Israel's forests have been declining for years, and now a groundbreaking study has found a correlation between increasing tree mortality and the mounting incidence of drought.
During arid periods, the study pointed out, not only are forest fires more frequent: the trees are also more vulnerable to harmful insects.
The study, "A nationwide analysis of tree mortality under climate change" was published in the Elsevier journal Forest Ecology and Management. The research was conducted Dr. Tamir Klein of the Department of Plant and Environmental Sciences at the Weizmann Institute in Rehovot, with members of the Jewish National Fund and Prof. Gabriel Schiller of the Volcani Institute for Agricultural Research.
The researchers' sources to estimate the extent of tree mortality in Israel from its establishment in 1948 to 2017. include periodic JNF reports on damage to trees and its causes, surveys of the forests, and satellite images. They also gathered information on climate conditions, including rainfall, which they cross-referenced with data about the condition of the forests.
Worldwide there is a lack of precise information about the extent and causes of tree mortality: the Israeli study was one of the first to collate information at a national level, its authors say. What the reseachers found was a clear correlation between hot, dry years and tree mortality.
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The area of dying and dead trees totaled about 65,000 dunams, out of Israel's total forest area of 1.6 million dunams (about 7 percent of Israel's total area). Ostensibly that does not sound like much. But worryingly, mortality increased in the decade of 1990 to 2010.
Of all the trees that have died since Israel's founding in 1948, no less than 73 percent died during that decade - and under the conditions of climate change, the entire region, Israel included, is expected to become not only hotter, but drier, perhaps irrevocably so.
Meanwhile, the last 30 years have been the hottest and driest in recorded Israeli history and have featured the worst forest fires, including the conflagration in Sha’ar Hagai near Jerusalem in 1995, and the fire on Mount Carmel in 2010, in which 44 people were killed.
Analysis of the causes of tree mortality concluded that about a quarter was directly related to drought: two-thirds of all tree deaths were during drought years.
Fires caused 58 percent of the tree deaths, and snow caused another 13 percent, by leading to their collapse.
Another problem is pests, such as beetles that bore into and damage the bark. The drought apparently also affected the ability of trees to withstand insects: 54 percent of pest-related mortality occurred in years with little rainfall.
The researchers also examined the possible influence of forest fires during the Second Lebanon War, and concluded that in the final analysis it they affected only limited areas. In recent years there have been large fires caused during army training activities, but they didn’t harm forest areas, but mainly grazing areas and weeds.
Another finding is that forests with pine and eucalyptus trees were more badly damaged than trees with broad leaves, including mainly local species such as oaks and terebinths. This is not surprising, since it is known that the local species are better able to withstand dry conditions, to which they have adapted over thousands of years.
“The state of the forests in Israel is still good, and the local forestation project is unique in its scope. Still, the longer the present trends continue, the greater the risk for the forests in the future,” says Klein. “The findings indicate that we must prepare for a future with increased tree mortality in Israel. We have to thin out trees more often to avoid competition that weakens them in dry conditions. We should also plant more varied forests in terms of the composition of species. The JNF is already implementing these changes on the ground, and the lessons of the study may help to reinforce this activity.”