How Pecans May Help Lead to Net Zero Carbon

`Since ancient times, farmers have viewed the soil as a source of wealth to preserve and as an indicator of environmental health. For example, the Roman writer and politician Marcus Porcius Priscus (234-149 BCE), better known as Cato, noted in his famous essay “De Agri Cultura” many usual agricultural practices carried on vineyards and olive trees. One of them is particularly interesting: ancient Rome farmers used to grow a crop named Ocimum (many authors think that this was the name given to vetch, Vicia sp.), which was planted between the tree lines but turned under the soil as green manure before seed set. At that time, they did not know about carbon content in soil or carbon balance, but these kinds of practices said much about their sense of keeping the soil accounts in order—I will come back to this point later.

Fertility was extracted from the soil (and contained in the fruit) and had to be somehow restored. This concept was also discussed in a Chinese agricultural treaty written in the Middle Ages by Chen Fu (1075-? CE), the “Nongshu.” In this book, there are four chapters linked to the improvement of the soil and the environment, and they are entitled: Dishi (The strength of the soil), Fentian (Fertilization), Jieyong (Sparing use of resources), and Nianlü (Long-term planning). All this background knowledge on using natural resources was disrupted in about 1750 when a revolutionary device appeared: the steam engine, kickstarting the Industrial Revolution. The Industrial Revolution also brought us agriculture mechanization in the late 1800s, the use of fuel energy, more aggressive soil labor techniques, and later on, in the ’60s, genetically improved crops that allowed larger yields but also caused more nutrients to be extracted from soil. This turned into soil exhaustion in some regions, but not only that, soil organic matter was being actively consumed, which meant more soil respiration and, in turn, more carbon dioxide emissions into the atmosphere. Both industrial and agricultural activities have been emitting carbon dioxide ever since, contributing to climate change. From 1750 on, carbon dioxide would increase nearly 150%, triggering climate change consequences, or in other words, a carbon imbalance that provoked an energy imbalance on a global level. Given this context, pecan cultivation becomes even more important.

A pecan orchard’s first contribution to mitigating carbon dioxide emissions is its capacity to sequester carbon in trees’ biomass and return this carbon to the soil through leaves, husks, bark, and pruning debris. Despite the importance of knowing the amount of carbon sequestered by pecans, few studies exist showing that. In an effort to synthesize that information, some researchers in the Americas (Prof. Emeritus Esteban Herrera from NMSU, Prof. Enrique Frusso from INTA Argentina, Dr. Roberto Zoppolo from INIA Uruguay, Dr. Carlos Martins from Brazil, Dr. Marcelo Beltrán from INTA Argentina, Dr. Carlos Mendoza from UAER Argentina, Ph.D. student Fernanda from UFFRJ, Brazil and myself), as part of a review about pecan, joined forces to estimate how much carbon the current area of pecan was sequestering. Initially, we made the calculation only for the Americas, but in this article, I have broadened the calculation a bit to contain the most important areas planted with pecan around the world. We used several sources of data to build the table you can see below:

Pecan planted area by region and country.

Region	Country	Planted Area 2022 (ha)	Mean tree Density	Mode tree age (yrs.)	Estimated number of trees in 2022 (M, millions)	Estimated carbon stock in biomass (Mt C )	Estimated C stock in Mt CO2 eq
North America	USA1	165,992	70	10 to >20	11.62	7.46
	Mexico2	144,649	70	10 to >18	10.11	5.84
	Subtotal	310,641			21.73	13.30	48.76
South America	Brasil3	12,000	70	8 to 10	0.84	0.42
	Argentina4	8,000	70	8 to 10	0.56	0.25
	Peru5	2,950	70	7 to 10	0.21	0.09
	Uruguay	1,000	70	7 to 10	0.08	0.04
	Subtotal	23,950			1.69	0.80	2.93
Africa2	South Africa	38,800	100	10 to15	3.88	1.99	7.3
Asia2	China	34,000	3008	10 to 15	10.20	5.35	19.62
Oceania	Australia7	1,800	1007	8 to 10	0.18	0.09	0.33
World		409,191			37.50	21.53	78.94

* Mt C = Megatonne Carbon

¹Calculations made considering 70 trees per hectare as mode density and the information of planted area from Wells (2014), Herrera-Aguirre and Lopez Díaz (2018), and data of NASS USDA (2022).

²Planted area taken from Herrera-Aguirre and Lopez Díaz (2018) and Herrera-Aguirre (pers. comm.)

³Data from Martins et al. (2018) and Martins (pers. comm)

⁴ Planted area based on Sec. Agroindustria (2019), Frusso (2020) and Frusso (pers. comm.) and Lavista Llanos (2020)

⁵Planted area based on the data of Ysmael García from Consorcio Pecanero Peruano (pers. comm.)

⁶ Planted area according to Tanaka Vidal and García Pintos (2013)

⁷ Planted area and tree density based on the data of the Australia Pecan Association webpage (2022)

⁸Tree density based on the data of Zhang et al. (2015)

Methodologically, we multiplied the planted area by an estimated mean tree density per country. Then we considered the biomass of the pecan trees as 1.2 tonnes of biomass per tree per year, with a weight gain of 0.2 tonnes per year per tree (values taken from Kraimer et al. 2001) for trees planted before 2012 and for those bearing fruits during the period 2012-2022. For immature or not bearing trees, we considered half those values. Thus, cohorts were adding up biomass progressively to the tree population. Carbon stored in biomass was considered to be half the biomass, and carbon dioxide equivalents removed from the atmosphere were calculated by multiplying carbon stored in biomass by 44 and dividing it by 12.

The results were astonishing because pecan represents around 4% of the global area devoted to dry fruits such as almonds, walnuts, cashews, etc., and this “tiny” surface was capable of storing 21.5 megatonnes of carbon and removing the equivalent of 78.94 megatonnes of carbon dioxide between 2012 and 2022. This total relates to 0.21% of annual carbon emissions at the global level, considering the 36.6 gigatons emitted globally during 2022.

However, this calculation considers only the carbon sequestered by pecan biomass. If we take into account the soil’s cover around the trees, the carbon sequestration total enhances. Using Eddy’s covariance towers, Rodriguez et al. (2021) estimated that the annual net ecosystem exchange (the difference between carbon fixed by photosynthesis and carbon respired) varied between 7.88 and 10.06 tonnes of carbon per hectare according to the year. When extrapolated to our calculation, this exchange makes a carbon retention ranging from 77.8 to 99.3 megatonnes of carbon dioxide equivalents from 2012 to 2022 (0.21-0.27%). Pasture and soil add up to the matter.

With this in mind, I return to the point in the first paragraph about the Ocimum. Contributing to a good carbon budget is crucial not only for the pecan but also for the whole system, which highlights the importance of depending on a healthy, well-managed cover crop. The legacy of the ancient farmers is worth revisiting.

Thus, the amount of carbon removed by pecan orchards worldwide from 2012 to 2022 (considering planted area variations and only bearing trees) would equal to:

• 5.3 to 6.6% of carbon dioxide emissions produced by terrestrial vehicles in the United States annually (emissions produced by road transportation: 4.486 kilograms of carbon dioxide per capita according to the International Energy Agency)
• 2.2 to 2.84% of carbon dioxide emissions emitted by coal mining annually worldwide.
• 0.94 to 1.2% of methane emissions emitted annually by cattle as well as other activities worldwide.

Of course, these numbers and calculations are far from perfect and will be subjected to further improvements and modifications. Still, at least they let us see how significant growing pecans is for the environment. Additionally, piecing together the cohorts of trees planted between 2012 and 2022 shows us that 37.5 million pecan trees standing around the world can surely make some noise and garner more attention every year.

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Editor’s Note: The article was updated on March 1 with the correct numbers for the carbon removed by pecan orchards worldwide.