Variability of Growth-Climate Relationship at the Timberline

Research project P 14554 Variability of Growth-Climate Relationship at the Timberline Sigmar BORTENSCHLAGER 09.10.2000 In mountainous areas the upper timberline defines an important climate-related ecotone stretching from closed forests to the treeless alpine zone. The purpose of the proposed dendroclimatological study is to investigate radial growth variability of Cembran pine (Pinus cembra L.) within this ecotone and the extent to which this variability is based on differences in response to climate factors. Growth variability of Pinus cembra, which is the dominant coniferous species at the timberline in the central part of the Eastern Alps, will be determined along several topographically distinct transects reaching from the timberline up to the treeline and at various habitats at the treeline on Mt. Patscherkofel and Morgenkogel near Innsbruck. To accomplish this, numerical methods like principal component analysis, non-metric multidimensional scaling, cluster analysis and discriminant analysis will be applied. Based on a local climate data set growth-climate relationships will be determined from clusters of stands and trees found. in numerical analysis and showing high similarity in year-to-year variation of cambial activity. Differences in dominant climate factors, which limit annual growth increments, as well as sensitivity to extreme climatic conditions will be assessed at established transects. The effects of climatic extremes on -several tree ring parameters (total and latewood ring width, frost rings, light rings etc.) and prevailing climate-in extreme growth years will also be determined at the various habitats. Since temperature is regarded as the primary growth limiting factor at the alpine timberline, air and soil temperature will be recorded at selected stands throughout the project period to evaluate the response of temperature variations and temperature extremes on growth processes. Results of the proposed dendroclimatological study will increase our understanding of the spatio-temporal extent of growth variability, the within-species variability of growth response to dominant climate factors and the individual response of trees to climatic extremes in alpine timberline ecotones. Determining spatial variability of tree growth at present timber- and treeline locations will also be helpful to interpretate paleoenvironmental records preserved in fossil woods and to quantitatively reconstruct past climate from tree past climate from tree ring series.

Radial growth variability and response of Cembran pine (Pinus cembra L.) to short- and long-term climate variability were studied within the timberline ecotone on Mt. Patscherkofel (2246 m a.s.l.). The study area, which is situated within the inner-alpine dry region of the Central Austrian Alps, is characterized by a seasonal precipitation minimum in winter and frequent occurrence of dry winds in early spring. Time series of annual radial increments were analyzed with respect to growth-climate relationships, extreme growth depressions and long-term growth trends. It was found that spatial variability of Pinus cembra growth is influenced by local site factors of elevation and slope aspect and radial growth at the timberline is positively correlated not only with temperature in July but is as strongly favoured by warm temperatures in previous autumn and high winter precipitation (January-March). At the tree line temperatures in previous autumn and precipitation in late winter (March) also control radial growth, whereas July temperature is not significantly correlated with ring width. Analysing climatic conditions in extreme growth years confirmed the high sensitivity of tree ring growth to precipitation in late winter (March) at the tree line plots investigated. Furthermore, irrespective of variations of local site conditions due to effects of topography (duration of snow cover, wind, insolation), radial growth of timberline stands during the last 200 yr showed several similar periods of increase, whereby trees growing on north-facing slopes showed a stronger response to climate warming compared to south-facing stands. On the other hand, climate extremes do not inevitably induce growth responses as would be expected from growth-climate relationships, which is explained by the occurrence of synergistic and/or compensating effects of growth limiting climate variables and preconditioning of tree growth in previous years. Comparison of growth reductions with other Pinus cembra timberline chronologies from inner- Alpine dry locations in the Eastern Alps revealed that investigated stands show the highest climate sensitivity during the last 200 yr. This difference in growth response to climate variability is most likely related to the special climate situation at Mt. Patscherkofel, which is exceptionally windy throughout the year and frequently exposed to warm dry winds (Föhn).