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Desert Trees of the World Database (@ Tropicos)

Thibaud Aronson1 Edouard Le Floc’h2, and James Aronson1   

1Center for Conservation and Sustainable Development, Missouri Botanical Garden, St. Louis, Missouri 63110, USA, E-mail: ja42014@gmail.com.

2 Center for Functional and Evolutionary Ecology, C.N.R.S. Montpellier, France.

Introduction

Welcome! Desert Trees of the World is a database of trees and tree-like plants found in arid and semi-arid regions of the Earth. It is based on herbarium studies, literature review, our extensive field work over the past several decades on all the continents except Antarctica.

In the database we describe 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.


In the Table below, we summarize some of the contents of the data base.
 
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
South Africa 196 81 88 45
East Africa 124 1 68 36
South America 103 66 66 34
South Asia 91 8 54 32
West Africa 68 5 40 22
North Africa 60 11 34 25
Central America 52 0 41 26
Central Asia 36 7 14 10
South Europe 14 2 8 8

       * Endemic to the country or region indicated


Criteria for inclusion, 1. What is a desert tree?

Given that none of the numerous existing definitions perfectly fit our purposes, we made up our own, combining elements from the definitions offered by Shreve and Wiggins (1951), Felger et al. (2001), and Schatz (2001). Our definition is as follows:

In the context of drylands, trees are long-lived plants that develop one sturdy long-lived trunk, from one to 20 meters or more in height. However, individuals subjected to cutting, burning, or browsing, may develop a more multi-stemmed, shrubby appearance.

Examples includes many species of well-known genera such as Acacia (s.l) and Prosopis (Fabaceae), Eucalytpus (Myrtaceae), Commiphora (Burseraceae), Tamarix (Tamaricaceae) and Terminalia (Combretaceae).

In the evolutionary ecology of desert and dryland trees, succulence also merits special attention as it occurs in leaves and young stems, but also in tree trunks (and roots), producing bottle trees (also called pachycaul trees). To underscore the spectrum of life forms and taxonomic groups in which they occur, we use names such as ‘monocot tree’, ‘dwarf tree’, ‘bottle tree’, or ‘arboreal cactus’ when a dryland tree deviates from standard forestry or colloquial notions of trees.

Dwarf trees, candelabra trees, tree succulents, and monocot trees—those without lignified stems—are all included as unusual, but valid categories of desert trees. Typical examples are Adansonia, Aloidendron, Euphorbia, Pachypodium, Phoenix, and Welwitschia.

Criteria for inclusion, 2. Bioeographical factors.

We only included species that have been recorded in areas that receive less than 400 mm of average annual rainfall (Keeping in mind the fact that in drylands, inter-annual variability of rainfall increases in correlation to increasing aridity). Precise rainfall information for remote desert areas are often hard to come by, so we separated areas receiving 0 to 150 mm (hyper-arid), 150 to 250 mm (arid) and 250 to 400 mm (semi-arid). And while many of our species are exclusively found in such arid or semi-arid areas, many others also occur under wetter conditions. In particular, many are also found in desert fringes, which receive between 400 and 600 mm of rainfall. Finally, some trees, particularly in Africa and Australia, range from true deserts to wet or very wet areas, with more than 800 mm of annual rainfall.

 
We also provide the elevational range at which our species can be found. Desert trees have been recorded from sea level (and even below, in the areas around the Dead Sea) to over 4000 meters above sea level, in the high Andean deserts.

Besides elevation, we provide further information about the major habitat types that our species favor, from coastal lagoons (as several mangroves occur on desert coasts), along river courses, to dune fields, and outcrops and inselbergs, or even special formations such as termite mounds. We also indicate the types of substrates on which our species can grow, with further information on underlying rock types. Equally important are the types of vegetation communities in which the different species are typically found, from grasslands to wet montane forests. species, as well as occasional discussion of species whose native range is not known with certainty.

As for geographical range of distribution, we provide large regions of continental or subcontinental scale, such as East Africa, Central Asia, or Australasia. We also include a more detailed account of the countries in which a species occurs, with only a mention of the countries at the edges of the range for particularly widespread

We also indicate whether a species is endemic to a single country and a general assessment of its abundance, as well as the official IUCN Redlist threat assessment. NB. Only 661 of our 1578 species have been assessed as of August 2019).

As for morphological characteristics that are typical of arid lands, we provide mention of species that are spiny, as well as some discussion of succulence and typical leaf duration (i.e., deciduous or evergreen).

We also indicate all the known uses that the trees can have for desert-dwelling human populations, given in broad categories, from timber and fuel, to medicinal uses, to nitrogen fixing and soil protection. 

We describe the average height range, as well as occasional exceptional heights that can be reached by individuals in favored positions. We also indicate where a species has the capacity to resprout and whether it is tolerant to salt, frost, and / or fire. When available, we also indicate broad categories describing the species’ lifespan, “Short” refers to species that typically live less than 50 years; “Medium”,  for species that live between 50 and 200 years, “Long” for species that reach 200 to 500 years of age, and finally “Very long” for species that live over 500 years.

For managers, we indicated known effective propagation techniques, such as direct seeding, cuttings, or suckers. We also indicate where species respond well to lopping, coppicing, or pollarding.

Finally, we indicated whether the species has the potential to be invasive outside of its native range.

We do not provide full bibliographic references consulted as that would take up a huge amount of space and readers can find up to date info and most or all of the references we used on the internet with little difficulty. Some of the score of useful websites that we consulted include Trees of Namibia, Living Atlas of South Africa, Flora del Norte de Chile, Useful Tropical Plants, and of course Tropicos….

We do not intend for this database to be static. Indeed, we invite any and all visitors to reach out to us if they have knowledge about any tree species, or characteristics thereof that we might have overlooked.

Please contact James (ja42014@gmail.com) if you have questions or comments, or updates for the database.

Photo 1: Baobab trees (Adansonia digitata; Malvaceae) on the Ile aux Lamatins, in southwestern Niger. © T. Aronson.
Photo 2: Hatem Taifour and James Aronson with an enigmatic stand of Dalbergia sissoo (Fabaceae), in Wadi Hasa, in Jordan, far from its center of distribution in northern India. © T. Aronson
 
Going beyond the database

Over the past five years, Thibaud and James have taken field trips to various of these arid and semi-arid lands, all of which you can read about on the MoBot website “Natural History of Ecological Restoration”, (see direct link to our posts here). Also, for those who read French, see Les Arbres des Déserts, a semi-popular book on this same subject, published back in 2013, with special emphasis on the Old World deserts.

Together with other colleagues, including Pete Phillipson, administrator of this site, we’ve published several scientific articles on desert trees that you may find of interest. Two of these are available for downloading at these links: Desert trees of SW Madagascar and Enigmatic trees of the Middle East.

Now, we are midway through the writing of a more comprehensive, richly illustrated book in English, to accompany this database.

 

That book is intended for a wide audience, not just for botanists and other specialists. We devote that book and attempt to speak to restoration ecologists, naturalists, explorers, and everyone working for biodiversity, sustainability, resilience, and human health around the world in these wildly changing and worrisome times.

In addition to referring to our database of desert trees, we write about the tree canopies once common and extensive in some arid and semi-arid lands, and still found in some areas, such as parts of Peru and Namibia, and more extensive tracts in relatively well-conserved areas like the Great Western Woodlands of Western Australia (Photo 3). We discuss this history of transformation and degradation as well as efforts currently underway to restore healthy dryland ecosystems for present and future generations.

 

Photo 3. Eucalyptus salubris (Common name - Gimlet) and Cratystylis conocephala (Blue-bush Daisy) are two species common across the Great Western Woodlands, where mean annual rainfall is well under 400 mm (15.7 inches). © T. Aronson.

Interestingly, there appear to be three age groups of Gimlet in this photo, presumably as a result of different fires. According to our colleague Keith Bradby, ED of Gondwana Link, with whom we travelled to see this canopy while in Australia,  one tree in this canopy is 300-400 years old at least, whereas the main foreground trees he’d put, roughly, at 100-200 years, and the ones in the background, particularly on the left, at probably below 100 years old. Thanks again Keith, and thanks to Malcolm French, for confirming identification of the Gimlet.

References cited:

Felger, R. S., Johnson, M. B. and Wilson, M. F. 2001. The Trees of Sonora, Mexico. Oxford University Press, New York.

Schatz, G. E. 2001. Generic tree flora of Madagascar. Royal Botanic Gardens, Kew and Missouri Botanical Garden, USA.

Shreve, F. and Wiggins, I.L. 1951. Vegetation and flora of the Sonoran Desert. Carnegie Institute of Science, Washington, D.C. 

 
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