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Ecosystems are dynamic by nature. Throughout history biomes worldwide have changed. Currently multiple dry forests are in the midst of (or have already) altering their structure and losing their function as a dry forest ecosystem (such as in Ethiopia and Ghana). Together with the degradation of dry forests to savannahs there is a loss of ecosystem services that they provide.
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Still, dryland ecosystems, such as Savannahs, are of major importance to human societies. According to the Research program on dryland systems (CGIAR) around 2. 5 billion people (about 30% of the human population) live and rely on dryland ecosystems for the natural resources these provide. These ecosystems are home to 50% of reared livestock, 44% of cultivated lands (a third of all cultivated crops originate from dryland areas) and store about 36% of global carbon in their soil (CGIAR). One can thus understand the importance of maintaining, as well as improving, the dryland ecosystem and the ecosystem services that they provide through regeneration of lost dry forest ecosystems. Resilience thinking is one tool that can be used to increase the productivity of dryland ecosystems. Resilience is a concept in ecology that illustrates the ability of an ecosystem to remain intact in a stable state whilst disturbances are taking place. If disturbances are too big for an ecosystem to remain within a stable state, there are two ways that an ecosystem can react to the changing (a)biotic factors, namely through adaptation and through transformation. Resilience thinking, as described by Folke et al, will be used to investigate which factors have caused the degradation of dry forest ecosystems, and how this could be used to transform savannah/semi-arid ecosystems back to dry forest.
As such this paper will summarize the findings of a literature review, 2 experimental papers and a paper reflecting on a success story in the Maradi and Zinder region. The findings of these papers will be used to reflect on which factors cause degradation of dry forests and maintain the resilience of savannah/semi-arid ecosystems. Lastly, a conclusion and discussion is provided that attempts to answer the research question, and integrate the knowledge gained from the papers reflected upon, with that of resilience thinking.
Yirdaw et al (2017) wrote a review article on the rehabilitation of dryland ecosystems. This paper gives an overview of factors that influence dry forest degradation and offers options for rehabilitation.
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One reason for degradation of dry forests is the occurrence of fires, which although beneficial at times (increase soil fertility and favouring species composition to fire-tolerant species), prevents natural succession of savannahs to dry forest ecosystems. Additionally, factors such as overgrazing, herbivory, land conversion to agriculture and collection of firewood are causes for dry forest/savannah degradation.
With regards to rehabilitation, site conditions require different management strategies. Areas that have a low degree of degradation (where degradation is a result of fire, overgrazing, collecting firewood and/or cultivation) can be restored through “passive restoration”. Passive restoration entails excluding human and animal disturbances through isolation, allowing for natural regeneration of dry forest. Strongly degraded dry forests have shown not to restore under passive restoration, and instead active restoration is required. Active restoration is practiced when an area is thus far degraded that the natural seedbank in the soil has diminished. Besides isolating the area from animal and human disturbances, replanting of multiple species is done. This should include the late successional and rare species for a quick recovery of the natural succession process. In order to do so, local knowledge of tree species is often required as scientific knowledge on which tree species grow in sub-tropical areas often lacks.
These two papers look at how an ENSO-event could trigger tree establishment. By doing so they reveal multiple (a)biotic factors that affect resilience of semi-arid areas. As such, herbivory, soil conditions, precipitation and temperature affect resilience.
During an ENSO-event (El-Nino Southern Oscillation event), climatic conditions change leading to higher rainfall patterns in some areas (such as the West coast of South America), and droughts in other areas (such as in the Philippines). The increase in rainfall has a significant influence on semi-arid vegetation, and can be used as a trigger to transform vegetation in a way that there are long-lasting effects on local water and nutrient cycles.
The paper by Squeo et al (2007) looks at three sites, namely Piura, Mejia and Fray Jorge, located North, Centre and South of the Atacama Desert respectively (the area itself has a semi-arid ecosystem). To simulate an ENSO-event, all the plants were watered additionally to the natural rainfall. The three sites had different rainfall patterns (accounting for the different effect ENSO has on the rainfall patterns), average temperature, as well as slightly different soil types (sandy, loamy and sandy-loamy). These three factors influenced the growth rate of the Prosopis Pallida (in Peru) and Prosopis chilensis (in Chile) trees significantly. The researchers concluded that these trees have better growth rates in areas where it would rain in the summer season (when temperatures are higher), and where the soil is sandy, allowing for easier root establishment. A tree-ring analysis conducted by Holmgren et al (2006) further supports the idea that rain during the summer season is a cause for higher growth rates. The experiment also revealed that in Peru, the P. pallida tree established twice as much during ENSO years, compared to non-ENSO years, however the same was not found for the P. chilensis. Moreover, the tree-ring growth rate doubled during ENSO-events for the P. pallida tree. Similarly, there was a significant correlation between tree-ring growth rate and annual precipitation for the P. chilensis tree. Differences between the species did not play a role, as when planted together both species grew better in Peru than in Chile. Temperature and soil differences likely explain the better survival and growth rate in Peru than in Chile.
In the paper Holmgren et al (2006), the influence of herbivory on tree establishment during ENSO events was tested. The study area once again was in Fray Jorge, and Piura. Herbivory had a clear effect on seedling survival in both Peru and Chile. Without herbivory 85% of the P. pallida seedlings survived (even under low water treatments), whereas only 31% survived when herbivores were present. P. chilensis fared less well, especially under low water-treatments (no seedlings survived after 1 year with a water availability lower than 450mm, even without herbivory).
The paper written by Sendzimir, Reij and Magnuszewski (2011) describes deforestation of the Maradi and Zinder regions in Niger after the 1920’s, and the socio-ecological policies that were implemented to re-green these bush-grass savannah areas. Deforestation in Niger has mainly occurred due to a large increase in population (and therefore land conversion from forest to agriculture), natural resources being centralised under colonialism in 1935, a high export of natural resources in 1950, as well as deforestation following a drought in the early 1970’s where locals were desperate for food and fuel.
In the 1980’s a new farming technique (farmer-managed natural regeneration) was introduced, where particular tree and shrub species (that were drought resistant) would be planted amongst crops. After another drought in 1985 it became clear that this strategy prevailed compared to previous farming techniques. However, due to corrupt forestry officers and paramilitary forces, this farming technique was not adopted by farmers nationwide until 1987, when political upheaval following the death of Niger’s president allowed rural farmers to experiment with the new farming technique.
Farmers realised that the agro-forestry farming technique provided multiple benefits, including rainfall increases of up to 30%, reinforcing a positive feedback loop that increased tree densities and crop yield. Additionally, the trees provided shade for their livestock, along with more fodder. Soil fertility increased naturally due to the litter of the trees, as well as the dung of birds and livestock. The trees’ roots fixed nitrogen through the rhizobium bacteria, further increasing soil fertility. The sanctuary that the trees provided for the birds also enhanced seed dispersal, which meant that farmers saved costs as they had to replant crops less frequently. Farmers, seeing these benefits, were motivated to implement the new farming technique. Besides increasing their crop yield, they would receive additional benefits from the ecosystem services, such as protection from drought, as well as naturally fertilizing their crops, and getting firewood.
Savannah ecosystems seem to be an alternative stable ecosystem as opposed to dry forest. To transform a savannah ecosystem back to dry forest the following (a)biotic factors have shown to be relevant: fire occurrences; land conversion for agriculture; overgrazing/herbivory; climate (temperature, precipitation, and ENSO-events); soil conditions. If one would want to transform these areas back to their original state (there is a debate whether this is morally/culturally appropriate or not, for more information, then these factors need to be addressed properly by multiple actors.
Farmers can have a significant influence on the restoration of an areas’ vegetation biomass, soil properties (both physical and chemical) and plant and animal diversity. The willingness of farmers to implement these strategies in Niger is a great example of how these actors, globally, could be stimulated to integrate nature conservation practices with agriculture practices, potentially saving them costs (less fertilizers and firewood bought) and decreasing externality costs (less environmental damage). Moreover, farming areas that have integrated nature conservation in semi-arid areas provide an optimal starting point to further transform degraded landscapes to their original state. The results of these three papers can be combined to offer management strategies that integrates resilience thinking to transform previously degraded dry forests back to their original state. For example, although overgrazing can prevent the establishment of seedlings, Sendzimir shows that livestock can increase soil fertility benefitting growth conditions for both trees and crops. Balancing overgrazing by livestock, and excluding herbivores could potentially stimulate a positive feedback cycle that enables the transformation of savannah/semi-arid ecosystem to dry forest through soil fertilization (from both dung and rhizobium bacteria). Additionally, in areas where there is a low seed bank, active restoration practices could be combined with farmer managed natural regeneration, to increase seed dispersal as the arrival of birds increase. In this way another positive feedback cycle is reinforced, helping the spread of seedlings naturally. This shows that there are management options to transform savannah/semi-arid ecosystems back to dry forest. However, on a global scale many different site-specific factors become relevant that influence these management options, and therefore there is no single management option that is guaranteed successful. More research needs to be done to see which management options could be feasible in different areas, while being more specific about soil and climate conditions.
This paper did not take vegetation type into account. Therefore, more research is required to understand the comprehensive relationship that resilient savannah/semi-arid ecosystems have with species composition, climate, soil and (a)biotic factors (e. g. fire and herbivory) in relation to rehabilitation of dry forest ecosystems.
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