Allometric equations coupled with remotely sensed variables to estimate carbon stocks in date palms

Highlights

• Allometric equations of date palm (Phoenix dactylifera) created with high accuracy.

• Crown area and trunk height are the best estimators of date palm biomass.

• Carbon stock for date palm was estimated at 53% of the above ground biomass.

• Mean soil organic carbon added to the area dominated by date palms was 22 t. ha⁻¹.

• Remote sensing variables are important predictors of date palm biomass/carbon stock.

Abstract

This study aims at developing specific allometric biomass equations that can be integrated into a remote sensing (RS) based model for assessing carbon sequestered in date palms. Assessing the potential of date palms to improve soil carbon sequestration was another objective. In a previous study, the authors demonstrated that combinations of visible and short wave single bands (Red, SWIR1, SWIR2) besides vegetation indices (DVI, NDI and RVI) derived from Landsat imagery correlated significantly with date palms’ biomass. Here and based on field and lab work, relevant structural variables were identified and used in the development of allometric equations. Results showed that the crown area (CA) best estimated both crown biomass (CB) and soil organic carbon (SOC). Likewise, the trunk height (Ht) was the best estimator of trunk biomass (TB). Using these variables, allometric equations were developed for date palms at different age stages and were used to estimate CB, TB and SOC with coefficients of determination (R²) of: 0.884, 0.835 and 0.952, respectively. Furthermore, the average ratios of below ground biomass (BGB) to above ground biomass (AGB) varied with palm maturity stages averaging 0.332, 0.925 and 0.496 for young, medium and mature palms, respectively. Moreover, the present study demonstrated that the amounts of organic carbon (OC) stored in date palms were considerable with values of: 15.88 kg/palm for young, 96.62 kg/palm for medium, and 225.58 kg/palm for mature palms. Substantially higher amounts of SOC were measured compared to other local plants with values of: 18.092 kg/palm, 62.594 kg/palm, and 92.908 kg/palm under young, medium and mature palms, respectively. The main achievement of the current study was the development of new and unprecedented allometric equations for date palm species in arid land. Such equations allow the development and calibration of a RS-based model for estimating biomass and Carbon Sequestration (CS) of date palms in the region with high accuracy.

Introduction

Some palm species are considered keystone and provide multiple ecosystem services, such as CS (Van der Hoek, Solas and Peñuela, 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, Sanquetta 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 more sustainable ways 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 and Guha, 2004; Lal, 2003, 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 measurement and can cause severe destruction to the forests as well as a risk of environmental deterioration (Khalid and 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 Deans et al., 1996 used 15 and 14 trees, while Brown et al. (1995) and Khalid, Zin, & Anderson, (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 et al., 2016) (Appendix I). Hensen 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 (H) and Ht (Dewi et al., 2009; Khalid et al., 1999a; Thenkabail et al., 2004), while Corley and Tinker (2008) Corley et al. (1971) used diameter at breast height, 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; Issa et al., 2019; Dahy 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 multi-species, have been developed for tropical rainforest ecosystems because of their relevance to the global carbon cycle (Basuki et al., 2009; S. Brown, 1997; Chave et al., 2005; Cole and Ewel, 2006; Makinde et al., 2017). A few plant species biomass assessment equations are available for desert ecosystems (Chao and Krueger, 2007). Nonetheless, none of these were used to fit one of the most important fruit crops in the arid regions, Phoenix dactylifera, date palm.

According to Food and Agriculture Statistics Database (FAOSTAT, 2003) and reported in Abd Rabou (2017), the world's total number of date palms is about 120 million, distributed throughout 30 countries and producing nearly 7.5 million tons of fruit per year. Over two-third of this amount are produced in the Arab World (El-Juhany, 2010). Three of the top 10 date producers worldwide are in the Arabian Peninsula, namely: Saudi Arabia, United Arab Emirates (UAE), and Oman (Kader and Hussein, 2009; AOAD, 2008). The UAE has the largest number of date palms for any single country in the world. In 2008, the UAE had more than 16 million date palms producing around three quarters of a million tons of dates (El-Juhany, 2010). By the year 2020, the estimated number of date palms in the UAE has reached 40 million, as reported by the UAE University (UAEU) – based, Date Palms Research and Development Unit (UAEU, 2020). Furthermore, the UAE possesses at least 200 cultivars, 68 of which are the most important commercially (Jaradat and Zaid, 2004).

Date palms possess multipurpose advantages, including environmental benefits, especially for the Arabian Peninsula nations, such as the UAE, where date palms 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. Date palm species are a good alternative for CS in such arid ecosystems as they require minimum water supply and tolerate harsh growth conditions: high temperatures, drought and high levels of salinity. In fact, it is the most salt tolerant plant of all fruit crops (Alhammadi and Kurup, 2012; El-Juhany, 2010; Zohary and Hopf, 2000). To estimate date palms 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 aboveground and belowground biomass in estimating the CS, as both are available for recycling in the ecosystem at replanting (Khalid, Zin and Anderson, 1999b).

The objectives of the present study are: (1) to identify most relevant structural field variables for the estimation of date palms biomass; (2) to develop specific allometric biomass equations integrable with RS; (3) to estimate CS in date palms; and (4) to assess the potential of date palm species to improve soil CS in such desert ecosystems.