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.

Saguaro and boojum

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.

jordan woods

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.

 

baobabs Mada - pete

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.

spiny thicket - pete

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.

macuira stream

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

wayuu family

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.

Dhofar: Oman’s vertical fog oasis on the edge of the unknown

James and Thibaud Aronson report from Dhofar, in southern Arabia. This remarkable region is a biogeographical and historical crossroads at a critical moment of change.

The Sultanate of Oman doesn’t figure on most people’s mental map. But once told that it sits on the southeastern rim of the Arabian Peninsula, between Yemen to the west, and Saudi Arabia and the Emirates to the north, they may visualize an endless sea of sand with pack camels ambling towards an oasis dotted with date palms. A large part of the country – which is half the size of France – does look like this and is part of the fabled Empty Quarter of Arabia – Rub al Khali.  But we have come here to see Dhofar, the monsoon-affected and heavily wooded fog oasis that harbors some of the most unexpected and spectacular landscapes and ecosystems in arid lands worldwide.

DSC02426 - Copy

One of the beautiful landscapes of Dhofar, southern Oman, in the remote Wadi Ahjawt.

From mid-June to mid-September, every year, Dhofar is thoroughly drenched by the southwesterly monsoon – khareef in Arabic. Not what you expect in a desert region! The steady stream of drizzle and fog, aka “horizontal precipitation”, during this period recharges hundreds of springs and temporary rivers (wadis in Arabic), supporting dense, lush woodlands and forests on the south-facing slopes, dominated by large and varied trees more typical of east and north-east Africa. This constitutes a giant vertical oasis of sorts, but as the monsoon goes over the ridgetops, it is stopped by the hot desert winds coming from the interior and the vegetation transitions to a sparse desert scrub in a matter of meters, in one of the best illustrations of an ecotone – or ecological frontier – imaginable.

The stark contrast between the slopes which receive the monsoon fogs, and the plateaux beyond the escarpments, where the moisture is stopped by the desert wind. Djebel al Qamr, near the Yemeni border.

The stark contrast between the slopes which receive the monsoon fogs, and the plateau beyond the escarpments, where the moisture is stopped by the desert wind. Jebel al Qamr, near the Yemeni border.

It is there, right on the threshold of the desert, that are found the Boswellia sacra trees that produce the finest frankincense in the world. This tree resin was one of the most prized commodities in ancient times, throughout Asia, Europe, and Arabia, and it is still highly prized today for many uses, including incense, ritual ceremonies, bath oils, and perfumes.

A wild Boswellia sacra tree in Wadi Aful.

A wild Boswellia sacra tree in Wadi Aful.

Indeed, two thousand years ago, Oman was the meeting point for maritime traders and spice caravans, who traded between the Mediterranean region, east Africa, India, and China. They came to Dhofar to buy the Frankincense harvested by the Jibbali tribes of the escarpments, who to this day manage the Boswellia trees in a sophisticated agro-forestry system, where the ownership of each tree is carefully recorded. You can read about this in detail in Gary Nabhan’s excellent book, Camels, Cumin and Caravans: A Spice Odyssey.

Moreover, as one would expect in such a major historical crossroad, there are many intriguing questions here, for those with an interest in human ecology. For example, Wadi Hinna, connecting the coastal foothills east of Salalah and Jebel Qara, hosts a beautiful population of very old baobab trees, as well as equally ancient tamarind trees. Are they relicts, the last remnants of once much larger populations, or were they brought here as seeds or young transplants by traders and returning travelers hundreds of years ago? Together with our colleagues at the Oman Botanic Garden, we are working on a study of these and other enigmatic trees of Arabia, on which we will report more in due course.

Ancient baobab trees (Adansonia digitata) in Wadi Hinna, eastern Dhofar, the only population of the species found in Oman.

Ancient baobab trees (Adansonia digitata) in Wadi Hinna, eastern Dhofar, the only population of the species found in Oman.

Equally ancient Tamarind trees (Tamarindus indica) in Wadi Hinna,

Equally old tamarind trees (Tamarindus indica) in Wadi Hinna,

and in Wadi Ahjawt. These are often the sign of ancient human settlements.

and in Wadi Ahjawt. Both these trees are often found near ancient human settlements.

Besides its value for traders, Oman’s geographic position also makes it a genuine biogeographical crossroad for fauna and flora. Indeed, thanks to the monsoon, the humid slopes and wadis of Dhofar may be home to one of the last remnants of a belt of seasonally dry tropical forest that once stretched from sub-Saharan Africa to India, but which was almost entirely severed as Arabia became drier. Indeed, among the 900 species of plants found there, Dhofar hosts quite a few species whose nearest relatives – or populations of the same species, are more than 1000 km away, across the Red Sea in Africa or otherwise found only in India, Pakistan and Iran. There is also quite a large number of plants that are unique to the region, with over sixty endemic species and 2 genera endemic to Dhofar.  For animals, it is also a haven for many species, such as the critically endangered Arabian leopard, and the fascinating Arabian chameleon.

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The beautiful desert rose (Adenium obesum, Apocynaceae), which is in fact related to the common oleander.

The beautiful desert rose (Adenium obesum) which is related to the common oleander.

The Arabian chameleon (Chamaeleo arabicus), found only in Dhofar and southern Yemen.

The Arabian chameleon (Chamaeleo arabicus), found only in Dhofar and southern Yemen.

However, despite its lush appearance, Dhofar’s vegetation is not doing well, for many reasons. This – like much of the world – is a region in transition. Over-grazing is worse than ever. Throughout the Middle East – and Africa, it is a sign of status for a man to have a large herd of animals. However, many Omanis today actually hold sedentary jobs in the cities and simply employ shepherds – often from Bengal, Pakistan or Bangladesh – to look after their livestock for a very small wage. Further, they feed the animals fodder, which is subsidized by the government and which is of dubious value long-term. This enables them to maintain their herds at unsustainably large numbers, and as they roam over the slopes, the cattle, goats and camels graze the vegetation well past the threshold of sustainability. As a result, in many areas, you will see trees with no leaves under 3 m (the height camels can reach), with additional branches cut, bent and often broken to bring them within reach of the domestic animals, and an understory cropped to less than one cm above ground level.

The effect of large, essentially uncontrolled herds of camels and other browsers on an Omani landscape.

The effect of large, essentially uncontrolled herds of camels and other browsers on an Omani landscape.

Note the 3m high browse line, marking the limit of the camel’s reach into the tree canopy and the all but extinguished understory.

Note the 3m high browse line, marking the limit of the camel’s reach into the tree canopy and the all but extinguished understory.

Farmers then go further by breaking higher branches to bring them within reach of the animals. This technique, which can be applied with varying degrees of care, is illustrated here with a particularly unsustainable example.

Shepherds then go further by breaking higher branches of trees, in this case the dominant species Anogeissus dhofarica, to bring them within reach of domestic animals. This technique, which can be applied with varying degrees of care, is illustrated here with a particularly regrettable example.

When the animals are kept out, as we saw in a few fenced off plots maintained by the Department of Forestry, you see instead rapid regrowth of tall native grasses, often over a meter high, hopping with insects and birds, in amazing contrast to the silent, rocky moonscape outside the exclosure. This is actually an interesting problem, from a restoration perspective. Indeed, as recently as the 1980s, parts of Dhofar were dominated by tall grasslands, the easternmost extension of a habitat type more characteristic of East Africa. These are the ecosystems most affected by the increasing grazing pressure and today they are all but gone. However, according to Dr. Annette Patzelt, Director of the Oman Botanic Garden, it is likely that these grasslands were anthropogenic, and only arose following local clearing of woodland and forest. Therefore, did they exist for long enough to become an integral part of the Dhofari landscapes, which would make them a valid target for restoration, or should they instead be abandoned, to focus on the woodlands they replaced?

The only way to prove the effects of livestock on a landscape is to keep the animals out. The result speaks for itself.

The only way to prove the effects of overstocking livestock on a desert landscape is to keep the animals out. In Dhofar, the result speaks for itself.

Dhofar faces other problems as well, such as rampant road building and urbanization. But it is conceivable that better policies relating to further development could be implemented and accepted with relative ease.  This issue of livestock, however, is a systemic problem pertaining to people’s perception of themselves and their relationship to land, which is a lot more complex to address and change.

One ray of hope is the terrific work in education and outreach being done by the team at the Oman Botanic Garden, who are documenting the flora and vegetation of the whole country and tracking its changes over recent times. They are also learning how to propagate hundreds of native species that have never before been cultivated as they prepare a monumental desert park and garden that will demonstrate all the ecosystems found in the country and help Omanis to better understand the richness of their country’s natural heritage. It is a first step for desert ecosystem restoration, which is emerging not only in the Middle East, but throughout the world.