29 May 2015, Mongabay news - Scientists at the Woods Hole Research Center in Massachusetts have developed  vegetation height maps for the entire tropics at very fine spatial scales. These  first-of its-kind high resolution maps can help researchers estimate forest  cover, monitor biodiversity and wildlife habitats, and manage and monitor  timber, according to a recent press  release by WHRC.

The WHRC researchers, led by senior scientist Josef Kellndorfer,  combined two active remote sensing systems – radar and lidar – to create these  maps.

These active remote sensors send out pulses of electromagnetic  waves – microwaves in the case of radar and light waves in the case of lidar - to map the earth's surface. Passive remote sensors, on the other hand, depend on  the sun to illuminate parts of earth, and then detect the energy reflected or  re-emitted by the different objects on earth.

Radar has the advantage of  being able to acquire images at fine spatial scales anytime of the day. It can  also penetrate through cloud cover, which is normally a handicap for most  passive remote sensors. Lidar, too, can capture earth's information anytime of  the day. Moreover, lidar emits pulses of laser beams, which can be used to  measure heights of objects on earth. This is similar to how bats use  echolocation to find food.

Height of tropical forests around the world. Image courtesy of Woods Hole Research Center.

Kellndorfer and his team combined more than 17,000 fine-scale radar images  obtained by the Japanese ALOS satellite, with height measurements obtained by  lidar on NASA’s ICESat/GLAS mission, and developed continuous vegetation height  maps for North and South America, Africa, and Asia for the year 2007.

“So we had tens of terabytes of data, using which we produced some 20,000  satellite scenes, each over 1 gigabyte in raw storage,” Kellndorfer told  mongabay.com. “So there was a big computational effort behind this. And in the  end, we were able to develop statistical models to combine both lidar and radar  observations to generate this kind of fine-scale, consistently produced  dataset.”

WHRC’s pantropic maps are at a spatial resolution of 30 meters  and can detect tree heights of up to 15  to 20 meters, the press release notes.  This is an improvement over the previously existing vegetation height maps that  were at coarser spatial scales, Kellndorfer said.

For instance, two  previous global vegetation height maps developed for years 2005  and 2004,  have spatial resolutions of one kilometer. 

According to Nancy  Harris, a researcher at the World Resources Institute, the finer scale maps  developed by WHRC can help researchers get a better sense of "texture" in tree  height data, such as how uniform the tree height cover is over the landscape. “That might tell us something about what types of trees are growing there and/or  how they are being managed,” she added.

In fact, detailed vegetation height maps have numerous applications. For  instance, they can be used to assess global forest cover.

“Many forest  definitions include a height criterion in addition to crown or canopy cover. For  example, trees must be taller than 5 [meters] in height,” Harris said. “From my  understanding, many remotely sensed products of tree or forest cover use canopy  cover as the main criterion with height much more loosely defined. This height  dataset could be a useful supplement to those efforts.”

Kellndorfer  added, “The nice thing with this dataset is that it offers a consistent,  unbiased way to check forest cover.”

Vegetation height data can also be  useful for various other forest management actions, such as monitoring areas  that have natural forests, and those that have plantations. For example, an area  where tree height is uniform is more likely to be plantation, Harris said. And  where it is more varied, it is more likely to be a natural forest.

Vegetation height data can also be used to measure biomass, assess quality of  wildlife habitats and corridors, and measure and manage timber, Kellndorfer  added.

Despite the advantages of the fine spatial resolution and global  coverage, WHRC’s maps have some limitations. For instance, lidar sensors have  incomplete spatial and temporal coverage, capturing information from only a  small part of the earth’s surface over a multi-year mission. Moreover,  environmental effects like rain and other seasonality can lead to uncertainties  in the dataset, Kellndorfer said. “So as people start using it, they will find  that it works better in some regions than others.”

But these limitations  can be overcome in the future through missions such as the NASA-ISRO  SAR Mission, which aims to acquire global radar images rapidly. A similar  mission for lidar called the Global Ecosystem Dynamics  Investigation Lidar or GEDI, will have much denser observation points for  lidar, Kellndorfer said. “And with such missions, there is potential to create  global vegetation height maps annually.”

Another constraint with the vegetation height dataset is that it works well for  trees with heights up to 15 to 20 meters, depending on location. “This is  because of the sensitivity of radar at those heights with the particular  approach we’ve used," Kellndorfer explained.

This could limit the  applicability of the global height maps, Harris noted. “The vast majority of  tropical forests, and even other types of woody vegetation, are taller than 15  meters, even when young,” she said. “So the new data for the tropics are likely  to be most useful for monitoring trees outside forests or regenerating forests,  or trees that may be associated with restoration or plantation activities than  monitoring the status of much taller primary forests.”

However, Harris  added that WHRC’s vegetation height data could be useful for WRI’s Global Forest  Watch, an online forest cover monitoring platform.“Currently we have a previous  version of tree height as a basemap layer on GFW,” she said. “But it could be  useful to overlay the new height data with a tree plantations layer we are  finalizing and releasing over the next few months to see if this information  could shed additional insight as to where plantations exist as opposed to  natural forests.”

Source:  http://news.mongabay.com/2015/0529-mrn-dasgupta-new-maps-show-tropical-forest-heights.html#ixzz3dNQXyRgG