Yesterday, Karin and I took a winter walk through one of the local woodlands, our feet crunching on the iced-over crust which is all that remains of last week’s snowfall. No doubt more snow is on its way as we approach the deepest part of winter here in Denmark. But seeing this holly tree reminded us that some parts of the woodland might remain snow-free no matter what the conditions.
Although I’m no meteorologist (and any who are reading this can correct me if I’m wrong), I suspect that two things are going on here. Firstly, the tree is sheltering the ground and reducing the amount of snow that falls below it. That umbrella effect is fairly obvious. But secondly, and more subtly, the small amount of warmth that there is in the soil is being prevented from radiating off into space by the presence of the holly leaves. So the warmer soil and surface layer of vegetation melts any snow that manages to make it through or under the holly’s canopy.
In both of these ways, the evergreen holly is affecting the microclimate of this part of the woodland. That in turn adds to the ecological heterogeneity of the habitat, proving greater access to food for animals, affecting the phenology of the ground flora, reducing local soil moisture, and so forth. All of these, in turn, will potentially lead to greater diversity of species with the local area.
On this blog and in my book Pollinators & Pollination: Nature and Society I’ve often written about evergreen trees, shrubs and climbers such as ivy, holly and mistletoe, including both their cultural associations (especially with Christmas) and their ecological importance. As so often is the case, the English poet John Clare thought about all of this two centuries ago. The quote I used in the title of this post is from his poem Winter Walk:
The holly bush, a sober lump of green,
John Clare (1793-1864)
Shines through the leafless shrubs all brown and grey,
And smiles at winter be it e’er so keen
With all the leafy luxury of May.
And O it is delicious, when the day
In winter’s loaded garment keenly blows
And turns her back on sudden falling snows,
To go where gravel pathways creep between
Arches of evergreen that scarce let through
A single feather of the driving storm;
And in the bitterest day that ever blew
The walk will find some places still and warm
Where dead leaves rustle sweet and give alarm
To little birds that flirt and start away
This might be my last post of the year, and so it only remains for me to wish a Glædelig Jul and Merry Christmas to all of my readers!
Prof. Jeff Ollerton - ecological scientist and author 