Saturday, 15 May 2010


As was pointed out graphically by Sarah in her blog Our Gossamer Planet the forests of the world are disappearing at an alarming rate. The highest proportional rate of forest loss is in the United States, with a huge 6% loss of their trees from 2000 to 2005. However, the greatest total loss was in Brazil, a massive 165,000 sq. km.

Many people around the world are interested in rehabilitation of tropical forest, but despite this there is no real agreement on the best way to go about it. To look at some of the nitty gritty involved here is one detailed study that illustrates the problems (Jose Camargo et al, 2002, Rehabilitation of Degraded Areas of Central Amazonia Using Direct Sowing of Forest Tree Seeds, Restoration Ecology, 10, 636-644).

How does one make a forest?

The usual method in forestry is to plant seedlings, and this has been tried in forest reclamation, but it is expensive and time consuming, and not practical for large areas. Cheaper and easier would be use to use seeds, and they are also less susceptible to injury during planting by unskilled hands, but would they germinate? Part of the problem is the poor quality of the soil, made worse by poor agriculture. Also, which species? If you just want something growing there, one of the most successful on abandoned ex-forest land is Eucalyptus, but it is hardly a native flora. Renovating a temperate oak forest is relatively simple, just plant lots of oak (well ok, it´s a bit more than that), but the Amazon forest is characterised by an extreme diversity of plants, all in intense competition for very limited resources. Which to use?

Camargo and his team chose 11 native species, ranging from pioneer species to those of more established forest. This in itself is a problem, as the very number of Amazon trees means that most are not characterised yet. Also, some are dormant until they receive the right stimulus such as hot water or perforation.

They then chose four sites, intact forest, with trees about 20-30m tall, secondary forest dominated by shrubs and saplings of pioneer trees, up to 5m tall, abandoned pasture consisting of grasses and herbs, and, most hostile of all, bare earth from a highway project, compacted and stripped of top soil.

So, what happened?

After one year only one, the Piquia (Caryocar villosum), survived on all sites. The problems of a seed begin as soon as it hits the ground. Seed predation was high and seedlings suffered from fungi, predation and low light in the shade (germination was much higher on bare earth). Of the pioneer species, not one seedling was alive at any site after one year.

Caryocar villosum is a very large tree, 40-50 m tall. One reason for the success of the Piquia was it´s hard spiny shell defeating predation, whilst others suffered heavily. It also grew spectacularly fast, reaching 100 cm after just a year in some conditions, allowing it to escape shade very quickly.

The 2nd success was Parkia multijuga. P. multijuga has the advantage that it is a member of the legume family like peas and beans, and so, in effect, makes it´s own fertiliser. It´s therefore much less reliant than other trees on finding symbiotic mycorrhizal fungi in the soil, and in fact it actually renovates poor soil, adding nitrogen.

One of the surprises of this study was the failure of the pioneer species. These are plants that rely on producing huge numbers of very small seeds that drift on the wind, which spreads them over a huge area. This means that when new habitats become available they will usually be there ready, a big advantage. It might be expected therefore that these are exactly the trees to chose when starting a new forest. In contrast, it was those trees that produce huge seeds that more or less drop straight down and rely on animals to move them a little way from the truck, that survived. Why? Well the main benefits of the small seed/pioneer strategy are increased dispersal range and access to new germination sites, both of which are negated by direct planting. But also, the sheer hostility of the environment was unexpected. Large seeds are not a guarantee of success, as other studies have shown, but they do give the potential to grow fast escaping the shade of your rivals, and also the spare capacity to produce toxins, important when there are so many herbivores about. In the Amazon, any sort of ground cover harbours massive numbers of insects that chop up seedlings, one reason why germination was more successful on the bare, safe, earth.

In the end, out of 11 species tested, the authors could only recommend Caryocar villosum and Parkia multijuga for rehabilitation work, both non-pioneers with very large seeds. This is a still a big step forward, and gives a place to start.

There will be, hopefully, a lot of reforestation in the future. What this paper, and others like it, show is that it has to be done only after careful study, or a lot of effort will be wasted. There is a price to pay for the convenience and saving from avoiding nurseries and planting direct, and that price is paid by the plants. Without the care and protection of a nursery, life is very risky for seedlings.

Lastly, the long term future of any "new" forest has to be considered. For example the timber of Piquia is know to be excellent, and is used for housing and boats in the Amazon region. A forest mainly consisting of Piquia would be very tempting to loggers in the future.

Camargo, J., Ferraz, I., & Imakawa, A. (2002). Rehabilitation of Degraded Areas of Central Amazonia Using Direct Sowing of Forest Tree Seeds Restoration Ecology, 10 (4), 636-644 DOI: 10.1046/j.1526-100X.2002.01044.x

1 comment:

Sarah Stephen said...

A very interesting study which can be of use for the reforestation in tropical forests elsewhere in the world. At Ponmudi, the reforested areas features conifers, eucalyptus, and Casuarina. It wasn't exactly considered a success, mainly because the latter two are not native (plus eucalyptus drains the moisture from the soil).


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