The Relationship Among Desert Patchiness, Resources, Production and Diversity: Implications for Rehabilitation of Desertified Environments
Investigators (most current known information)
Proposal Abstract
Our conclusions refer to: 1) The effects of patch manipulations on patchiness, water flow and plant species diversity; 2) The effect of annual rainfall and patch type on water flow and plant species diversity; and 3) Implication of patch manipulations for management.
Patch manipulation
In the course of the three years study we note the following trends, in relation to the treatments, in the relationship among patchiness, water flow, and plant responses in terms of species diversity:
- Harvesting: The general trend is an increase in the size of the macrophytic and decrease in microphytic patch size. We do not know the reason for this. However, one of our hypotheses is that this trend is dependent on the dispersal pattern of the seeds in the area. We harvest the shrubs at the end of the summer. Since most of the seeds are produced in the spring, we think that a large seed bank accumulates in the macrophytic patches from the spring to summer. Without the presence of the shrubs in the macrophytic patches, there is high germination of the herbaceous plants on the soil mounds of the macrophytic patches and even on the interface between the macrophytic and microphytic patches. All this leads to a relative increase in the macrophytic and decrease in microphytic patches. Our model predicts that the increase in the macrophytic and decrease in microphytic patches should result in an increase in runoff generation. Our runoff data confirm the above prediction. Above ground removal of the shrubs alters patch quality and increases patch size, and thus increases the total surface area covered by macrophytic patches. This in turn results in an increase in water retention and availability for plant growth. The net effect of mowing is an increase of species richness and the variation in composition in both patch types. Species with a northern or southern distribution benefit from these changes, as opposed to general species.
- Scraping: After scraping that removes all the above ground vegetation and exposes the soil, succession processes start. From our field observations it appears that the successional pathway at our field site is as follows. Atractylis serratuloides is the first shrub that colonizes the area. When they grow in size, soil accumulates and forms a mound under the shrub. This soil mound enables the establishment of herbaceous plants and germination of Noaea mucronata shrubs. The development of the two shrubs allows for more soil accumulation and thus an increase in the size of the macrophytic patch. Since, in our experiment, we are at the start of the successional process, the macrophytic patches are still small and consist only of A. serratuloides. Therefore, in the scraping treatment the microphytic patches are large and the ratio between the microphytic and macrophytic patches is large. The high proportion of the micophytic patches explains the relatively high runoff generation and lower water retention. Patchiness modification by scraping and its effect on water flow can explain the decrease in species richness in both patch types, decrease in compositional variation and decrease in patch type differences. Nevertheless, even after removal of the top soil, patches are still recognizable, and retain some of their differences in diversity. Scraping favors general species relative to rarer edge species.
- Spraying: As a result of spraying with Simozine, all the herbaceous plants and A. serratuloides were eliminated. The area is now inhabited by only one plant species, N. mucronata. Since there aren't herbaceous plants on the soil mounds, the mounds are disintegrating and the soil is being eroded due to runoff water. The net effect of these processes is a decrease in the size of the macrophytic patches and an increase in the size of the microphytic patches.
Based on our model on the relationship among patchiness, flows, productivity, diversity and our results on patchiness our predictions are as follows. For the harvesting treatment we assume that runoff and nutrient leakage will decrease since there are more macrophytic patches capable of absorbing runoff. The effect is that more resources are available for the plants on the macrophytic patches. Thus, their productivity and diversity should increase.
In the scraping treatment the macrophytic patches are still small. Thus, they are able to absorb only a small portion of the runoff generated by the microphytic patches. Our assumption is that during the early stages of succession, the leakage from the plots should be high. However, since the macrophytic patches are in the process of increasing in size we assume that leakage will decrease through time. The plant response to this process will be an increase in productivity and diversity with time.
As for the spraying treatment, we envision a process that will bring about maximization of leakage. This is due to the total disintegration of the macrophytic patches. Productivity and diversity will obviously be low since we removed the vegetation and decreased resource availability for the only remaining species.
Annual variation and patch type
The results of our three-year study show annual variation in runoff generation and that herbaceous plant community diversity, both in terms of species richness and species composition, differ between patch types and years.
- Annual variation in runoff: For the three years we did not have significant differences in percent runoff generation. In the control the average percent runoff generation the first year was, 16.6 + 7.1 (SD) with a coefficient of varience of 42.0%. The second year it was 10.9 + 4.3 (SD) with a coefficient of varience of 18.7%. The third year it was 14.8 + -4.1 (SD) with a coefficient of varience of 27.5%.
- Patch type differences: Microphytic patches were poorer in species than macrophytic patches. Macrophytic patches trap seeds, and provide a greater variety of microsites conducive for germination, establishment and growth of herbaceous plants. Microphytic patches, in contrast, are more homogeneous, and provide less sites for arrival and germination of seeds.
- Between year differences: The changes in time during the three years of this study show the importance of variation in rainfall. The lower rainfall in 1994 not only resulted in lower species richness, but also in smaller patch type differences, and smaller differences among treatments. The study period was not sufficiently long to differentiate between climatic variation and successional processes. Not only are these processes slow, they can be masked by fluctuations in annual rainfall.
Implication for management
The objectives of arid land ecosystem management are:
- To preserve the productivity and diversity of the landscape with little or no human induced damage; and
- To restore desertified areas and to increase their productivity and diversity.
Our study suggests that patch manipulation can be developed into a management tool for the achievement of the above two objectives. Patch manipulation implies either a change in the ratio between the microphytic and macrophytic patches or the addition of human made patches. Since the present study deals with manipulation that changed the ratio between the microphytic and macrophytic patches we can suggest how change in patchiness can be used for productivity and diversity management.
Management by mowing can be useful in cases that the aim of the manager is to increase plant production for native and domestic grazing animals. In addition it can be used as a restoration tool if the objective is to increase plant species diversity.
Scraping is a problematic management tool in comparison to mowing. The benefit of scraping is an increase in runoff harvesting that can be used for water enrichment of human-made catchments. The cost is a decrease in productivity and diversity of the ecosystem that contributes the runoff. Scraping can be used in the early stages of afforestation when runoff havesting after every rain is an important source of water for the seedlings. In addition, scraping can be used as a restoration tool when the aim of the manager is to restore early succession species and to create a landscape mosaic.
We do not recommend herbicide usage as a management tool to increase water harvesting in spite of our short-term results showing that herbicide treatment increased runoff generation.
This is because of two reasons:
- We found that the herbicide kills the microphytic community. In our experiment the crust structure that was formed by the microphytic community was not destroyed, therefore the crust continued to generate runoff. However, our prediction that in the long run the structure of the soil crust will disintegrate, because of the termination of polysaccharide production by the microphytic community that binds the soil particles together. Crust disintegration will increase soil infiltration and decrease runoff generation; and
- We found the the cost of herbicide spraying is very high in terms of ecosystem productivity and diversity. Most of the plants and animals were eliminated from the plots sprayed with herbicides.
- The effects of patch manipulations on patchiness, water flow and plant species diversity;
- The effect of annual rainfall and patch type on water flow and plant species diversity; and
- Implication of patch manipulations for management.
Patch manipulation
In the course of the three years study we note the following trends, in relation to the treatments, in the relationship among patchiness, water flow, and plant responses in terms of species diversity:
- Harvesting: The general trend is an increase in the size of the macrophytic and decrease in microphytic patch size. We do not know the reason for this. However, one of our hypotheses is that this trend is dependent on the dispersal pattern of the seeds in the area. We harvest the shrubs at the end of the summer. Since most of the seeds are produced in the spring, we think that a large seed bank accumulates in the macrophytic patches from the spring to summer. Without the presence of the shrubs in the macrophytic patches, there is high germination of the herbaceous plants on the soil mounds of the macrophytic patches and even on the interface between the macrophytic and microphytic patches. All this leads to a relative increase in the macrophytic and decrease in microphytic patches. Our model predicts that the increase in the macrophytic and decrease in microphytic patches should result in an increase in runoff generation. Our runoff data confirm the above prediction. Above ground removal of the shrubs alters patch quality and increases patch size, and thus increases the total surface area covered by macrophytic patches. This in turn results in an increase in water retention and availability for plant growth. The net effect of mowing is an increase of species richness and the variation in composition in both patch types. Species with a northern or southern distribution benefit from these changes, as opposed to general species.
- Scraping: After scraping that removes all the above ground vegetation and exposes the soil, succession processes start. From our field observations it appears that the successional pathway at our field site is as follows. Atractylis serratuloides is the first shrub that colonizes the area. When they grow in size, soil accumulates and forms a mound under the shrub. This soil mound enables the establishment of herbaceous plants and germination of Noaea mucronata shrubs. The development of the two shrubs allows for more soil accumulation and thus an increase in the size of the macrophytic patch. Since, in our experiment, we are at the start of the successional process, the macrophytic patches are still small and consist only of A.serratuloides. Therefore, in the scraping treatment the microphytic patches are large and the ratio between the microphytic and macrophytic patches is large. The high proportion of the micophytic patches explains the relatively high runoff generation and lower water retention. Patchiness modification by scraping and its effect on water flow can explain the decrease in species richness in both patch types, decrease in compositional variation and decrease in patch type differences. Nevertheless, even after removal of the top soil, patches are still recognizable, and retain some of their differences in diversity. Scraping favors general species relative to rarer edge species.
- Spraying: As a result of spraying with Simozine, all the herbaceous plants and A. serratuloides were eliminated. The area is now inhabited by only one plant species, N. mucronata. Since there aren't herbaceous plants on the soil mounds, the mounds are disintegrating and the soil is being eroded due to runoff water. The net effect of these processes is a decrease in the size of the macrophytic patches and an increase in the size of the microphytic patches.
Based on our model on the relationship among patchiness, flows, productivity, diversity and our results on patchiness our predictions are as follows. For the harvesting treatment we assume that runoff and nutrient leakage will decrease since there are more macrophytic patches capable of absorbing runoff. The effect is that more resources are available for the plants on the macrophytic patches. Thus, their productivity and diversity should increase.
In the scraping treatment the macrophytic patches are still small. Thus, they are able to absorb only a small portion of the runoff generated by the microphytic patches. Our assumption is that during the early stages of succession, the leakage from the plots should be high. However, since the macrophytic patches are in the process of increasing in size we assume that leakage will decrease through time. The plant response to this process will be an increase in productivity and diversity with time.
As for the spraying treatment, we envision a process that will bring about maximization of leakage. This is due to the total disintegration of the macrophytic patches. Productivity and diversity will obviously be low since we removed the vegetation and decreased resource availability for the only remaining species.
Annual variation and patch type
The results of our three-year study show annual variation in runoff generation and that herbaceous plant community diversity, both in terms of species richness and species composition, differ between patch types and years.
- Annual variation in runoff: For the three years we did not have significant differences in percent runoff generation. In the control the average percent runoff generation the first year was, 16.6 + 7.1 (SD) with a coefficient of varience of 42.0%. The second year it was 10.9 + 4.3 (SD) with a coefficient of varience of 18.7%. The third year it was 14.8 + -4.1 (SD) with a coefficient of varience of 27.5%.
- Patch type differences: Microphytic patches were poorer in species than macrophytic patches. Macrophytic patches trap seeds, and provide a greater variety of microsites conducive for germination, establishment and growth of herbaceous plants. Microphytic patches, in contrast, are more homogeneous, and provide less sites for arrival and germination of seeds.
- Between year differences: The changes in time during the three years of this study show the importance of variation in rainfall. The lower rainfall in 1994 not only resulted in lower species richness, but also in smaller patch type differences, and smaller differences among treatments. The study period was not sufficiently long to differentiate between climatic variation and successional processes. Not only are these processes slow, they can be masked by fluctuations in annual rainfall.
Implication for management
The objectives of arid land ecosystem management are:
- To preserve the productivity and diversity of the landscape with little or no human induced damage; and
- To restore desertified areas and to increase their productivity and diversity.
Our study suggests that patch manipulation can be developed into a management tool for the achievement of the above two objectives. Patch manipulation implies either a change in the ratio between the microphytic and macrophytic patches or the addition of human made patches. Since the present study deals with manipulation that changed the ratio between the microphytic and macrophytic patches we can suggest how change in patchiness can be used for productivity and diversity management.
Management by mowing can be useful in cases that the aim of the manager is to increase plant production for native and domestic grazing animals. In addition it can be used as a restoration tool if the objective is to increase plant species diversity.
Scraping is a problematic management tool in comparison to mowing. The benefit of scraping is an increase in runoff harvesting that can be used for water enrichment of human-made catchments. The cost is a decrease in productivity and diversity of the ecosystem that contributes the runoff. Scraping can be used in the early stages of afforestation when runoff havesting after every rain is an important source of water for the seedlings. In addition, scraping can be used as a restoration tool when the aim of the manager is to restore early succession species and to create a landscape mosaic.
We do not recommend herbicide usage as a management tool to increase water harvesting in spite of our short-term results showing that herbicide treatment increased runoff generation.
This is because of two reasons:
- We found that the herbicide kills the microphytic community. In our experiment the crust structure that was formed by the microphytic community was not destroyed, therefore the crust continued to generate runoff. However, our prediction that in the long run the structure of the soil crust will disintegrate, because of the termination of polysaccharide production by the microphytic community that binds the soil particles together. Crust disintegration will increase soil infiltration and decrease runoff generation; and
- We found the the cost of herbicide spraying is very high in terms of ecosystem productivity and diversity. Most of the plants and animals were eliminated from the plots sprayed with herbicides.
Outcome
Please see project 01R-15 for a complete list of outcomes for this and related projects.