There’s been much discussion in the news and online recently about seed collecting, habitat restoration, and tree planting as a way of storing carbon in an effort to reduce the effects of climate change.  This is one of the (many) elements proposed by the recent Drawdown Framework.  In fact their “Table of Solutions” ranks tropical forest restoration in the top 5 to 10 ways of reducing CO2 in the atmosphere, and temperate forest restoration and planting in the top 20.

At one end of the spatial scale, Markus Eichhorn relates the story of his father’s obsession with collecting oak seedlings to reforest the local countryside.  At the other, there’s some very high profile forest restoration schemes going on at the moment; here’s a couple that immediately come to mind:

Grain for Green – China’s attempt to restore vegetation to abandoned farmland to reduce soil erosion and flooding.  According to the Wikipedia entry “Grain for Green has involved 124 million people in 1,897 counties in 25 provinces……. By 2010, around 15 million hectares of farmland and 17 million hectares of barren mountainous wasteland were converted back to natural vegetation”.

Great Green Wall – a multinational initiative in Africa aimed at restoring the vegetation on the southern edge of the Sahara to combat desertification and mitigate climate change.

Several countries have also made a great deal of noise about marshaling huge public efforts to plant hundreds of millions of trees in a single day,  for example India and Ethiopia.

These big schemes are all well and good: they generate a lot of publicity for actions on climate change and a warm, fuzzy feeling that governments and people are Doing Something.  But there’s a couple of problems.  First of all, planting trees is not enough: we could not plant enough trees in the world to reduce CO2 to pre-industrial levels.  Secondly, planted trees require nurturing.  It is not enough just to put in some young plants and hope for the best; a high proportion of trees die even when well looked after.  If they are just planted and ignored, who knows how many will survive?

However habitat restoration is important; it’s not a silver bullet solution to climate change, but it is part of our toolbox of Things We Can Do.  Just as importantly, restoring habitats provides more opportunities for species to move in response to changing climates, and to recolonise areas from where they have been extirpated.  And of course diverse, functioning ecosystems support human societies in ways both tangible and unquantifiable.

With all of this in mind I was interested to read a piece by John Carey in PNAS entitled   “The best strategy for using trees to improve climate and ecosystems? Go natural“. There’s some really inspiring stories in here, it’s well worth taking a look.  The main message of the article is that allowing forest vegetation to naturally regenerate, from seeds, and dormant roots and stumps, is by far the best way to ensure that trees survive and the restoration is successful.  However there’s something fundamental missing from that article: the role of species interactions in determining the survival of these forests over long time scales.

The vast majority of the world’s plants are animal pollinated; this includes trees.  Even in the UK where we often associate trees with wind pollination, about 65% of our native species are insect pollinated.  In the tropics this can rise to 100% of species within a community.  Although many of those trees can engage in some self-pollination, in the long term this is likely to result in genetic problems associated with inbreeding.  Outcrossing sex is common in plants for a good reason.

Similarly many trees require animals to move their offspring away from the parent plant.  This avoids competition between parent and offspring, and the impacts of diseases and pathogens caused by the Janzen-Connell Effect.  I don’t have any comparable statistics on the proportion of trees, regionally and globally, that use animals as seed dispersers (does anyone?  Please comment below if I’ve missed something).  But I’m willing to bet that it’s a high proportion.

Without pollinators and seed dispersers, restored forests will not flourish in the long term.  There seems to be an implicit assumption that once the forests are established, the pollinators and seed dispersers will follow.  That may be true up to a point, but it shouldn’t be taken for granted, particularly for isolated fragments of forest with no ecological connections to more established areas of woodland.  These are the aspects that are missing from John Carey’s (otherwise fine) article, and indeed from wider discussions about forest restoration and tree planting.  As so often when we talk about the conservation of biodiversity we neglect to consider the role of species interactions.  I’ve been trying to press home that point for years, on the blog and in papers, and I was pleased to see an interesting contribution to this topic by Pedro Luna and colleagues from Mexico on “Measuring and Linking the Missing Part of Biodiversity and Ecosystem Function: The Diversity of Biotic Interactions“.

Let’s not forget: species do not occur in isolation, and the biodiversity of species interactions in fundamental to the ecology of the planet.