Carbon storage and water use efficiency in Abies alba as affected by simulated browsing and genetic heritability

Evidence suggests an increase in temperature and a decrease in summer precipitation. Since biotic interactions have the potential to alter the carbon allocation of seedlings, ungulates may affect the drought tolerance of trees. As ungulate numbers increase, there is an urgent need to address the joint effects of drought tolerance and herbivory. Silver fir provides an excellent system to study such effects as it is an important but preferentially browsed forest species and has been suggested to cope with induced drought. The main objective of this study is to reveal the roles of genetics, environmental and biotic stress in shaping the variation of drought tolerance in fir. We shall use stable isotope measurements of fir seedlings to determine the extent to which water use efficiency and C storage are influenced by genetic factors and browsing. Our study will help to understand the genetic and environmental drivers in determining how stress factors modify the growth of fir seedlings.

The major challenge of terrestrial plant life is to perform photosynthesis without drying out. CO₂ fixation via photosynthesis is necessary for tree growth; however, inevitably water is lost during the process due to transpiration through the open stomata. Recent evidence suggests an increase in mean and daily maximum temperature and a decrease in summer precipitation. In the same time, a strong increase in ungulate browsing has been observed, which can further challenge the establishment of future tree generations. Interactions between biotic and abiotic factors are generally poorly understood, nevertheless, their understanding is essential to make future predictions  and to develop sustainable forest management plans.

Carbon storage is a main feature of long-lived plants such as trees allowing growth to be at least partially uncoupled in time from assimilation. Trees may store large amounts of C pools as non-structural carbohydrates. Such C reserves can be used until exhaustion by trees coping with various stress factors to sustain vital processes such as respiration and for recovery after leader loss due to browsing However, depleting the storage pool decreases the resistance towards carbon starvation. Little is known how the combined effect of drought and herbivory modify the carbon storage and allocation of C in trees. Without understanding of such process, predictions under changing climates cannot be achieved for tree species preferentially browsed by ungulates.

It has been recently suggested that Abies alba Mill. is less vulnerable to future maladaptation due to rapid climate change than Norway spruce, but currently ca. 19% of the fir and only 2.4% of the spruce seedlings suffer from ungulate browsing in Switzerland. Moreover, herbivory has the potential to influence not only the limiting resources but also to increase the N concentration in trees, leading to trees even more preferred by ungulates. In addition, effects of browsing on carbon allocation belowground seem to be phenology dependent. There is thus an urgent need for adaptive forest management to address the joint effects of climate change and ungulate herbivory.

In order to improve our knowledge on the mechanistic basis underlying browsing and drought stress, we will combine data on seedling morphology, eco-physiology and genetics. To assess how C storage and water use efficiency are affected by browsing, we will use the Abies alba saplings of the common garden experiment of the Project ‘ADAPT’ that were subjected to heavy, light or no (control) simulated browsing in 2015. We will measure δ¹³C, N content and non-structural carbohydrates (NSC) in needles of saplings of 19 fir provenances originating along a drought gradient and of which we possess phenology, growth and wood quality measures (large-scale provenance trials of the project ADAPT) with and without browsing (clipping treatment).

In this project, we will estimate the extent to which water use efficiency assessed by δ¹³C and C storage are influenced by the climate of origin, genetics and ungulate browsing. The main objective of this study is to reveal the respective roles of genetics, environment and biotic stress in shaping the natural variation of drought tolerance in silver fir and to identify populations with high evolutionary potential for drought and browsing adaptation or populations that contain - independent of biotic stress - drought resistant genetic variants.