Functional balance between leaf and xylem tissues is maintained under different soil water availability in Pinus sylvestris and Picea abies

Plants are composed by tight connected different tissues and organs that work in synchrony among each other guiding all physiological processes. The functionality of the entire organism depends on the resources allocation for the different organs that have to balance the cost and the benefit of the plant. The ratio between leaf biomass and xylem biomass is an extremely important plant trait since it links photosynthesis to transpiration efficiency and to respiratory costs. Resource availability have been reported to significantly affect the growth of trees. In limited resource environment trees present smaller leaves and shorter braches than plants grown in non-limiting conditions. The aim of this work is to evaluate if the resource allocation is maintained constant during ontogeny and if the ratio between leaf and xylem is preserved in different environmental conditions to guarantee the functionality of the system. We sampled branches of Pinus sylvestris and Picea abies grown on arid and mesic soils. For each the branch we measured the xylem volume and the leaf biomass produced each year and how they cumulate from the branch apex to the base. Our results showed that the cumulated leaf biomass and xylem volume scale linearly with the distance from the branch apex and the branches from the wet site, especially P. sylvestris’ s, had more leaf biomass and xylem volume. This confirms that carbon allocation is conserved during the ontogeny and that the trees grown in non-limiting conditions have higher production. Moreover, the relation among leaf biomass and xylem volume showed a conserved allocation pattern in the two species with no effect of the environmental conditions. This demonstrates that these two traits are highly correlated and dependent on each other and that their functional balance is highly preserved to sustain the functionality of the tree independently by the resource availability.