Time for some nature watching a little closer to home now!  I’ve been visiting the oak tree in High Shincliffe which I talk about in my post ‘My real oak tree’ every two or three weeks since I first went there in January but it’s only just beginning to show real signs of change.

My oak tree and its neighbouring Swedish Whitebeam trees on 2^nd and 25^th March and 1^st and 9^th April, left to right

To start with, most of the changes were in the understorey vegetation.  Heavy frosts in late March highlighted leaves of hogweed, nettle, white dead nettle, vetch and cleavers amongst the grass.

Just a month later, the hogweed, dead nettles and bush vetch are in flower.

Heraceum sphondylium, Lamium album and Vicia sepium, left to right

The Swedish Whitebeams either side of my oak are coming into leaf.

But, more excitingly, so is my oak.  Buds which were just starting to lengthen on April 9^th…

…are now bursting open.

My friend Sue Antrobus, whose ‘Oak Obsessions’ blog follows a year in the life of three very different oak trees in and around Guisborough, asked me about what controls bud burst in oak and why it is getting earlier as our spring weather gets warmer.  She set me thinking about why oak buds seem much more sensitive to increasing temperature than the buds of other native species, such as ash.

What is it that tells the tree when it is time for buds to burst and new leaves to appear?  Maybe it makes sense, first, to think about what buds are and why they form.  In deciduous trees such as oaks, the tree sheds its leaves in the autumn to avoid the expense of maintaining those leaves when light levels and temperatures are too low for efficient photosynthesis. Just like humans, plant growth is controlled by hormones – one called Abscisic Acid (ABA) accumulates at the end of the growing season, causing leaves to drop (senescence).

This begs the question of how the tree ‘knows’ that it’s getting colder. ABA can pass messages around the plant but is not a ‘sensor’ molecule.  When temperatures drop, the lipids (fats) which make up cell membranes start to solidify, just like if you were to put olive oil in the fridge.  Many of the proteins embedded in membranes can no longer function normally, which causes problems for plants (see What about the cold?)  It seems that plants sense temperature by a way of a molecule in the membrane, which is activated when the membrane becomes more rigid.  This, in turn, activates expression of a set of genes whose job is to help the plant prepare for the cold. One such preparation is the accumulation of ABA, which stops the production of new leaves and causes dormant buds to form.  Bud scales are simply thick, modified leaves which form a protective layer around the sensitive meristem tissue at the shoot tip (see How doth your garden grow?)

ABA accumulates in buds, keeping them dormant until, after a period of exposure to cold, the balance between ABA and other, growth-promoting, hormones dictates whether the bud remains dormant or starts to grow – it is this part of the process that seems to be changing in response to warmer spring weather.

When light levels and temperatures pick up, the tree can capitalise on this quickly.  It’s more efficient to have embryonic leaves protected within a bud than it is to have to make leaves from scratch.  In oak trees the buds come in clusters so the tree isn’t putting all its eggs into one basket.  Leaves are emerging from the large, apical bud now but, if a hard frost was to damage these, the other buds would, in turn, have their dormancy broken and produce leaves.

What about the role of light in this process? I’ve always understood that the increasing length of spring days is also a signal for plants to restart growth. It makes sense that temperature alone should not be enough – after all, starting to grow after an unseasonably warm snap mid-winter could prove fatal to a plant. However a paper published last year in Nature Climate Change by Constantin Zohner and colleagues finds that daylength is only important in regulating but burst in about one third of Northern Hemisphere woody species, mostly from lower latitudes where winters are milder.  Whereas daylength-sensitive species such as Beech (Fagus sylvatica) will only react to increasing spring temperatures once daylength increases, bud burst in other species is regulated by temperature alone.

It seems surprising that most daylength sensitive species come from lower latitudes – it would seem logical for daylength to be more important at higher latitudes, where day length changes more across the seasons.  Maybe this is because, at these higher latitudes, days lengthen long before the risk of freezing temperatures has passed and so daylength is not a good indicator of a safe time to grow?  What will happen as our climate warms?  At the winter period shortens, maybe the range of these daylength sensitive species will be able to increase poleward.

Oak turns out to be sensitive to both daylength and temperature and another recent paper, by Richard ffrench-Constant and colleagues, points out that light pollution, as well as increasing spring temperatures, may be bringing forward the time when buds burst.  As no organism lives in isolation, this has all sorts of ramifications for the hundreds of insect species such as the winter moth caterpillar (Operophtera brumata) which rely on young oak leaves for food.

As with all interesting science, these papers raise more questions than they answer!

—

ffrench-Constant, R.H. et al.,  (2016) Light pollution is associated with earlier tree budburst across the United Kingdom.  Proc. Royal Soc. B, 283 20160600; DOI: 10.1098/rspb.2016.0600. Published 29 June 2016

Zohner, C.M., Benito, B.M. Svenning J-C. & Renner S.S. (2016) Day length unlikely to constrain climate-driven shifts in leaf-out times of northern woody plants Nature Climate Change http://dx.doi.org/10.1038/nclimate3138; published online 17 October 2016