The influence of mesquite trees (
Worldwide, invasive alien species are considered as direct drivers of ecological change and biodiversity decline [
The species was introduced into Bahrain for landscaping and amenity purposes in the 1930s. The oldest specimen of the species, which is believed to be 500 years old, locally known as the “Tree of Life,” is considered a focal point in the landscape of Bahrain [
Several authors studied the influence of mesquite trees on soil properties [
On the other hand, adverse effects of mesquite on biodiversity and ecosystems including rangelands, pastures, wetlands, and native flora have been controversial [
The effect of mesquite trees on soil properties and understory plants has been altered by many parameters including the climate of the region [
Bahrain is an archipelago of 36 islands occupying a total land area of 779.39 km2. It is located between latitude 25°32′ and 26°20′ north and longitude 50°20′ and 50°50′ east. The study area is chosen to be in the middle of one of the largest mesquite stands in the country at 26°10′20″N and 50°29′36″E (Figure
The location of the study area.
Bagnouls and Gaussen’s ombrothermic diagram for Bahrain [
Soil samples were collected from the understory (2 m from the main stem of the tree) of four randomly distributed mesquite trees and the uncanopied adjacent areas (2 m distance from the edge of the tree crown). A core was taken at four depths of 0–5, 5–20, 20–40, and 40–60 cm in four replicates. The samples were air-dried and passed through a 2 mm sieve. Bulk density (BD) (g·cm−3) and soil moisture content (SMC) were estimated [
The heights and diameters of mesquite trees were measured in four randomly quadrates of 50 × 50 m. Canopy cover was estimated using a 100 m line intercept running along the middle of each quadrat [
The results of the study indicated that sand comprised 96.4% of soil composition, while clay and silt constituted 3.4% and 0.2%, respectively. Soil BD ranged from 0.92 to 1.55 g·cm−3. An increasing trend in BD with depth was detected in the understory and the uncanopied adjacent areas. Significant differences in BD were observed between the understory of mesquite trees and the uncanopied adjacent areas at a depth of 0–5 cm (Figure
Soil bulk density in the understory of mesquite trees and the uncanopied adjacent areas.
Moisture content in the soil profile ranged from 3.41% to 5.26% in the understory compared to 3.10%–21.9% in the uncanopied adjacent areas. No significant differences were observed in SMC among various depths of the soil profile in the understory and the uncanopied adjacent areas, except for the 40–60 cm layer (Figure
Soil moisture content in the understory of mesquite trees and the uncanopied adjacent areas.
Soil pH was moderately alkaline, ranging from 7.9 to 8.1. The maximum pH was 8.1 ± 0.03 in 0–5 cm depth in the uncanopied areas compared to 7.9 ± 0.04 in the 20–40 cm depth understory. No significant differences in pH were observed between understory and the uncanopied areas. Electrical conductivity ranged from 1.44 to 3.56 mS/cm. Likewise, no significant differences in EC were found beneath and beyond the canopies of mesquite trees (Table
Mean values and standard errors (
Depth (cm) | Area/soil properties | EC (mS/cm) | pH | N (%) | P (mg·L−1) | K (mg·L−1) | Na (mg·L−1) | Ca (mg·L−1) | Mg (mg·L−1) | SOM (%) |
---|---|---|---|---|---|---|---|---|---|---|
0–5 | Understory | 2.43 (1.09)AB | 8.08 (0.06)AB | 0.24 (0.05)A | 805 (233)A | 190 (122)A | 63 (36.8)A | 345 (125)A | 77.6 (36.2)ABC | 1.47 (0.30)A |
Uncanopied | 1.77 (0.57)AB | 8.10 (0.03)A | 0.11 (0.01)B | 347 (42.2)BC | 59.5 (31.6)A | 89.1 (75.4)A | 303 (60.6)A | 35.8 (9.66)BC | 0.67 (0.07)B | |
5–20 | Understory | 3.56 (0.89)A | 7.98 (0.07)AB | 0.18 (0.04)A | 511 (55.1)B | 182 (54.3)A | 107 (36)A | 572 (156)A | 96 (25)AB | 0.74 (0.11)B |
Uncanopied | 1.93 (0.44)AB | 8.02 (0.03)AB | 0.07 (0.01)BC | 209 (33.3)C | 55.5 (18.2)A | 73.9 (40)A | 382 (117)A | 38.7 (5.79)BC | 0.31 (0.07)C | |
20–40 | Understory | 3.41 (0.63)AB | 7.9 (0.04)B | 0.10 (0.01)BC | 234 (40.3)C | 155 (31.1)A | 123 (39)A | 547 (126)A | 98.9 (22.5)A | 0.41 (0.07)BC |
Uncanopied | 1.44 (0.45)B | 8.10 (0.07)A | 0.05 (0.01)BC | 175 (27.8)C | 50.6 (8.06)A | 24.2 (14.1)A | 350 (169)A | 30.5 (3.81)C | 0.12 (0.02)C | |
40–60 | Understory | 2.52 (0.77)AB | 8.03 (0.11)AB | 0.04 (0.004)BC | 137 (33.2)C | 121 (22.2)A | 104 (45.1)A | 395 (120)A | 84.7 (20.4)ABC | 0.12 (0.02)C |
Uncanopied | 2.75 (0.66)AB | 7.96 (0.06)AB | 0.04 (0.01)C | 147 (19.2)C | 69.1 (14.9)A | 98.4 (63.8)A | 585 (135)A | 62.7 (19.9)ABC | 0.21 (0.03)C |
Significant differences existed in SOM in 0–5 cm and 5–20 cm depths between the understory and the uncanopied areas. The SOM was more than twice the amount in the first two topsoil layers than those of uncanopied adjacent areas. However, a decrease in SOM was noticeable from the topsoil downwards in both understory and the uncanopied adjacent areas. The low level of SOM in this study compared to other ecosystems could be attributed to variations in moisture, pH, vegetation cover, land management, and land-use history [
Likewise, significant differences existed in N percentage between the understory and the uncanopied areas, down to a depth of 20 cm. Nitrogen was twice the amount in the 0–5 cm and 5–20 cm depths in the understory compared to uncanopied adjacent areas. Our results confirmed those of Tiedemann and Klemmedson [
A similar trend was found for the phosphorus, where concentration was higher in the depths of 0–5 cm and 5–20 cm in the understory than the comparable depths of uncanopied adjacent areas. Equally, significant differences existed in Magnesium (Mg) between the understory of mesquite trees and the uncanopied adjacent areas, starting from topsoil to 20–40 cm depth. The result is consistent with Tiedemann and Klemmedson [
No significant differences in the levels of Na and Ca were observed between the understory and the uncanopied areas. Furthermore, no clear trend in these nutrients was detected with soil depth. Comparable results were reported by El-Keblawy and Al-Rawai [
The results of the study showed that mesquite tree heights and diameters averaged 7 ± 1.57 m and 20 ± 8 cm, respectively. Trees were of a shrubby multistemmed form (Figure
The effect of mesquite trees on the annual understory plants.
Mesquite-associated plant species in the study site.
Position | Family | Understory | Uncanopied | ||||
---|---|---|---|---|---|---|---|
Species | No. of plants | R. Frequency (%) | R. Density (%) | No. of plants | R. Frequency (%) | R. Density (%) | |
|
Aizoaceae | 83 | 12.5 | 42.8 | 2 | 10 | 0.9 |
|
Poaceae | 2 | 12.5 | 1.03 | 6 | 20 | 2.69 |
|
Polygonaceae | 6 | 12.5 | 3.09 | 0 | 0 | 0 |
|
Geraniaceae | 4 | 12.5 | 2.06 | 44 | 20 | 19.7 |
|
Malvaceae | 17 | 25 | 8.8 | 67 | 20 | 30 |
|
Compositae | 82 | 25 | 42.3 | 99 | 20 | 44.4 |
|
Amaranthaceae | 0 | 0 | 0 | 5 | 10 | 2.24 |
194 | 223 |
The Shannon-Weaver index of diversity was higher in the uncanopied areas compared to the understory, calculating 1.27 and 1.18, respectively. Species evenness (Shannon Equitability (
Finally, since its introduction into the country, mesquite widely spreads in the rangelands of Bahrain due to seeds dissemination by camels. Recently, mesquite was categorized as slightly invasive with low invasion risks [
This study presented the influence of mesquite as an invasive species on some physicochemical properties of sandy soils and annual plants in Bahrain. The study showed mixed-effects of mesquite trees on soil’s properties. No differences in the pH, EC, K, Na, and Ca were found between the understory and uncanopied adjacent areas. However, significant differences existed in soil organic matter, N, P, and Mg between the two categories in the upper 20 cm of the soil. Mesquite trees did not affect species richness; nevertheless, species diversity was higher in the uncanopied areas compared to the understory.
Based on the results of this study, it can be concluded that mesquite enriches upper soil layers with some nutrients and organic matter. Moreover, its adverse effect on annual plants in the arid environment where sandy soils prevail is minimal. Mesquite proved to be an invasive and highly competitive species in many ecosystems of the World. Nonetheless, it did not function as an invasive species in the low-resources sandy ecosystem of Bahrain. Mesquite tolerates drought and fixes nitrogen in the soil. It stabilizes sands and nurses some annual understory plants. Therefore, we recommend planting the species to reclaim similar sandy ecosystems in the arid environment. In the meantime, we believe that critical knowledge gaps still exist regarding the effects of mesquite invasion in particular ecosystems. Further research could be conducted to value the socioeconomic and environmental benefits of mesquite in light of climate change and the threat of widespread drought and desertification.
Data are available upon request from the corresponding author.
Ahmed A. Salih was formerly affiliated with Natural Resources & Environment Department, Arabian Gulf University. Jameel A. Alkhuzai was formerly affiliated with the University of Bahrain.
The authors declare that there are no conflicts of interest regarding the publication of this work.
This study was conducted within the framework of the research plan of the Department of Natural Resources and Environment, Arabian Gulf University.