Woodlands create their own special conditions and the plants living there have to be adapted to cope with them. Many woodland wild flowers have become so well adapted to this environment that they are rarely found anywhere else. Others which are less specialised are found more on the margins of woodland or in open glades and can also often be found in hedgerows.
There are several environmental factors which are different in a woodland to outside of the woodland, but one factor is by far and away the most significant and this is light.
Fairly obviously the light underneath the tree canopy will be much less than it is outside a woodland. Woodlands vary in density and type of species and so the range of light energy will show considerable differences. Light is measured in units called Lux. There is a massive range in the lux values that we experince, and adapt to. Unlike temperature where at the extreem we can tollerate a range of maybe 80 degrees centigrade (minus 40 to plus 40) We can easily see with light levels as low as 20 lux ( a dimly lit room) through to 25,000 lux (a bright sunny day) Our eyes are very well adapted and able to cope so we are not so aware of this massive range.
At the East Anglian Field Study Center, the students would often carry out investigations into the light levels in different woodlands. One study was to compare the light levels in different areas of a coppiced woodland and then relate that to the field layer species growing there. A similar study was undertaken in Thetford forest where they have some ‘experimental plots’ planted up with different tree species, to monitor their growth, suceptibility to disese and their effect on the rest of the ecosystem. Light meters were used and readings of lux levels recorded. It was quite difficult to get meaningful results, as light varies so much. A day with sun and cloud was a bit of a nightmare, a clear blue sky was best, uniform cloud was OK but even then a set of results taken at midday would obviously be different to a set of results taken at 2.30 pm So we used all sorts of techniques to try and get accurate comparisons. This sometimes involved getting the student to disperse to the different experimental plots and then getting them to all take a set of readings at the same time thus all getting the same weather conditions and the same position of the sun in the sky. I mention all this just to illustrate how varried and difficult measuring light can be…. most photographers are aware of this.
This is the type of light meter we would use, the light was measured in different ranges 0 to 10, 10 to 100, 100 to 1000, 1000 to 10,000 and over 10,000 Lux. This was quite fiddly to deal with but is a product of the huge range of light that is experienced.
OK back to the woodland environment. Not only is the light affected by the density and type of trees but in a deciduous woodland it is effected by the presence or absence of leaves and the number of leaves. Here is a graph which I used to present to the students in my morning lecture before we went off to the woods to carry out our studies. The two lines on the graph show the amount of light received at ground level outside a woodland (red line) and inside a woodland (blue line).
As you can see at any time of year the woodland is always darker than outside. This is because even during the winter when there are no leaves on the trees there will still be twigs and branches which will prevent some light getting through. So as the days get longer and the sun gets higher in the sky the two lines follow a parallel upward path for the first 3/4 months of the year. Then the leaves start to open and so the woodland graph starts to diverge from the outside. Still there is an increase in the amount of light because the day lengths continue to increase and the strength of the sun continues to improve. Also at this time of year the leaves are few, they are small and have relatively little chlorophyll in them. Leaves in Spring are that wonderful light green/yellow colour, looking pretty but not that efficient at absorbing light. As we proceed into summer the leaves get bigger and darker green and perhaps more important the trees and bushes grow and more and more leaves are produced which increases the light absorption and so despite there being more light available it actually gets darker inside the woods. In the autumn the leaves fall of the trees and so there is a slight increase in light levels and then during the late autumn and winter the graph is once again following a parallel path the to graph for outside the wood.
The tree to the left is an Oak growing at the RSPB reserve called Nag’s Head in the Forest of Dean, it is quite magnificent and I would guess that its is about 200 years old. This winter (2018) many of the trees in its vicinity have been felled, a mix of Oaks and Conifers but for the moment this one has been left. I hope this remains the case. You can see the light green leaves which will not be absorbing much light at this stage.
The plants living underneath the trees, the ‘field layer’ plants have to be able to deal with the light conditions and there are many strategies which can be employed.
Obviously it is essential to make maximum use of the period of the year when most light is available and this is March, April and May. To this end woodland plants need to get their leaves in position early in the year, it is no good starting the year as a germinating seedling with two tiny leaves and then steadily growing leaf by leaf because by the time you achieve a reasonable surface area for light absorption the bright (photosynthetic period) is over and you have blown it. Woodland plants are perennials, this means that they have a large amount of stored up food in the form of a bulb, corm, tuber, rhizome or swollen root stock, whatever, but it means when the days first start to warm up and the light begins to increase in February then BANG they can convert all that food into leaf, not two tiny little cotyledon that you get from a seed but a large amount of fully functional leaf surface which can then make maximum use of the next three months.
One of the best at doing this is the Lords and Ladies or wild Arum. It is often the first leaves to push up, even in late January but certainly they are by mid February. Other plants that are quick out of the blocks are Dogs Mercury and Wood Anemones. The bulb plants like Bluebells and Dafodills are also good exponents of this strategy. Quite a few woodland plants do not bother to retain their leaves after the ‘photosynthetic period’ is over, one of the first to give up is the Wood anemone, its leaves will start to turn yellow and die as early as late May. Bluebells also give up quite quickly. If you go to a bluebell wood in July, all you will see is lots of white dried up dead leaves lying flat on the ground with the flower stalks still sticking up, the ripening seed pods at the top. Some plants retain their leaves throughout the summer like the Dogs Mercury and no doubt manage to carry out some low level of photosynthesis during this period. The photo of the wood anemone leaves was taken in May.
There is of course another reason why most woodland wild flowers are perennials at this is because if you are an annual plant then you have to produce sufficient seeds to make sure at least some seeds will survive and grow next year, otherwise if you mess up then you become extinct. Also you have to do this every year, its no good being quite good at it because if you don’t produce enough seeds one year then that’s it. No second chance. Its almost the same for biennials because although they live for two years it is generally only in their second year that the seeds are produced. These types of plants then have to produce vast numbers of seeds in just one or two years, Poppies are annuals and it is estimated that one poppy plant produces between 10,000 and 60,000 seeds. I got this from ‘Interesting facts about Poppies’ published by the Daily Telegraph…. so don’t quote me on it , Foxgloves are Biennials and they produce 70,000 seeds per plant, again my source for this is a wild flower seed distributor Emorsgate Seeds.
So woodland plants simply do not have enough light energy available in one year to produce this many seeds, not even in two years. So they do produce seeds, but not enough to ensure survival through the winter as a seed every year. Indeed many woodland wild flowers will only produce seeds every few years. like lots of Orchids. After flowering and producing some seeds, they take a break for a couple of years and build up their food resources and then maybe produce another flower in a few years time. This maybe accounts for the disappearance and then reappearance of some rare woodland species like the famous Ghost Orchid which has at times thought to be extinct only to pop up again.
Being perennials is perhaps the most dramatic effect the reduced light has on woodland wild flowers, but there are other devices which many employ. If light is in short supply then making best use of it is quite sensible. To get best use, there are lots of simple things that you can do.
Have large leaves. Big leaves are a mixed blessing, more light, but more transpiration, ie you are going to loose more water. This is why desert plants have reduced leaves or no leaves, however in a woodland there is usually quite a lot of water available and it is often more humid and less windy than outside, so transpiration is not such a problem…just as well.
- Have lots of leaves.
- Grow taller. This is not so as to get closer to the sun as some of my less intelligent students occasionally said because being 60 cm closer to the sun is not going to make any difference. No if is obviously a competitive thing…. get above your neighbor.
- Make sure that one leaf does not shade a leaf located below it. If you take a time lapse film of woodland wild flower leaves they are constantly shifting position to get in brighter places, not shaded by leaves above them, also adjusting the angle because a more vertical angle is suitable when the sun is lower in the sky and more horizontal is best when the sun is high in the sky. This is called Phyllotaxis Phyllo, leaf and Taxis is movement. Also the leaves are generally arranged in a spiral pattern up the stem.
- Adjust the structure of the leaf (morpology) to make it more efficient at capturing the limited amount of light. For example reducing the thickness of the cuticle, and no layer of hairs, both reflect light. Then they may adjust the shape of the upper epidermal cells to make them more lens shaped which will focus the light onto the Palisade cells below them.
7. Adjust the amount and types of pigments. The chlorophyll is the green stuff which absorbs the light, it is contained in little cell organelles called chloroplasts. Most of the chloroplasts are in the cells called the palisade cells and most plants do not have any chloroplasts in the top layer called the Upper Epidermis. This is because although chlorophyll absorbs light it is also quite sensitive to light and if there is too much it can be destroyed, so plants that live in bright conditions outside a woodland do not risk it by having chloroplasts in the upper layer. Indeed the Palisade cells are long and narrow which means that the chloroplasts can move up and down thus getting themselves into the optimum position based on how bright it is. Now woodland plants do various things to modify this basic leaf structure. First of all they may have a higher density of Chlorophyll in the chloroplasts also the amount of Chlorophyll b may be increase in relation to Chlorophyll a, as the quality of light in a shady woodland is different to what it is in the open. Nicotiana, Tobacco plants may opt for a different strategy and have chloroplasts in the upper epidermis. Some plants can have a Palisade layer which is two cells thick not one. All of these things will make the leaf darker green and will absorb more light. Another option is to increase secondary pigments which will make use of the increase in red wave length light which manages to filter through to the lower levels of the woodland.
These are some of the different strategies employed , sometimes in combination, sometimes singularly by various woodland wildflowers…and it is all because light is the main limiting factor in their environment. Probably the one adaptation most used is to be a perennial.
There are many other environmental factors affecting woodland plants like the quality of the soil, the humidity, wind speeds etc butthe one factor which has the most effect is light which is why I have dealt with it in more detail. All the other factors will be dealt with together in a separate article.