Environmental and ecological aspects of limans in a desert environment

How Limans help to combat desertification

 

The ecosystem of the northern Negev is characterized by loess subsoil with variable natural areas of topsoil crust and patches of woody shrub vegetation. The topsoil crust is inhabited mainly by debased microphytic vegetation composed of algae, lichens and cyanobacteria.

 

These cyanobacteria and other bacteria secrete substances high in sugar that cause the particles of soil to stick together and form a flat, firm crust. The crust prevents rainwater from permeating the ground and so creates surface run-off that can lead to flooding.

 

Areas of low shrubby vegetation, on the other hand, are characterized by knolls of friable soil rich in nutrients and organic materials created by the breakdown of vegetation and animal droppings. The natural dynamic between these two types of area is one of source and sink, as runoff water, soil and nutrients interact. Under natural conditions, this relationship is vital to the functioning of the ecological system and conserves the agricultural potential of the soil.

 

Desertification in the Negev:
Desertification is a process of environmental deterioration that includes the erosion and eventual destruction of the soil’s fertility potential as a result of either naturally-occurring or man-made processes.


Limans as desertification retardants:

A possible interface solution to the problem of desertification and dwindling resources, especially in view of the prospect of global warming, is the creation of artificial “sink” areas possessing two characteristics: firstly, a method of preventing the loss of surface runoff, soil and nutrients; and secondly, a place to store these resources. This solution simulates the activities of the natural areas of vegetation destroyed by the desertification process.

 

The runoff-prevention component moderates surface flow and reduces soil erosion, while the storage area allows the natural resources to be reintegrated into the ecosystem rather than escaping from it. Throughout several periods of history desert dwellers have made use of a variety of methods to retain resources in danger of “escaping” from the desert ecosystem; Nabatean agriculture in the Negev is one example of this. Methods in use today, such as shichim (terraces), limans and streambed terraces are interface solutions for the creation of artificial “sink” areas in the desert landscape.

 

These methods differ from one another in their location, form and extent, and, as a result, their moisture interface is also different: limans receive large quantities of runoff water infrequently, while shichim (terraces) located on hillsides receive smaller quantities of water at more frequent intervals. All these methods can slow the desertification process, depending on the degree to which they are implemented, and they are used as afforestation techniques throughout the Negev today.

How limans encourage biological diversity

As we have already seen, the ecosystem of the northern Negev is regulated by a source-sink dynamic between areas of topsoil crust and areas of vegetation. The topsoil-crust areas generate runoff water, erosion, nutrients and seeds, which are absorbed and stored in the vegetated areas, where they contribute to increased fertility, productivity and biological diversity.

 

The limans can be likewise regarded as large sinks that absorb water, soil, nutrients and seeds from their arid surroundings, thereby increasing their own productivity and biodiversity, together with the biodiversity of the desert landscape as a whole. It should be emphasized that no ecological research has been conducted on biodiversity within limans, but from research carried out in areas of extensive desert afforestation in Yatir and Sayeret Shaked Park, which uses various methods of runoff harvesting, we can perhaps draw conclusions regarding possible processes.

 

In these areas, as a result of the runoff-harvesting and afforestation techniques used, an increase was found in the productivity and diversity of herbaceous vegetation, arthropods (i.e., insects, arachnids, etc.) and birds.

 

The creation of areas of differing microtopography, where ground moisture levels are higher than in the surrounding arid topsoil-crust desert environment, provides a trap for runoff and seeds, and allows denser germination of herbaceous plants whose large seeds are spread by runoff and wind. As a result, a clear difference in the composition of annual plant populations was found to exist between the natural areas and the artificial areas that benefit from runoff harvesting.

 

Photos: KKL-JNF Photo Archive
 

The pits used for runoff harvesting were found to contain a greater wealth of common varieties and Mediterranean species that are usually found in the desert only in areas where resources and humidity are greater.


In view of anticipated climate change and increasing desertification, the contribution these artificial areas can make is even greater: they can now be regarded as providing a form of sanctuary for these species and for the conservation of biological diversity in the desert landscape. 

  
Another possible role for the limans, as sites comparatively rich in resources and flora amidst an arid environment, is their high potential for attracting migratory birds, which may find them to be sources of food in the spring and autumn. The conservation and support of vital biological processes such as bird migration is, therefore, another important potential liman contribution to biodiversity.

 

On the other hand, there are fears that phenomena such as changes in the composition of the ecological population (changes of this kind have been discovered by research into flora, birds and reptiles in areas where runoff harvesting is used in forestry), may lead to the disappearance of desert-dwelling species that may be preyed upon or pushed out by competition. For example, research on reptiles conducted in Sayeret Shaked Park showed that local desert variety of lizard had disappeared as a result of greater predation by species of raptor that use trees as vantage points.

 

These desert varieties were replaced by widely-distributed Mediterranean species that appear to be better adapted to an environment where trees and raptors abound.  The Beersheba valley, where the soil is loess mixed with quartz sand, serves as a habitat for a wide variety of species whose numbers are dwindling because of destruction and restriction of their natural habitat due to increased building, infrastructure extension and agriculture in the area. Local varieties of plants, reptiles, rodents and birds, such as the Negev iris, the Gilead iris, the leopard fringe-fingered lizard, the marginated tortoise, the lesser Egyptian jerboa and the houbara bustard are today in danger of extinction both regionally and worldwide.


The intrusion of widespread Mediterranean varieties into these newly created habitats is liable to result in competition or high predation pressure for local species already under the threat of extinction.

 

The hooded crow, for example, is a notoriously invasive Mediterranean species that has spread throughout the Negev in the wake of environmental change, causing serious damage to local species because of the pressure of high predation. Hooded crows prefer to nest in eucalyptus trees and they feed off the abundant food scraps often left in limans and forests. In order to conserve sensitive local species, spot treatments for invasive species like these have to be devised.

 

As the limans are intermittent features of the landscape, the chance that one of these Mediterranean species will use them to penetrate and settle the Negev, thereby pushing out the local desert species, would appear to be small.

 

We should bear in mind that settlement, agriculture, afforestation and roadside environments probably have a very much greater impact in this regard than the limans do. These issues have not yet been examined with regard to the limans; it would probably be a very good idea to monitor and research their ecology, perhaps with the help of the hydrological monitoring stations already in operation.

 

Planning vegetation in limans as a support for biodiversity
The different types of vegetation (herbaceous plants, bushes and trees) have a decisive effect on the animal population that thrives within them, as they determine a large proportion of the conditions of that particular habitat; thus it is vital to keep biodiversity in mind when planning the combinations of flora to be grown in the limans.

 

The encouragement of local varieties of bushes and shrubs such as thorny saltwort (Noaea mucronata), distaff thistle (Atractylis comosa), mitnan / hairy thymelaea (Thymelaea hirsuta) and Arabian Boxthorn (Lycium shawii), together with a wider variety of herbaceous plants, should be considered, and planting density and composition should be changed.

 

Native species of tree such as the terebinth / Mount Atlas pistache (Pistacia atlantica), buckthorn (Rhamnus disperma), prunus (Prunus korshinskyi), sumac (Rhus tripartita) and different varieties of acacia are common throughout the Negev; they are resistant to aridity and may help promote biodiversity in the limans (and some of these trees give shade, too), as they provide habitats or food sources for a wide variety of creatures.

 

Increased use of orchard vegetation and resistant local fruit trees in suitable locations should also be considered. Using deciduous trees at limans would introduce seasonal variation into the environmental conditions, and so further encourage biodiversity. In addition, deciduous trees would help to create a more comfortable environment for winter visitors to the limans.

Limans as potential providers of a variety of services

Limans’ possible contribution to the carbon cycle:
In recent years there has been increasing evidence that the afforestation of semi-arid areas can make a number of valuable environmental contributions. The pine forests of Yatir were found to have a higher carbon dioxide absorption rate than the pine forests of Europe – i.e., a rate only slightly lower than the average for pine forests worldwide. 
 
These figures show desert afforestation to be a very significant potential sink for carbon dioxide, and thus a vital part of the global carbon cycle and the struggle to reduce global warming, which is caused in part by the accumulation of greenhouse gases such as carbon dioxide. In addition, it was found that judicious planting of tree species that provide grazing in damaged areas considerably raised biological productivity.
 

 
Afforestation for grazing and agricultural purposes was found to be likely to aid the rehabilitation of damaged areas, prevent soil erosion and raise biodiversity and productivity levels by providing an abundance of stubble that increased fertility. Methods such as these supply the soil with vast carbon reserves stored in organic material in the ground.
 
On the global level, the researchers claim that the rehabilitation of land in desert areas by means of these afforestation techniques can create a sink for billions of tons of carbon dioxide per year and provide employment for millions of residents of these areas, while biodiversity is carefully maintained. We recommend promoting research to examine these aspects of the precise contribution made by the limans to their desert environment.

Limans as regulators of over-exploitation of the desert environment

Overgrazing: The spreading Bedouin presence throughout the Negev is causing over-exploitation of the natural environment because of extensive overgrazing. Overgrazing destroys the vegetation that serves as sink areas, and hastens desertification. If some of the limans were to be used as grazing forests (silvopasture), they could perhaps play an important role in relieving grazing pressure. We should examine the desired ratio of tree-planting as opposed to encouraging the development of the natural shrub population (shrubs are the most common form of woody vegetation in the desert) and the natural covering of annuals that enrich grazing potential.


Cutting down trees for firewood: In recent years more and more protected trees growing in the wild (such as Acacia pachyceras and Acacia raddiana) have been cut down for firewood by Bedouin in natural areas of the Negev. Management of some of the limans as agricultural woodland (agroforestry) could play an important role in reducing the number of trees cut down in Bedouin areas.


Stress caused by tourism and visitors to the natural desert region: In recent years the Negev in general, and its northern expanses in particular, have become more attractive to Israelis. Increasing awareness and greater access to information about natural and recreational sites from books, maps and the Internet have caused the Israeli public to “invade” certain areas of the Negev during the winter flowering season. The excursion culture has changed: bicycles and all-terrain vehicles take people to campsites where they can sleep and eat out of doors. Dance parties, weddings and birthday parties are now also regularly held in natural surroundings.

 

These leisure and recreational activities damage the desert ecosystem and place it under stress. This is aggravated by the small number of overnight camping sites available in the Negev and the prohibition against holding activities of this kind in recreational areas inside nature reserves. The series of limans along most of the roads of the northern Negev has a great potential for use as venues for a variety of recreational activities, thereby reducing pressure on more ecologically sensitive areas.