Why do trees die

Return of normal rainy weather in summer following an extended drought period of a year or more is often associated with tree death.

This apparent parody can be explained in the following manner. Roots die back and reserves stored energy in the form of starch and fatty acids are used up during the drought. As a result, some foliage may drop during this period of drought but the tree remains alive.

Return of normal soil moisture as rains return stimulates soil pathogenic fungi. The fungi are able to gain a foothold and overpower the tree because it is in a weakened state.

Although admittedly hard on our native woods, it also presented an opportunity for researchers at the University of Basel to closely study the reaction of trees to this weather phenomenon. Their aim was to study the tree canopy 30 meters above ground using a crane to determine how native tree species such as the Norway spruce respond to climate change.

Shortly afterwards, the heatwave of summer descended. Matthias Arend, a member of Kahmen's research group and the lead authors of the study. As part of their study, the researchers studied 10 randomly selected Norway spruces, all more than years old and about 30 meters tall, in order to measure the seasonal fluctuations in the water balance in the canopy.

With its flat root system, the Norway spruce is particularly susceptible to collapse, says Arend: "The tree dies because the hydraulic system that transports the water upwards from the soil collapses. Arend emphasizes that the observation that trees suffer during drought is not new. What is much more important, he says, is to understand the processes that lead to this tree death, and this is exactly what the researchers have achieved in the study published in the scientific journal PNAS.

The study also made a surprising finding: "The hydraulic system collapses extremely quickly," he says. The researchers assume that this critical point is reached when a large proportion of the roots in the drying soil lose contact with the soil moisture.

The new results diverge from the previous mortality threshold values identified in the lab, which means that the hydraulic system of a tree collapses much sooner than previously thought.

This happens because dehydration does not progress linearly and the tree cannot recover from the hydraulic collapse, and dies as a result. The researchers conclude that the Norway spruce in particular responds more sensitively to drought than previously assumed. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content.

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Steve Nix. Forestry Expert. Steve Nix is a natural resources consultant and a former forest resources analyst for the state of Alabama. He is a member of the Society of American Foresters. Updated August 14, Featured Video. Cite this Article Format. Nix, Steve. What Causes a Tree to Die? Hemlock Wooly Adelgid - Identification and Control.