Nearly all the vegetation biomes identified in the global map at the top of page 3-1 are found in North and Central America, as evident in the next illustrative map:
Some of these biomes are predominately composed of evergreens. There are two types of evergreens: Evergreen Needle Leaf (pine, spruce, fir, etc.) and Evergreen Broadleaf (laurel, magnolia, etc.). The pines are usually coniferous (cone-bearing) and in North America are widespread. Pine habitats fall into three main groups: Boreal Forests (mainly in Canada); Mountain Forests (further to the south); and Southern Forests (southeastern U.S.). Although examples from all three are included on this page, the concentration will be on Boreal Forests. In Asia, mainly Siberia, these are also called Taiga. These are the second most widespread plant biome type in the world, as indicated in the next map.
In western Canada's Boreal Forest the most common type of tree is the Lodgepole Pine. Its distribution is vast and many forests are almost continuous stands of these trees (next figure). They are a main source of lumber (as are the other pines mentioned above) and a very valuable commodity. Threats to their normal existence, such as fires and disease, can have a huge economic impact. On this page, we shall dwell upon just one specific and serious problem - the infestation of the Mountain Pine Beetle (MPB) - as a case study showing how remote sensing is helping.
Various species and genera of beetles are known to attack pines and other evergreens. Among common ones known by their non-taxonomic names are the Southern Pine Beetle, the Douglas Fir Beetle, the Spruce Beetle, the Turpentine Beetle, annd the Pinion Ips. Here are five different species in two genera.
The beetle ravishing the western Canadian boreal forest is the Mountain Pine Beetle, Dendroctonus ponderosae, a small (less than 0.5 cm) shown here:
A good, reasonably brief webpage that provides an overview of the MPB problem is this Beetle Infestation website prepared by the Alberta (Canada) Forest Service. The main stage in the killing of a pine tree by the MPB is known as a Red Attack, a name evident from the color of this Lodgepole pine after the disease runs its course.
The MPB usually begins as an early stage known as the Green Attack. The Red Attack is concentrated during the second growing season (sometimes over two years in high elevations). The life cycle during the main season is shown here:
Here is a tree trunk surface showing larva evolving into pupae:
During much of the attack, the MPB dwells in the inner bark. It then proceeds into the inner tree by tunneling to deposit its eggs. This is a sawed surface exposing the interior with borings and eggs:
The tree fights the MPB by secreting resin on its surface producing pitch tubes seen in this outer surface view.
Later in the process further damage is done by the invasion of Bluestain fungi, which respond to the activity of the MPB, as evident in this exposed cut into a dead pine:
The final stage, in which the needles become inert and fall is known as the gray stage.
Now to field views of MPB infestation. The next group of field aerial photos shows stands of Lodgepole pine in British Columbia that have been infested by the MPB. Read the captions for further information:
Once the MPB damage becomes widespread, the main recourse is to cut down the diseased trees, thinning the forest or even making it barren. Some of the trees can be partially salvaged for lumber or other uses.
Others are too far gone and must be burned:
The "trick" is to catch the onslaught of the MPB disease before it progresses too far. One obvious approach is to spray individual trees. The two most effective sprays are Carbonyl (Sevin) and Permethrin (Astro). An alternative that often helps is to inject the tree with chemicals that provide some resistance.
But the humongous numbers of pines involved make individual tree spraying labor-intensive and usually impractical. The standard solution is aerial spraying, preferrably in the early stages of the MPB infestations.
British Columbia at present is the most threatened part of the Boreal Forest in Canada. Here is a map showing the extent of the region affected.
As stated above, infestations by various beetle species is ubiquitous in North America and worldwide. Here are two examples of damaged pine forests in several regions of the United States:
Prior to the plethora of earth-observing satellites, detecting and monitoring the onset and spread of MPB tree-destroying activity was carried out through aerial surveys and reconnaissance flights. Here is what a forest agent would see from a plane passing over early stage infestation.
Experience with color IR film showed a greater sensitivity to the progress of the disease inflicted by the MPBs; diseased trees in blue.
Landsat has been a major means of watching the progression of MPB blight over large areas. Here is a Landsat subscene in which Maximum Likelihood classification has pinpointed areas suffering MPB damage at the time.
Terra's ASTER sensor provides broad area coverage of MPB infestation, shown in shades of olive brown, in part of British Columbia.
Terra's MODIS sensor has been used to produce and periodically update maps enclosing areas experiencing beetle damage:
IKONOS high resolution imagery covers smaller areas but provides high detail, so that these images are comparable to aerial photos.
This pair of images compares areas identified as in the Red Attack phase as extracted from an aerial photo (left) and an IKONOS subimage right.
Satellite imagery is used to calculate NDVI (see page 3-4) whose values deviating from normal are indicative of tree stress. For the British Columbia infestation, Canadian foresters have devised the Enhanced Wetness Difference Index (EWDI), which uses temporal change detection (multidate) imagery to determine MPB damage. Here is a typical result:
The current infestation in Canadian pines has been intensifying in the past decade. This has been attributed to warmer average winters (intense cold can kill the beetles) and less summer moisture (which decreases wetness). This is now a general condition in western North America. Foresters in the U.S. began to note MPB problems in some of the western states:
By 2005 this condition had affected the entire western U.S.
Coincident with this increase is a conclusion reached by experts monitoring drought conditions that the U.S West is experiencing the worst sustained drought in 500 years. Consider this map:
Many experts attribute this drought situation to global warming - the MPB infestation being one of many indicators. We shall examine other evidence for global warming on.