The quest for a better tree

By Jonathan Horst 

A major limitation to the long-term success of desert restoration work is the need to establish trees that can grow without irrigation.  Naturally growing mesquite trees develop a long tap root as small seedlings that will, ideally, eventually find its way down near to the water table.  If the tap root is damaged or its downward growth is impeded the tree will not be able to adequately harvest deep water – a requirement for long-term survival of large mesquite trees in the desert.  Most trees purchased in greenhouses have a tap root that has circled the pot many times and will not be able to regain its normal downward growth trajectory in a sufficient fashion.  These trees therefore need either irrigation or supplemental water, in the form of harvested rainwater or roadside runoff, in the drier months in order to survive—easy enough to provide in town but not practical in the open desert.  Tucson Audubon attempts to limit the irrigation volume on restoration sites and to have plants fully established to survive without irrigation after two years. Even though trees are planted in basins utilizing berms to capture any additional water possible, the average nursery mesquite is likely doomed to a smaller size and shortened lifespan when used in a desert restoration setting compared to trees growing naturally from seed.

The Restoration Crew at Tucson Audubon has decided to start a trial method to grow seedlings that will be able to be planted with next-to-zero taproot disturbance using biodegradable pots made from gift wrapping-paper tubes that can be directly installed in the soil and will biodegrade rapidly.  This will be part of a larger experiment to compare growth and long-term survival rates of trees grown and transplanted using a variety of established and novel techniques. We expect that the minimal disturbance to natural tap root development utilizing the gift-wrap tube pots will allow faster overall growth rates, increased survival, and greater overall tree size once plants are taken off irrigation than standard nursery trees. Whether they can outperform trees grown from seed on site is another question!

Interesting facts about mesquites

• By the time a mesquite seedling is 3” tall and has its first pair of true leaves, the tap root is already over a foot long – deeper than a 1 gallon nursery pot.
• Mesquite trees have the deepest roots known, one live root measured over 160 feet deep. Arizona Sonora Desert Museum
• Mesquite seeds need to be scarified (scratched up) before they will sprout. This can be accomplished by rolling along in a flood, repeated freezing and thawing, being eaten by cattle or deer, or being driven over by car tires along the road.
• Mesquite trees increase the fertility of the soil around them. Nitrogen is a major nutrient that plants need. However, they can’t utilize the nitrogen in the air. Mesquites, and other plants in the bean family, have bacteria in their roots which convert atmospheric nitrogen into a form usable to plants.