Vegetation and Agriculture

After water, vegetation is the next most common land cover, including natural, agricultural, and other human-modified plantings and ranging from formal gardens, to tropical forests, to tundra. Given that most active, emergent vegetation is green, visible color is often less important for interpretation than are other visual clues such as size (height), shape, pattern, texture, and context. These factors are especially important for interpretation of wetland vegetation (Fig. 10-17).

In the case of forest canopies, for example, the size, shape, and pattern of tree crowns are most noteworthy in addition to seasonal phenological characteristics (Murtha et al., 1997). Furthermore, it should be understood that nearly all vegetation is managed, manipulated, or altered by human activities in various obvious or subtle ways, which impart additional distinctive aspects for photo interpretation.

Consider this view of mixed conifer and deciduous forest in north-central Poland (Fig. 10-18). Both conifer and deciduous trees are present in distinct stands. The conifers are dark green to blue-green in color and conical in shape. Deciduous trees, in contrast, are light green to yellow-green in color and have billowy or tuffed crowns; some display autumn colors (right background). In addition, linear and rectangular boundaries mark some stands, which represent artificial plantings. The same view taken in winter would show even greater contrast between the conifers and bare deciduous trees.

As another example, regard this view over bottomland forest of the Missouri River floodplain taken in early spring (Fig. 10-19). Cottonwood (Populus deltoides) trees display green flowers, but leaves have not yet sprouted. The dark appearance of the forest floor is because the understory was intentionally burned a few days before the picture was taken. The cottonwood trees are arranged in curved, linear patterns that reflect natural morphology, soils, and drainage on the floodplain surface. To the right, an open, green strip marks an abandoned river channel that is too wet for trees to grow. Knowing the vegetation phenology, human activity, and geomorphic context are all essential for interpretation of this image.

The same applies for the interpretation of the vegetation patterns in Figure 10-20, where shrubs can be seen encircling a grassland area in wavy formations. The image was taken in a small catchment in the mountains of the Spanish Pyrenees, which was arable land until the second half of the twentieth century. This particular field, which is bordered by grove-hemmed brooks and stone walls, was abandoned in the 1970s and only occasionally used for sheep grazing since. Subsequently, a spiny broom common in Spanish matorral (Genista scorpius) began invading the enclosure from the side, divulging its ballistic dispersal mechanism in a wavy encroachment front.

Agricultural fields are human-managed vegetation plots, ranging from fruit orchards to rice paddies, that typically have sizes, shapes and patterns determined by property boundaries as well as drainage and contour of the land. Where soil is fertile and land is relatively flat, people tend to impose various geometric field shapes—square, rectangle, circle, etc. (Fig. 10-21). Where land is steeper, terraces and

FIGURE 10-14 Rill erosion on a field tilled one year previously in the Bardenas Reales, Province of Navarra, Spain (field of view ~ 60 m across). The tillage pattern of ridges and furrows curving around the sloping edge of the field is dissected by dendritic rills that merge into deeper rills following the circular pattern left by the tractor. In the corner of the field, the two rills upslope of the crawler transporter (loaded with water cans for soil erosion experiments) are redirected diagonally to the gradient by the furrows. These rills are significantly deeper and more linear. In the lower left corner, sheep trails can be seen between the patchy vegetation cover. The trails converge at the field border where the sheep have to cross a trench. Kite aerial photograph by IM, JBR, and S. Plegniere, February 2009.

FIGURE 10-15 Rill erosion on the field adjacent to Figure 10-14. (A) Complex pattern of tillage furrows resulting from repeated crossings by the tractor. The field slopes slightly from top to bottom in the image; the upper part of the gully and the tributary rills from the right all run more or less oblique to the inclination and follow the furrows of the last tillage operation. The lower gully part not only follows the slope direction but is also predetermined by an underlying tillage pattern. A chain of piping holes (circled in red) indicates the subsurface rill course in this older tillage direction. Kite aerial photograph by JBR, M. Seeger, and S. Plegniere, March 2007. (B) From the ground perspective, the destructive effect of gully erosion becomes clear. Such erosion forms are already too deep to be leveled by plowing. The junior gully researcher measures ~70cm. Photo by JBR.

FIGURE 10-16 Close-up vertical shot of steep mountain slope with turf-banked solifluction terracettes in the Spanish Pyrenees near Tramacastilla de Tena, Province of Huesca, Spain. Slope descends from right to left; blimp flyer near scene center for scale. Hot-air blimp photograph by IM, JBR, and J. Heckes, October 1995.

FIGURE 10-16 Close-up vertical shot of steep mountain slope with turf-banked solifluction terracettes in the Spanish Pyrenees near Tramacastilla de Tena, Province of Huesca, Spain. Slope descends from right to left; blimp flyer near scene center for scale. Hot-air blimp photograph by IM, JBR, and J. Heckes, October 1995.

FIGURE 10-17 Plant heights, patterns, textures, and color variations are visual clues for recognizing distinct vegetation zones in these coastal wetlands. (A) Mississippi Sound, United States. (B) Island of Vormsi, Estonia. Kite aerial photos by JSA and SWA, March 2004 and August 2000.
FIGURE 10-18 Low-oblique, autumn view over mixed forest near Mlawa, north-central Poland. Kite aerial photo by JSA and D. Galazka, October 1998.
FIGURE 10-19 High-oblique, early-spring view across the Missouri River valley floodplain forest at Fort Leavenworth, Kansas, United States. Kite aerial photo by JSA, April 2000.
FIGURE 10-20 Abandoned farmland in the Arms Catchment in the Spanish Pyrenees near Jaca, Province of Huesca, Spain. Note wavy vegetation front marked by dashed white lines. Hot-air blimp photograph by IM, JBR, and M. Seeger, August 1998.
FIGURE 10-21 Rectangular shapes and patterns are most common in agricultural fields. (A) Blue Hills of west-central Kansas, United States, May 2006. Rectangular fields cover 80 acres (~ 0.2 km2) each. (B) Foreland of the Tatra Mountains, Slovakia, August 2007. Kite aerial photos by JSA and SWA.

contour plowing follow hill slopes to minimize erosion and retain water (Fig. 10-22). Rocky, sandy, steep, or erosion-prone lands, as well as lands too dry or wet for crops, are normally left in pasture or range for hay or livestock grazing (Fig. 10-23).

Most agricultural crops are annual monocultures, for which knowledge of crop phenology and local agricultural practices are essential for successful interpretation (Ryer-son et al., 1997). As an example, consider winter wheat, a major crop in the High Plains of southwestern Kansas.

FIGURE 10-22 Terraced fields on sloping land under fallow conditions. Dark stripes represent water and wet soil behind terraces following heavy rain. Smoky Hills, central Kansas, United States. Kite aerial photo by SWA and JSA, May 2006.

FIGURE 10-23 Grazing lands in the High Plains of western Kansas, United States. (A) Dry rangeland in a drought year, June 2006. (B) Sand hills terrain in a wet year, May 2007. Kite aerial photos by SWA and JSA.

The wheat is planted and begins growing in early autumn, overwinters, grows rapidly in the spring, and is harvested in early summer. From autumn until late spring, it is the only active crop in the region. Furthermore, the semi-arid climate limits winter wheat growth in non-irrigated fields to every other year. In other words, each field is used for growing wheat one year and stands fallow the next year in order to accumulate soil moisture. This practice results in a patchwork of fallow fields intermingled with crop fields, which alternate every other year (Fig. 10-24).

Active, green vegetation has quite distinct spectral characteristics (see Fig. 4-16), which are most obvious in color-infrared photography (Fig. 10-25). The strong near-infrared (NIR) reflectivity of active "green" leaves was discovered a century ago, and is known as the Wood Effect after Prof. R.W. Wood who first photographed this phenomenon in 1910 (Finney, 2007). NIR radiation (~0.7-1.3 mm wavelength) is strongly scattered by leaf cell walls, and some NIR energy passes through an individual leaf and may interact with adjacent leaves or soil (Fig. 10-26). Red and blue light are absorbed by chloroplasts for photosynthesis. Thus the ratio of NIR to red is an indicator for

FIGURE 10-24 Winter wheat fields, fallow fields, and wind turbines on the High Plains of southwestern Kansas, United States. Kite aerial photo by JSA and SWA, April 2006.

photosynthetically active vegetation. This gives rise to several vegetation indices, such as the normalized difference vegetation index (NDVI), which are important for ecological studies of biomass, leaf area index, and photo-synthetic activity (Tucker, 1979; Murtha et al., 1997).

Aerial photography is widely employed for interpreting and monitoring vegetation conditions (e.g., Baker et al., 2004; Imeson and Prinsen, 2004; Lesschen et al., 2008) and making estimates of crop yield, forest production, and similar purposes (e.g., Grenzdorffer, 2004; Jensen et al., 2007; Berni et al., 2009). SFAP is used increasingly by farmers to help increase crop yields in conjunction with precision agriculture. Impacts of drought and flooding

FIGURE 10-25 Color-infrared photograph of the Arkansas River flood-plain and recreation area near Pueblo, Colorado. Active vegetation appears in bright red, pink, and magenta colors. Kite aerial photo by JSA and SWA, May 2003.

FIGURE 10-26 Schematic illustration of sunlight interaction with a healthy green plant leaf. Most near-infrared is reflected from leaf cell walls, and is not affected by chlorophyll. Blue and red light are absorbed by chloroplasts (black spots). Adapted from Murtha et al. (1997, fig. 5.7) and based on Colwell (1956).

FIGURE 10-26 Schematic illustration of sunlight interaction with a healthy green plant leaf. Most near-infrared is reflected from leaf cell walls, and is not affected by chlorophyll. Blue and red light are absorbed by chloroplasts (black spots). Adapted from Murtha et al. (1997, fig. 5.7) and based on Colwell (1956).

FIGURE 10-27 (A) Floodplain of the Hornad River. Overflow channels (c) are revealed clearly in this fallow field, which was flooded one year before. The white building on the floodplain is a water well for the city of Kosice. Southeastern Slovakia with Hungary across the river in right background. Kite aerial photo by JSA, SWA, and J. Janocko, August 2007. (B) Ground view of Hornad River flood in 2006; photo courtesy of J. Janocsko.

FIGURE 10-27 (A) Floodplain of the Hornad River. Overflow channels (c) are revealed clearly in this fallow field, which was flooded one year before. The white building on the floodplain is a water well for the city of Kosice. Southeastern Slovakia with Hungary across the river in right background. Kite aerial photo by JSA, SWA, and J. Janocko, August 2007. (B) Ground view of Hornad River flood in 2006; photo courtesy of J. Janocsko.

(Fig. 10-27), disease and insect infestation, and many other factors can be evaluated to various degrees in all types of SFAP—panchromatic, color-visible, and color-infrared.

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