Impact of Environmental Conditions on Wood Anatomical Traits of Green Alder (Alnus alnobetula) at the Alpine Treeline

Due to land use change, green alder (Alnus alnobetula), formerly restricted to moist slopes, is now expanding to drier sun-exposed sites at the alpine treeline. The highly productive shrub is forming closed thickets, establishing nitrogen-saturated species in poor shrublands. To evaluate wood anatomical adaptations to changing environmental conditions, we analyzed vessel characteristics (mean vessel area, MVA; vessel density, VD; and theoretic conductive area, TCA) and axial parenchyma abundance, as well as their distribution in the annual ring at a moist north-facing and a dry south-facing site at the alpine treeline on Mt. Patscherkofel (Central European Alps, Austria). Results revealed that lower soil water availability and enhanced evaporative demand did not affect MVA while VD and TCA were significantly reduced at the dry south-facing site. This suggests that in green alder, vessel size is a static trait whereas vessel number isresponds plastic. Limited water availability also triggered a significant increase in axial parenchyma, confirming the important role of xylem parenchyma in water relations. Harsh environmental conditions at the distributional limit of green alder may have affected xylogenesis, leading to a near semi-ring-porous distribution of vessels and an accumulation of parenchyma in the late growing season. We conclude that in a warmer and drier climate, growth limitation and physiological stress may set limits to the distribution of Alnus alnobetula at drought-prone sites in the alpine treeline ecotone.