Urban Green Infrastructure Assessment for Their Regeneration in Tehran Landscape

Introduction
“Green infrastructure” is a term that describes the abundance and distribution of natural features providing ecological services necessary for community wellbeing specially in urban areas where these are limited. Green infrastructure is increasingly espoused by landscape conservation and metropolitan planning because it is an effective way to surrogate ecological network functions where landscapes are highly impacted by anthropogenic changes. 
In this study structural characteristic of open and green patch network called green infrastructure was central in deciding urban management strategies because of their life support services. Finally, applicable strategies have been presented for a gradual regeneration according to the intensity and extent of degradation these patches and other natural elements face in each urban district. Decision making regarding selection of each strategy and priorities are estimated in correlation to the current condition of green infrastructure as well as the speed of alteration they are experiencing in Tehran (with very rapid change or alteration).
 
Materials and Methods
City of Tehran, with 730 km² area is located in the southern slope of Alrborz Mountains and Northern margins of central desert of Iran, and has 22 urban districts.
Tehran's urban landscape structure, viability and environment have incredible speed of fluctuation in time and in space. This pace of change forced us to focus on conservative policies and protection of natural conditions, in order to prevent landscape structure destruction before any other objective (for rehabilitation). 
The method used in this study based on structure - function interdependency is basic concepts of landscape ecology such as patch features are considered to evaluate the degree of fragmentation and connectivity of green infrastructure network at the landscape scale while considering its alteration speed.
In order to present optimized ecological services and effective function at the scale of urban landscape, these infrastructures are required to have sufficient presence, high stability with appropriate composition and configuration regarding their spatial distribution. Therefore, landscape metrics were used to evaluate these features of green infrastructure quantitatively. Selected metrics regarding evaluation of composition and configuration of landscape elements related to our research goal were selected to be PD, MPS, MNND and CAP. Their quantities were then used at landscape and patch type levels as finding for discussions and conclusion.
 
Discussion and Results
Landsat satellite image was used for derivation land cover information and then landscape metrics was estimated based on time series land cover maps (Fig. 1 and 2). 
Structural analysis of the current conditions of greenspace: Districts in the north of Tehran specially district one and three had better conditions in terms of many parameters (like presence of green patches, their stability and distribution) rather than the other ones. In turn, central districts specially districts 10 and 17 and also those located from center to east and west had inappropriate conditions in terms of most metrics. The presence of green patches in these districts was very low, and the patches were fine grain with low stability.
 Structural analysis of the current conditions of open space: In terms of open space of Tehran, it can be noted that in districts located on the west edge like 19 and 22 and specially 22, the condition of the patches was better than the others. The result showed highly presence and stability of large open patches in these areas. Nevertheless, the conditions of open patches distributions in these districts were low and the north districts had better distribution of open patches. In turn, conditions of open patches in central districts especially 10, 17 and also 8 in northeast were inappropriate and presence of open patches of fine with low stability and relatively inappropriate distribution were observed.
Total analysis of changes in Tehran green space (2003-2013): In green space of Tehran, during the studied period, the process of all metrics changes except CAP was similar in all districts such that MNND and MPS have decreased and PD has increased in all districts. Therefore, it can be concluded that decrease of the patch size and their fine size has decreased their stabilities in all districts. Despite of MNND decrease in all districts which shows a relatively high connectivity among green patches, this change cannot be considered as a positive one as regarding to PD and MPS changes, it can be concluded that green patches have fragmented. CAP of green patches had been increased in 12 districts and decreased in 6 districts. This trend was constant in four districts.
Total analysis of changes in open space of Tehran (2003 -2013): During the studied time, in terms of open space metrics, CAP has decreased in all districts except districts 7, 12 and 14, PD had been increased in most districts and constant in some ones. MPS and MNND had been a decreased in all districts. Regarding to the process of open patches changes, it can be noted that open patches have had a destructive process (decrease presence and stability of open patches) in most districts. The trend of PD and MNND changes also showed the fragmentation of open patches in Tehran.
 
Fig. 1: Classified image of study area in 2003
 
Fig. 2: Classified image of study area in 2013
Planning Strategies:

Protective strategy: where the existing landscape supports sustainable process and patterns, a protective strategy should be employed (even along with defensive strategy in some cases suffering a regressive tendency). This strategy can be recommended for greenspace protection in districts located in north, west and southwestern of Tehran and for open space protection, northern and northwester districts are prioritized, too.   
Rehabilitation strategy: for where the existing landscape is disturbed and fragmented, such as in those districts of northern and southwestern Tehran. These should have rehabilitation of natural elements as priority task.
Development (offensive) strategy: is to be recommended for those urban districts with a low presence of green and open patch as well as with low connectedness. Districts located in south, east and central of Tehran, are required to development of greenspace.
Defensive strategy: was most needed in districts where these contain valuable natural elements (such as north and west of Tehran) that provide desire function. Utilization strategy can be employed for these districts but defensive attitudes are needed to sustain the present situation.

Conclusion
Findings show that the main problem of green infrastructural network in Tehran may be its uneven distribution, with low connectivity and almost non-existing in the central city matrix. This along with the much reduced natural matrix connectivity, have created an unsustainable urban texture with very low livability and polluted atmosphere.
It can be noted that in terms of green space, expanding the area has been the only plan with no consideration regarding advantageous use of all natural spaces of Tehran at micro up to macro scale planning. Hence a better configuration of natural patches with connectedness to each other (specially to 9 river valleys and the row of hills crossing them in perpendicular) should be also part of a comprehensive green structural renovation and development strategy for the future of Tehran.
In order to overcome landscape structural degradation an strategic perspective is needed to be based on pattern-process principles and to be implemented based on landscape linking concepts. This planning may well follow strategic perspectives found by this research which is also based on several years of research by others along with frequent revisions. Added value of all these efforts is here presented by our final completion looking for a comprehensive synthesis.