Spatio-temporal variation of ecosystem service values adjusted by vegetation cover:a case study of Wuyishan National Park Pilot,China

Sen Lin·Xisheng Hu·Hong Chen·Chengzhen Wu·Wei Hong

Abstract Ecosystem service values (ESV) are strongly influenced by the vegetation cover,which is heterogeneous across different vegetation types.We develop a dynamic evaluation model of ESV for Wuyishan National Park Pilot adjusted by the rate of inflation and the fractional vegetation cover,which is calculated by an enhanced vegetation index from 2000 to 2018.The spatio-temporal variation of vegetation was also examined.The results demonstrated that:(1) the unit area of ecosystem service values adjusted by vegetation cover (ESV VC) shows a gradient of forest >tea plantation >grassland >cropland,and the major ecosystem services provided by forests include soil formation and conservation,climate regulation,and biodiversity maintenance;(2) the ESV VCincreased to 2.1 billion yuan (The reference rate announced by the People’s Bank of China is the US dollar to 6.42 Yuan per dollar.) from 2000 to 2018.Higher and lower ESV VCare predominant in the northwest and southeast region,respectively.In addition,changes of ecological protection structures and human disturbances negatively affected vegetation cover,leading to a decreased ESV VCfrom 2000 to 2005 in the Jiuqu Stream Ecological Protection Area and the Wuyishan National Scenic Spot.The implementation of ecological protection policies from 2010 to 2018 enhanced the ESV VCin the study area;and,(3)the ESV VCis highest in the southeast and 25°–35° area with altitudes of 800–1000 m.Our model can provide timely and helpful information of changes in ESV for use in ecological corridor design and ecological security monitoring.

Keywords Vegetation cover·Ecosystem service values(ESV)·Topographic factor·Dynamic value model

Introduction

Ecosystem service functions refer to all the benefits provided directly or indirectly by ecosystems to support human life,including tangible ecological products and intangible services (Daily 1997).This includes not only products and services (such as product supply,regulation of water supplies,cultural benefits,and ecological support services)but also disturbances accepted by humans (such as snow disasters,diseases and pests infestations,and drought) (Li 2019).As a natural resource,vegetation is the cornerstone of the natural environment on which humans and other living creatures depend for survival and development.Long-term vegetation cover change is the main component of global energy transmission and environmental change,representing the change in land use types and patterns (Gitelson et al.2002).A change in vegetation cover will inevitably affect the structure and function of the ecosystem and affect the ecological products and services (Zhang et al.2017).An important correlation has been identified between vegetation cover and ecosystem services (Bi and Ge 2004).With rapid development of the global economy,human activities of various intensities and scale have aggravated the damage to the natural environment,resulting in changes in vegetation cover and loss of biodiversity (Westman 1977).Approximately 60% of global ecosystem services are currently being degraded or being used unsustainably (Hu et al.2015),and the conflict between the environment and development has gradually escalated (Gretchen et al.2013).

The establishment of a nature reserve system is internationally recognized as an effective way to protect biodiversity,improve ecosystem services,and protect the natural environment (Maxwell et al.2020).The Chinese government has adopted the measure of“building a nature reserve system with national parks as the core.”Research on the relationship between ecosystem services and vegetation cover changes in nature reserves can identify the impact of human activities on the environment,provide theoretical decision-making support for ecosystem service resilience,the design of ecological corridors,security pattern construction,the development of nature reserves,and the promotion of the sustainable development of regional ecosystems.

The Wuyishan National Park Pilot (WNPP) is not only a national park system pilot but also a World Biosphere Reserve included in the World Cultural and Natural Heritage List,and its conservation value is indispensable to China and the world.Many scholars have carried out several studies on the Wuyishan National Nature Reserve,Wuyishan National Scenic Spot,and WNPP.These studies have covered plant community structure (He et al.2007),landscape pattern evolution (You et al.2017a,b),ecological quality (Yang and Xu 2020),vegetation cover change (Lin et al.2020),community interests (He et al.2020),management method(Sheng et al.2020;Li et al.2021),ecotourism (Guo et al.2019),and ecosystem service values (ESV) (He et al.2018;You et al.2012).However,since the establishment of the WNPP,the relationship between ESV adjusted by vegetation cover and topographic factors has not been reported.Costanza et al.(1997) used currency to evaluate ecosystem functions_ENREF_23,which can directly reflect the changes in ecosystem service benefits (Xie et al.2 010) and provide essential support for the theory and method of ESV evaluation.ESV has become an important research topic in ecological and environmental economics.Chinese scholars have been actively searching for more suitable ESV evaluation methods for the actual situation in the country.Xie et al.(2003,2008) produced a table of ecosystem service equivalent values per unit area of the Chinese terrestrial ecosystem,based on Costanza et al.(1997).This is widely used in the evaluation of ecosystem services of different scales and types (Fan et al.2019;Li et al.2021).The equivalent table is based on the national average and focuses on static ecological value estimations,while the strength of the ecological service function will change with a change in biomass (Hu et al.2011).This is insufficient to reflect the temporal and spatial heterogeneity of the same type of ecological community (Li 2019).Therefore,Xie et al.(2015) established a correction coefficient improvement table based on net primary productivity,precipitation,and soil adjustment factor.Other researchers have proposed an improved method of equivalent scale based on Xie et al.(2003),combined with remote sensing,to produce a regional ESV map (Sutton and Costanza 2002) that effectively explains spatial and temporal changes in structure and function of the ecosystem.For example,Tang et al.(2010) and Hu et al.(2013) created a dynamic estimation model of ESV from the aspect of resource scarcity and social development coefficient (Wilson and Stephen 1999).Zhang et al.(2018) calculated the fractional vegetation cover (FVC) using a normalized difference vegetation index (NDVI) to establish the spatial and temporal heterogeneity index for estimation of the ESV.However,NDVI tended to become saturated in dense vegetation,which may show a bias in representing vegetation coverage (Matsushita et al.2007).Therefore,we adopted the enhanced vegetation index (EVI),one more appropriate than NDVI for densely vegetated areas to calculate FVC.

In a previous study,the normalized difference mountain vegetation index (NDMVI) (Wu and Xu 2011),EVI,and NDVI were used to estimate vegetation cover (Lin et al.2020).The results showed that EVI was most appropriate in capturing the dynamics of vegetation cover in high biomass areas of the WNPP.This study has proposed a dynamic model of ESV estimation suitable for WNPP areas.We attempt to:(1) Assess the fractional vegetation cover (FVC)by using the enhanced vegetation index (EVI) to modify the equivalent table of ecosystem service values (ESV);(2)Examine the spatio-temporal heterogeneity and evolutionary trend of ESV under different vegetation cover types in the WNPP from 2000 to 2018;and,(3) Clarify the relationship between the ecosystem service values adjusted by vegetation cover (ESV VC) and topographic factors in the WNPP.

Materials and methods

Study area

The WNPP (27°31′20’’–27°55′49’’N,117°24′12’’–117°59′19’’E) is in the northern part of Fujian Province and has an area of approximately 1027.8 km2(Fig.1).It is hilly with altitudes from 153 to 2158 m a.s.l.and slopes up to 70°.The annual average precipitation is 1684 to 1780 mm,average annual relative humidity of 78%,and annual average temperatures of 17–19 °C.The WNPP includes all vegetation types of China’s mid-subtropical zone plus 290 km2of undisturbed native forest vegetation,which is the most intact and largest natural forest ecosystem in the mid-subtropical zone in the world (Lan 2003).Wuyishan County has a tea planting history going back more than 1500 years and is also the main production area for bamboo.More than 80% of the communities in the WNPP are engaged in tea and bamboo production,and are supplemented by eco-tourism,forming distinctive agricultural and forestry production projects in the study area.

Fig.1 a Location of WNPP in Fujian Province; b Wuyishan National Park Pilot.Wuyishan National Nature Reserve includes a biodiversity protection area in the west,the Jiuqu stream ecological protection area in the center,and the Wuyishan National Scenic Spot in the southeast

Data

The primary data used in this study included Landsat remote sensing data and a digital elevation model (DEM).Landsat imageries (tile number 120/041) were obtained from the International Scientific and Technical Data Mirror Site,Computer Network Information Center,Chinese Academy of Sciences (http:// www.gsclo ud.cn).The imageries were chosen from the growing season (April–May) of five individual years (2000,2005,2010,2015 and 2018),with cloud cover less than 5%.The data were geometrically and radiometrically corrected,and projected using the software of ENVI5.3.Additionally,the grid maps of altitude,slope,and aspect were derived from a 30-m DEM map in ESRI ArcGIS 10.2,which were then reclassified and re-sampled to 90 m.Altitude,aspect,and slope were re-classified as follows:(1) altitude reclassified into ten categories with an interval of 200 m:<200 m,200–400 m,400–600 m,600–800 m,800–1000 m,1000–1200 m,1200–1400 m,1400–1600 m,1600–1800 m,and >2000 m;(2) aspect reclassified into eight categories:East (E,67.5°?112.5°),Southeast (SE,112.5° ?157.5°),South (S,157.5°?202.5°),Southwest (SW,202.5°?247.5°),West (W,247.5°?295.5°),Northwest (NW,292.5°?337.5°),North (N,0°?22.5°,337.5°?360°),and Northeast (NE,22.5°?67.5°);and (3)slopes reclassified into six categories:0°?5°,5°?15°,15°?25°,25°?35°,35°?45°,and 45°?70°.

Methods

Ecosystem service values were determined by the land cover type classification adopted by Xie et al.(2010) and Hu et al.(2013),and included forests,croplands,grasslands,and tea plantations (rivers and wetlands were excluded).From the Millennium Ecosystem Assessment (2005),nine service functions were selected,including regulation of atmospheric gases,climate amelioration,hydrological functions,soil development and conservation,waste treatment,biodiversity maintenance,food production,raw material production,and esthetic value.The vegetation cover of the ecosystem service equivalent values were determined (Xie et al.2010).

A standard unit of the ESV equivalent factor is defined as 1 ha of the average output of China’s cropland every year for the economic value of natural food production (Xie et al.2010).Thus,the ESV equivalent factor of the other vegetation cover types is represented by the cropland ESV unit area.The calculation was improved by considering the inflation rate.The benchmark year for the study was 2005,and the cropland ESV unit area in the base year was discounted for each evaluation year through the inflation rate,expressed as:

where,tis the evaluation year,t0the base year,the discount of ESV per unit area in the base year,rthe inflation rate of 10% (Hu et al.2013).

Owing to the spatial and temporal disparity of ecosystem service functions,the values will change with biomass (Shi and Wang 2008).The same vegetation type cover is particularly different,and the ESV per unit area also differs (Su et al.2006).To expressESV VC, a dynamic estimation model of ESV is proposed that is more suitable for areas with high vegetation cover.

where,RED,NIRandBLUEare surface reflectance in the red,near-infrared,and blue bands,respectively,Gis a gain factor,Lis a soil-adjustment factor,andC1andC2the coefficients of the aerosol resistance term,which uses the blue band corrects aerosol interference in the red band according to Jiang et al.(2008),and the values of each parameter areG=2.5,C1=6,C2=7.5,andL=1.Based on the dimidiate pixel model of the Gutman model (Gutman and Ignatov 1998),for an area with high biomass,FVC EVIcan be calculated as follows:

where,FVCEVIis the vegetation cover based on EVI,EVIminandEVImaxwhich correspond to the minimum and maximum values of EVI within the confidence interval [0.5%–99.5%],andFVCEVIrevised the ESV in WNPP.

where,A iis the area of vegetation cover typei,andEis the totalESVVC.

Results

Application of the dynamic evaluation method of ecosystem service values adjusted by vegetation cover and inflation

The total income unit area of cropland in China in 2005 was 3,629.43 yuan ha?1,and the equivalent values factor was 449.1 yuan ha?1(Xie et al.2008).The ESV unit area of cropland was revised by the inflation rate in each period(Fig.2) and rose from 278.6 yuan ha?1in 2000 to 1150.41 yuan ha?1in 2018.Equation 4 was used to calculate theESVVCand create grade maps for each year (Table 1).

Fig.2 Cropland ESV per unit area in each year

After the fractional vegetation cover (FVC) correction,the forest ESV was the highest,followed by the tea plantation,and the cropland (Table 2).The forest ESV represents soil formation and conservation,climate regulation,and biodiversity maintenance.In 2018,the highest forest ESV was 27,236.3 yuan ha?1,five times that in 2000.In comparison,the lowest ESV was 7641.4 yuan ha?1found in cropland,and the grassland ESV was 8591.9 yuan ha?1.The tea plantation ESV was mainly contributed from hydrological regulation (3568.8 yuan ha?1) and raw material production (2379.2 yuan ha?1).Overall,the total tea plantation ESV increased from 1822.7 to 12,235.1 yuan ha?1from 2001 to 2018.

Table 1 Ecosystem service equivalent value per unit area of the ecosystem in WNPP

Table 2 Ecosystem service values of different land cover types adjusted by FVC in 2000 and 2018

The changes ofESV VCin the WNPP from 2000 and 2018 are similar (Fig.3 a,e),with a spatial pattern of highESV VCin the northwest and low in the southeast.The highest ESV was found in the northwest region which is the majority of the national nature reserve and protected under the ecological red line area less disturbed by humans.

Fig.3 Ecosystem service values adjusted by FVC from 2000 to 2018

Spatial distribution of ESV VC

The ESV of the Wuyishan National Park Pilot significantly increased from 438.9 million yuan per year in 2000 to 2.5 billion yuan in 2018,with a rate of 113.7 million yuan per year (Fig.4,Table 3).Among all the vegetation types,the ESV increment was largest in forest ecosystems from 417.2 million yuan per year in 2000 to 2.3 billion yuan in 2018,with an average rate of 106.8 million yuan per year (Fig.4,Table 3).Moreover,the forest was also the largest contributor (>90%) to the rise of total ESV in the study area.From 2005 to 2015,the contribution rate of forest ESV decreased from 95.4 to 93.7%,cropland and grassland remained stable,however the tea plantation ESV increased the most rapidly.This is a result that part of the forest was transformed into tea plantation because of the high economic value of tea.Extreme drought,possibly as a reflection of climate change,and human disturbances (such as harvesting and planting tea,have led to reduced forest cover.From 2015 to 2018,the forest ESV increased rapidly,indicating that ecological restoration projects such as the establishment of protected areas for water conservation and mountain closure protection on both sides of the Jiuqu Stream Ecological Protection Area(JSEPA) are effective.

Fig.4 Contributions of different vegetation types to the total ESV in WNPP from 2000 to 2018

From 2000 to 2018,croplands and grasslands were minor contributors (2.2%,1.2%) to the increase ESV.The ESV cropland increased from 5.3 to 35.8 million yuan per year,and grassland ESV increased at an average annual rate of 2.6 million yuan per year.

Value of different ecosystem service functions adjusted by vegetation cover

From 2000 to 2018,change ofESV VCwas:soil formation and conservation >gas regulation >biodiversity maintenance >hydrological regulation >climate regulation >raw materials production >waste treatment >providing esthetic values >food production (Table 4).In 2018,the ecosystem service values of soil formation and conservation was the highest (444.9 million yuan per year),three times that of the‘providing esthetic value’ function and 20 times that of the lowest ESV of food production (22.4 million yuan per year).

Table 3 Total ecosystem service values adjusted by vegetation cover from 2000 to 2018

Table 4 Values of different vegetation covers to ecosystem service functions from 2000 to 2018

Ecosystem service values adjusted by vegetation cover at different altitudes

To further study the change of vegetation cover,the ESV adjusted by FVC was estimated at different altitudes (Fig.5).The vegetation cover is distributed in five horizontal bands:evergreen broad-leaved forest;mixed forest;temperate coniferous forest;moss dwarf forest;and,montane meadow forest.The statisticalESV VCof the different altitudes from 2000 to 2018 were from high to low:(800?1000 m) >(1 000?1200 m) >(600?800 m) >(200?400 m) >(1200 ?1400 m) >(400?600 m) >(1400?1600 m) >(1600?180 0 m) >(1800?2000 m) >(<200 m) >(>2000 m).The evergreen broad-leaved forest is the most widely distributed forest cover in the WNPP,occupying 1/4 of the park’s forest area.In 2018 (Fig.6),the totalESV VCof the (800–1000 m)zone was the highest (388.4 million yuan per year).Cropland (29.2 million yuan per year,occupying 81.7% of the same type),grassland (17.9 million yuan per year,occupying 31.8% of the same type),and tea plantation (33.9 million yuan per year,occupying 62.9% of the same type) in the 200–400 m zone were at the peak and,with the increase in altitude,the percentage decreased rapidly.There are mostly extreme habitat conditions in the 1600–1800 m zone (such as strong winds,low temperatures,excessive rainfall,and high humidifies),the vegetation is make up of forest (89.7 million yuan per year),cropland (0.008 million yuan per year) and grassland (6.6 million yuan per year).

Fig.5 Ecosystem service values adjusted by vegetation cover at different altitudes from 2000 to 2018

Fig.6 Ecosystem service values adjusted by vegetation cover at different altitudes in 2018

Above the 1600 m zone,the species composition is mainly moss dwarf forest (1600–1800 m) and montane meadow forest (above 1600 m),suitable for growing at high altitudes,and the forestESV VCdecreased from 89.7 to 1.1 million yuan per year.The grasslandESV VCoccupies 63.5% of the totalESV VCin (>2000 m) zone.

Ecosystem service values adjusted by vegetation cover at different aspects

From 2000 to 2018,the overallESV VCof different aspects in the WNPP showed an upward trend and a rank of:southeast >east >northwest >south >west >southwest >northeast >north.

All aspects of the tea plantation ESV increased from 2000 to 2015,and decreased from 2015 (2.2%) to 2018 (1.6%)in the northwest.From 2015 to 2018,the proportion of grassland and cropland ESV in this area was maintained at 0.8% and 1.6%,and forest ESV increased by 0.5%.Before 2015,the forests were converted into tea plantations.However,the illegally cultivated tea plantations in the northwest were gradually restored to forests after 2015,which helped increase the overallESV VCin the WNPP (Fig.7).

Fig.7 Ecosystem service values from 2000 to 2018 adjusted by vegetation cover at different aspects

In 2018 (Fig.8),theESVVCin the southeast was the highest (42.1 million yuan per year) in the type of forest cover,mainly distributed as evergreen broad-leaved forest;in the north,theESV VCwas the lowest (164.7 million yuan per year) and was mainly distributed in Huangshan pine forest communities alongside other forest communities.The grasslands are mainly in the southeast (14.0 million yuan per year),east (9.64 million yuan per year),and south (8.8 million yuan per year).Croplands and tea plantations are mainly in the southeast (8.4 million yuan per year) and east(7.5 million yuan per year) with moist soils and moderate temperatures.

Fig.8 Ecosystem service values in 2018 adjusted by vegetation cover at different aspects

Ecosystem service values adjusted by vegetation cover at different slopes

From 2000 to 2018,the overallESVVCof different slopes displayed an upward trend,especially the rapid increases from 2010.ESV VC, from high to low:(25°?35°) >(15°?25°) >(5°?15°) >(35°?45°) >(0°?5°) >(45°?70°) (Fig.9),increased to a peak and decreased to zero (Fig.10).

Fig.9 Ecosystem service values from 2000 to 2018 adjusted by vegetation cover on different slopes

Fig.10 Ecosystem service values adjusted by vegetation cover on different slopes in 2018

The majority of grassland ESV is contributed by slopes in the 5°?15° range (18.6 million yuan per year),and in 15°?25° range (18.7 million yuan per year).For croplands,the ESV is mainly contributed from slopes of 0–5° (12.9 million yuan per year) and 5°?15° (16.1 million yuan per year).Croplands are close to areas with frequent activities and ESV descends rapidly in areas with slopes above 15°.The ESV of tea plantations is contributed from area with slopes of 5°?15°,15°?25° and 0–5°.Forest ESV was dominant in the WNPP,increasing from areas with slopes 0–35° and peaks at slopes of 25°?35° (795.1 million yuan per year,97.3% of 25°?35° slope areas).From areas withslopes of 35°?70°,the proportion of forest ESV in all vegetation types increased to 98.6% and 98.9%.

Discussion

Revised ecosystem service values adjusted by vegetation cover per unit area.

The ESV of a vegetation type may differ due to spatial heterogeneity (Su et al.2006).The evaluation of ecosystem services should consider vegetation cover changes.Several studies,using NDVI-based FVC,have revised the evaluation of ESV.In spite of the use of NDVI in vegetation cover change studies,there are limitations related to soil background brightness and easy saturation in high biomass regions (Zhang et al.2007).However,EVI provides improved sensitivity in high biomass regions (Jiang et al.2008).Therefore in this study,the ecosystem service values calculation was improved using EVI-based FVC (Lin et al.2020).The rising rate of inflation in China should be considered in ESV evaluations.Therefore,a dynamic model was created with consideration of the inflation rate and the enhanced vegetation index (EVI)-based–fractional vegetation cover (FVC) to improve the ecosystem service values estimation.

Wuyishan National Park Pilot is on the list of world cultural and natural heritage sites (Song et al.2020).Many governments and environmental protection agencies place great importance on the protection of world cultural and natural heritage sites.Therefore,for further study in the evaluation of ecosystem service values adjusted by vegetation cover,we need consider the different distances between the same type of vegetation and the cultural or natural landscape.For instance,the mother tree of Dahongpao in Wuyishan is more than 360 years-old and should have higher ecosystem service values than others (Song et al.2020).

The driving forces of ESV VCchange in WNPP

The ecosystem service values,as adjusted by vegetation cover (ESVVC) is highest in the northwest and lowest in the southeast in the WNPP.The driving force for changes of ESV includes constraints set by the geographical environment,by economic benefits that lead to changes in production behavior patterns,and by development and construction resulting from population and tourism growth (You et al.2017a).With an increase in altitude,aspects and slope,temperature,precipitation,biological environment,soil moisture,and light intensity changes are apparent.Topography is an essential element for determining vegetation habitat(Song et al.2005).

From 2000 to 2005 (Fig.3 a,b),the structure and function of ecological resources were not affected in the upper reaches of the Jiuqu Stream Ecological Protection Area(JSEPA) and the Wuyishan National Scenic Spot (southeast of WNPP).Therefore,theESV VCdecreased because of the change in vegetation cover and function induced by extreme drought;our findings are similar to You et al.(2017a).The results reveal that there was a favorable environment for tea growing in this area at 200?400 m a.s.l.,with slopes below 30°,and humid surroundings.Croplands are distributed near settlements and the Jiuqu stream.In this area,the ESV contribution of tea plantations increased and the forest ESV dropped.This demonstrates that there could be other factors,such as economic benefits,than geography to influence the distribution of tea plantations.With the socio-economic development that occurred in the 2000s,many local farmers became aware of the economic benefits associated with tea plantations.As a result,many areas of bare land,as well as some areas of cropland and low altitude natural forests have been exploited and developed into tea plantations.

With rapid economic development and efforts devoted to fight poverty,the demand for a better ecological environment is increasing,and deforestation in the JSEPA area has been banned.From 2005,the ESV per unit area of the southeastern region gradually increased,indicating the overall improvement of ecosystems (Fig.3 b).From 2015 to 2018,the ESV of grasslands and croplands in the northwest was constant at 0.8% and 1.6%,respectively,while the ESV of tea plantations decreased from 2.2% to 1.6% and forests increased by 0.5%.This is mainly due to the government’s water conservation efforts,ecological protection of the river source,and ecological restoration projects such as returning tea plantations to forests in Xingcun Town to gradually improve vegetation cover.The illegal tea plantations in the northwest were gradually restored to forests after 2015.

In the northwest of the WNPP,the government persisted in protecting the natural resources,ecological environment,history,and culture.Natural coniferous broad-leaved transitional forests and coniferous forests on Huanggang Mountain and the natural evergreen broad-leaved forests and coniferous broad-leaved transitional forests on Zhumu Mountain are typical primary vegetation.Therefore,the vegetation cover in the region was generally >80% (Lin et al.2020).The region contributed to the annual forest ESV >90%,indicating important ecosystem services from soil,climate,water quality and habitat.Thus,the government has promoted environmental protection and eco-tourism by trading-off the functions of various ecosystem services of the national park which help improve the overall ESV.

Conclusion

We developed a dynamic model for the estimation of ecological services values in the Wuyishan National Park from 2000 to 2018 and examined the spatial heterogeneity and change between topographic factors andESV VC. We found that the overallESV VCis increasing and the value of each service function from high to low is:soil formation and conservation >regulation of atmospheric gases >biodiversity maintenance >hydrological regulation >climate regulation >raw materials production >waste treatment >providing esthetic values >food production.From 2000 to 2018,the ecosystem service values of each vegetation cover showed a strong altitude gradient.Additionally,it significantly differed by aspect and slopes.Through the contrast analysis of ecosystem service vales over different times,the land use should be regulated,the policies of forest protection be implemented,and tea plantations restored to forests in order to optimize the vegetation cover for sustaining development.