I have just returned from a week's visit to Lope National Park in Gabon, Central Africa. The reason was to kick off a couple of new projects (a DPhil project by Anabelle Cardoso on how fire and elephants mediate the transition between forests and savannas, and a Royal Society-Leverhulme project by Kate Parr on how large mammals and soil termites and ants interact to affect soil function. We also had a workshop on our long-term ecosystem monitoring in Gabon, where we spent two days with our Gabonese colleagues working through the data we have collected over the past three years, to come up with the first measurements of productivity of Central African forests.
It is a gorgeous place, one of the loveliest landscapes I have experienced in the tropics: a landscape of ancient soft curving hills, decked by a patchwork of deep green forests, soft green-brown grasslands and bright green fern savannas on some hill slopes. The lowlands between the hills are a similar mosaic, but also interspersed with damp marsh grasslands, forest island "bosquets" that represent the sites of long-abandoned villages, and green fingers of riverine gallery forests that are snaking through the grasslands. The savannas are always hot in this time of year, April, when an relentless equinox sun beats down overhead and noon. But this year is particularly unusual as we are experiencing the unusual heat and drought of a strong El Niño. On top of the long term warming trend this makes it potentially one of the hottest month this landscape has experienced for a century and probably more.
This is a fascinating landscape, where the forests of Central Africa are encroaching on the remnants of forest grasslands that once stretched across much of the current forest zone of Gabon. But what makes this landscape special is that it has been intensely studied as a labour of love by scientists over 30 years. These scientists - Lee White, Kate Abernethy, Kath Jeffery, Riczard Oslisly and many others - have dedicated much of their lives to unpicking the story of this landscape, and through this understanding trying protecting it for future generations.
The picture that has emerged is of a dynamic dance between savanna and forest that plays back over hundreds of thousands of years. Early hominids appear to have occupied this landscape for at least 700,000 years ago and more modern Neolithic farmers arrived several thousand years ago, farming palm oil and bananas, opening the forest and smelting iron, but also undergoing periods of mysterious population collapse, such as in the period 700-1400 years ago, when they disappear from the landscape.
Since the ice age the overall story has been of forest advance into the savanna, leaving now only pockets of savanna hugging the might Ogoué river in the drier northern fringes of the park. Occasionally in history human activity as slowed or stopped this advance, and at the moment fire management in the park aims to preserve these last pockets of unique savanna.
This landscape gives us a chance to understand in exquisite detail the processes that lead forest to advance in to or retreat from savanna. One way to do this is to walk from the savanna into the forest zone.
We walk towards an advancing front of this forest. The savanna around is dominated by metre-high grass, and only two or three woody, contorted savanna tree species 1 or 2 metres high: Nauclea latifolia with broad leaves golf-ball sized round, spiky fruit, Crossopteryx febrifuga with pale grey bark and a twisted form. The savanna is surprising species poor in trees - is this because this is an outlying zone and isolated pocket of the vast savannas that stretch across much of Africa?
We move into the transition along an elephant trail about a foot wide. Local experienced researchers stay at the front and back of our group, alert for a hint of elephant. The forest elephants are beautiful here, but are particularly aggressive, perhaps because they are the survivors of hunting episodes in the 1970s and 1980s. The front troops of the march of forest trees are the wonderful Lophira alata (Azobe), its young leaf tips flushed bright red. The grasses peter out in the shade of forest trees, taking this risk of fire with them, but the twisted savanna trees linger on, increasingly crowded out and shaded out by their youthful new forest neighbours. Then we are in older forest, dominated by surprising large trees including Lophira alata (azobe), Aucoumea klaineana (okoumé) and the elephant-dispersed Sacoglottis gabonensis (ozouga). What happens to these early forests in terms of fire or elephant events seems to shape the path of these forests for centuries to come.
Moving beyond the colonising forest we enter regions a few hundred years old, dominated now by towering stands of mainly okoumé. One fascinating aspect of okoumé's biology is that the root systems of different trees are often joined below the ground and nutrients are exchanged between individuals, helping a damaged tree to survived through a support of its neighbours.
As we walk on through the okoumé forest, eventually we reach a phase where the forests are more than 400 years old. This first cohort of okoumé trees has begun to die, opening up the forest canopy. But instead of younger trees surging up to fill the gaps, the ground is smothered by an almost impenetrable thicket of wild gingers and Marantaceae (arrow roots), which also climb up form towers which engulf dead stumps and small trees, and which suppress the emergence and growth of young trees. These plants provide valuable and accessible food for elephants, gorillas and others animals - the air is thick with the odour of giant creatures in a way I have never experienced in a forest, and everywhere we see signs of plants and fruit being eaten, trees heavily damaged by elephants feeding on their bark, and the dung of elephants, gorillas and chimpanzees. There is a tinge of excitement, mixed with an edge of alertness, listening out for every sound in the bush and being prepared to run quickly if necessary.
A troop of mandrills rustles the bushes nearby, and a hidden gorilla lets of a warning cry. Why does this Marantaceae forest occur? There may be a mutually beneficial interaction between the Marantaceae and the large animals - the plants attract animals to feed on them but can regenerate leaves and shoots quickly. The animals trample and damage small trees, preventing their ability to grow and shade out the Maranaceae. Whatever the reason, the Marantaceae freeze the process of forest regrowth, large trees die but are not replaced, and the canopy becomes more open.
Another amazing example of a tree-animal interaction is the tree Cola lizae, a species that is common in Lope but was only scientifically described in the 1980s. It produced bright crimson fruit that are particularly delicious for gorillas, bur fairly unpalatable for elephants. The gorillas then tend to nest in clearings with a dense growth of herbaceous vegetation, deposit their dung nearby, and in the process create conditions that let the young Cola trees establish and thrive.
Eventually, after at least 500 more years, the Marantaceae appears to be edged out. Maybe by chance a few trees to manage to escape the Marantaceae and grow and shade it out, which the enables more shade-tolerant trees to grow. Even 1500 years after colonisation of the savanna, the forest appears still some way from a stable old-growth form.
If we climb up the hills we come across a much more ancient forest, a forest that has been continuous since the last ice age and probably much longer. A low layer of cloud bathes these forests in the dry season, and the cool and humidity ensure that these hill tops remained forest even in the most arid and carbon dioxide starved phases of the ice age. The forest has a very different feel, with an open understorey easy to walk around, and many magnificent giant trees. Small termite mounds are everywhere. Most magnificent of all must be the mighty moabi tree (Baillonella toxisperma), it's elephant-scarred trunk an astounding 2.5 metres in width, its branches arching out across the forest. In the fruiting season it drops fruit from a great height. The strong odour and maybe the thud of these fruit attracts animals from miles around, but only elephants are able to swallow the fruit whole and disperse the seeds.
It is this mosaic of savannas and forests of different ages and histories, combined with intense interactions between animals and trees, that makes Lope such a fascinating landscape.
In both the Marantaceae forest and the ancient forest, we have installed our GEM ecososyem monitoring plots and have been tracking the growth and carbon cycling of these forests. Up to now, our only published studies are from South America (some from Borneo are imminent). There has never been a direct measurement of the productivity of an African forest, and in both Gabon and Ghana we have been working hard over the last few years (with post-doc Sam Moore and a large host of local collaborators and research assistants) to develop the first data for African forests.
In Lope we run a two day workshop for our local research assistants who have been diligently collecting field data every month for several years. The aim of the workshop is to use Excel to show how to move from field data to estimates of productivity, and to gain a greater understanding about what are the most important aspects of data collection. A key output of the workshop is to produce the first estimates of net primary production for African forests. Below shows the crowning moment. The bars on the left are from Amazonia, the bars on the right are from Borneo. The six bars in the middle are from Gabon. In the next month we hope to add another 16 bars from Ghana, and another two from Ethiopia. Within a few months, from a starting point of zero, we will have more data points in Africa than in any other continent. Then we will pore over the data and work out how they relate to climate, soils and history. Africa arise, your time has come!
Yadvinder Malhi is an ecosytem ecologist and Professor of Ecosystem Science at Oxford University