Doctoral defence: Bikash Kharel "Effects of environmental and competitive conditions on carbon and nitrogen use and specialised metabolites in Norway spruce and silver birch"

On September 10 at 10:15 Bikash Kharel will defend his doctoral thesis "Effects of environmental and competitive conditions on carbon and nitrogen use and specialised metabolites in Norway spruce and silver birch" for obtaining the degree of Doctor of Philosophy (in Plant Ecology and Ecophysiology).

Supervisors:
Associate professor Arvo Tullus, University of Tartu
Associate professor Priit Kupper, University of Tartu

Opponent:
Professor Line Nybakken, Norwegian University of Life Sciences (Norway)

Summary:
Carbon (C) and nitrogen (N) are essential elements that enable trees to grow and protect themselves from environmental stress. Forest trees use N in their photosynthetic machinery to bind atmospheric C. Captured C is further used for growth, reproduction, maintenance, as well as to produce chemical compounds known as “specialised metabolites” (SM).

However, how trees balance growth and defence when the environmental conditions change is poorly understood. This study examined how Norway spruce and silver birch respond to different soil moisture and light conditions, and competition with other trees. Data were collected from the Free Air Humidity Manipulation experiment, and forest stands differing in site type (Carex-Filipendula, Filipendula, Hepatica, Oxalis, Oxalis drained swamp) and species composition.

The results showed that, in the driest Hepatica site type, spruce trees had lower stomatal conductance, net photosynthesis, and photosynthetic nitrogen-use efficiency. In the Hepatica site, the concentrations of several SMs, including total phenolic content, flavonoids (in young needles), and lignans (in bark), were lower than in wetter Carex-Filipendula and Oxalis sites. At the same time, silver birch growing in drier soil had higher concentrations of potential defence compounds in roots, including flavonoids, benzoic acids, and total root SM. However, increased air humidity and soil moisture reduced defence compound concentrations in birch while promoting greater fine-root growth.

Within tree canopies, birch leaves in the upper part of the crown, which get more sunlight, produce more protective chemical compounds. In spruce trees, the older needles had higher levels of defence compounds than the younger needles. Mixed forest spruce trees that faced less competition for light increased photosynthetic performance in the early growing season. In pure spruce stands, characterised by stronger competition, trees invested relatively more resources into SM production. Since climate change alters environmental conditions and species interactions, this study helps to understand how forests will function and adapt to future environmental changes.