Where Did Trees Go?

Deforestation is really serious at this time and most of
the land leave few trees with other soil erosion and desertification.
http://www.skogur.is/english/forestry-in-a-treeless-land/

Sheep: the most common livestock in Iceland,
also the culprit of eliminating birch forests
http://www.flickr.com/photos/43021596@
N00/234876383

When Vikings first arrived in the late 9th century, they found arctic birch forests, coastal sea mammal and sea bird colonies, migratory nesting birds, and green grass that could feed their imported domestic livestock(Ashwell 1970)[4]. According to Iceland’s historical records, the first Viking visitors found, to their surprise, that the island had fertile soil and large forested areas, which made it resource-rich territory. In Norway and Sweden, where Vikings used to live, the forest recovered from overexploitation (Kissane.1988)[5]. So they mistakenly started Norwegian traditional practices of farming in this new island: cutting down trees for firewood, exploiting land for agriculture, and grazing livestock imported from Europe, especially sheep. Because they let the livestock find food for themselves, the birch forest was totally denuded. Since livestock have neither predators nor competition, they reproduced rapidly. Sheep ate all the vegetation they could get, including grass, dwarf branches off the tree, as well as bark. Without branches and bark, birch trees almost died out. Meanwhile, the climate became cooler because of a mini ice age, and a volcanic eruption left ash on the soil, so desertification became a serious problem as it changed soil properties in Iceland. Although volcanic soil is known to be very fertile and extensively good for vegetation growth, it is also vulnerable to physical disturbance, pesticide pollution, and heavy metals (Oskarsson, 2007)[7]. Because of these specific features of volcanic soil, the land could not grow any trees or even weeds.

Sorry about the resolution but I cannot find an original figure.
This is the comparison of vegetation in Iceland between the time
when first settlement and present times.
We can see from the first picture, most places are dark green, which means
the land covers by forests. From the second picture, most of the land change to
light yellow, which is almost barren land.
http://www.cbd.int/doc/world/is/is-nr-01-en.pdf

Besides the destruction caused by Vikings’ livestock, the forests were also heavily exploited for firewood and timber (2000). Excavated ruins of Viking settlements indicate that the wasting of timber was serious. Much of the wood was either burned for no reason or left in unused piles. They had no concept of "multi-using," which pertains to the efficient use of resources. Many of those farms were abandoned, and are still today, and so three-quarters of Iceland's land is affected by soil erosion, with 18,000 km2 so seriously damaged as to be useless, which is about the area of 10 New York Cities (Montgomery, 2007)[1]. Before humans arrived in the island, about 25 to 40 percent of the land was covered by forest. Now, only 1 percent of the forest is left, and only a few small birch trees exist in isolated reserves.