Mapping the urban phenological footprint
Urban areas are renowned for their admixture of species and vegetation types that can change from one parcel to the next. Yards and woodland parks intermix with road medians--all of which may be dominated by an irregular mix of native and exotic trees, shrubs, herbs and grasses. In cities, the vegetation of nearly every block is compositionally complex.
These species green up at different times and at different rates. Because of this, it can be difficult to decide when spring occurs. As most ground-based observations of greenup take place where people live, this raises questions about the representativeness of spring indicators as a measure of greenup across the broader vegetational landscape.
ForWarn's definition of greenup presumes that there is a rise in greenness, and in dense evergreen vegetation, that is by definition a non sequitur. Some climates and vegetation types simply don't have the four classical seasons of winter, spring, summer, fall, so ForWarn's model green-up profile may not always apply. In addition, ForWarn's definition is vegetational rather than species-specific, so the day-of-year provided may not correspond to individual herbs, shrubs or trees. Third, ForWarn's definition is locally adapted, such that the vegetation present in each MODIS pixel can essentially define its own start of greenup according to the dominant species' adaptations to the local climate. Because of that built-in flexibility, ForWarn detects remarkably fine patterns of temporal variance within cities and in their surroundings.
Despite having a large assortment of native and exotic species, grass represents no small part of most urban vegetation cover (and therefore greenup). Greenup patterns can reflect the importance of grass across parks, lawns and roadways relative to the urban forest. In many cities, patterns in greenup dates reflect distinct "phenological neighborhoods" that result from a combination of cover types and microclimates. Microclimates grade from the warmer downtown Central Business District to the Wildland Urban Interface or Intermix. Urban heat from buildings and asphalt may accelerate the start of greenup compared to the surrounding landscape.
These maps show remarkable urban phenological footprints based on ForWarn's start of greenup date. This date is based on when the Normalized Difference Vegetation Index (NDVI) reaches 20% of greenup from its late winter minimum toward its mid growing season maximum. Differences within and among cities and their surroundings reflect a combination of the seasonal response of different mixtures of vegetation, topography or the urban heat island effect, although these are often difficult to disentangle.
INDIANAPOLIS, IN--The greater Indianapolis metropolitan area extends well beyond Marion County, as evidenced by the remarkable contrast between corn-soybean fields that green up late (in blue) and developed area (in green). The riparian deciduous forest shows up mostly as shades of yellow, orange and red.
CENTRAL ILLINOIS--Much like central Indiana, central Illinois shows sharp contrast in greenup dates among urban areas and surrounding croplands. Note the structure in the crop-dominated landscapes in shades of blue that results from the road system and different vegetation along roadsides and in the fields.
CHICAGO, IL--The phenological footprint of greater Chicago extends over several counties adjacent to Lake Michigan. While most of the residential area starts to green up in early April (dark green), the denser areas are delayed by a week or two. Remarkably, this delay (in red) is also associated with areas of more natural vegetation, most prominantly the sand dunes along the south edge of Lake Michigan near Gary. To the west, corn and soy fields are most delayed as is expected with annual crops that need to be mechanically planted (shown in blue).
COLUMBUS, OH--Columbus falls near an ecoregional zone that includes more forested vegetation to the east and more extensive cropland to the west. The city itself occupies most of Frankin County and shows small inliers of late greenup within an otherwise homogenous sea of green.
MINNEAPOLIS-ST. PAUL--Minnesota lies farther north than the other cities in the Midwest described above, so it is no surprise that later-season orange and reds replace the March-April greens of the urban landscape. Subtle structures within this urban area reflect differences in density of vegetation and impervious surfaces. Note the phenology of the gridded field-road system in the lower left that is probably due to distinct roadside fencerow and drainage ditch vegetation.
OMAHA-LINCOLN, NE-The urban vegetation of Omaha greens up considerably earlier than the forests of the Missouri River bluffs, the regional riparian vegetation and croplands. The red area west of Columbus is an extensive area of sand dunes along the river.
LITTLE ROCK, AR--Arkansas is farther south than the cities highlighted above, and lower latitude of Little Rock brings a mid-March date to the start of greenup there. The deciduous forest to the north, west and south greens up later, but not nearly as late as does the agricultural land to the east (in blue).
BATON ROUGE and NEW ORLEANS, LA--It is difficult to even detect where Baton Rouge is from this phenological map, as its vegetation blends in with its surroundings. These two southern cities green up early, with large portions of New Orleans greening up in late March (inside the loop of the Mississippi River). Note how early the bottomland wetland forest is between them and amid the agricultural areas along the river shown in red.
ATLANTA, GA is barely distinguishable from its surroundings starting greenup in late March to early April, although the densest part of downtown Atlanta shows up in pink, weeks earlier than the heavily forested residential areas.
ASHEVILLE, NC sits in a mountain valley between Mt Mitchell--the highest peak east of the Mississippi at over 6,600 ft.--and lower peaks to the southwest. At 2,000 feet elevation, downtown Asheville starts to greenup by late March, but the deciduous forest that lies below the high elevation spruce fir zone doesn't start green-up until late April to early May, over a month later.
WASHINGTON DC--The most developed portions of Washington and Arlington show up as pink, being particularly early, but the phenological footprint of this urban area is far less distinct from the surrounding vegetation than is seen in the Midwest.
NEW YORK--The complex development of the greater New York City area contributes to the fragmented start of greenup seen here. The densest areas are neither particularly early or late.
STATE COLLEGE, PA--Both State College and Altoona green up earlier than the surrounding fields, and substantially earlier than the oak dominated forests of the ridgelines, sandy soils southwest of State College and the higher elevation Allegheny Plateau.
MISSOULA, MT--This valley city is surrounded by strong phenological gradients that relate to the timing of snowmelt and greenup with elevation. While these causes can be hard to separate using NDVI history alone, there are at least three months of greenup onset dates represented in this landscape.
ALBUQUERQUE, NM--The urban vegetation of Albuquerque differs from the area around it. In desert areas, greenup is not so dependent on warming temperatures as it is in the east and northern parts of the West, but on precipitation. Climate, elevation, irrigation and vegetation types all contribute to this remarkably complex landscape pattern.
To explore the typical greenup date for other cities, click on Track this Event below.
Start of greenup in natural vegetation:http://forwarn.forestthreats.org/highlights/917
Start of greenup in agricultural areas: http://forwarn.forestthreats.org/highlights/918