Agro-ecological landuse transformation in oasis systems of Al Jabal Al Akhdar, northern Oman

Sayh Qatanah

Since 1978 the town of Sayh Qatanah has experienced a strong physical expansion, initially driven by the building of secondary houses by families from the oases below. This was increasingly followed by population transfer, family growth, tourism facilities, and general expansion of urban infrastructure. The number of developed plots within the town area rose from 276 in 2009 to 534 in 2018 (+ 90%). During the same period the total plot area increased from 41.6 ha to 73.5 ha (+ 77%). This lead to an increase in the urban area from 206 ha in 2009 by 24 ha in 2014 (+ 13.6%³) to 252 ha in 2018 (+ 8%). At the current rate of growth, the planned urban space of 268 ha will be reached by 2023, likely followed by densification of the built-up area (Fig. 3). To the east of the city centre a new settlement of 8.6 ha has been established, which, in addition to the typical residential buildings and home gardens, contains a new mosque and an olive grove of 0.7 ha.

In 2018 the town’s 56.3 ha non-governmental land comprised 19.3 ha private green spaces, 15.2 ha private buildings, and 4.0 ha public green areas. The total irrigated area thus amounted to 23.3 ha (Fig. 4). The size of individual homegardens ranged between 7 and 3590 m² with an average of 368 m². Some homegardens were partly outside the property wall and contained fruit trees and annual crops. In total 33 perennial and annual plant species of 16 families were identified (Table 1). Abundance was highest for pomegranate, olive, rose bushes, and vine, but also peach, apricot, pear, and fig trees were encountered. Garlic was cultivated in 14 of the 25 homegardens studied, followed by onion, maize, and some fodder barley.

Our surveys indicated that besides some private cisterns of unknown capacity for rainwater collection, most residents of Sayh Qatanah used tap water from local borewells for irrigation whereby little attention was given to crop-specific water needs. Average monthly water consumption varied from 43 to 213 l m⁻² (mean 97 l m⁻² ± 49 SD). This translated to a total irrigation water use in the 19.3 ha private homegardens of 224,652 m³ in 2018. Including the public green areas, the annual estimated water consumption of all green areas of the town amounted to 272,054 m³.

Terrace gardens

Excluding the newly created terrace areas southwest of Ash Sharayjah and the information-free plots of Al ‘Ayn, by 2018 the actively used area of all five oasis systems had declined from 20.3 ha in 2007 to 19.9 ha (− 2.0%). Fallow land increased by 3.5%, while the use of non-perennial crops decreased by 1.9%. The share of perennial crops without underplanting decreased by 5.1%. In contrast, the share of land under agroforestry increased by 2.1% (Table 2). The 2018 plant census yielded an N_S of 13,739 with 25 different perennial species from 12 families. The 2007 count resulted in 1150 individuals less, with 24 different species from 14 plant families.

In 2007 D_N was highest for pomegranate (51%), rose (21%), date (9%), true lime (5%), peach (4%), and banana (3%). By 2018, D_N increased for pomegranate (52%) and rose (28%), but decreased for date (7%), banana (2%), lime (1%), and peach (1%). The establishment of drip-irrigated olive yielded a D_N of 4% in 2018, while this crop was non-existent in 2007. Over the past decade olive has thus become the third most common crop species in the study region.

In 2018 the information-free plots of Al ‘Ayn had a similar composition than the other ones, with the three most common species being pomegranate (51%), rose (27%), and olive (6%). Also the newly established Ash Sharayjah terraces were dominated by pomegranate (38%), rose (28%), and olive (23%).

From 2007 to 2018 the N_A of most species declined. Sapodilla, pigeon pea, almond, prickly pear (Opuntia vulgaris Mill.) and lemon were no longer recorded in the oases. Instead, prickly pear was identified on the newly created terrace areas of Ash Sharayjah and a young almond tree was spotted in Al ‘Ayn. In addition, a sorb tree (Sorbus domestica L.) was discovered in Al ‘Ayn. The stand of pome fruits such as apple and pear decreased by 89% and 86%, respectively, and stone fruits recorded a similar decline. The N_A of apricots decreased by 88%, while the decline of peaches was 71% and of plums 64%. Bitter orange, true lime, orange, and Palestinian lime were decimated by 91%, 71%, 63%, and 22%, respectively, while date and banana stocks decreased by 14% and 16%. In contrast, the N_A of pomegranate increased by 11% and of rose by 50%.

Al ‘Aqr

At a constant total terraced area of 1.7 ha the actively used land declined by 3.4% (Fig. 5). Thereof the proportion of agroforestry systems increased by 3.8%, woody plant alone areas declined by 4.8% and annual crops by 3.0%, and fallows increased by 0.8%. Pomegranate and rose were the dominant species in both years (Fig. 5). While the D_N of pomegranate decreased from 63.6 to 58.0%, that of rose increased from 22.8 to 39.4%. Whereas the D_N of peach fell from 4.5 to 1.6% and bitter orange, orange, lemon, pear and plum completely disappeared, barley, maize, eggplant, and Rhodes grass (Chloris gayana Kunth) continued to be cultivated on the terrace areas.

Al ‘Ayn

Also Al ‘Ayn’s total terraced area of 1.9 ha remained constant. For the 2007 investigation period, information on landuse of ~ 0.3 ha was missing. This was taken into account in the data on relative landuse changes by not considering information-free plots from 2007 which in 2018 contained 20.5% agroforestry systems, 52.0% woody plants only, 1.4% annual crops, and 26.1% fallow land (Fig. 6, Appendix 1).

During the decadal study period the active cultivation area of Al ‘Ayn declined by 0.2%. Areas with agroforestry systems were expanded by 9.3%, while the use of woody plants only recorded a decline of 16.4%, fallow land increased by 25.1%, and the annual cropping area declined by 18.1% (Fig. 6).

Between 2007 and 2018, the D_N of rose increased from 54.6 to 61.8% and of pomegranate from 28.2 to 30.5%. The D_N of peach decreased from 4.3 to 2.1%, and of papaya, lime, and apricot to less than 2.0%. In contrast to 2007, no records of apple and lemon were obtained in 2018. However, barley, garlic, onion, sweet potato, sorghum, and oats continued to be cultivated.

Ash Sharayjah

In 2007 Ash Sharayjah’s total area was 15.2 ha to which, by 2018, 1.7 ha of newly developed farmland were added and included in our digital mapping (Fig. 7, Appendix 2). For the determination of relative area changes, however, these newly established terraces areas were not taken into account. During the transformation decade the agriculturally used area of Ash Sharayjah decreased by 4.5%. The total area with agroforestry systems increased by 0.2%, woody plants only declined by 4.5%, areas with annual crops decreased by 2.1% and fallow fields expanded by 3.1% (Fig. 7).

Until 2018 the D_N of roses increased from 21.4 to 26.7%, while if fell for pomegranate from 63.9 to 61.7%, for true lime from 5.6 to 0.9%, and for apricot and peach it declined to < 2.0%. Also the D_N of date (− 83%), apricot (− 81%), bitter orange (− 80%), true lime (− 78%), pear (− 78%), peach (− 65%), walnut (− 44%), and grape (− 44%) declined compared to 2007, while only a single specimen of the bitter orange was sighted in 2018 and prickly pear was only found in the newly established terraces. The latter, located in the southwest of Ash Sharayjah, were built with cement and some of them had a large field structure. In contrast to the traditional terraces they were often drip-irrigated and planted to perennials such as pomegranate (38%), rose (28%), and olive (23%). Overall agroforestry systems occupied 9% and annual crops 1.4% on this new land.

Qasha’

Qasha’ covered a total area of 4.9 ha. In 2018, 56% of the terrace fields were abandoned while the agriculturally used area increased from 1.8 to 1.9 ha (+ 4.7%). Areas with agroforestry systems increased by 6.1%, those with woody plants declined by 6.3%, annual crops increased by 1.9% and fallow by 1.3% (Fig. 8). The D_N of pomegranate increased from 56 to 74%, of rose from 9.1 to 29.7%, and of banana from 5.7 to 5.9%. In contrast, D_N of peach declined from 12.1 to 0.9% and D_N of grape and walnut were each < 2.0%.

Decreases in N_A were noted for peach (− 92%), walnut (− 86%), apricot (− 85%), date (− 70%), lime (− 47%), sweet lime (− 43%), fig (− 38%), and grape (− 33%), while the N_A of guava (+ 450%) and banana (+ 3%) increased. Individuals of lemon, mango, olive, and plum were resettled, whereas mash apple, bitter orange, orange, papaya and pears were not further cultivated. In 2018 on the terraced areas the annual species maize, garlic, tomato, oats, onion, Rhodes grass, potato, coriander, carrot, and pepper were cultivated.

Masayrat ar Ruwajah

The total area of Masayrat ar Ruwajah (3.4 ha) remained unchanged over the past decade, while the agriculturally used terraces increased by 0.3% from 3.05 ha to 3.06 ha. Relative landuse by agroforestry systems increased by 1.3%, areas with woody plants experienced a slight decline (0.2%), while annual crops and fallow areas remained largely unchanged (+ 0.1% each; Fig. 9).

With respect to the N_S of all perennial crops the fruit stock in Masayrat decreased by 7.7%, while date, banana, and true lime remained the most common species across time. The D_N of date increased from 67 to 70%, although its N_A declined by 15%. The D_N of banana fell from 14.5 to 14.1% and that of lime from 8.3 to 5.2%. Declines in N_A were also noted for peach (− 84%), lime (− 48%), grape (− 25%), and banana (− 19%) while the abundance of guava and Palestinian lime remained unchanged. In contrast the N_A of papaya (+ 54%), fig (+ 73%), and mango (+ 167%) increased. In contrast to 2007, no apricot and lemon were recorded and Rhodes grass was the most widespread annual crop followed by barley, sorghum, maize, and oats as fodder crops on smaller areas. Alfalfa and vegetables such as onion, garlic, chili, and eggplant were rarely cultivated.

Qanfarah

In 2018 landuse of the largely newly established terraced areas of Qanfarah was recorded for the first time. Therefore no comparison with the 2007 data is possible. The agricultural area amounted to 0.7 ha largely composed of homegardens (Fig. 10). They are surrounded by high walls, a protection against unauthorized entry and browsing livestock. We recorded a total of 29 species from 20 plant families. The highest D_N had pomegranate (34%), followed by rose (29%), olive (6%), banana (5%), apricot (3%), guava (3%), and grape (3%). Also recorded were lemon, Palestinian lime, fig, and papaya as well as corn, barley, garlic, eggplant, helmet bean, potato, chili, strawberry, and sugar cane.

Spatio-temporal variation of ligneous species diversity

The Sørensen coefficient of similarity, used to compare the ligneous plant species in a given oases between 2007 and 2018, indicated that changes in species composition were lowest in Masayrat ar Ruwajah, where the coefficient exceeded 0.90 (Table 3). In the high altitude oases of Al ‘Ayn, Al ‘Aqr and Ash Sharayjah, similarity of species between 2007 and 2018 was also high with coefficients ≥ 0.8 in all cases. The most important temporal changes occurred in Qasha’ where the coefficient was only 0.72.

Due to the altitudinal gradient, species cultivated in Masayrat ar Ruwajah have always differed from those of the other oases and respective coefficients did not change considerably from 2007 to 2018. Ligneous plant stands in Qasha’ and Ash Sharayjah were similar in 2007 (0.87) and 2018 (0.84). The same applied to the ligneous species cultivated in Al ‘Ayn and Ash Sharayjah (2007: 0.80; 2018: 0.82). In contrast, ligneous stands in Al ‘Ayn and Al ‘Aqr were similar in 2007 (0.88), but differed considerably (0.64) in 2018. Species changes were most important between Al ‘Aqr and Ash Sharayjah where the coefficient dropped from 0.79 in 2007 to 0.58 in 2018.

Source: Ecology - nature.com