Which Family Has The Most CAM Species?

The correct answer is...the Orchidaceae! Here are a few references...

- 1983, Winter et all,

This is the reference that first introduced me to the concept of CAM. From it I got the impression that there are relatively few terrestrial succulents in Australia. However, approximately two thirds of the epiphytic/lithophytic orchids studied were CAM plants. Orchids with thicker leaves generally had a greater degree of CAM...as did those from more exposed habitats. Heat plays an important role so lower elevation orchids tend to be more CAM than those at higher elevations.

After a brief breakdown of the number of species in the largest terrestrial succulent families...the authors concluded with the following..."Thus, from the standpoint of species numbers it may be that more CAM plants exist as epiphytes in tropical and subtropical rainforests than exist in arid terrestrial habitats. If this is so, we should perhaps reconsider the validity of the commonly accepted notion of a "typical CAM plant", characterized as a cactus-like stem succulent growing in a desert environment."

Incidentally, I was quite surprised when several years later I ran across an entire book on Australian succulents. It wasn't much of a surprise though when I browsed through it and found that quite a few of the plants included were orchids.

- 1989, H. Griffiths,

This paper provided a table with the following numbers...

Orchidaceae / 30,000 species / 20,000 epiphytic species / ~60% CAM species
Cactaceae / 1,800 species / 120 epiphytic species / 100% CAM species
Bromeliaceae / 2,500 species / 1,144 epiphytic species / ~50% CAM species
Crassulaceae / 1,000 species / 5 epiphytic species / 100% CAM species

- 2010, Luttge, Ability of Crassulacean Acid Metabolism Plants to Overcome Interacting Stresses in Tropical Environments

This is the most recent paper that I've found. Here are some of the numbers it provided...

Orchidaceae - 9500 CAM species
Cactaceae - 1500 CAM species
Bromeliaceae - 1250 CAM species

Wikipedia puts the Crassulaceae at 1400 species so it seems that the Cactaceae is the second most CAM rich family at around 1800 species.

Basically, even though an epiphytic orchid might live in a rain forest, its micro-habitat on the tree can possibly be considered xeric...depending on where on the tree it's growing. Orchids on the outer branches tend to be more exposed to sun and wind so they'll dry out faster than orchids in crooks or low down on the trunk.

Here's where it gets interesting. While the large majority of epiphytic orchids occur in rain forests...quite a few have radiated into dry forests. It's the same concept with Cactus and other succulent families. The large majority of Cactus occur in drier habitats but a few have adapted to wetter habitats. For example, here's an in-situ photo of Miltonia flavescens growing with Epiphyllums and what looks like a Ric-Rac Cactus.

To try and illustrate this concept I created this diagram...

Cactus and Orchid Moisture Continuum by epiphyte78, on Flickr

The proportion is probably off...and I'm not exactly sure in which type of habitat Cactus or epiphytic orchids have the most species. If somebody wants to give a whack at it feel free to PM me for the MS PowerPoint file.

As faulty as my diagram may be...it helps illustrate that if you purchase a random cactus...chances are pretty good that it comes from a drier habitat. On the other hand, if you purchase a random epiphytic orchid...chances are pretty good that it comes from a wetter habitat. But!...in absolute terms, in comparison to Cactus...there are just as many, if not more, epiphytic orchids that are from "drier" habitats.

Where it gets difficult is that there are books on miniature orchids, several articles on fragrant orchids and 100s of orchid monographs...but no single reference that consolidates the epiphytic orchids from dry forests. I've scoured 100s of references and come up with a reasonably comprehensive list. Still haven't quite decided how much accompanying info to include for each species.

But if I had to pick one single reference to recommend...it would be the

by Hagsater et all. The very first picture in the book shows a large Agave with a Laelia blooming on a branch above it! It's the only orchid book that is organized by habitat type...tropical rain forests, tropical dry forests, arid zones and scrubs, etc. It's also unique in that gives the scientific names for the succulent plants that share habitats with orchids...Neobuxbaumia multiareolata, Pachycereus pecten-aboriginum, Plumeria rubra, Pseudobombax ellipticum, Pseudosmodingium perniciosum, Stenocereus weberi, etc. Plus, there are numerous really nice habitat shots.

Mexico probably has the most species of dry forest orchids...but they can also be found throughout the Americas, Africa, India, Asia and Australia. Besides Mexico... Bolivia and Madagascar are also extremely species rich. I linked each country name to the satellite view in Google Maps. You'll notice that Bolivia and Madagascar both have a green side and a brown side while Mexico has a large brown area sandwiched in by two green sides. As I'm sure you're all aware...these 3 countries are extremely rich in succulent plants as well.

So which are the driest growing epiphytic orchids? These are the orchids that I'd put my money on...

Africa - Ansellia africana, wide distribution in Africa. It's the only epiphytic orchid not only in Botswana but in Namibia as well. Sometimes found growing on top of Baobab trees.

Americas - Oncidium cebolleta, small pseudobulbs and fleshy leaves. It has hallucinogenic properties so it's kinda like the peyote of the orchid world.

Australia - Cymbidium canaliculatum, I believe that this is the Westernmost occurring epiphytic orchid in Australia.

Madagascar - Hard to say...maybe Angraecum decaryanum or Aerangis decaryana. If you like plants from Madagascar, then I'd highly recommend the

by Phillip Cribb and Johan Hermans. Each entry has a distribution map and habitat description.

Here's a photo of Oncidium cebolleta and Cymbidium canaliculatum in the Huntington Desert Conservatory...

Oncidium cebolleta and Cymbidium canaliculatum by epiphyte78, on Flickr

Culture wise, all CAM epiphytic orchids are from summer rainfall areas so you probably want to grow them with your summer rainfall succulents. That being said, all my orchids received 30 inches of rain this past winter and were perfectly fine. They are all mounted though so they have perfect drainage.

If you attach your CAM orchids to branches then it's really important that you fasten them as securely as possible. I use 15 to 20lb fishing line and tie a slip knot on one end so I can tightly cinch the fishing line around the orchid and branch without losing tension. With rain forest orchids it's ok to place a little moss between the orchid and the mount but with dry forest CAM orchids I wouldn't recommend using any moss.

If you grow them in pots, it's safer to go with inorganic media. Large bark works fine as long as it is replaced as soon as it starts breaking down. Once it starts breaking down then the orchid can be very susceptible to rot. The Santa Barbara Orchid estate grows many of their orchids in 3/4" gravel and they water 1 to 2 times a week during summer.

Here in Glendale I water most of my orchids 2 to 3 times a week at night during summer. My backyard is tropical but my front yard is all drought tolerant plants...

Front Yard by epiphyte78, on Flickr

When the trees and shrubs get larger I plan on moving more of my CAM orchids to the front yard in order to see which ones adapt the best to my once a week summer watering schedule.

Part of the fun of succulents is the very wide range of variety and diversity...there's always a new group of plants to learn about and try. That's the same reason I like orchids! If you're looking to expand your horizons I highly encourage you to consider the epiphytic CAM orchids from dry forests.

Incidentally, epiphytes are a diverse bunch as well. My very first epiphytic Rhododendrons just arrived in the mail today from Bovees Nursery...I'm going to run outside now and attach both of them to my tree.