Remembering to See the Forests as more than Trees

Gary Paul Nabhan, W.K. Kellogg Chair in Borderlands Food & Water Security, University of Arizona

The Role of Perennial Plants other than Trees & Herbaceous Perennials in Redesigning Agroforestry Systems to Weather Climate Change in North America

Picture by Gary Paul Nabhan

Greetings, Earthlings! Welcome “a Braised New World” in which more than 2000 U.S. counties (33% of Continental U.S.) have suffered severe drought, exhausting heat waves, water scarcity and crop failure within the last three years.  Throw in the various levels of drought currently felt in 2% of Canada and in 30% of Mexico, as well as 64 % of the area of the United States, and it is fair to claim that half the continent’s food-producing arable lands have already joined the ranks of Planet Desert. There is the smell of danger in the air.

These climate stresses are already becoming major factors in reshaping North American agriculture, as farmers dependent of Colorado River recently lost 1 million acre feet routinely needed for their crop production. Climate shifts will inevitably make or break the successes of various forms of agricultural food systems in a hotter, drier world.  Although many herbaceous annual food crops are now facing climate thresholds that are triggering floral abortion and poor fruit set, tree crops are also vulnerable to reduced chill hours, high summer temperatures, reduced soil moisture levels, and catastrophic kill events like the 2020 derecho storm in Iowa.

These climate shocks beg the question: are we still seeing the agro-forest only as tree crops, or simply as trees alley-cropped with annual seed crops? Most of us involved in agroforestry clearly wish to focus on the forest, not just individual trees. But has our protracted attention and investment sufficiently extended to other perennial lifeforms? Most of them are not really plants, but holobionts, really, long-lived phyto-montages intricately allied with beneficial bacteria and fungi in both the rhizosphere and endosphere.

Picture by: Gary Paul Nabhan

The Trouble with Trees in an Era of Rapid Climate Change

No one wants to throw trees out of our toolkit for dealing with climate change; rather, we want to make sure that trees are not treated like the proverbial hammer that insists its capacity for pounding in nails is the only solution needed. Coming from Planet Desert to land back in temperate zones now and then, I am struck by the fact that there are virtually no trees at all with the C4 metabolic pathway (other than African Euphorbias), and few wood perennials with the CAM metabolic pathway other than arborescent yucca, aloes, and columnar cacti, most of which have never qualified as trees by the gatekeepers of “million tree” initiatives to combat climate change. Why is it concerning that nearly all trees use the C3 metabolic pathway? Well, C4 and (even more) CAM plants can produce substantially greater biomass below and above ground than C3 trees can on same amount of energy and water on a decadal, and therefore may be better adapted to a water-scarce world. CAM plants like agaves and prickly pear cacti use only a fifth to a half of the water used by temperate crops but produce more edible (and drinkable!) biomass per unit water on a decadal scale than any C3 crop, tree or annual. They are essential elements in any future slooow agriculture to produce slooow food.

Climate anomalies in winter or summer can be hard on trees, taking them out after many years of investment in their growth. Take the August 2020 multi-state derecho catastrophe: NOAA estimates the derecho caused over $11 billion in damage across the Midwest. In the Hawkeye state alone, the storm damaged or downed over 7 million trees, according to the Iowa Department of Natural Resources.  In my own backyard, summer 2021 heat waves during the flowering time of my 30 varieties of pomegranates left me with just 5 ripened pomegranates to savor for the season!  And a sudden deficit of winter chill hours may keep orchards trees from budding out and reproducing at all for an entire year.

Trees are Not the Only Woody Perennials That Agro-foresters Need    

My final point is that in a world fraught with climate-uncertainty, agro-foresters in at least a third (and maybe a half) of the US should not put all their eggs in one basket with trees. There are geophytes, woody vines, columnar cacti, stem succulents, root succulents and leaf succulents (like domesticated prickly pears). There are growing commercial markets for the fruits of many of these crops. There is emerging research to suggest that grafting some fruit-bearing varieties onto desert-adapted rootstock from crop wild relatives might confer more drought and heat tolerance than genetic engineering of the scion wood. Succulents are rather easy to graft and have long fruit-bearing lifespans.  What’s more, the co-location of crop plants of multiple growth-forms in vertical intercropping or alley-cropping is an ideal way to confer structural heterogeneity to an agro-forestry polyculture. I would go so far as to say that a polyculture comprised of only trees is not a true permaculture as Bill Mollison envisioned it. We are planting three such desert polycultures of multiple perennial growth-forms in Arizona and Sonora this year for evaluation of yield stability and carbon pull-down.  There are already commercially viable desert agroforestry permacultures in the Canary Islands, Spain and in Jalisco and Guanajuato, Mexico.

They too are not silver bullets or miracle crop success stories but have much to teach us. And yet, there has never been a non-timber forest products lab in the desert regions of the U.S. like nearly every other region of the U.S. has had, as if agaves, cacti, and mesquite” cannot really be considered trees. For that reason, we are calling for $3 million startup funds for a new Consortium Center for Desert Agriculture and Climate Resilience to be hosted by the USDA/ARS Walnut Gulch Long Term Agroecosystem Research Site near Tombstone Arizona, and are seeking $4.8 million over 3 years from the USDA NRCS Climate-Smart grants competitions.  

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