Hollies are Native Trees, but Under the right conditions, they can become a Pest!

Friend or Foe?
Holly is one of our most well known and documented native trees.  It has evolved alongside our other native vegetation and wildlife, and it usually forms a natural balance with in our woodlands, heaths and hedgerows.

Normally there is just the right amount of it and it is a fairly common.  A well behaved little tree, giving shelter in the winter and berries for the Birds.

Under certain condition, however; Holly can quickly become a rampant nuisance, spreading mostly from seed, often engulfing the entire under-canopy, in a similar fashion to many invasive foreign weeds, such as Rhododendron, Bay or Cherry Laurel.

There seems no clear indication of what exactly triggers the Holly to behave in this way, but my observations, over the past few years; have lead me to conclude with two main theory's.

Hollies Take over in some wild areas, But not in others

What ever could be causing this strange phenomenon?
I will go more into my own conclusions in a bit, but firstly, I would like to show some examples of a few wild woodland areas on Dartmoor national park, where Hollies have really made problems and become a dreadful invasive weed, in their own right.

The caption on the right, shows just how quickly Hollies seed themselves, when they become troublesome.  From other pictures on this post; you can see how they rapidly take over.

These photos were taken on three sites.  Two of the sites were completely taken over with Holly, but in the third; Holly was common, but behaving normally.

Other observations have shown me that often coastal areas are particularly at risk from such natural unrest.

Theory's 

I have, as I already mentioned got two main theory's for this unnatural Holly growth, but I shall mention a third theory here as well:

1. Young Woods - One clear observation as shown that Hollies don't seem to be anything like as invasive in a fully mature woodland and younger half grown trees perhaps do not block the light from the under canopy quite as much as so make conditions ideal for them to take over, which would perhaps mean that when such wood do reach maturity, perhaps the problem would solve its self.
2. Climate Change - Perhaps the change of our climate over recent decades has created conditions, which favour our Hollies and they have had a population explosion as a direct result of this.
3. Garden Hybrids - Its possible that our wild Hollies could have become hybridises with many new and exotic garden varieties and foreign species and have developed into a disease resistant super-weed.

What ever is causing this problem, it really dose need some further investigations.  I aim to keep an eye on the behaviour of these plants and hope that the problem subsides and this plant goes back to its normal balanced pattern of growth.