Stem growth and respiration of Pinus cembra at timberline

In high altitude timberline environments low temperature during the growing period is suggested to be the major factor restricting tree growth. However, the physiology of cambial activity, i.e. onset, cessation, year-to-year variability and relation to local environmental factors, has not yet been elucidated for Stone pine (Pinus cembra L.) along a temperature gradient. Pinus cembra will be chosen because of its dominance at timberline in the central part of the Eastern Alps. An altitudinal gradient from the closed forest (c. 1700 m a.s.l.) up to the tree species limit ("krummholz"-limit; 2150 m a.s.l.) will be established at Mt. Patscherkofel (Tyrol, Austria) and daily stem growth and seasonal dynamics of wood formation of P. cembra are proposed to be followed by installation of automatic band dendrometers and continuous extraction of micro-cores, respectively (n = 3 trees/site). Additionally, records of cambial activity will be compared with stem respiration, photosynthesis, local microclimate and tree phenology, because radial stem growth is closely related to the cambial activity and hence also to carbon efflux from the number of living and newly built cells in the xylem and ploem. By this way, along an altitudinal transect (i) the seasonal dynamics in stem respiration will be related to the prevailing temperature conditions and total stem CO 2 efflux will be separated into growth and maintenance respiration, and (ii) and we will also measure carbon uptake by the foliage because woody tissue strongly depends on the supply of carbohydrates from photosynthesis. Hence, this proposal primarily aims at determining the dynamics of cambial activity and wood formation in relation to stem respiration of P. cembra and their dependence on photosynthesis and microclimate along a natural gradient of increasing temperature limitation for tree growth within the timberline ecotone. The determination of dynamics of cambial acitivity will also allow a more detailed interpretation of growth-climate relationships, within the timberline ecotone. Such studies provide an opportunity for spatio-temporal studies in the context of climate warming because in contrast to short-term artificial warming experiments the ecosystem as a whole had time to adjust to long-term, gradual changes in temperature. Furthermore, based on the results of the proposed study an optimisation of climate-growth models used for this species to reconstruct summer temperature based on tree-ring widths is rendered possible.

Growth processes involve cell division, expansion and differentiation and result in increment of dry mass, volume, length or area. There is extensive evidence that at high altitudes low temperature during the growing period limits formation of new tissues and tree growth. There are also reports that cell division and extension are generally more sensitive to temperature than is the rate of photosynthesis. The purpose of this study was to determine the relation of intra-annual dynamics of radial stem growth and cambial activity to stem respiration and photosynthesis in Stone pine (Pinus cembra L.). Stone pine was chosen because of its dominance at timberline in the central part of the Eastern Alps. An altitudinal gradient from the closed forest (c. 1950 m a.s.l.) up to the tree species limit (`krummholz`-limit; 2180 m a.s.l.) was established at Mt. Patscherkofel (Tyrol, Austria) and daily stem growth and seasonal dynamics of wood formation were followed throughout two growing seasons (2006-2007) by installation of automatic dendrometers and continuous extraction of micro-cores, respectively. Additionally, records of cambial activity were compared with stem respiration, photosynthesis, local microclimate and tree phenology. Results of this study revealed (i) that at the beginning of both growing seasons highest numbers of cambial and enlarging cells were observed at the treeline, suggesting that a threshold root-zone temperature is involved in triggering onset of above ground stem growth. (ii) During seasonal variations in cambial activity non-linearity between stem respiration and xylem production was found. We concluded that besides temperature, the number of cambial and living xylem cells influenced stem respiration. (iii) Daily radial increments were significantly correlated with air temperature at the timberline and treeline only, where budburst, cambial activity and enlargement of first tracheids also occurred quite similarly, suggesting that the relationship between climate and radial stem growth within the treeline ecotone is dependent on a close coupling to atmospheric climate conditions. Furthermore, we also provided evidence that initiation of cambial activity and radial growth in spring can be distinguished from stem re-hydration by histological analysis. (iv) Additionally, we found that a long-term adaptation of the photosynthetic apparatus to the general temperature conditions with respect to elevation exists, which is combined with a short term acclimation to the prevailing temperature regime. Summarizing, results of this study confirmed the necessity that a combination of dendroclimatological, ecophysiological and histological analysis are required to be able to assess the influence of temperature on tree growth throughout the treeline ecotone in detail.