Hydraulics of juvenile trees: Effects of seed provenance

Trees are long-lived plants which strongly depend on well-balanced hydraulics and the ability to adjust to changing conditions. A prerequisite therefore is a high plasticity in relevant hydraulic and anatomical traits. There is increasing evidence that stress factors not only have a direct impact on the plant but also on its progeny. The aim of this project is (1) to study the influence of hydraulic conditions during seed maturation on the drought tolerance in the progeny, and thus (2) to estimate the plasticity in hydraulic and anatomical traits and the acclimation potential of seedlings and juvenile trees. The project consists of three parts in which various hydraulic (water potential, hydraulic safety and efficiency, stomatal behaviour, drought tolerance of living leave tissues) and anatomical measurements (xylem anatomy), as well as analyses of biomass and growth will be carried out. Measurements will be based on well-proved and new techniques, whereby especially for the youngest stages special methods (e.g. rehydration technique, ultrasonic technique, x-ray tomography) have to be applied. Some of these have already successfully been tested on tree seedlings. In the first part of the project, measurements will be done on three-year old plants of Larix decidua, Picea abies and Pinus sylvestris. Plants have been grown from seeds collected in different years (dry/warm and humid/cold) and on different sites in Austria. Part of the plants will be subjected to drought stress so as to include the acclimation potential of the different trials (i.e. species x geographical provenance x seed year). In the second part, the developmental stage of trees will be considered. Therefore, seeds of Picea abies, Pinus sylvestris, Fagus sylvatica and Sorbus aucuparia will be collected in two subsequent years on a dry and a humid site in Tyrol and grown in the Botanical Garden of Innsbruck. Drought tolerance will be analysed on newly emerging seedlings as well as at the end of the first and the second vegetation period. The third part is connected to a long-term irrigation experiment in an inner-Alpine dry valley (Valais). Seeds from irrigated and non-irrigated Pinus sylvestris trees will be collected and grown in the Botanical Garden of Innsbruck. After the second growing season, hydraulic and anatomical measurements will be performed. This study will provide new and important insights into tree hydraulics: It will be the first study dealing with the influence of hydraulic conditions during seed maturation on hydraulic traits in the progeny and one of the few ones dealing with drought tolerance in the course of seedling establishment. It will also provide valuable information on the plasticity of hydraulic traits in juvenile trees. Results are not only of great importance for basic research but also relevant in forestry, particularly with respect to expected climate change.

Successful seedling establishment is decisive for survival and future distribution of plant species, particularly for long-lived woody plants which need long time periods to get reproductive. Seedlings though are very sensitive to many biotic and abiotic stress factors, including drought. Due to the small root system and limited internal buffers drought-related seedling mortality is high. An optimally balanced water relationship with an adequate drought tolerance and the ability to adapt to climate conditions is thus crucial for seedlings and juvenile plants. Due to methodical problems though, hydraulic studies on seedlings are rare. Hitherto it was also unknown if and how climatic conditions during seed maturation might impact efficiency and safety of the hydraulic system in the progeny. To gain important insights into seedling hydraulics, seeds of different maturation years (warm-dry versus cold-humid) and seeds of mother plants with differing water supply, respectively were germinated, and subsequently drought tolerance of the resulting juvenile plants was assessed. Furthermore, in different species and under varying environmental conditions, the development of relevant hydraulic and wood anatomical parameters was monitored during establishment of few weeks to several months old plants. Influence of climatic conditions during seed maturation on hydraulic safety and wood anatomy, as well as the acclimation potential of the progeny was found to be negligible. This contradicts resent studies showing that parent plants can influence their offsprings phenotype non-genetically via epigenetics or maternal effects and indicates a great conservatism in hydraulic and anatomical traits. On the other hand though, it could also be shown that few weeks old seedlings are already able to acclimatise to prevailing climate conditions: Six weeks old sycamore seedlings grown in the glasshouse differed considerably in various hydraulic and anatomical parameters from seedlings developed at their natural site under mother plants. It could also be shown that first leaves are particularly well water-supplied to enable fast growth, even though this increases the vulnerability of the hydraulic transport system. Outcomes of the project provide important new insights into hydraulics of seedlings and juvenile plants, highlighting the peculiarity of early ontogenetic stages. While already youngest plants were able to hydraulically adjust to prevailing conditions during establishment, climatic conditions during seed maturation were of minor importance. Overall, findings clearly reveal the importance of an optimal balance between fast growth and adequate drought tolerance in the first year of a trees life.