Research Article |
Corresponding author: Amin U. Khan ( akeco1111@gmail.com ) Academic editor: Ji-Zhong Wan
© 2023 Amin U. Khan, Asad Abbas, Faiza Sharif, Asma Mansoor, Zafar Siddiq.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Khan AU, Abbas A, Sharif F, Mansoor A, Siddiq Z (2023) Conserving the threatened woody vegetation on dune slopes: Monitoring the decline and designing adaptive strategies for restoration. Nature Conservation 53: 165-182. https://doi.org/10.3897/natureconservation.53.106406
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The southern tip of the Thal desert in Pakistan harbors the remnants of the original tropical thorn forest, amounting to two percent, which covered the province of Punjab a hundred years ago. In the past three decades, there has been a progressive decline in woody species cover on dunes, which is directly related to the increase in population in the surrounding area. Stabilized and destabilized dunes were subjectively selected followed by quantification of cover and diversity of woody species on the top and lower slopes. Dunes closely resembling the overall cover were grouped to suggest corresponding restoration measures. The results suggest that trends of decrease in cover and diversity of woody species were evident in the upper slopes of some stabilized dunes having less than 50% cover. The destabilized dunes with less than 20% cover are highly vulnerable to erosion. A general trend observed among dunes was that with a decrease in the cover of upper slopes, there is a decrease in the cover on lower slopes. The number of destabilized dunes is increasing without effective restoration measures against the prevailing trends of disturbances. Ranking dunes on the basis of cover could help in proposing simple restoration measures as a first step towards developing an understanding of designing adaptive strategies to restore the woody cover.
Anthropogenic factors, conservation, desertification, restoration, threatened tropical thorn forest
Although pervasive economic incentives to support human settlement are regarded as underlying causes of destruction and vulnerability in fragile arid lands, they are still being unabatedly pursued in developing countries (GOP 1992;
The bi-species association of Salvadora oleoides and Prosopis cineraria is considered to be the ultimate climax of the tropical thorn forests of the Indus Plain; stretching from the foothills of the Himalayas to the Arabian Sea (
In addition, piloting restoration of four woody species on a sand ridge by enrichment planting showed 40% survival of S. oleoides, of the four woody species planted (
Most of the work conducted in the deserts of this region is based on investigating the characteristics of soil and vegetation parameters, and socio-economic and developmental factors (
The Thal desert (Fig.
General view of RK and the surrounding landscape: dark grey color represent natural forest cover in which stabilized (SD) and destabilized dunes (DS) are located, and intervening light grey color shows the cultivated area with seasonal cropping. Dark grey patch on the left is the rangeland area in which the restoration site is located near the town of Chaubara. Green color shows the private arable canal irrigated zones. The road (orange) across the desert between two main towns. Map powered by google imagery 2023 CNES / Airbus, Maxar Technologies, Map data 2023.
General view of the KP with grove of P. cineraria, S. oleoides and C. polygonoides with grass cover on the flats and ridges, where sheep can be spotted, and a dune in the background showing slopes occupied by woody patches.
Research (
Studies (
A survey (
Over the past two decades pumping of ground water by turbines has made agriculture more reliable in non-irrigated areas, resulting in an increase in the cultivated areas and a corresponding increase in population in the adjacent villages. In addition, this is because sugar cane is the most popular crop grown in irrigated areas and bagasse an agricultural by-product (
Studies conducted at RK (
Obtaining true replication was difficult at the landscape-scale level for this management-based study, therefore it was hoped that comparisons based on pseudoreplicates would effectively help in interpreting the trends of the decline of woody vegetation on the dune slopes. Priori categorization of dunes was made by selecting assemblages of one square kilometer area in stabilized and destabilized dunes located in less exploited and over-exploited areas, respectively. The assemblage of stabilized dunes was located about 3 km away from the metaled road connecting the two main towns whereas assemblage of destabilized dunes was located adjacent to the metaled road. The stabilized dunes generally have a greater height (>4 m) and dimension as compared to destabilized dunes (<3 m).
The criteria of selection of dunes were slightly different in the two assemblages. In the assemblages of stabilized dunes adjacent ten dunes having approximately 50 m2 cover of woody species on both slopes were demarcated for study. In the assemblages of destabilized dunes where most upper slopes are devoid of woody species, dunes having at least 5 m2 cover of woody species on upper slopes were selected. GPS coordinates of the selected dunes were noted. The cover value of the following woody species was made: P. cineraria, S. oleoides, C. polygonoides, and C. decidua. Grasses and herbs were not included in the cover estimates as they have poor representation on the slopes.
Upper and lower slopes were visually bifurcated; as the upper slope is the top of the dune and the steep slopes whereas lower slopes are gentle slopes before it merges with flats dominated by grasses and herbs. It varies from dune to dune; they are more defined in the stabilized dunes as compared to destabilized dunes.
A rope was used to demarcate a circle in order to estimate the cover of woody species. Since the cover of woody species was very scattered, ranging from a single plant to patches of various sizes, therefore measurements were based on the proportion of the ground cover occupied by the perpendicular projection of the aerial crown (
Hierarchal cluster analyses (HCA) using Ward method was applied to provide classification of upper and lower slopes of both stabilized and destabilized dunes. Two-way ANOVA was applied to find out the statistical mean difference of vegetation cover between the upper and lower slope of both stabilized and destabilized dunes. Shannon Diversity Index was used to determine the vegetation diversity trends with cover among the groups. Linear regression analysis was applied for grouping dunes on the basis of cover. In this research, the dependent variable represented by average coverage of plant species and independent factor is the condition of the dune in terms of stabilized and destabilized dunes. And dunes having close resemblance of overall cover were grouped to suggest corresponding restoration measures. All statistical methods were applied using software SPSS (SPSS 21). Species nomenclature followed the taxonomic criteria of
The results are analyzed on the basis of the cover of upper and lower slopes separately and also dunes as a unit by combining cover on upper and lower slopes and then grouping them in order to suggest appropriate techniques for restoration and conservation of the woody plant cover.
The result of vegetation hierarchical clustering (Fig.
Hierarchial cluster analyses (HCA) showing classification of upper and lower slopes of both stabilized and destabilized dunes into distinct communities’ groups. Slopes of destabized dunes were placed in group A where as slopes of stabilized slopes were placed in group B the group B at a lower distance splits into C representing upper slopes of stabilze dunes and D representing their lower slopes.
These units formed by cluster analysis also indicate a kind of interconnection among the upper and lower slopes of a dune as a decrease in the cover of upper slopes is reflected by trends of decrease on lower slopes. Therefore it was considered that by ranking dunes on the basis of overall cover and then suggesting restoration measures for the two slopes would be more practical in restoration of the woody plant cover. Results of two-way ANOVA show that the difference in woody cover (m2) between upper and lower slopes is more significant in the stabilized dunes compared to the destabilized dunes. This trend is also obvious in the difference between mean cover of upper and lower slopes which is significantly higher in stabilized dunes compared to destabilized dunes (Fig.
Mean cover (m2) of woody vegetation on upper and lower slopes of stabilized and de-stabilized sand dunes. Upper bars show ± S.E. *Significant difference between means (P < 0.01), Two-way ANOVA.
Similarly, linear regression showed that expected cumulative cover (Fig.
Linear regression analysis showed grouping of dunes on the basis of cover, destabilized groups have lower expected cum prob as compared to stabilize groups.
Shannon Diversity Index applied to the groups indicating trends of increase in diversity with increase in cover among the groups.
Dune no. | Group | Upper | Lower | |
---|---|---|---|---|
Stabilized | 1,2,3 | 1 | 0 | 0.175 |
Stabilized | 4,5,6 | 2 | 0.92 | 0.93 |
Stabilized | 7,8,9,10 | 3 | 1.06 | 1.06 |
Destabilized | 1,3,4,5,9 | 4 | 0 | 0.11 |
Destabilized | 2,6,7,8,10 | 5 | 0.42 | 0.48 |
The following dune groups were segregated after ranking them on the basis of overall cover for both stabilized and destabilized dunes and corresponding restoration measures were suggested for the two slopes.
Ranking of stabilized dunes on the basis of overall vegetation cover (Table
Mean overall cover of woody patches in the three groups on the upper and lower slopes of stabilized and destabilized dunes.
Dune Rank and no () | Slope | Area (m2) | Plant species (m2) | Total cover (m2) | Mean cover (m2) | |||
---|---|---|---|---|---|---|---|---|
S. oleoides | P. cineraria | C. polygonoides | C. decidua | |||||
1 (SD 1) | US* | 120.5 | 41 | 0 | 0 | 0 | 41 | 51.5 |
LS** | 156.7 | 62 | 0 | 0 | 0 | 62 | ||
2 (SD 2) | US | 125.6 | 44 | 0 | 0 | 0 | 44 | 53 |
LS | 206.2 | 54 | 8 | 0 | 0 | 62 | ||
3 (SD 3) | US | 121.1 | 43 | 0 | 0 | 0 | 43 | 54 |
LS | 288.5 | 65 | 0 | 0 | 0 | 65 | ||
4 (SD 4) | US | 130.5 | 35 | 5 | 8 | 0 | 48 | 54 |
LS | 250.7 | 49 | 1 | 10 | 0 | 60 | ||
5 (SD 5) | US | 140.9 | 25 | 11 | 6 | 0 | 42 | 54 |
LS | 192.7 | 40 | 14 | 12 | 0 | 66 | ||
6 (SD 10) | US | 109.4 | 24 | 9 | 13 | 0 | 46 | 57.5 |
LS | 201.9 | 36 | 12 | 19 | 2 | 69 | ||
7 (SD 6) | US | 108.9 | 29 | 12 | 14 | 0 | 55 | 60 |
LS | 192.8 | 38 | 19 | 8 | 0 | 65 | ||
8 (SD 9) | US | 131.5 | 28 | 11 | 13 | 0 | 52 | 60 |
LS | 226.6 | 37 | 16 | 15 | 0 | 68 | ||
9 (SD 7) | US | 162.2 | 30 | 10 | 21 | 2 | 63 | 64 |
LS | 289.1 | 34 | 10 | 18 | 3 | 65 | ||
10 (SD 8) | US | 185.1 | 26 | 8 | 23 | 0 | 57 | 64.5 |
LS | 311.7 | 35 | 11 | 26 | 0 | 72 | ||
Total ± S.E. | 775 ± 2.85 | 157 ± 1.32 | 206 ± 1.90 | 7 ± 0.19 | ||||
Frequency (%) | US | 100 | 70 | 70 | 10 | |||
LS | 100 | 80 | 70 | 20 |
The group I includes dunes 1, 2 and 3, the overall cover is less than 55 m2 although they have 60 m2 cover on lower slopes but the upper slopes have less than 45 m2. In addition, upper slopes have low diversity as compared to lower slopes. The presence of lone P. cineraria on dune 2 seems to be a vestige from the original cover. This group of dunes could naturally restore their cover on protection from further exploitation but at the same time active restoration based on enrichment planting of saplings P. cineraria and C. polygonoides on upper slopes by soil amendments could assist in increasing stability of the patch.
The group II with dunes 4, 5 and 10, have overall cover of less than 60 m2 although lower slopes have 60 m2 cover but the upper slopes have less than 50 m2 (Fig.
Close view of stabilized dune showing decrease in the cover of upper slopes while still retaining the diversity of species.
The group III with dunes 6, 7, 8, and 9, have more than 60 m2 overall cover; their upper slopes have slightly less than 60 m2 cover. The presence of C. decidua, indicates that the woody patches are stabilized as they are harbouring species preferring more stable sandy loam soils. This group represents a low level of exploitation and are therefore less susceptible to natural disturbance. These dunes are recommended to be officially designated as model dunes, barring them from any form of extraction.
Ranking of destabilized dunes on the basis of overall vegetation cover (Table
Dune Rank and no () | Slope | Area (m2) | Plant species (m2) | Total cover (m2) | Mean cover (m2) | |
---|---|---|---|---|---|---|
S. oleoides | P. cineraria | |||||
1 (DD 1) | US* | 91.9 | 8 | 0 | 8 | 10 |
LS** | 143.5 | 12 | 0 | 12 | ||
2 (DD 5) | US | 86.3 | 7 | 0 | 7 | 11 |
LS | 95.8 | 13 | 2 | 15 | ||
3 (DD 9) | US | 79.9 | 8 | 0 | 8 | 11 |
LS | 153.1 | 14 | 0 | 14 | ||
4 (DD 3) | US | 58.3 | 5 | 0 | 5 | 11.5 |
LS | 95.8 | 18 | 0 | 18 | ||
5 (DD 10) | US | 51.9 | 11 | 0 | 11 | 13 |
LS | 95.8 | 15 | 0 | 15 | ||
6 (DD 4) | US | 90.9 | 11 | 0 | 11 | 15 |
LS | 163.5 | 19 | 0 | 19 | ||
7 (DD 6) | US | 86.6 | 9 | 5 | 14 | 16 |
LS | 95.0 | 13 | 5 | 18 | ||
8 (DD 7) | US | 49.4 | 12 | 0 | 12 | 15.5 |
LS | 155.6 | 19 | 0 | 19 | ||
9 (DD 8) | US | 114.7 | 13 | 5 | 18 | 18 |
LS | 237.3 | 11 | 7 | 18 | ||
10 (DD 2) | US | 76.9 | 12 | 0 | 12 | 24 |
LS | 239.4 | 27 | 8 | 35 | ||
Total ± S.E. | 257 ± 1.12 | 32 ± 0.60 | ||||
Frequency (%) | US | 100 | 20 | |||
LS | 100 | 40 |
The group I with dunes 1, 3, 4, 5, 9, have low cover (< 10 m2) and surviving S. oleoides on upper slopes have either exposed root system (Fig.
The group II with dunes 2, 6, 7, 8 and 10 have higher cover (>18 m2) and S. oleoides appears less stressed and presence of occasional P. cineraria, showed increase in diversity as compared to group I. Proposed amendments for upper slopes should be similar to those suggested for group I; at the same time enrichment planting of woody sampling on lower slopes with bagasse amendments could help to expand the woody patches.
Trends of decline of cover exhibited in the stabilized and destabilized dunes indicated that the woody patches on top of the slope are more vulnerable to destabilization and once its cover decreases below a minimum on the top slopes it could destabilize woody patches on the lower slopes. Signs of initiation of the process of destabilization are evident in the low ranked stabilized dunes in the form of decrease in cover and diversity on the upper as compared to lower slopes (Fig.
A stabilized dune, showing both slopes having more than 50% cover B stabilized dune showing decrease in the cover of upper slopes C destabilized dune showing decrease in cover on both slopes. Map powered by google imagery 2023 CNES / Airbus, Maxar Technologies, Map data 2023.
This study shows that large woody patches on the slopes retain the ability to naturally expand their cover on sandy slopes but once they are destabilized they are increasingly impacted by sand mobilization and therefore cover seems the most influential factor in suggesting control measures. At the moment there is virtually no data available on the economic cost of wind erosion, including, site damage, loss of biodiversity and soil productivity in terms of organic matter removal. Neither have any measures ever been adopted to monitor and restore the woody patches. The simple restoration measures proposed here could be considered as a first step in developing an understanding towards designing adaptive strategies on conservation of the woody cover.
The decline of woody species on dune slopes reflects the lack of recognition and understanding of values and attributes of species in influencing physical, chemical and biological characteristics of fragile soils at societal and official levels. Effective knowledge of vegetation dynamics has played an important role in identification of disturbance factors and developing protocols in categorizing biological features of dunes in terms of plant growth, stabilization process and vegetation improvement, (
The significance of study findings in combating loss of threatened woody species on sand dunes emphasizes that management strategies in a spatially heterogeneous edaphic system, where the stability of dunes is of great significance to the basic ecology of this region, needs to be adapted to an understanding of the factors and processes. Suggesting high-ranked stabilized dunes as protected representative benchmarks is compatible with the objectives of the range land. This would discourage leveling of dunes as it would affect the productivity of grasses on flats and therefore can be adopted on a large-scale restoration plan. Whereas conserving stressed plants on destabilized dunes can be proposed to be implemented at a small scale to develop a protocol for large-scale restoration. In addition, the indigenous threatened species can be popularized as a productively chosen species by proposing it as a multiple-species shelter belt for sustainable agriculture, which will effectively control erosion and provide food and fodder, and pest control, and as a seed production area for recruitment. But in the absence of laws encompassing the conservation of natural vegetation on sandy land, these proposals seem to be ahead of their time.
We are also thankful to the rangeland officer and livestock officer at Rakh Khairewala for helping us during our visits to the site.
The authors have declared that no competing interests exist.
No ethical statement was reported.
No funding was reported.
Amin Khan conceived, designed the research and wrote the manuscript; Asad Abbas contributed in data collection and performing experiment; Asma Mansoor applied the all statistical analysis and edited the manuscript; Faiza Sharif and Zafar Siddiq reviewed the manuscript
Amin U. Khan https://orcid.org/0000-0001-8521-2579
Faiza Sharif https://orcid.org/0000-0002-5142-9136
Asma Mansoor https://orcid.org/0000-0002-6786-3236
Zafar Siddiq https://orcid.org/0000-0002-1693-1649
All of the data that support the findings of this study are available in the main text.