The drought experiment

Innovative methods for measuring drought explored in Kostelec forests

The roofs covering the area of approx. 700 m2 are expected to slow down the stem diameter and reduce transpiration, which started to decrease immediately after the roof establishment and during the season dropped approximately to two-thirds of the control area. The soil water potential decreased slowly, and at the end of the first vegetation season, it decreased to the critical value of 1,5 MPa. At this point, the water is almost unavailable for the trees. The area was flooded twice a year to retain moisture for as long as possible, yet the soil water potential dropped to 5 MPa. After 4 years, the tree defoliation (plants losing all, or a large proportion of their leaves) is massive and we expect fatal bark beetle attack during late spring. It was surprising how the investigated trees survived for 4 years, which might be the result of horizontal water transfer and root interconnection. From a similar experiment conducted 10 years ago, researchers found that the amount of active mycorrhizae was about one-third lower, which proved the withdrawal of an important ally of trees in combating drought.

The drought experiment in the Kostelec forest. Credit: Ivana Tomášková

In our work, we focus on challenges triggered by climate change in forests of the Czech Republic, as this region became an epicenter of the devastating bark beetle outbreak. An insightful project we recently implemented in the SUPERB case study in Kostelec, was the Extemit-K project, where we built a spatio-technical background, focusing on the mitigation of the impact of climatic changes to forests from the level of a gene to the level of a landscape. The main goal of the project was the protection of forests as carbon sinks under climate change and maintaining the common forest management. At the gene level, we focused on the spruce bark beetle itself – its genome and physiology of the chemical communication during host selection. At the tree level, the crucial part was understanding the mechanism of susceptibility (vulnerability) considering the genetic, tree physiology and chemical messengers. At the landscape level, we developed models determining the stand species composition and structure leading to higher resilience. Our interdisciplinary approach did not only aim for scientific advancements but also addressed societal needs by developing practical applications. These included diagnostics for tree health, novel semiochemicals for beetle management, and methods like sniffer dogs integrated with remote sensing for early detection of beetle attacks.

Even though the current bark beetle outbreak peaked in 2020 and the situation in the last years is favourable, we expect large-scale bark beetle disturbances to increase in the near future. The Kostelec Forest site, with an altitude 430 m a.s.l., is facing the same challenges as any other site in Europe: higher temperature with increasing number of hot days and uneven distribution of precipitation (rainfall). Longer vegetation season and decreasing number of days below freezing point support more generations of bark beetle and low mortality of adults overwintering.

The drought experiment creates new knowledge about the resilience of Norway spruce in lowland altitudes. In cooperation with Sigrid Netherer from the University of Natural Resources and Life Sciences in Vienna, we found that the drought alters the tree defence chemistry, its composition, quantity and emission rates, making it more more challenging for trees to defend themselves.