Why big trees?

I've been involved with conserving rainforests and other habitats or over 30 years, and it has always been the accepted wisdom that the big, mature tropical forest trees are paramount, and we must conserve virgin forest whenever possible. I am not disputing this, but I am intrigued to know what the science is to back it. After all, nearly all the forests of Europe are secondary forests, and so are those of much of Central America, and a large part of Asia. And there is plenty of species diversity.



In England, almost every naturalist will know that hedgerows are among the most species rich areas for nesting birds -- and these effectively mimic forest edge habitats. I recall that when I was first working in Belize, the best place for seeing as many bird species as possible in as short as time as possible, was in a derelict milpa (slash an burn farm). In fact this is true in almost all parts of the world that I have visited. Most of the large mammals of South East Asia, such as Kouprey, Banteng, Gaur, Deer and even rhinos, all prefer forest clearings, and avoid dense closed-canopy forest. Dynamic, regenerating forests with a mixture of trees of different ages, almost always seem to have the greatest species diversity.



But if I was to postulate not worrying about old growth forest, there are some obvious flaws in this argument. There are some species that are only found in large stands of old growth forest. The best known is perhaps the ivory-billed woodpecker -- alas now extinct, largely through loss of its habitat. But it would be right to ask the question: How many species are restricted to old-growth forests with really big trees? And in the 21st century with all the pressures on the forest can forests really be maintained for these few species? I ask the question, because the alternative might be to selectively log forest, thereby providing a sustainable income, and increasing the numbers of other species, most of which thrive.



Conservationists do not have the funds to buy every bit of remaining forest, and neither do governments have the resources to protect from exploitation all the resources they control. Logging out big valuable trees reduces the value of land, in commercial terms, but does it reduce its value for wildlife? If it doesn't, then it makes sense to save heavily logged, cheap land, rather than virgin forest which may be more expensive.



Any ideas?