Do you like to go for a hike in the mountains on a sunny summer day? Do you then enjoy stepping out of the forest into one of those flowery meadows?
In many cases this would not be an actual alpine meadow [situated above the tree line] but a so-called subalpine meadow, formed in a local microclimate and microenvironment and therefore just as natural as alpine meadows – and perhaps even more important for ecological diversity.
Mountain meadow complex in Jefferson Park, Oregon, part of the Cascade Range, a volcanic mountain range (part of the Pacific Ring of Fire) stretching along the Pacific coast from British Columbia to northern California – including the famous Mount St Helens. Shown in the picture is Mount Jefferson volcano and both natural mountain meadows above (‘alpine’) and below (‘subalpine’) the tree line – where a researcher group led by Oregon State University monitors tree invasion in respons to climate change.
Alpine meadows are the secret landscapes of our planet. Hidden high above the tree line another world unfolds. Be there in summer and think the fairy tale of flowers and butterflies, the humblest of bumblebees, fast-running streams and happy nibbling marmots is never-ending.
But it does end, each autumn, which arrives early – and which is the actual secret to the existence of this flowery wonderland that is the summer hiker’s paradise.
The biome exists because of its harsh climate, dependent on the long and cold winters it is exposed to, that burry the shrub vegetation under meters of snow for months on end – and which demotivate any attempt of an invading tree by a short and slow growing season, biting frost, and grazing mountain goats that are permanently jumping around on the brink of a caloric deficit, that is very willing to eat anything that sticks out.
High mountain meadows
Serious people who like to be correct prefer to speak not of alpine meadows, but of alpine tundra – emphasising how dependent the high mountain grassland ecosystems are of the climate zone they evolved in. And indeed climbing up from the high-mountain pine forests to reach the meadows is very much like fast-forwarding to the North, from the Siberian taiga (or Canadian boreal forests) to the Arctic tundra.
No surprise then that climate change affects the mountain meadows as well – by melting and destabilising permafrost soils and altering the vegetation, just as birch trees invade the tundra [without adding any CO2 benefit btw!].
And just like climate warming for instance pushes Scandinavian tundra off the continent into the Barents Sea many mountain ranges run out of mountain, so the tree line moving uphill results in a net loss of alpine meadow area.
Intermediate mountain meadows
In practice the tree line is never a neat line. Differences in soils and bedrock, exposure to microclimatic factors like wind, sunshine, precipitation or snow accumulation sites mixes everything up in a zone that can stretch for many hundreds of meters uphill and downhill.
This means you can stumble upon so-called subalpine meadows – natural grassland ecosystems – right in the middle of the official tree zone, either on steep slopes or on surprisingly flat terrain.
Monitored tree invasion for different subalpine mountain meadow landscapes.
Exposed as these are by wind and grazing communities, (other) favourite spots for the subalpine meadows are the summits of lesser mountain ranges, enabling many a day-tripper to feel like a proper mountaineer – before indulging in the blueberry buffet.
Anyway, these mountain meadows are vulnerable to climate change too, a group of researchers of Oregon State University’s College of Forestry, the US Forest Service and the US Geological Survey have found out. And they deserve some credit for it. Problem is – as hikers can attest – many people tell personal tales that ‘the mountains are not quite as they used to be.’ But hey, isn’t everything?
Fortunately there are people actually measuring the retreat of mountain glaciers – a comparatively simple job. But who notices the forest is gradually, over decades, eating that beautiful mountain meadow you vaguely remember from your holidays? Well – leave that to the OSU-led research group.
Dramatic mountain meadow decrease
Thanks to their continuous plot measurements it is revealed these meadows have actually already responded dramatically to warming. The Cascade Range meadow complex shown above has seen a 328 percent increase in tree density between 1950 and 2007* – from 8 percent to 35 percent, they write in the journal Landscape Ecology.
[*) Shocking numbers, considering the effect of climate change on mountain plants is delayed – and will therefore likely accelerate over decades to come.]
The driving force their research has found out is not so much temperature rise directly, but a decrease in the annual snow-covered season. Just a few weeks less enable tree seeding to suddenly survive through the year.
In a different study the same researchers had found the ‘bald spots’ – subalpine meadows on low mountain peaks, had in an Oregon coastal mountain range been halved in size between 1950 and 2000.
Elsewhere in the mountain ranges of western North America researchers have found – oddly – the forested zone does not benefit from a prolongued growing season – as it worsens the pine beetle plague. As always in ecology the big rule for any biome is that ‘disturbance is bad,’ and no biome can really profit from the decline of any other – just like – as a whole – the taiga cannot profit from climate change, no matter that warming frees up space in the adjacent tundra.
© Rolf Schuttenhelm | www.bitsofscience.org