Growth response of green alder to climate at treeline

Green alder (Alnus viridis (Chaix) DC = Alnus alnobetula (Ehrh.) K. Koch) is an early successional shrub species that is widely distributed at the alpine treeline in the northern hemisphere and at high latitude areas. It is currently the most expanding shrub species in the Alps. Although green alder is naturally restricted to moist sites, expansion into subalpine grasslands and well-drained areas is enhanced by its clonal growth, high seed production and the capacity to establish symbiosis with nitrogen-fixing actinobacteria and ectomycorrhizal fungi. Expansion of green alder causes considerable environmental changes that have mostly a negative effect on ecosystem functions by decreasing avalanche protection, causing loss of biodiversity and drive grasslands and meadows into nitrogen saturated, species-poor shrubland. We will assess growth response of this deciduous pioneer shrub as a function of environmental and climate variability. Inter- and intra-annual stem growth will be determined by applying dendrochronological techniques (tree-ring analysis) and evaluation of high-resolution dendrometer records (i.e., reversible and irreversible stem diameter variations), respectively. Based on these data, long- and short-term influences of climate on growth of green alder will be evaluated. Monitoring of growth phenology of shoots and leaves and clonal spreading will complement analyses of climate effects on growth.

Additionally, ecophysiological data (stem sap flow, leaf water potential) and anatomical measurements of vessel parameters will be collected to determine stem water status and adjustment of the hydraulic system to varying environmental conditions. These data are essential to predict future spreading and competition with coniferous species within the treeline ecotone, an ecosystem which is strongly affected by climate warming and land-use change. Therefore, the foremost goal of the proposed study is to analyse effects of climate variability on short- and long-term stem growth along temperature and moisture gradients within the alpine treeline ecotone. We expect that (i) stem and clonal growth of green alder increase at moist sites in the course of climate warming, (ii) drought periods in climatically extreme years constrain growth and cause dieback of shoots, and (iii) due to its pioneer character and clonal spreading green alder shows high growth resilience enabling endurance under extreme environmental conditions.