| Measuring drought and drought impacts in Red Sea Province |
|8. Changes in tree density on five sites in Red Sea Province: early 1960s to 1989. Roy Cole|
Five sites in Red Sea Province were chosen to compare present tree densities with densities counted from vertical aerial photographs of the same sites at a scale of 1:40000 in the early 1960s. Map 8.1 below shows the distribution of the study sites around the province.
The first two sites were chosen because of their proximity to Port Sudan and urban demand. The third, fourth and fifth sites, on Khors Oko, Sitareb, and Nubahawayb were chosen for their distance from Port Sudan. The Khor Oko site is located on the other side of the Red Sea Hills northwest of Port Sudan in one of the most thinly populated, remote and arid parts of the Province. The Khor Sitareb site is located south-southwest of Port Sudan in a remote mountain valley on a minor tributary of Khor Sitareb. Site five, in a minor tributary of Khor Nubahawayb, is located on the western periphery of the Khor 'Udrus charcoal production zone west-southwest of Port Sudan. It should be emphasised that the study sites were not selected to be representative of the whole of Red Sea Province and any statements made in this paper refer only to the study sites.
Two criteria were used in selecting the sites from the 1:40000 photographs:
1. Individual trees had to be distinguishable on the photos but not too dense to cause clumping.
2. The location of the study sites had to be in areas where change in khor flow could not be responsible for changes in tree density. Expansive floodplains with wandering watercourses were avoided and sites were chosen in relatively narrow valleys where landforms controlled flow in a stable fashion.
The trees were counted on the photographs using a 30X illuminated magnifying scope. In the field all vegetation was enumerated with the exception of the ubiquitous Indigofera spinosa and Aloe abyssinica on Site Five. In the calculation of density, however, only trees with a crown diameter greater than one meter were included because trees one meter or less in diameter were not visible on the aerial photographs. The minimum resolvable diameter was calculated by measuring railroad carriages on the photographs (2.5 by 11 meters) and comparing the measured distance with visible tree crowns. Annual or perennial ground cover, where occurring, was not enumerated as being not pertinent to the study.