Are native Israeli oak trees suffering as much as others from the lack of precipitation in recent years? The last rainy season (winter 2013-14) was extraordinarily dry, bringing us only 54% of the annual average precipitation! This year seems to be continuing the pattern of relatively dry years, which have seen rampant dehydration and death among trees throughout the Nature Park. Needless to say, different plants have different reactions to the ongoing, worsening drought. A particularly high and widespread mortality rate, for instance, was observed in the park's planted groves of umbrella pines (Pinus pinea), while there were only moderate levels of dehydration in the natural garrigue (Michael Dorman, 2014).
In recent years, Ramat Hanadiv has witnessed extraordinary cases of desiccation among common Israeli oaks (Quercus calliprinos), and the phenomenon appears to be due to regional and global climate change characterized, among other things, by a significant decrease in rainfall. A similar phenomenon was reported in previous years on the Carmel and in the lower Galilee (Nava Sever, 2001). The common oaks at Ramat Hanadiv grow in a marginal habitat (in terms of rock and soil conditions) and in places where there is a relatively high amount of available water. After tracking the signs of dehydration among the trees, we decided to do a long-term monitoring project on the condition of the oaks in the Nature Park.
The study began in 2008, after a rainy season with very little precipitation (only 72% of the average annual rainfall). It focused on two parts of the north-facing slope of Nahal Hanadiv. Before the start of each rainy season, measurements were made of a number of variables that visually indicate the trees' condition: canopy density (i.e., the percentage of green leaves in the tree's canopy); the percentage of dry leaves; the presence of acorns on the trees; and the percentage of the tree that is covered by vines.
As early as 2010, it was clear that almost 10% of the oak trees were dehydrated (at least 85% of their leaves were dried out), and another 20% or so were partially dehydrated (at least 50% of their leaves were desiccated), meaning that approximately one-third of the oaks included in the study exhibited signs of serious dehydration. In the survey conducted in 2013, more than 20% of the trees in the sample were dry. Many oaks showed signs of stress, and there were only a few that had full canopies and no dry leaves. Besides the lack of rainfall, it is likely that the trees' condition was exacerbated by the dense vines growing on them and competing for the limited water supply.
This year, in the wake of last year's heavy drought, we are doing further monitoring to examine the oak trees' condition, and there is no doubt that we will see the impact that years of minimal precipitation, and last winter's extraordinary aridity in particular, have had. Analysis of the data has yet to be completed, but one conclusion can already be drawn: The long-awaited rains of winter are critical for our common oak trees, too.