RESOURCE ISLANDS OR BARRIERS
Robert M. Dixon, Ph.D and Ann B. Carr
This essay is an outgrowth of our many observations of natural and managed ecosystems in the Desert Southwest and Great Plains regions of United States . Generally, resource islands or barriers in drylands are small land areas where bioresources tend to collect or concentrate. This concentration develops and maintains an ecosystem that has much more biomass and biodiversity than surrounding areas with very sparsely distributed resources. Vegetation does not uniformly cover the landscape as in more humid regions but occurs in small patches surrounded by barren land in extreme cases. In the desert, every tree is an oasis; i.e., a circular island of concentrated resources beneath the tree's canopy.
Bioresources include rainwater/snowmelt, seeds/spores, plant litter/materials, and soil. Following land disturbances that create a barren landscape, resource islands or barriers begin to develop at a surface obstruction or barrier such as a small depression or swale, rock or gravel, and plant debris. Both surface microdepressions and microelevations or combinations such as side-by-side ridges and furrows concentrate resources by obstructing their movement under the forces of wind, water and gravity. Erosion and sedimentation/settling processes are primarily responsible for resource concentration at or near the initial obstruction or barrier. As the ecosystem develops in response to resource concentration, animals play an increasing role in transporting resources into the island or barrier.
Resources are transported to the barrier under the forces of wind, water and gravity by the process of soil erosion. The barrier reduces the velocity of both wind and flowing water (runoff) causing the solid resources to settle to the soil surface. Furthermore, differential raindrop splash erosion along the edges of the resource barrier moves resources into the barrier at surprisingly rapid rates. The barrier absorbs raindrop impact energy causing resource movement into but not out of the barrier.
Resource islands of plant communities develop through the process of ecological succession wherein short-lived and short-growing pioneer species are the first to establish, thereby paving the way eventually for long-lived, tall-growing species. Pioneer species are usually weedy exotic annuals along with some native annuals. Biomass production and biodiversity increase as the succession proceeds with animals being increasingly involved in resource transport. Birds, rodents, and ants carry seeds from outlying plant communities to enhance the diversity of the barrier community. Bees and other pollinators bring in pollen from the outside for greater genetic diversity of the next plant generation.
Well-developed resource islands typically display spatiotemporal succession with the interior being much more advanced than the exterior. The exterior edge communities consist largely of pioneer plant species. This edge or ring of pioneers advances outward during wetter-than-normal years and retreats inward during dry years.
Rate of plant succession depends on several environmental driving forces. Rate increases with amount of annual precipitation and water holding capacity of the soil which, in turn, depends on topsoil depth, soil structure and organic matter. Rate of succession also depends on the size of the upslope and upwind land areas that are contributing resources to the island or barrier. Rates decrease under natural and human disturbance including fire, drought, herbivory, livestock grazing, plowing (cultivation), etc. When disturbances become severe, succession can reverse and become regression with a return to pioneer species and even land barrenness in extreme cases.
Weeding of the ecosystem can either accelerate, decelerate or reverse succession depending on the method used. Careful hand pulling or line weeding within the central most advanced part of the island or barrier can eliminate competition with mid and late seral species. Cut or pulled weeds should be left in place as a mulch to minimize the disturbance. Weeding should be avoided along the edges of island because early seral plants or weeds at this location help to protect the interior from harsh edge effects. Generally, use of hoeing or herbicides for weed control should be avoided since such disturbances, being excessive, can regress the succession back to the early seral stage of weedy annuals. Enough patience to allow natural succession to displace the weeds is often the best approach.
Seeding of late seral species in the interior areas of the resource island can also help to accelerate succession if done with minimal disturbance. Hand imprinting and seeding when the soil is moist will make an indentation in the soil surface to funnel resources with minimal disturbance of the soil and surface litter. Conventional hand tools that dig into the soil will encourage weed growth and may thus be counterproductive. A well-formed imprint favors the establishment of perennial species because of efficient resource funneling and concentration.
Cultural practices such as the windbreaks of the Great Plains are essentially linear resource islands or barriers. By working like snow fences, windbreaks pile up huge snowdrifts in the winter to provide deep watering of the windbreak vegetation when the spring thaw comes. Other wind-borne resources such as dust and leaves settle out in the windbreak to rapidly build topsoil. Bramble barriers, which are described in the attached paper, are another example of linear resource islands. Such barriers should include enough spiny species to protect the barrier from excessive herbivory and provide safe roosting and nesting sites for birds that transport pollen and seeds into the barrier. These linear barriers also exhibit spatiotemporal succession of plant communities with the pioneer species being located on the outer fringes where seral driving forces are the weakest or disturbances are the greatest. Foraging and drought effects are the most severe along the edges.
Early development of linear resource islands can be accelerated by contour ridges of rock, soil, and woody plant material combined with seeding and transplanting. Land imprinting and furrowing on the contour or in a spiral pattern will accelerate the formation of resource islands by collecting and concentrating bioresources. Circular spiral patterns have the inherent advantages of being non-directional and having minimal edge effect.
Concurrent with the succession of plant communities is the succession of soil ecosystems of flora and fauna. This subterranean succession builds topsoil, thereby increasing water and nutrient availability for the plant community. The ability of mycorrhizal fungi to improve soil structure and increase the availability of soil moisture and phosphorous to plant roots has been well documented. Thus, these fungi are a strong driving force in the acceleration of plant community succession.
In conclusion, the concept of resource island or barrier not only helps to explain the patchyness of vegetation in drylands, but also can be applied to accelerating plant community succession in severely degraded land areas. Using appropriate strategies, sparsely distributed bioresources can be collected and concentrated to permit them to work in concert to germinate seeds, establish seedlings and accelerate plant succession for increased biomass production and biodiversity. Thus, this concept is expected to be useful in achieving the goals of sustainable agriculture and ecological restoration.
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