TREE LIFE
JANUARY 2011
MASHONALAND CALENDAR
Sunday 23rd January: Rydal Court, Ruwa. There is only one outing in January. Note that this is one week later than usual on the fourth Sunday of the month. Please bring a picnic lunch, a chair, a hat and water. We will meet at 9.30 a.m.
BAIKIEAS, BAPHIAS AND BAUHINIAS The Zimbabwe flora team goes to Hwange – Part 1
Monday, 4 January, 2010
Getting up rather early to go look at plants is not everyone’s idea of a sane way to pass the time. After all, plants, unlike birds or wildlife do not tend to fly off or disappear during the heat of the day. However, the aim of our trip is to reach Main Camp in Hwange National Park and that is at least a 10-hour drive from Harare in our more than middle-aged Landcruiser, so the Zimbabwe flora team gets going at the crack of dawn.
Two years ago, during our trip to Tuli (see Tree Life April 2008 or www.zimbabweflora.co.zw/speciesdata/outing-display.php?outing _id=4), we managed to survive serious car trouble, torrential rains and the total chaos that Zimbabwe was in at the time, and we feel confident that this trip will be a piece of cake in comparison. We resist all temptations of interesting plants and flowering trees, beckoning us along the way and manage to reach Bulawayo only slightly behind schedule. Despite the improved circumstances in the country, compared to two years ago, it still takes us three fuel stations to find diesel. Thoughts like ‘Oh no, not again!’ quickly come to mind but we do manage to fill up in the end. Somehow it always seems easier to get into Bulawayo than get out again, and it takes a couple of wrong turns before we find the right direction. However, we do arrive at Hwange Main Camp in good time, pay our dues and find our booked cottage ready for us. We also find that we are probably the only visitors in the area. Apart from the staff, the camp appears completely deserted and we may well find we have more than 14.500 square km (about half the size of Belgium) of undisturbed wilderness to ourselves.
A quick tour of the grounds mainly gives us a series of cosmopolitan weeds, such as Gomphrena celosioides, Alternanthera pungens and Guillemimea densa. All are well adapted to any place seriously disturbed by man. Where other plants disappear because they get trampled by too many feet, these species actually thrive. With hooked seeds and sticky burs they clamp themselves onto fur, skin, socks and trousers and spread themselves wherever we go. The next days will show well what perfect agents of dispersal humans are. At every camp, picnic site or viewing platform, wherever people are allowed to leave their vehicles, the same series of weeds have taken over in an otherwise diverse and almost pristine wilderness.
It is not only weeds at the camp of course. Under the majestic thorny crowns of camel thorns, ————————, with their broad, curved pods covered in grey velvety hairs, we find several small creeping plants. One is Tribulus terrestris, with its lovely yellow flowers and the rather ominous popular name ‘Devil’s Eyelashes’—referring to the small viciously spiny fruits. It uses a similar way as the weeds to disperse its seeds by sticking to the hooves of animals or piercing the soles of your boots. Another find is Hermannia quartiniana with lovely bell-shaped flowers. It’s a tiny herbaceous relative of the enormous African Star-chestnut or Tick-tree, Sterculia africana.
Electricity is as absent as other visitors so we tackle the “braai” for our cooking while it is still light. After dinner, in the glow of the campfire, we enjoy the sounds of the African bush: crickets, hyenas whooping and giggling nearby and even the distant roar of a lion in an otherwise silent and pitch black night. A perfect start to our trip.
Tuesday, January 5, 2010
On our first full day we want to find, record and, where possible photograph, as many species of typical Kalahari sand flora as we can, so we decide to do the large loop to the Kennedy Pans.
Leaves of Guibourtia coleosperma, false Mopane. Photo Bart Wursten; https://www.zimbabweflora.co.zw/
This very quickly turns out to be a rather over-ambitious plan. There is so much to see that we will never be able to do this in time, and we decide to do the shorter Dopi Pan loop instead. It is a perfect display of the flora we want. Densely wooded areas, interspersed with open grasslands and seasonal pans. At drier times, these pans attract both the wildlife and the people who come to watch them, but now they are devoid of both. At this time of the year animals are able to find food and water everywhere and have no need to stay close to these more permanent waterholes.
The woodlands are dominated by large trees of Zambezi Teak, Baikiea plurijuga, and an understorey abundant with Sand camwood, Baphia massaiensis, and the Kalahari bauhinia, Bauhinia petersiana var. macrantha. All are in flower and sparkle the landscape with small and large white flowers. Large False Mopanes, Guibourtia coleosperma, are also flowering but are more conspicuous because of the distinct orange-yellow bark with large black patches, giving the trees their freshly burnt appearance.
Having only been to Hwange during the dry season, it is amazing how lush, green and different it is now. Masses of flowering herbaceous plant species colour the roadsides. Many are typical for this environment and we are seeing most of them for the first time in the wild. Acanthosicyos naudinianus is a creeping species of wild cucumber. Since it usually has nothing to climb into in the almost bare sandy patches it inhabits, it has no use for tendrils, which are modified into spines. Dicerocaryum eriocarpum is a close relative of the more widespread Devil Thorn or ‘Boot-protectors’ (D. senecioides). It looks very similar but has broader, unlobed leaves and the typical double-spined woody fruits are more rounded and hairy.
Harpogophytum zeyheri flowers. Photo Bart Wursten; https://www.zimbabweflora.co.zw/
Another relative takes its dispersal mechanisms to even further extremes. Harpagophytum zeyheri, aptly named the Grapple Plant, has large flattened woody fruits armed on the margins with several rows of curved extremities bearing recurved spines. Even the largest mammals, buffalo, giraffes or elephants, cannot avoid getting hooked by these fruits and help disperse them. The plants we see show spectacular trumpet-shaped flowers, at least 6 cm in diameter, deep pink with yellow throats. Even one of the more familiar sights in Zimbabwe manages to look somewhat different. The Flame Lilies (Gloriosa superba) of Hwange are the brightest of orange you can imagine. You can see them from far away when the strings of flowers climb into the otherwise green vegetation. Common as they are in Zimbabwe, one tends to forget how extravagantly shaped and stunningly beautiful these almost alien-looking flowers really are.
In the afternoon, many dozens of new species later, we get back to camp and even manage to see some zebra and giraffes to remind us there’s more to life than just plants. A fantastic storm soon turns the whole camp into a temporary marsh. There is nothing quite like rain in Africa. One moment it is a hot and sunny day, then suddenly the floodgates open up and in less than an hour more rain falls than London sees in several months. The thunder and lightning, the enormity of it all, the sudden downpour temporarily cooling everything for a short while. An area such as Hwange may receive only a handful of these torrential showers, all in a period of a few months, so many plants, in fact all life forms, need to be ready and make the most of these brief periods when everything is plentiful.
There is not much point in going out again so we go through the findings of the day, download pictures and wait for the rain to stop. When it does, everything is soaking wet, including the firewood; and still no electricity. Many attempts, matches, candles, and much newspaper later we do manage to get enough of a fire going to cook some supper. The weather has obviously woken up every species of frog in the region as their orchestra of calls form the soundtrack for this evening. At 9:30 the electricity attempts an appearance lasting literally one single second. Recharging cameras, GPSs and computers will have to wait. Definitely time for bed.
Wednesday, 6 January, 2010
While two of the team are still dreaming their dreams, Bart gets back into his former guiding mode and manages to get the fire going, earning points by waking up the others with a nice cup of tea. Today’s plan is to visit similar habitats as yesterday but to venture outside the official boundaries of the National Park. This offers the relative advantages of legally being allowed to leave the vehicle and have a good look around and also collect some specimens of species that need closer inspection to identify. The main road outside the National Park offers perfect opportunities to do so.
Our first stop, at a railway crossing, immediately proves to be a treasure trove. The area has been partly cleared of trees, and we find many of the herbaceous species we saw the day before as well as many new ones. As usual we are confronted by several species of Indigofera. With more than 80 species, it is the largest genus in the country, ranging from tiny annuals to large woody shrubs. There is hardly a fieldtrip possible without finding at least a couple of species, and it is always a daunting task to get them identified. There is no recent comprehensive literature on them and, while some are quite distinct, others are deceptively similar. Trawling through all the specimens of a genus at the National Herbarium may work fine for most genera, but the numbers in Indigofera make it nearly impossible. Still, as always we record and photograph all possibly relevant details, collect a specimen and promise ourselves, as always, that one day we’ll get all our Indigofera specimens together and go for it.
At the edge of the woodlands, we get a chance to take a good look at some of the woody species such as Acacia fleckii, a species similar to Acacia erubescens. It is most easily distinguished from the latter by the much shorter petiole, bearing a much more conspicuous saucer-shaped gland and longer leaves with more pairs of pinnae.
Croton pseudopulchellus flowers. Photo: Bart Wursten; https://www.zimbabweflora.co.zw/
We found a nice comparison between the Lavender Croton, Croton gratissimus and its smaller cousin C. pseudopulchellus which were growing right next to each other. As both species were flowering, it was easy to see the difference between the long spikes of C. gratissimus and the short cluster of C. pseudopulchellus. Both species have very discolorous leaves, the undersides shiny silvery-white. The leaves of C. pseudopulchellus are considerably smaller and much more densely dotted with reddish-brown scales than its larger relative.
We slowly move on, making several more stops to inspect the woodlands. Fire has played an important role in the ecology of this environment, even since before the influence of man. Many species have adapted to overcome the dangerous effects of fire. Most trees either have thick layers of corky bark to protect them or have several thin layers of peeling bark, which can be easily shed and replaced by new growth. Many species, even tiny annuals, produce hardened fruits and seeds, which are able to survive fires or even need fire to break their shells and germinate. Many plants have evolved to largely growing underground, protected from fires, occasional frosts or seasonal flooding, only to send up annual shoots with flowers and fruits whenever the time is right. These suffrutex species, or underground trees, have evolved in many different families of plants and are particularly numerous in areas of Kalahari sand. We encounter several of these interesting plants today. Some are well-known and widespread in Zimbabwe, such as the Marama Bean (Tylosema fassoglense) a yellow-flowered relative of the Bauhinias. From an enormous woody, tuberous rootstock, it grows masses of creeping stems each year, which flower profusely, bear fruit and die back again. Other species we see for the very first time. Ancylanthos rubiginosus is a member of the Coffee family (Rubiaceae). It sends up several shoots every year bearing pretty orange-yellow flowers and fruits crowned with the golden-velvety remains of the calyx. Dichapetalum rhodesicum is member of the Poison-leaf family (Dichapetalaceae), a small family with only a handful of species in Zimbabwe. In both species, the shoots are normally only about 60 cm tall but in some years, when fires floods and frosts are absent, they can grow somewhat taller.
As we get closer to the main road between Bulawayo and Victoria Falls, signs of human disturbance increase, but the Kalahari sand vegetation still dominates. To our surprise we even find a small store seemingly in the middle of nowhere. It is proudly named the ‘Save-The-Nation Store’. It certainly saves us. The shops at the National Park camps have long since closed, as have the restaurants and bars.
Schinziophyton rautanenii, Manketti tree. Photo: Bart Wursten; https://www.zimbabweflora.co.zw/
We’re very happy to find some bread and other supplies to last us through the rest of our trip. Right outside the shop we also find another interesting tree, the Manketti Tree, Schinziophyton rautanenii,—a fascinating and unusual species in almost every aspect. It can be an impressively large tree with smooth, pale, golden-brown bark; it has very discolorous, palmately divided leaves, densely covered in stellate hairs; it has very conspicuous glands on the petiole where the leaflets are joined; it exudes copious clear sap; it has unisexual flowers with sexes on different trees and the female trees bear large velvety fruit with a single, very hard seed containing a bright yellow edible oil. If Meg Coates Palgrave says “All trees have labels to advertise their names,” than the Manketti Tree has billboard-signs to do so.
On our way back to camp, we stop at a small seasonal pan, obviously visited more by cattle than wildlife, but it still offers new interesting plants. Petra gets particularly excited when she discovers her speciality: a small fern. It is a species of Marsilea, an inconspicuous aquatic fern, one of the very few fern-like plants able to survive in this generally hot and dry, very fern-unfriendly environment.
In the late afternoon, we drive to Nyamandhlovu platform. Often the busiest place in Hwange at sunset, today there are no other people and few animals. Lots of birds around the pan, four hippos splashing about; a curious black-backed jackal having a look, and a herd of impala grazing in the distance. All very peaceful and quite enjoyable. A quick survey of plants largely yields the same weeds—which are becoming the standard. Back in camp there is still no power, nothing new there. Tomorrow we move toward Sinamatela. It promises to be botanically very different but do we dare to hope for improvement on the electricity front…?
All sorts of information, photos from this trip and the plants we saw, can be found on www.zimbabweflora.co.zw
[To be continued]
Petra Ballings & Bart Wursten
ANOTHER GREEN WORLD
At the top of Green Mountain, the central peak of Ascension Island, there is a small pond, dotted with lilies, shadowed to one side by the fronds of a pandan tree. It is the only open body of fresh water on the island—and for a thousand kilometres in any direction. Around Dew Pond grows a grove of towering bamboo, beyond which the trade winds blow incessantly from the southeast. Within the grove the air is still and damp.
Dew Pond, Ascension Island.
Along the trailing ridge of the summit are fig trees, Cape yews and a garland of remarkably vigorous ginger. Below, on the mountain’s lee side, trees and shrubs from all parts of the world spread down the hillside to a landscape of casuarina trees-ironwood, or she-oak—and thorny chaparral around its base. Even on the bleaker windward slope, grasses and sedges are dotted with Bermuda cedar and guava bushes. Above, the bamboo scratching at their bellies, are the clouds the trade winds bring; some days they cover the mountain top.
Once seen as too dry to be worth inhabiting, Ascension Island is becoming greener at an increasing rate. People are responsible. In part, their contribution was unwitting: the thorny mesquite that anchors a lot of the island’s scrub was introduced for a landscaping project just 50 years ago. But the forest on the peak of Green Mountain represents a deliberate attempt to change the island’s climate to make it more habitable. It is the centrepiece of a small but startling ecological transformation which is part experiment and part accident, part metaphor and part inspiration.
Ascension was discovered by the Portuguese in 1501. Just to the west of the mid-ocean ridge that separates South America’s tectonic plate from Africa’s, it is the top of a volcano which rises steeply from abyssal plains more than four kilometres below the surface of the ocean. The volcano made it above that surface only a million or so years ago, since when the island has grown to about 100 square kilometres. Before people arrived it was home to just a flightless bird, a land crab and no more than 30 species of plant, none as big as a bush. It was so barren and isolated that during the following three centuries of assiduous empire building neither the Portuguese nor any other nation bothered to claim it. When Captain Cook passed by in 1775, Georg Forster – later to become renowned for his accounts of exploration – wrote it off as a “ruinous heap of rocks”, drearier even than Tierra del Fuego and Easter Island. But Forster’s naturalist father Johann saw something more promising.
This barren island, with very little trouble, might be settled and made a very useful place of refreshment… I am persuaded that if the common furze, which thrives so well on St Helena, were planted on this island, it would no doubt equally thrive here, and were these Furzes everywhere growing, grass and other plants would no doubt immediately grow between them … The more the surface of the earth is covered with plants the more would they not only evaporate but even attract the moisture of the air… after grass and water were more plentiful in the isle certainly many a tree would soon grow and thus afford fuel.
Islands had a particular hold on the imaginations of explorers like Forster. It had long been widely held that the varieties of humankind reflected the action of different climates. In the late 18th century the opposite notion began to take hold among sailors, scientists and administrators: that humankind might itself act to change the climate, either for the worse or for the better, mainly through what it did or didn’t do to trees. A decade after Cook and the Forsters, a French explorer, La Perouse, visited Easter Island. Noting the island’s “dreadful aridity” in the midst of an immense ocean, he blamed the ancestors of the island’s inhabitants, who had cut down the trees.
Those imprudent ancestors have become symbols for mankind’s short-sighted carelessness with his environment. As environmentalists began to preach the gospel of finite resources, and satellites sent home images of the Earth looking like a small island in a vast dark sea, the fate of Easter Island seemed like a fearful parable. In his jeremiad, Collapse, Jared Diamond described Easter Island’s story as “the closest approximation that we have to an ecological disaster unfolding in complete isolation”.
Yet it would be a mistake to place too much weight on this tale. The familiar story – deforestation leading to environmental degradation; subsequent population collapse, possibly including cannibalism; eventual endemic misery – has been revised in recent years. Some suggest that the Easter Islanders’ fate was not purely self-inflicted: seed-eating rats, European slavers and climate change were in part responsible. And although apocalyptic stories have a power that brighter tales lack, mankind’s record is more nuanced than the Easter Island story suggests. People have created fertile ecosystems as well as destroyed them. Ascension Island is a supreme example.
A kaleidoscope of connections
Ascension’s key advantage over Easter Island is that it is remote, but not entirely isolated. Once it was eventually settled, it remained connected to the rest of the world for all sorts of purposes and in a succession of different ways. Britain first took possession of it in 1815 lest it be used as a staging post to rescue Napoleon Bonaparte from exile in St Helena. Later it became a supply base for the navy’s campaign against the slave trade; the steady warm air of the trade winds meant it also made a good sanatorium for sailors and freed slaves. For administrative purposes it was treated as a vessel, HMS Ascension, “sloop of war of the smaller class”. Subsequently it provided succour to ships, both naval and merchant, that found themselves in distress. The fact that the island could supply magnificent turtles – they migrate from Brazil to lay eggs on, or in, the beaches – as a delicacy to the lords of the Admiralty probably helped justify its garrison, too.
As the 19th century waned, steam and the Suez Canal meant that there was less and less call on Ascension for services to shipping. Then, in 1899, a telegraph cable connecting Britain to Cape Town came ashore amid the jagged rocks of Comfortless Cove. It was soon joined by cables from Sierra Leone, Buenos Aires and Rio de Janeiro. During the First World War radio receivers were strung over the lava like washing lines to provide communication with ships at sea. In 1922 the “stone frigate” HMS Ascension was decommissioned, and the island became in law a dependency of St Helena; in reality it became a fief of the Eastern Telegraph Company, which was subsequently absorbed by Cable & Wireless. Two decades later, it found itself part of a different connection – one that ran from the aircraft manufacturers of America to the North African and Mediterranean theatres of the Second World War. American troops built an airstrip on the lava plains in the south of the island where the wide-awake terns nested; 20,000 fighters and bombers flew in from Brazil, refuelled and went on to Africa. Forty years after that, Ascension provided an air bridge when Britain fought for the Falkland Islands.
These days it is a communications hub. Wires strung between two sets of tall towers transmit the BBC World Service’s broadcasts to 85m listeners in Africa and beyond. Nearby, strange geometries of short-wave systems connect the British and American armed forces to ships and aircraft. Aerials that look a bit like fish-skeletons are used by the spooks at Britain’s GCHQ, a strategic eavesdropping organisation. Dishes track space launches from Cape Kennedy and European space launches from French Guiana, and monitor tests of submarine-launched missiles.
Not all the information that washes across Ascension is picked up by electromagnetic means. The Met Office station measures greenhouse gases. Seismometers listen out not just for earthquakes but also for illicit nuclear explosions. Infrasound monitors do the same job for the atmosphere, picking up the inaudible but remarkably persistent sound waves that circle the world when a bomb goes off. Offshore instruments near the American base listen out for the underwater sound of such blasts.
Building a new Eden
In the mid-19th century one Joseph Hooker visited Ascension Island. He was the son of the director of Kew Gardens, a job he later took on himself. Hooker advised the Admiralty to plant trees over the top of the mountain, encourage brambles, aloes and briar rose in the ravines and establish acacia, casuarina and eucalyptus on the lower slopes. Shipments of plants from the Cape and from Kew started soon thereafter, drawing on the entire botanic inventory of empire. In four months of 1860, John Bell, the island’s horticulturalist, is reported to have supervised the planting of some 27,000 trees and shrubs. Surveying the results 140 years later, a British ecologist, David Wilkinson, turned to science fiction for the appropriate metaphor. Like an alien planet rearranged for human life, he wrote, Ascension Island had been “terraformed”.
The chief aim was to provide more rain and soil for the farm that had been established when the garrison was founded. In this, the scheme proved a long-term failure: the farm is no more. With two flights from Britain and one flight from America every week as well as regular visits by the Royal Mail ship St Helena, it is now cheaper to import food than to grow it.
Nor is it clear that the foresting of Green Mountain has increased rainfall. Precipitation varies on Ascension; in the years that Bell was expanding the plantings it was particularly heavy. Later it fell back and the farm, as well as some of the plantings, suffered. No one has documented any long-term trend in rainfall in response to the plantings, which is not surprising. The degree to which forests encourage rainfall is a matter of considerable debate. Those mechanisms that can plausibly be called into play at larger scales – such as the way that transpiration through leaves recycles water to the air, allowing the same moisture to fall as rain repeatedly – seem unlikely to apply on Ascension. The constant trade winds ensure that air passing over the mountain is back over the ocean in less than an hour.
In practice rainfall, like farming, is not much of a problem anymore. The BBC, which has ended up producing most of Ascension’s electricity because its transmitters are the biggest energy users, puts some of that power to work desalinating seawater. The American base has a desalination plant, too. Ascension could get by with no rain at all, if it had to. What the trees certainly do, though, is catch moisture directly from the clouds. As air climbs the mountain it cools, encouraging water vapour picked up from the warm ocean to condense. The trees provide copious surfaces on which that condensation – “occult precipitation”, to ecologists – can take place. That is what provides the water for Dew Pond, for the moist air under the spreading yews and figs below it, and for the soil. The more trees, the more moisture, the more trees. This explains the success of the plantings on the mountain. It may also explain some of the greening that has swept down the mountain’s south-eastern flank; water from higher up may be percolating through rock and soil. But not entirely. Stemson Stroud, the island’s conservation officer, first arrived from St Helena to work at the Apollo tracking centre in 1967. He and others contend that the island’s subsequent greening has been far more widespread than the slopes of Green Mountain.
The thorny mesquite is undoubtedly another factor. It was introduced to the island in the 1960s, when the BBC built a new village, Two Boats, for the people working on its World Service transmitters. Intended as decorative erosion-proofing the mesquite quickly took off, helped by the fact that its seeds pass happily through the digestive tracts of the island’s small population of feral donkeys.
Around Two Boats, which is near the foot of Green Mountain, the mesquite has teamed up with acacia, yellowboy (a shrub in the jacaranda family) and prickly-pear cactus to make thick scrub. It has also spread to the west and down to sea level. Mr Stroud and his colleagues spend a fair bit of time hacking it back and poisoning the stumps – and through them, they hope, the prodigiously deep roots – in order to preserve the lifeless volcanic splendour of at least some parts of the island. Goats, Mr Stroud speculates, might help them in their task. To hear a conservationist speak warmly of the notoriously omnivorous and disruptive goat is to get a sense of how potent a foe the mesquite has become.
But the greening is not just an invasion. Nor is it merely a result of increased soil moisture. Just look at Mountain Red Hill, an impressive cinder cone that lies southwest of Green Mountain. Contrary to its name, Mountain Red Hill is increasingly green, but not with mesquite, and not thanks to groundwater, unless it is a special type that flows uphill.
The green side of global warming
One possibility, far from proved, is that Ascension is benefiting from global warming. Warmer seas impart more moisture to the winds blowing across them: more mists, more clouds, more condensation. Although the temperature on Ascension has not changed appreciably in the past 30 years, sea-surface temperatures upwind of it jumped by more than a degree in the 1980s before levelling off. This warming may be a natural variation; it may well not. Rainfall measured by the Met Office has not increased over 30 years; but its rain gauge, at the southern tip of the island, is in one of the drier spots.
If there is more moisture in the air condensing as dew, you might expect to see the effects high up and to windward, on somewhere like Mountain Red Hill. If there’s more rain, you might expect to see it in the lee of Green Mountain’s central peak – and there is indeed a rainier strip, the locals say, stretching across the island from Two Boats to Comfortless Cove, a frequent source of teatime drizzle in the rainier months. It is along that strip that the mesquite and yellowboy grow most strikingly. If nearby ocean temperatures climb higher still, as climate projections would have them do, Ascension will probably become ever moister and greener. All those Victorian plantings mean that there are dormant seeds, both of plants that prospered and of those that didn’t, all over the island biding their time. Euan Nisbet, a Zimbabwean geologist and climate scientist, speculates that after a century or two of further warming the island may be green from top to toe.
With plants in place and seed banks built up in the soil, such a greening might continue unassisted. It may, in time, have to. The fact that Ascension has always found new uses to replace old ones does not mean the trick can be carried on indefinitely. And the electromagnetic connectedness on which much of the island’s usefulness now rests allows it to get by with fewer and fewer inhabitants. Even on Ascension Island, which is about as far off shore as you can get, jobs can still go offshore; the contracting companies that run the island’s many antennae are all looking to reduce their costs and their presence when possible. And since the crown allows no right of abode to anyone not working or dependent on a worker, nor the right to own private property, no jobs means no people.
If left to itself, Ascension would probably decline into dull, scrubby simplicity. Humans can help avoid that by creating the sort of balance that cannot evolve for itself on human timescales. To do so, though, is to make choices. Should the mesquite be allowed to kill the casuarina trees by drilling its roots deeper and depriving them of water, as in some places it seems to be doing? Which cinder cones should keep their bleak red beauty? What new elements should be introduced into the ecology in attempts to reinforce it? Would goats be OK? Would giraffes?
Such questions are easier on Ascension, where the ecological canvas was almost empty to begin with, than in the other novel ecosystems that humans are, mostly by accident, setting up around the planet. Yet decisions must be made. In 2002 the island set about eradicating its population of feral cats. Introduced to control the rats that had arrived with sailors, they had instead chosen to prey on the vast colonies of sea birds that roosted on the lava plains, wiping them out. Every species on the island retreated to a small islet offshore, Boatswainbird Island, except for a few individuals that held on to the most inaccessible cliffs, and the phenomenally scrappy wide-awake terns that visit the southern plains to breed. With traps, poison and guns, over 500 cats were wiped out. The birds have started to return.
Not all indigenous species are so easily accommodated. The grasses, sedges and shrubs that have been brought to the island handily out-compete the native species, many of them ferns, which were making such a poor fist of greening the island before people came. This is hard to regret, seeing the result. What’s more, the new painting need not cover up all of the original, almost bare, canvas beneath. The creation of the new can, with care, make room for the conservation of the old.
Restoring the balance
Beneath the summit ridge on Green Mountain, on the lawns of a small garden, Mr Stroud nurtures indigenous plants. He discovered one fern only a couple of years ago – a species hidden for centuries. He plants the successes under a huge fig tree on the ridge. When they flourish he takes them further out into what on other islands would be the wild, but here is the artifice, returning occasionally to check up on them and take more seed. While he and his successors are here, those ferns and grasses will be safe from extinction. And a few are taking the initiative themselves. Xiphopteris oscensionis, a tiny endemic fern, had never seen a tree before the Victorian planters came. Now it lives in and on them, nestled in their moist bark, pioneering the epiphytic way of life familiar from ancient forests around the world and discovered afresh in their youngest cousin. Life, with helping hands, adapts.
The lesson that Easter Island teaches humanity is bleak. Ascension Island’s story has a more hopeful message. It shows that environments not remotely natural in their origins can become lovely to inhabit. People like Mr Stroud can and will act not just to preserve the environment but to improve it, making it more, not less, than it otherwise would be.
Winding down the flank of the mountain, there is a graceful fluttering in the woods off to the side of the road. Free from the threat of cats, fairy terns have returned to the island – and forsaken their ancestral cliffs for a new life among the leaves and branches. They flash bright white and beautiful against the green.
[Reprinted in the interests of science from The Economist.]
EXTREME TREES
The world’s tallest, thickest, biggest, and oldest trees can be an extreme challenge. For the National Geographic “Extreme Trees” poster, featured in several of the magazine’s international editions, we took a look at what counts when measuring. Trees grow, limbs fall, ways of measuring change, and new trees are discovered, so figuring out which trees to highlight wasn’t easy.
To accurately measure these behemoths, scientists need knowledgeable tree climbers and high-tech-equipment. Redwoods have long been known as the tallest trees on Earth, yet the current champion, 115.6 m tall, was found just four years ago in California’s Redwood National Park. In 2008, a new tallest flowering plant was discovered in Tasmania – a 99.6 m mountain ash tree.
It’s just as hard to decide which is the world’s thickest tree. Bald cypresses, African baobabs, and giant sequoias vie for the title. Should you just take the measurement of the diameter at the base? Or is that measurement skewed by the fluted nature of the trunk? Robert Van Pelt, a renowned tree expert at the University of Washington, calculated that the diameter of the bald cypress, which holds the current record, in Oaxaca, Mexico, was more than 10 m. Others give an even greater figure. Van Pelt based his on a cross-sectional survey of the tree, rather than just its widest diameter.
The biggest tree in the world is the giant sequoia, though this title needs a bit of qualifying too. It’s actually the world’s largest single-stemmed tree in terms of total wood volume. The 83.5 m tree in California’s Sequoia National Park, with 1,489 cubic meters in the main trunk alone, has held the size record for almost eight decades.
Even getting information about the oldest living tree in the world is difficult – the location of the Great Basin bristlecone pine, the oldest known individual tree, is a carefully guarded secret. Scientists say it’s at least 4,800 years old. Oldest certainly doesn’t mean biggest. Bristlecones grow only about 9 m high, with strips of living stem surrounding mostly dead trunks.
Posted on April 9, 2010 by Christy Ullrich at http://blogs.ngm.com/blog_central/2010/04/extreme-trees.html
