Ozone and Drought – Root Responses to Experimentally Manipulated Environmental Conditions in a Mixed Forest

Kranzberg Forest has been subjected to a series of large-scale, long-term, experiments since 1998 with the aim to manipulate the resource availability of adult forest trees to better understand whole tree performance and specifically root responses to predicted changes in ozone and water availability. The managed forest stand consists of two dominate tree species, Norway spruce (Picea abies) and European beech (Fagus sylvatica). To study competition effects between species and how competition is influenced by environmental stress. Select trees were exposed to enhanced ozone concentrations between 2000 and 2008 (CASIROZ). At the same site, but in a neighboring stand, precipitation was completely excluded from the soil surface during the growing seasons of 2014 to 2017 (KROOF), with continued experimentation to date. Two exceptionally hot and dry years, 2003 and 2015, reinforced the relevancy of the experimental set-up and questions surrounding forest responses to climatic shifts.

Surprisingly, elevated ozone stimulated root growth. Most likely this increase in growth occured through ozone disruption to the transport of cytokinins normally signaling the nitrogen supply status of the trees. The extended drought stress of the KROOF experiment resulted in reduced above- and belowground biomass production as well as a shift of the fine root distribution in the soil profile.
The rooting depth proved to be crucial in calculating the water balance of the six drought-stressed and six control plots at the experimental site. Despite the type of stress our results support the Optimal Partitioning Theory (OPT), i.e. coordinated growth between the root system and aboveground parts of the trees.