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.

The Desierto Central of Baja California: past, present, and future.

James and Thibaud Aronson, and Edouard Le Floc’h, are working on a book on lost and regained tree canopies and ecological restoration in desert and dryland regions of the Earth. Here is their second report from Baja California, Mexico in March 2015.

In our last posting, we described Guadalupe Island and the NGO “Group for Ecology and Island Conservation” or Grupo de Ecología y Conservación de Islas (GECI), which is doing fine restoration work on more than two dozen Mexican islands since 1995.

After spending a week on Guadalupe Island, with Luciana Luna and Julio Montoya of GECI, Luciana and Antonio Alcaraz accompanied us to cross the bumpy and still quite remote Central Desert, which includes 77,700 square kilometers (30,000 square miles) of some of the most beautiful desert in the world.

We crossed from El Rosario, on the Pacific coast, to Bahia de los Angeles on the coast of the Sea of Cortez, also known as the Gulf of California. This is the enchanted subtropical desert that Joseph Wood Krutch described so memorably in The Forgotten Peninsula based on his visits to the region in the late 1950s, when “Baja” was still incredibly difficult to visit and almost no scientific work had been undertaken.

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Luciana Luna and Antonio Ortiz Alcaraz of GECI, the Grupo de Ecología y Conservación de Islas.

Desert landscape between Cataviña and Punta Prieta, Baja California.

Desert landscape between Cataviña and Punta Prieta, Baja California.

Our focus of course is on trees, of all sizes and shapes, including the “queer, queerer and queerest” as J. W. Krutch called them, including the Boojum tree, known in Spanish as cirio. We are also keenly interested in the animals that use them, including birds, bats, and invertebrates.

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A 6-meter tall Boojum (Fouquieria columnaris) providing a nesting site for a red-tailed hawk (Buteo jamaicensis), at the gateway to the Valle de los Cirios National Park, Baja California.

Gila woodpecker (Melanerpes uropygialis) on a Cardón, Pachycereus pringlei. Cataviña, Baja California.

Gila woodpecker (Melanerpes uropygialis) on a Cardón, Pachycereus pringlei. Cataviña, Baja California.

The spectacular desert palm trees merit our full attention as well, of course, wherever they still occur in washes, or arroyos.

Mexican Fan palm (Washingtonia robusta) and the Blue fan palm (Brahea armata) in an arroyo at the entrance of Cataviña, Baja California.

Mexican fan palm (Washingtonia robusta) and the blue fan palm (Brahea armata) in an arroyo at the entrance of Cataviña, Baja California.

Also the giant cacti, which we consider as trees of a special kind, not only because of their height and longevity, but also because of their multiple ecological roles in the desert canopy.

Large stand of Cardón, or Elephant cactus (Pachycereus pringlei) as a “Forest”, between Punta Prieta and Cataviña, Baja California.

Large stand of cardón, or elephant cactus (Pachycereus pringlei), mixed with Boojum as a “Forest”, between Punta Prieta and Cataviña, Baja California.

But, we wonder, where are the large hardwood trees that formerly contributed to the gallery or riparian forests canopies, judging from historic records? Where are the mesquites (Prosopis glandulosa) from which so many roof beams and kitchen tables were made, and the incredibly hard-wooded Ironwood?

Ironwood (Olneya tesota) in the Tucson Mountain Park, near Tucson, Arizona.

Ironwood (Olneya tesota) in the Tucson Mountain Park, near Tucson, Arizona.

Both species were heavily over-used in the past for firewood and charcoal production as well as construction wood, but Ironwood also appears to be surprisingly sensitive to drought and can die suddenly. In much of Baja California, except on the islands, what is left in the canopy is the softwoods, of little or no energetic value, such as the charming but soft-wooded, swollen-trunked elephant tree, often growing with columnar cacti, and the boojum.

The beautiful white-stemmed Elephant tree (Pachycormus discolor) growing with Cardón and Cirio, near Cataviña, Baja California.

The beautiful white-stemmed elephant tree (Pachycormus discolor) growing with cardón and cirio, near Cataviña, Baja California.

The take-home message is this: let us never forget the forest for the trees, in the compelling and urgent context of desert and dryland restoration.

Daniel Pauly’s Shifting Baseline Syndrome is more than ever relevant nowadays, not only to fisheries but also to the restoration of desert canopies. If we want to get serious about restoring desert and dryland ecosystems, we have to imagine what the canopies – however spatially constricted and reduced in diversity they are today – could have looked like 100 years ago, and think how they could look 100 – 300 years from now.