leaves.

A solitary rowan tree, with orange leaves, grows out of a crevice in a high-altitude gorge in Glencoe.

There is also a question of what to record. Currently the highest recorded sycamore is one found at 1,045m on the Ledge Route of Ben Nevis in 2016. However, an accompanying note states that it was a ‘single seedling, looking unhealthy’ – the chances of it having survived are very low. Whenever relevant, it is worth noting when a record is of a seedling rather than an established plant. For anyone who has ever watched a sycamore ‘helicopter’ spin through the air, the thought that the wind could carry a seed so high onto Ben Nevis is incredible. In fact, apart from rowan, all the tree species with records above 900m have seeds dispersed by wind. Rowan berries are dispersed by birds – it is easier to imagine a bird like a fieldfare perching on a rock near a summit, depositing a fertile package containing the remains of its last meal. It is intriguing that (so far) none of the other trees that could potentially be spread in such a way have been found anywhere near as high in the hills.

A fieldfare sits on a branch of a rowan tree, which is laden with red berries. The sky behind is bright blue. The bird is a similar size to a blackbird but is brown with a speckled tummy and a grey head.

A fieldfare in a rowan tree

Is this branch of plant recording more than a bit of fun, albeit with an element of competition included in the mix? Starting from a low baseline, it would be difficult to use this as another example of climate change or changes in grazing affecting the vegetation of our hills. However, with more people recording what they find, the greater the chances of the data increasing in value over time and becoming useful for research at some point in the future. Just taking a photograph and recording an accurate grid reference with a GPS will allow someone to return in the future and see how that individual tree has fared. Which reminds me, I’d better get back up Am Bodach and see how that rowan is doing!

Many thanks to Sarah for sharing the data that she has collected. You can read more about this research in her paper in the British & Irish Botany journal.

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