Long term monitoring of common oak (Quercus calliprinos) population in response to drought conditions

Yael Navon, Liat Hadar, Hussein Muklada, José M. Grünzweig

The oak stands found in Ramat Hanadiv are an important resource and constitute a unique habitat in the landscape of the park, which should be known and preserved. In light of the fact that the number of oaks in Ramat Handiv is limited and in light of the continuation of climate change processes that may lead to the drying out and death of oak trees, it is recommended to monitor the condition of the trees, especially in extreme years and during successive years of dryness and heat.

Common oak groves are prevalent in Israel from the Upper Galilee Mountains in the north to the Judean Mountains in the south. In Ramat Handiv, the oak grows only at a few suitable sites in the park, in areas where water availability is relatively high (Perevolotsky 2001) and, hence, might be vulnerable even to small changes in the growing conditions. During most of the last years, following a sequence of years in which average amounts of precipitation fell in Ramat Handiv or less than that, an alarming phenomenon of the drying out of the common oak trees was observed. Following this, it was decided to conduct a long term monitoring of the condition of the oak trees in the nature park.

The monitoring, which began in 2009 and continues to this day, focuses on two plots where there are dense stands of common oak, located at the north facing slope of Hanadiv Vadi. The monitoring is conducted once a year, every autumn, before the onset of the rainy season.

The monitoring method includes the use of visual indicators through which the condition of the trees is defined, such as canopy density, percentage of dry and green leaves in the canopy, level of acorn bearing, level of coverage by climbers, and resprouting of dry branches. The monitoring indicators were chosen according to tree monitoring standards in Switzerland and the European Community and the United Nations (CEC-UN/ECE 1994, Dobbertin et al. 2009). The selected indices well express the condition of the trees, according to the changes in the amount of rain over the years.

From the results of the long-term monitoring of almost 12 years and from other studies conducted in Ramat Handev (Väänänen et al. 2020), it appears that the mortality of the common oak trees is due to a combination of low water availability, low tolerance to extreme dryness and the fact that it is a relatively large tree that consumes large amounts of water.

In an overall view on the years of monitoring, there is a clear relationship between the amount of annual precipitation and tree mortality, leaf drying and the percentage of green foliage density of the entire canopy.

It was found that common oak trees are sensitive to dryness, especially when the average amount of precipitation is less than 450 mm of rain. Thus, for example, an extreme drought occurred in the 2013/2014 rainy season, in which only about 50% of the average amount of rain fell and the temperature values in the winter were relatively high.

According to the survey conducted in the fall of 2014, it appears that a minority of rainfall in the previous years and the unusual dryness of this winter had a negative effect on the condition of the oak trees in the park. Canopy density (the amount of foliage in a tree) decreased in about 70% of the trees. Over 20% of the trees sampled were dry. Many oak trees showed signs of stress, and only a few trees had a full canopy and no dry leaves. Apart from the dryness, there may be a load of climbing plants weighing on the trees, due to competition for the amount of available water.

At the same time, at the end of the dry season of 2015, after a particularly wet winter, most of the trees recovered, the canopy density increased again in 70% of the trees and the relative cover of dry leaves decreased. New growth of the foliage with the improvement of the conditions, indicates plasticity in the response of trees to environmental conditions. However, the lack of recovery of those trees that lost all their foliage following the extreme dry year suggests that the plasticity is partial and that beyond a certain threshold of water stress the tree may not survive.

The good news is that in recent years, in compliance with the gradual increase measured in the average amount of precipitation, the average foliage density of the trees has also increased. It seems that it is very important to continue long-term and continuous monitoring of the condition of the trees, in parallel with monitoring the amounts of precipitation throughout the years.

The information about the state of the trees gathered over the years contributes to increasing awareness of the importance of preserving trees in an era of climate change. The data collected so far can support decision-making regarding the management of the park and it seems worthwhile to consider intervention in some of the trees, in order to facilitate their dealing with dry conditions and improve their visual status. Considerations of accessibility and usability for the public, visibility (dry trees), fire hazard (accumulation of understory vegetation, dead trees and/or decomposing trunks and branches that constitute fuel), function as a habitat for animals, etc., must be taken into account.

Here is a ‘story map’ showing the story of the oaks in Ramat Handiv in a unique web interface. To activate, click on the image. Turn on the speakers…

Additional links:

Oak monitoring final report 2013-2021 (in Hebrew)	Oak monitoring final report 2015-2009 (in Hebrew)
Visual guide for evaluation of leaf canopy density	Oaks dashboard