Similarities and differences in biomass allocation to xylem and needles in branches of Scots pine trees from two sites differing in soil development

The ability of a species to survive and adapt to adverse environmental conditions is determined by the vulnerability of its xylem to embolism and the availability of carbon resources. The vulnerability to embolism is complex, and species-specific within the trait of tree hydraulic architecture. Drought limits long-distance water transportation of the water due to reduced leaf-to-soil water potential. In general, low soil water availability can reduce growth and be highly dangerous to plant survival. Furthermore, biomass partitioning, carbon acquisition, and carbon resource utilization have great importance in adaptation and acclimation studies. The phenotypic plasticity of the xylem or the photosynthetic apparatus may play an important role in the adaptation of pioneer isohydric species like Pinus sylvestris to new/ harsh environments. How plants modify the biomass allocation to the different soil developmental areas, like wet and dry conditions affecting this coordination between biomass allocation to the xylem and needles of the branches, has been poorly investigated. In this experimental study, mature Scots pines from 2 different environmental conditions that are only a few hundred meters apart in the Italian Dolomites, were evaluated for their xylem anatomical hydraulic traits and biomass resources. One of the study areas is a mature mixed forest with high water availability, and the other is a pure pine forest growing on a landslide (1814) with low water availability due to the soil conditions. In this study, we selected the topmost apical branches 150 cm from the apex and selected several sampling positions at different distances from the stem apex in two groups of trees. We measured the annual Ring area, the xylem hydraulic diameter (Dh) and leaf (needle) biomass (LM), the branch biomass (BM), leaf mass (LM), and leaf mass area (LMA). The experimental results show that pines in the landslide area have significantly reduced growth, but with contrasting xylem anatomical results for hydraulic diameter, which show smaller conduits in the first 50cm from the branch apex and become larger afterward. Needle biomass in the two areas is not statistically different, with the same pattern of cumulated mass along the branch axis. Low-water-availability pines have the same stomatal density as high-water-availability pines but have higher leaf mass per area (LMA) due to a higher number of smaller, denser, and heavier needles. Scots pine is an early isohydric species that likely closes stomata to avoid excessive transpiration at the cost of reduced growth.