Chapter 3: Development of Date Palm Biomass Allometric Equations and
3.1 Overview
Some palm species are considered keystone and provide multiple ecosystem services, such as CS (van der Hoek et al., 2019). The amount of carbon that can be sequestered in palms is relatively high compared to some other plant species. In their study of the relationship between land use and CS in northeastern Brazil, Carlos et al.
(2015) found that land planted with palms provided 40 t. C ha-1 while lands used for pasture and agriculture provided only 8 t. C ha-1 and 5 t. C ha-1, respectively. In another study in Northeast India, Singh et al. (2018) recorded considerably higher amounts of carbon in oil palm plantations than in shifting cultivation fallows. They concluded that a 10 years old oil palm plantation could sequester up to 3.7 t. C ha−1 year−1. Hence, palms generate economic benefit and contribute to carbon storage in a more sustainable way especially when planted in areas of low productivity or on degraded lands.
Afforestation projects can be used to earn carbon credits and reduce the carbon footprint. This type of supportive efforts has a growing interest among policymakers and governments (Baral & Guha, 2004). Therefore, estimation of CS in forests and plantations is an important measure towards assessing mitigation effects on global change (Ebuy et al., 2011). Many destructive techniques (felling or harvesting) exist to directly estimate CS (Gibbs et al., 2007). Although these techniques provide the most accurate measure of biomass, they ultimately rely on ground measurements and can cause severe destruction to the forests as well as a risk of environmental deterioration (Khalid & Hamid, 2017; Maulana et al., 2016). In addition, such methods are tedious and time consuming (Ebuy et al., 2011), hence they cannot be used routinely. Therefore, developing biomass equations (allometry) that rely on non- destructive measurements, is very essential in estimating biomass. Subsequently, allometric equations have been developed and used to estimate tree biomass and CS from dendrometric measures, such as tree diameters and height (Ebuy et al., 2011;
Picard et al., 2012). Notwithstanding, the number of trees destructively sampled to build allometric equations is not constant and differs from one study to another.
Currently, there is no consensus on that number, as this is often dependent on resource availability and permission to harvest trees (Yuen et al., 2016). For example, Russell (1983), and Moran and Grace (1996) used 15 and 14 trees, while Brown et al. (1995) and Khalid et al. (1999a) used only 8 and 10 trees, respectively to build their allometric equations.
Different quantitative variables were considered when building oil palm biomass allometric equations (Korom & Mastuura, 2016) (Appendix 1). Henson and Chang (2003) used age as a predictor to estimate the standing biomass of oil palm in tons per hectare. Others used structural variables such as total height and trunk height
(Dewi et al., 2009; Khalid et al., 1999a; Thenkabail et al., 2004), while Corley et al.
(1971) used DBH, number of fronds, leaf area, rachis and petiole length, rachis and petiole cross-sectional area at intervals, and volume of petiole sections in their pioneer study to estimate the average yield of oil palms. More recently, allometric equations have been used, coupled with RS and field-based structural variables measurements (Fonton et al., 2017; Salem Issa et al., 2019). Furthermore, Cheng et al. (2014) recommended to develop more equations with different field structural variables that can be linked to RS predictors. Likewise, Jucker et al. (2017) suggested in their review of allometric equations to develop a new generation of allometric equations that estimate biomass based on attributes which can be remotely sensed.
Most biomass equations, whether species-specific or multispecies, have been developed for tropical rainforest ecosystems because of their relevance to the global carbon cycle (Basuki et al., 2009; Brown, 1997; Chave et al., 2005; Cole & Ewel, 2006; Makinde et al., 2017). A few plant species biomass assessment equations are available for desert ecosystems. Nonetheless, none of these were used to fit one of the most important fruit crops in arid regions, Phoenix dactylifera, date palm (DP). Over two-third of dates production amount worldwide are produced in the Arab World (El- Juhany, 2010). Three of the top 10 date producers worldwide are located in the Arabian Peninsula, namely: Saudi Arabia, UAE, and Oman (Kader & Hussein, 2009; AOAD, 2008). On the other hand, the UAE has the largest number of DP for any single country in the world. In 2008, the UAE had more than 16 million DP producing around three quarters of a million tons of dates (El-Juhany, 2010). Furthermore, the UAE possesses at least 200 cultivars, 68 of which are the most important commercially (El-Juhany, 2010)..
DP possess multipurpose advantages, including environmental benefits, especially for the Arabian Peninsula population including the UAE, where DP have been an integral part of the farming system. More than 90% of the UAE territory is covered by desert ecosystems representing more than two-thirds of the country's land area. DP species are a good alternative for CS in such arid ecosystems. To estimate DP biomass and its carbon content, it is necessary to quantify the biomass in all palm components. Moreover, it would be more accurate to include both the AGB and BGB in estimating the CS, as both are available for recycling in the ecosystem at replanting (Khalid et al., 1999b).
The current chapter meets objectives no. 1 and 2 of the dissertation (see Chapter 1, Subsection 1.3 Aim and Objective). Specifically, this chapter aims at: (1) Identifying the most relevant structural field variables for the estimation of DP biomass; (2) Developing specific allometric biomass equations that can be correlated with RS variables; (3) Estimating CS in date palms; and (4) Assessing the potential of DP species to improve soil CS in such desert ecosystems.