#urban_vegetation
#urban_rural_differences
#vegetation_cover
(The vegetation cover was quantified as Enhanced Vegetation Index (EVI)).
In 2018, Shuqing Zhao analyzed the direct and indirect effects of urbanization on vegetation growth. The direct effect of urbanization is through land cover change. The indirect effect is through the urban environment. On average, the indirect vegetation growth enhancement in the U.S. offset 29.2%, 29.5%, and 31.0% of the growth reductions caused by impervious area replacement.
Background
There has been a debate on whether the urban environment enhances or suppresses vegetation growth. High air temperature, low soil water content are stress factors on vegetation. But fertilization, irrigation, introduction of non-native species, urban heat island, climate change, and atmospheric chemistry change like ozone and carbon dioxide were found to enhance vegetation growth by later studies. The authors previous study [Shuqing Zhao et al. PNAS 133 (22) 6313-6138 https://doi.org/10.1073/pnas.1602312113] already demonstrated that vegetation growth enhancement existed in 32 major cities in China. In this study, they would like to verify the same effect existed in USA cities.
Some useful information
The Northeast (NE) and Southeast (SE) U.S. are mainly covered by forests. The East North Central (ENC) and Central (C) are covered by a large cropland area. The West North Central (WNC) are the South are heavily covered by crops, pasture, and grassland. The Southwest (SW) is dominated by shrubs. The West (W) and Northwest (NW) are mostly covered by forests and shrubs.
Data
377 metropolitan statistical areas in CONUS
EVI 250 m product (MOD13Q1) from MODIS, averaged over the growing season (frost-free days) in the years 2001, 2006, and 2011
Impervious area was originally 30 m and interpolated to 250 m
The pixels that were water body, or had elevations > 50 m above the highest elevation of urban core (impervious percentage > 50%) were excluded to remove the effect of water body and elevation
Methods
(1) Definitions of the relative direct effect, the relative indirect effect, and the growth offset
The conceptual decomposition of the indirect and direct effects of impervious area is:
V_{obs} = (1 + \omega)(1 - \beta)V_v + \beta V_{nv}
, where V_{obs} is the observed vegetation index, V_{v} is the background vegetation index without urbanization, V_{nv} is the vegetation index of the pixel when it is completely impervious, \beta is the percent impervious area, and \omega is the effect of urbanization on vegetation growth. Note that the pixels with no impervious area may still have enhanced vegetation growth in an urban area, but the number of these pixels is relatively small.
V_{nv} was obtained as the mean EVI of the fully urbanized pixels over all the MSAs, and found equal to 0.0064. V_v was obtained by polynomial regression between V_{obs} and \beta (V_{obs} = V_v + a_1 * \beta + a2_ * \beta^2 + a3 * \beta^3).
The “background” vegetation growth, without vegetation impacts, can then be interpolated from V_v and V_{nv} as:
V_{zi} = (1 - \beta)V_v + \beta V_{nv}
, with V_{nv} being equal to 0.0064 in this study. So the relative direct urbanization effect on vegetation growth is:
\omega_d = (V_{zi} - V_v) / V_v * 100%
, and the relative indirect urbanization effect on vegetation growth is
\omega_i = (V_{obs} - V_{zi}) / V_{zi} * 100%
Finally, “growth offset” is defined as the ratio of the absolute indirect effect to the absolute direct effect, in order to quantify how the indirect effect compensates for or worsen the direct effect:
\tau = (V_{obs} - V_{zi}) / (V_v - V_{zi}) * 100%
(2) Definition of urban intensity
The urban intensity of a pixel is defined as the percentage of developed imperviousness surfaces in the pixel.
It seems they derived V_v separately for each city and each year, based on Fig. 2b and Section 3.2. However, it is not clear if they controlled for the land cover effect by separately deriving for each land cover type.
Results
Fig.2c shows quite clearly that in the vast majority of pixels, the V_{obs} is higher than the expected value from impervious area (V_{zi}) - i.e. “urban pixels are often greener than expected given the amount of paved surface they contain”.
The estimated \omega_i values appear to increase slightly with \beta, whereas the \tau values decrease slightly. That is, as the impervious areas become higher and the vegetation becomes sparser, the compensated growth becomes larger relative to the actual growth, but becomes smaller relative to the amount of missed growth due to the direct effect.
In terms regional differences, unsurprisingly, the unperturbed vegetation (V_v) is the highest for the forested NE and the cropped ENC and C, and the lowest for the arid SW and W.
In all the individual regions, the growth offset (\tau) decreases with higher impervious area (\beta). The SW has the highest \tau, whereas the C, NE, and SE have the lowest. This may be because the western U.S. is more heavily irrigated?
In the eastern regions (ENC, S, SE, NE, C), the relative indirect effect (\omega_i) increases with impervious area (\beta), but in the western regions (NW, WNC, W, SW), the relative indirect effect is the highest at medium impervious levels.
Discussion
Their results demonstrated the indirect effect, which was rarely done by past regional scale studies. Also, their results were consistent with most ground observations that showed urbanization to enhance vegetation growth. A few studies (e.g. in temperate zone cities in Europe) showed negative impacts of urbanization on vegetation growth.
The urban to rural gradient include factors like terrain, soils, species, air pollutants, temperature, CO2 enrichment, N deposition, ozone, and traffic volume. Therefore, the potential causes of influences are myriad and complicated.
The unique hump-shaped curve in the west may be because there are too little effort spent on vegetation planting on city fringes, and the amount of impervious area is too high to allow vegetation in the urban center. As a result, the medium dense areas, intentional planting and urban management effectively increase the vegetation index. On the other hand, man-made changes in species and management in humid cities would not cause any obvious increase in vegetation index, since the background is already quite high.
