Assessment of forest community response to environmental variability by using an integrated approach from tree-ring anatomy to allometry of tree structures

Climate change is the biggest challenge of this century and is exerting pressure on high altitude forests. Increase in global temperature along with the rise in CO2 in the atmosphere may change the structure and function of treeline species. Several studies showed the range shifts of trees towards higher altitude affecting growth, mortality, and composition of the forest. There are few studies carried out in Nepalese Himalaya at tree ring level but still miss the inter and intra annual information. To enhance our knowledge, the main objectives of the thesis is to understand the response of treeline species to climate change. This study provides knowledge on the competition between trees for the resources used in the natural forest which alters the structure and pattern of the forest ecosystem. The target species for wood anatomical and isotopic study were Abies spectabilis D. Don Mirb. and Betula utilis D. Don which is dominating in upper treeline of Himalayas. I used the dendro-anatomy to assess the growth responses of xylem anatomical traits to climatic constraints. This allowed retrieving the information at a cellular level with longer time resolution. Further, the results were complemented by isotopic measurements that were inscribed in wood cellulose during their formation. Moreover, dendrometric data (DBH, crown radius, tree height) were collected from forest permanent plots located from different geographic locations (Nepal, Italy, and Romania). The data were used to test the crown allometries and their effects on natural forest structure and dynamics using crown area and crown volume models. Wood anatomical studies of B. utilis showed mean ring width, mean vessel area, and ring specific hydraulic conductivity to positively correlated with summer temperatures. However, fibers were negatively correlated with same season temperature suggesting that fiber get narrower when the vessel is wider to maintain the xylem hydraulic system. Another, study based on dual isotope (carbon and oxygen) showed growing season water availability could be a supplementary limiting factor for this treeline species though high altitude species are mainly limited by low temperature. In such a condition, A. spectabilis, a high altitude conifer could benefit from its higher water use efficiency during the drier period taking the competitive advantage to gas exchange compare to B. utilis. The last part of a study on the crown geometry of natural forest showed trees are site-specific determining the structure of forest ecosystem through growth, mortality, and recruitment. The predicted number of trees calculated based on crown area/volume models suggested that natural forest is oriented towards a condition of space equivalence between tree-size classes, showing in parallel that the use of soil resources increment in higher tree classes. In conclusion, this thesis provides information on wood anatomy and physiology of treeline species in response to global warming. Integration of crown models opens the idea how crown allometries contribute to a better understanding of forest communities and dynamics.