Accuracy Data on Gain and Loss of Canopy on GlobalForestWatch.org

Does Pugu Forest Recover after all development efforts have failed and the forest seems forgotten? Or does the remote sensing technology create illusions?

Read more: Accuracy Data on Gain and Loss of Canopy on GlobalForestWatch.org

Global Forest Watch Interactive Map

A powerful tool to monitor forest canopy Gain and Loss worldwide is provided by Global Forest Watch.The Gain and Loss of of canopy on the interacctive map for the period 2001 - 2012 is investigated on basis of the historical imagery on Google Earth and the results are discussed here

Pugu Forest Recovering?

Pugu Forest Recovering?  

PuguForestSouthernSectionJune2010 PuguForestSouthernSectionMay2014
The aerial photographs of the southern section of the Pugu Forest from June 2010 and the same area in May 2014 suggest that notwithstanding the adversities challenging this small oasis of greenery just outside the exploding Megacity of Dar es Salaam is actually getting greener. It would be a thrilling development after all planned development efforts have failed to save the Pugu Forest, to see Nature bouncing back.

The most tempting explanation could be the success of pioneer invaders like the Cassave Tree and Iron Wood still planted by the authorities in combination with some good rainfall.

Analysis of satellite images from April 2004 and July 2014 support the suggested recovery since 2010. The data from the Terra Spacecraft which flies a sensor called the Moderate-resolution Imaging Spectroradiometer, or MODIS. greatly improve scientists’ ability to measure plant growth on a global scale. Every 16-days an image is collected up to a resolution of 250m.

Surprisingly the Enhanced Vegetation Index (EVI) data for April 2004 and July 2014 also suggest increased vegetation in the rainy season of 2014 compared to the rainy season of 2004. The EVI values of 0.8 and above can be considered to represent dense forest and are the darker blocks equivalent to 250m squares. In the images below not only the EVI values for the Pugu Forest in 2004 and 2014 are illustrated, also the difference between the EVI or increase and decrease in vegetation per pixel or 250m square. Google Earth imagery an other free source for aerial photographs covering the Pugu Forest during the same period as the MODIS imagery is compared for counter checking obtained data. Unfortunately the suggested increase of vegetation as suggested by the EVI values is not fully consistent with the observed development on the Google Earth historic imagery covering the period 2004 - 2014.

normal_09483-Trema-orientalis
The Successful Pigeon Wood (Trema Orientalis)
PuguForestEVI2004JulyLegend
PuguForestEVI2014AprilLegend
IncreaseDecreaseEVI2004-2014Legend
 

Checking areas with high and low EVI values on the Google Earth imagery of the same periods as the satellite images from Terra results in some dissappointment. Although the location of EVI pixels correspond well with features on the ground like the urban development around Pugu Forest which clearly provides increasingly low EVI values reflecting the continuous expansion of houses, the high density forest patch suggested by the EVI>0.8 and EVI<0.3 pixels selected for July 2004 do not match the condition of the forest according to the Google Imagery, a same conclusion is reached with counterchecking other EVI measurements in many locations on the historic imagery.

For example the vegetation "hotspot" in the middle of Minaki Settlement raises doubts about the MODIS accuracy. Also analysis of the areas of major vegetation increase on the image above with EVI value increase up to 300%, does not correspond well with the actually situation ground.

ExampleEVI0.3PuguForest2004July
An interesting interactive global map provided by Global Forest Map enables monitoring of Forest Loss and Gain at global level between 2001 and 2012. The analysis is based on the LANDSAT ETM+ imagery. A screenshot of the map section covering the Pugu Forest is at the left (to enlarge click on the image). The Julius Nyerere Airport is at the right and the Pugu HIlls Nature Centre in the centre of the image. The Pugu Forest Reserve is the blue polygon. For an interactive version of the map click Global Forest Watch Interactive Map. 2001-2012ForestGainLoss-Pugu-Forest
It may not be conclusive from the remote sensing technology available but the Pugu Forest is at least not as rapidly dissappearing as the trend between 2000 - 2010 predicted.