Woody Actinorhizal Plants

Woody Actinorhizal Plants
By Douglas Barnes

Dogs don’t eat dogs. At least normal, healthy dogs don’t eat dogs. So, if anyone tells you “It’s a dog-eat-dog world,” smile submissively and slowly back away – you are dealing with a sociopath. The world is, on the whole, a symbiotic dog-help-dog world. Why, even dogs help dogs! Nitrogen-fixing plants are one example of the general pattern of symbiosis.

Plants release an average of 40% of their photosynthates (the products of photosynthesis) out their roots. They don’t do this out of inherent inefficiency. These chemical compounds are doing tasks such as sending signals to call in mycorrhizal fungi and feed those fungi, to share with beneficial nitrogen-fixing and other bacteria, or to make soil nutrients more soluble and available for uptake by the roots.

Among woody nitrogen-fixing plants, there are two varieties: those associating with the bacteria Rhizobia; and those associating with the topic of this article, the actinomycetes Frankia. Actinomycetes are a type of bacteria that grow in long chains of filaments resembling the hyphae, or hair-like roots, of fungi.

Among the woody nitrogen-fixers in temperate regions, actinorhizal plants are an important group. These pioneering plants are able to grow in poor soils, enriching them with nitrogen and organic matter. This makes them very valuable in repairing disturbed or damaged soils.

The diagram below shows the 7 families of woody actinorhizal plants and their 23 genera (I left out the one herbaceous family, the Datiscaceae). A higher, 1 MB resolution image is available on request.
To rapidly revegetate a damaged landscape, be sure to include plenty of these species to help quickly build up the soils. In areas of very problematic soil, such as arid, tropical and subtropical regions, make 90% of your initial planting of trees nitrogen fixing, pioneer species (associating with either Frankia or Rhizobium), and 10% of species your long-term canopy overstory species. When the system reaches maturity, the proportions will be reversed with 10% nitrogen-fixing, support species and 90% canopy species. The same formula could be followed for temperate regions, but the soils in these area are not so fragile and can stand a lower percentage of nitrogen fixers. A 70/30 or even lower may suffice in these areas, as the seasonal cycles of death and regrowth feed these soils well.

Family	Genera
Betulaceae	Alnus
Casuarinaceae	Allocasuarina, Casuarina, Ceuthostoma, Gymnostoma
Coriariaceae	Coriaria
Elaeagnaceae	Elaeagnus, Hippopae, Shepherdia
Myricaceae	Myricaceae
Rhamnaceae	Ceanothus, Colletia, Discaria, Kenthrothamnus, Retanilla, Talguenea, Trevoa
Rosaceae	Cercocarpus, Chamaebatia, Cowania, Dryas, Purshia

Stay tuned for the gripping conclusion: Woody Rhizobium-Plant Symbiosis!

Species of the Month: Euphorbia tirucalli

This post marks the first in a new series. Each month, we will feature a new species from one of the six biological kingdoms. We will, look at their characteristics and possible duties they can perform for us.

Species of the Month: Euphorbia tirucalli
by Douglas Barnes

Other names that E. tirucalli has gone by include Arthrothamnus tirucalli, Euphorbia media var. Bagshawei, Euphorbia scoparia, Euphorbia media, Euphorbia rhipsaloides, and Euphorbia rhipsalioides.

Euphorbia tirucalli is an African tree that grows in semi-arid, savannah conditions. It is very drought resistant, withstanding long dry seasons. It is salt tolerant and can withstand to just under 5000 ppm arsenic. It will grow from 4 to 15 m tall and at altitudes to 2000 m elevation in hot savannah climates.

It is a coppiceable tree. When coppicing it, cut it at 20 to 30 cm from the ground. It makes good fuel wood with 17,600 kilojoules per kilogram of dry wood; and through pyrolysis, it makes not only charcoal, but also a high octane gasoline substitute. (One to two tonnes of fuel per hectare is what you can expect.) It can also be used as a diesel source.

The timber is useful. And it can be used as a living fence as it is not grazed by animals. Caution must be applied when planting this tree near any human settlement. It must not be in a location where it can contaminate wells or water collection sites as the tree contains co-carcinogens. Latex from the tree can be used as an insecticide and as a fish poison. As an insecticide, it is effective against Colletotrichum capsici, Fusarium pallidoroseum, Botryodiplodia theobromae, Alternaria alternata, Penicillium citrinum, Phomopsis caricae-papayae and Aspergillus niger and against the nematodes Hoplolaimus indicus, Helicotylenchus indicus and Tylenchus filiformis. The latex can also be used as a glue.

It has medicinal properties, though one would use caution obviously. The young twigs from the tree are roasted (presumably breaking down the poisons) and chewed to sooth sore throats. A poultice made from the greenwood is used to treat broken bones. Despite the toxins and co-carcinogens it contains, some of its compounds have been used to treat cancers.

The Instant Wadi Well for Arid Climates

The Instant Wadi Well
by Douglas Barnes

Rainfall in arid regions occurs in large and infrequent events throughout the year. Because the desert environment is a brittle one and because there is so much rain at one time, tremendous erosion occurs. This leads to the creation of scarps and wadis.

Wadi is simply the Arabic word for a riverbed. In an arid region, its water flow is likely to be ephemeral, particularly in headwaters (where permaculturists are most likely to be working). They are also places where a lot of erosion takes place. Looked at from another perspective, wadis are places where a lot of erosion can be stopped.

After teaching a permaculture course in Jordan, designer Geoff Lawton returned to the area a year later and found that someone had built a gabion (an uncemented rock wall, usually held inside a steel cage) across a particular wadi. Although the 8-foot-high gabion was less than a year old, it was already full of settlement and still had water trickling out of the base, despite the fact that there had not been any recent rains.



This natural tendency for gabions in wadis to quickly fill with water-retaining sediment provides us the opportunity to create what I am dubbing the Instant Wadi-Well (for lack of a better name). After determining where the gabion is to be placed, start with a shallow hole about 2 feet (60 cm) deep in a teardrop shape with the tapered end downstream. Place the first row of stones for the well inside the hole and cement them together, leaving the ground unmortared to allow water to seep in. Keep adding and cementing stones until you reach ground level. Once at ground level, continue up but add an extra 2 layers of mortared stone at the tapering end to allow stability in the face of the rushing torrent and sediment that is to come with the first rainfall. Continue this right up the entire height of the well shaft, making the top row at least 3 feet (90 cm) above the height the gabion will be. After all the sediment moves in, it is likely the well casing will only be 2 feet (60 cm) above the sediment.



All that is left is to build the gabion. Wire cages are not absolutely necessary for the gabion to perform, but they are recommended as they make it far less likely that the rock wall will blow out in the face of the torrent of water, sand and silt.

If the well casing is built strong enough, then there will likely be a well within a few major rain event is. The water will have to be tested as deserts tend to have salty soil. Baring salinity problems, this would be a quick and easy way to establish clean wells and combat erosion at the same time!

This technique compliments other systems as well. For example, if there are storage tanks or cisterns at the top of the scarp, a windmill can be employed to pump water from the Instant Wadi Well to this higher storage. From there, it can be gravity fed down to where it is needed.