Desert Trees of the World – A new database for ecological restoration

For the past five years, James and Thibaud Aronson have been traveling to the driest parts of the world to collect data about the distribution, ecology, uses by humans, and up-to-date systematic botany of  the soul-satisfying and mind-boggling trees that grow in Earth’s beleaguered, beloved, and mega-diverse drylands. Here they describe the content and purpose of their new Tropicos database. This work builds on more 3 decades of collaboration between James and Edouard Le Floc’h, who is also a co-author of the database and a book-in-progress on desert trees and their role in ecological restoration and allied activities.

Desert Trees of the World represents a multi-purpose, participatory database in which we have gathered a vast array of information about dryland trees, where and how they live, the communities they are part of, the many ways in which they are used by people, and some elements about their successful cultivation.

Our database brings together the most up-to-date botanical, biogeographical, ecological, and ethnobotanical information on 1576 species of trees from the arid and semi-arid regions of five continents and many islands. And because it is hosted on Tropicos, the Missouri Botanical Garden’s vast botanical database, a user can seamlessly access any supplementary information that may be available for a given species thanks to research carried out in other MoBot projects. Further, maps of collection sites, as well as full nomenclatural, bibliographic, and voucher specimen data accumulated digitally at MBG these past 30 years are available.

The data base is intended for students of natural history, practitioners, policy-makers, and scientists working in ecological and biocultural restoration, conservation, and sustainable and restorative environmental management.

Trees in the desert?

Most people think that deserts are – by definition – devoid of trees. Not true! Indeed, some of the strangest, oldest, and most remarkable tree species on the planet are found in drylands, a term often used to refer to deserts and semi-deserts, also known as arid and semi-arid lands.

For our purposes, drylands are all the lands of the globe that receive less than 400 mm (ca. 16 inches) of rain in an average year. In total, this concerns over 42% of all lands on Earth, so listing all the tree species that occur in them was no small task! But, we were drawing on decades of travel, research and residence in quite a spectrum of the world’s deserts and semi-deserts. We also pored over specimens housed in three dozen major herbaria, and read thousands of technical scientific articles and floras in several languages. And, as this is the 21st century, we used information already online in another Tropicos project, the Catalogue of the Flora of Madagascar as well as many other online sources.

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A cardón (Pachycereus pringlei) and a boojum (Fouquieria columnaris) in the Central Desert of Baja California, Mexico. In the harsh conditions of deserts, evolution has favored some of the strangest-looking trees on the planet.

Boswellia Oman

In southern Oman, we explored the remote Wadi Aful, where wild frankincense trees (Boswellia sacra) grow between sheer rock walls.

Astrotricha hamptonii

The irontree (Astrotricha hamptonii) is not among the most impressive-looking desert trees in our database. And yet, because it only grows on ironstone formations, clever prospectors used its distribution to discover some of the largest iron ore deposits in Western Australia.

Since there had been no previous attempts at documenting the trees of all the deserts in the world, we weren’t sure how many species we would end up with. And the end result was truly remarkable: a sum of 1576 species of trees native to deserts around the world, occurring in  422 genera and 100 families of flowering plants. Of course, new tree species are still occasionally being discovered, mainly coming out of Namibia, Somalia, and southern Arabia, but we are confident that we have captured the great majority of all extant dryland trees in this database.

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Then again, some desert trees are not so unfamiliar to visitors from Europe or North America, such as these junipers (Juniperus phoenicea) and oaks (Quercus calliprinos), growing in the central mountains of Jordan.

What does a desert tree look like?

If asked about what a desert tree looks like, you might think of spiny or resinous, sticky trees. And you would be right. Fabaceae, the legume family, make up just over a quarter (403) of all species, and of those, 217 are Acacia sensu lato. The next ‘big’ family is the Myrtaceae (the Eucalyptus family), with 133 species, all but one found in Australia, the exception, Myrcianthes ferreyrae, being restricted to the fog oases of Peru’s hyper-arid coast. And in third place are the Burseraceae, with 111 species. This is the family of myrrh and frankincense, two desert trees whose importance for humans dates back millennia, tied as they are to the great cultures of the Old World. For reference, people’s most common images of desert trees are palms (think – oasis) and tree cacti. But there are only 28 desert palm species, and 49 tree cactus species.

We also have some remarkable oddities, such as one arborescent member of the cucumber family (Dendrosicyos socotrana), and several rose relatives (Polylepis spp.) that grow above 4000 meters in the most parched areas of the Andean cordillera!

Where do desert trees grow?

Interestingly, the different desert areas of the world are not equal in terms of their contributions to our database (see the table below, the full version of which is posted on the homepage for our database).

Region Number of Species Endemic species* Number of Genera Number of Families
Australia 389 373 62 34
Madagascar 355 311 160 55
North America 272 222 126 55
Northeast Africa 233 80 87 42
West Asia 224 86 97 46

*Endemic to the country or region indicated.

Five regions alone account for two thirds of all the species in our database, with the deserts of Australia and Madagascar being almost preposterously rich in tree species. But of course the area of arid Australia is vastly greater than that of Madagascar, so that in fact the numbers of families, genera and species in the latter country are really the most impressive of all.

 

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Highly degraded spiny thicket vegetation at the edge of the Ranobe PK32 Protected Area near the town of Ifaty, in western Madagascar, with few trees other than the emergent baobabs, Adansonia rubrostipa (Malvaceae) remaining. Young plants of the spiny tree, Didierea madagascariensis (Didiereaceae) developing in the bare sandy soil around the baobab in the foreground. 11 September 2006. © Peter Phillipson, Missouri Botanical Garden. http://www.tropicos.org/Image/100624586.

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Secondary growth spiny thicket near the Ranobe PK32 Protected Area north of Toliara, in Madagascar, with occasional individuals of the locally endemic spiny tree Pachypodium mikea (Apocynaceae) – center image, but dominated by mature Didierea madagascariensis (Didiereaceae). 03 December 2018. © Peter Phillipson, Missouri Botanical Garden.

A zoom on the astonishing dryland tree species richness and diversity of Madagascar can already be found in an article we published last year, covering the remarkable assemblages of 355 tree species found in the driest part of Madagascar, of which no less than 311 are endemic to the country. This is all the more remarkable considering that they are all crowded into a narrow coastal strip in the Southwest, which is a mere 14,480 square kilometers (5591 square miles), or the same size as Connecticut.

For us, a key feature when discussing desert trees is the fact that even in the harsh areas where they found, trees can grow densely enough to form true woodlands, sometimes even with dense canopies, which has enormous importance for desert ecosystems and people. In previous blog posts we have reported on striking examples – in northeastern Jordan, and coastal Peru, among others, where evidence of former woodlands provide rays of hope and guidance for people attempting ecological restoration in desert lands.

Back in 2013, James and Edouard published a first book in French (Les Arbres des Déserts: Enjeux et Promesses) profiling desert trees and developing the subject of desert woodlands. We now have a more comprehensive book in preparation, called Desert Canopies: Reimagining our Drylands. Three chapters on animal-tree relations, and photos and drawings by Thibaud will help make this of interest for a wider audience, not just specialists. We also develop the theme of ecological restoration and provide profiles and virtual field trips from many restoration programs in drylands around the world.

 Where can one see living Desert Canopies today?

Unfortunately, most drylands are found in poverty-stricken regions of developing countries, where trees are an extremely valuable resource. In recent decades, desert canopies have been hammered by rising populations of people and livestock. As a result, today these canopies are so degraded and fragmented that it’s hard to imagine what they once looked like. Western Australia is one of the few places where reasonably intact desert woodlands still cover large areas.

Great western woodlands

A typical landscape of the Great Western Woodlands, in the semi-arid southwest of Australia (mean annual rainfall 250 – 400 mm), with gimlet eucalypts (E. salubris) growing over a beautiful understory of blue bush daisy (Cratystylis conocephala).

In our last blogpost, we reported on some notable trees, tree canopies, and indigenous peoples of the Guajira peninsula in northern Colombia.

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From looking at the tree cover, it is hard to believe that this area of Colombia is technically a desert!

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Young Wayuu and their donkeys, standing in the shade of a tree, on their family farm in the Serranía de Macuira, a mountain oasis in the middle of the Colombian desert. The Guajira, as the region is called, is a microcosm of the problems and drivers of arid lands everywhere, as well as a good example of the diversity and life and beauty that can be found in deserts.

Other striking tree canopies can still be found in diverse places today, including some of the driest places on Earth.

Prosopis cineraria

The Rub al Khali, the famous Empty Quarter of Arabia. Even there, trees can thrive amid the sand dunes (in this case, the venerable khejri, Prosopis cineraria), that we were lucky enough to observe in northern Oman.

Prosopis pallida Peru

On the arid coast of northern Peru, Prosopis pallida and other trees can grow in the ever-so-slightly richer soils at the bottom of gullies amid the plains.

As noted earlier, drylands make up more than two-fifths of all lands on Earth, at present. Furthermore, despite their harsh conditions, drylands are presently home to well over 2 billion people, and indeed many of these are among the poorest and most vulnerable populations on Earth. The United Nations, and many other organizations are working hard on the problems of drylands and their peoples, but it is very much an uphill battle… As we passed Earth Overshoot Day on July 29th this year– the earliest date ever – it is timely to stress once again that the restoration and rehabilitation of degraded ecosystems will be key if we are to hope for a sustainable future. Restoration is undeniably harder in arid lands than in many other places, but that only means that it is more necessary. We are happy to relate that the Society for Ecological Restoration’s scientific journal, Restoration Ecology, is launching a new initiative devoted to dissemination of scientific advances on ecological restoration and rehabilitation in arid lands. Our database is offered in that spirit.

isla guadalupe

The small, arid Isla Guadalupe, off the coast of northwestern Mexico, is home to several endemic tree species, which were almost extirpated by introduced goats. But now that the goats have been removed from the island, the trees are making a comeback. Pictured here is the endemic cypress Cupressus guadalupensis, and some of the people who’ve made this recovery possible.

A large number of the trees included – 932 out of 1576 to be exact – are endemic to a single country – and most are in urgent need of committed conservation, restoration, and better management. We hope that our database can act as a reminder of the wealth of life forms that can thrive in arid lands, and an exhortation to not give up on their desert homes, scarred and battered as they may be, but rather to try and help them flourish once again.

The Huarango and Algarrobo forests of coastal Peru: rays of hope

James and Thibaud Aronson report from coastal Peru, where they travelled in December, with Oliver Whaley, of RBG Kew.

Yesterday, at our local market here in France, we saw Peruvian avocados. We’d seen them there before, and we don’t buy them. But we’d never really given them more than a passing thought. Now, having come back from coastal Peru, where they are grown, we have a very different outlook.

Passing through much of Peru’s southern coast is perhaps more interesting to geologists than ecologists. The tenuous ecological balance, and its rather checkered history since humans arrived, needs time to reveal its secrets. But the earth’s surface here is simply naked and laid out as for a desert geomorphology text book. An extension of the famous Atacama desert of northern Chile, it is one of the driest places on Earth, with an average annual rainfall of 0.3 mm, or barely more than one tenth of an inch.

Here more than in nearly any other desert, life is almost entirely confined to areas with some amount of moisture. As a result, the few river valleys that come down from the Andes form spectacular green ribbons among the dunes. They carry some water down from the rainstorms high in the mountains but the critical driver here is El Niño. It brings down tremendous amounts of water and sediment once every 6 to 15 years, thereby rejuvenating the whole system, and generating extremely fertile alluvial soils.

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The last remaining stretch of Prosopis limensis riparian forest near Copara, Ica region, southern Peru.

500 years ago, the river valleys were occupied by dense woodlands or veritable forests of Prosopis limensis (local name – Huarango), which is often misidentified as Prosopis pallida, teeming with wildlife in the canopy and understories. Today, most of the Prosopis are gone, the excellent charcoal produced from their wood having been used up to fuel the stream engines of the now defunct coastal railway and, more recently, for fast food chicken restaurants ‘pollo a la brasa’ and millions of barbecue fires in cities and roadside restaurants. Instead, one now sees vast monocultures of asparagus, avocadoes, and table grapes, producing cash crops for the North American and European export markets.

This story of deforestation is in fact almost complete, with nearly 99% of the original vegetation having been removed.

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There once were countless Prosopis trees such as this one, which has probably lived for a thousand years on this sand dune, near Copara.

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Most of them fell to the charcoal burners’ axes, and even though it is now forbidden to harvest firewood from live trees, illegal cutting continues, as seen here, 20 meters from the tree pictured above.

Depressing? Yes, but as engaged ecologists, we were also encouraged that change is happening; there is cause for hope. There are program in community-based restoration led by Ecoan underway in the high Andes, near Cuzco and environs, and science and conservation efforts underway at the Missouri Botanical Garden’s station at Oxapampa.

We were lucky to spend 10 days with Oliver Whaley, from RBG Kew, who has been instrumental in many of the first initiatives attempting to reverse some of the damage done on this arid coast and to find a new path that explicitly includes restoration. Particularly noteworthy is a partnership he’s brokered with the large agroindustry supply chain, including a giant supermarket chain where almost half of the UK buys its vegetables, and which is the destination of a high percentage of the region’s produce.

The agro-industries overcome the lack of reliable rainwater by installing intensive irrigation systems, which although highly efficient, are driving unsustainable expansion even as they produce spectacular crop harvests.

However, the coast also experiences strong winds coming from the coast year-round, which require that rows of trees be planted as windbreaks to shield the valuable crops. Furthermore security hedging today is almost entirely composed of introduced water-guzzling African or Asian exotics. Under Whaley’s in-farm reforestation  program, some producers have begun planting native Prosopis, Parkinsonia, and Acacia trees yielding a mixed forest instead, which provides habitat for wildlife and also improves the neighboring soils through their capacity to fix atmospheric nitrogen in symbiosis with rhizobacteria. They require far less water than introduced trees, and will never become weeds. Agroindustry management has also donated land for restoration corridors, a vitally important undertaking at the landscape scale that has not yet been well-explored in coastal Peru or other drylands.

The Kew Peru team planted 7,000 trees and shrubs of 15 native species derived from the degraded tiny relicts. The results have been nothing short of astounding, with over 70 new native plant species, 45 bird species, various lizards, desert fox and wild guinea pigs recolonizing areas that were nothing but barren soil 9 years ago. (The full results of this work will be published in the spring). Whaley takes a practical and patient view – whereby to nurture back woodland and a cultural re engagement with what nature provides takes time and needs to show results.

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A patch of restored land on the edge of an asparagus field, showing the difference 10 years make. Santiago de Ica, southern Peru.

Whaley and the local NGO, Conservamos por Naturaleza that he helped found and works closely with, is also committed to education and communication, working in Ica, Nasca, and elsewhere, to change people’s perception of the key desert plants such as Prosopis and Capparis, and their ecosystems, and of native biodiversity in general. Cultural reengagement, he argues, is about changing perception from symbols of a rural, backwards environment to be left behind in the wake of progress, to highly useful and valuable trees and woodlands that are part of the local heritage. These ecosystems clearly are worthy of pride, not only for their inherent value, but also because they offer the prospects of  sustainable, profitable agriculture that is also conservation-friendly.

The Huarango Festival in Ica, which focuses on the numerous products that can be extracted from the tree, such as algarrobina, a sweet spread, a sweet drink, high quality honey, ink, and more has been a large success, and is now in its eleventh year. Whaley also works with schools, working to restore small patches of native vegetation inside the school compounds and promoting ecological consciousness through small nurseries and gardening projects.

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Seedlings in a native species nursery funded by the Royal Botanic Gardens, Kew, at the Faculty of Agronomy at the University of Ica.

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Middle school students in their school’s nursery, in a town near Ica, where they grow native species and food crops and have a great time doing it.

The other unique feature of the southern coast is an inland archipelago of sorts, made up of coastal fog oases, or lomas, in Spanish. Fed by the moisture provided by coastal fogs, they rise from the surrounding desert and harbor herbaceous vegetation and in some cases various kinds of trees, all of which show high rates of endemism. Most of the lomas are sadly threatened by mining and other uses today, but Whaley has already succeeded in helping protect the new national reserve of Lomas San Fernando. This and other efforts , such as drone surveys and modelling, are conducted with help from Kew GIS staff, and Whaley’s team in Peru that includes Alfonso Orellana, Consuelo Borda, Ana Juarez and other dedicated workers. They are currently doing the baseline research in two other lomas reserves to help strengthen the case for protected area status.

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The amazing vegetation supported almost entirely by coastal fogs in the Lomas de Atiquipa, home to an endemic very endangered Myrtaceae:  Myrcianthes ferreyrae.

 

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A female Andean condor (Vultur gryphus), in the Lomas de San Fernando reserve, the only place in Peru where this raptor roosts near sea level.

Whaley and his co-workers are also engaged in restoration efforts the tropical northern coast of Peru, where the circumstances are rather different. The north coast receives significantly more rainfall than the south, i.e., about 100 mm per year (!), that is 4 inches. This permits the vegetation to climb out of the river valleys, and in fact, the local Algarrobo (Prosopis pallida and in the extreme North only P. juliflora along with hybrids between the two), and Capparis species, among others, still form true woodlands. Traditionally, the trees were preserved by the local people as their leaves and in particular their pods are remarkably nutritive, and provide prime forage for free-ranging cattle, the peoples’ primary source of income.

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The remarkable woodlands dominated by Prosopis pallida (Algarrobo), in the Pomac Forest Sanctuary, Lambayeque Region, northern Peru.

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The Algarrobo trees have been preserved in large part because they provide excellent forage for the local people’s cattle.

However, a still poorly understood plague, apparently resulting from the combination of a small fly and a fungus, are decimating the woodlands; in some areas over 80% of the Algarrobo have apparently died. Just like in the South, El Niño is paramount in keeping the system healthy and, in particular, no Prosopis seedlings have been seen germinating in the last 50 years, except during El Niño years. What’s more, the next El Niño event is overdue: the last one came in 1997/1998, and now the system appears exhausted. Clearly the system is waiting for the next El Niño, and the long wait appears to have increased the Algarrobo’s vulnerability to the plague.

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Severe Prosopis pallida dieback, near Talara, Piura region.

However, El Niño is coming this year, and according to the climate experts, it’s going to be a big one. As a result, Whaley and his team are scrambling to prepare as many Prosopis seeds as possible, to be sown during or right after the heavy rains. As the trees slowly dying from the plague usually fail to set seed, Whaley and his collaborators fear that much of the soil seed bank is exhausted and this could truly be the last chance for this species in its wild state.

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A seed-ball with 4 native species, including Algarrobo, being prepared at a community nursery, near Salas, Lambayeque region.

Now there is an additional layer to the issue, which asks of would-be restorationists exactly what it is that they are trying to do. There hasn’t been nearly as much deforestation in Peru’s North coast as in the South, so there are still reasonably large tracts of native vegetation left intact. Further, apart from Algarrobo, none of the other five dominant native tree species, including two species of Capparis,  appear to be affected in any way by the plague.  Domestic cattle will also eat the leaves of these other trees and shrubs, though they aren’t quite as good as the Prosopis.

Therefore, some might say that this is just a natural transition occurring within an ecosystem, and it would be foolish, or even a case of trying to play God, to attempt to save the Prosopis at all costs. However, Whaley thinks differently, and we agree with him. The Algarrobo, like the Huarango, is a remarkable tree, fantastically well adapted to its environment, capable of living 1000 years of more, and clearly the keystone species of the riparian and related ecosystems where it occurs.

It forms remarkable canopies, and in areas where it is absent, we did not see the other tree species present produce anything like it, rather forming a much more open low savanna. Further, it has been a pillar of the various civilizations that have existed in the area for the last 4000 years. Therefore, Whaley is not yet ready to just let it go…

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A thousand-year old Prosopis pallida in the Pomac Forest Sanctuary, possibly the oldest of its kind still alive in northern Peru…