Cambial activity and wood formation of Scots pine

Dendroclimatological and -ecological studies within the inner Alpine dry valley of the upper Inn river (Tyrol, Austria) revealed a strong relationship between spring precipitation and radial growth (tree ring width) of Scots pine stands (Pinus sylvestris L.). Based on linear models of tree ring- climate relationships the amount of variance in annual radial increment accounted for by climatic variables (monthly mean temperature and total precipitation) reached up to 65 %. However, the physiology of cambial activity, i.e.onset, cessation, year-to-year variability and relation to local environmental factors (microclimate, soil water status), has not yet been elucidated for P. sylvestris exposed to soil dryness within this region. Therefore, daily stem growth and seasonal dynamics of wood formation of P. sylvestris are proposed to be compared between two sites, which differ in their soil moisture characteristics (shallow stony soil on south-facing slope vs. hollow), by installation of automatic band dendrometers and continuous extraction of micro-cores, respectively. Several trees (n = 5) will be studied to be able to take between-tree variation in response to variability in environmental factors into account. These records of cambial activity will be compared to measurements of microclimate, soil moisture dynamics and tree phenology. Hence, this proposal is linked to the physiological investigation in tree-ring research. It is primarily aimed at determining the dynamics of cambial activity and wood formation of P. sylvestris and their dependence on microclimate and soil moisture. Analysis of cambium activity and wood formation during the growing season will lead to a more comprehensive understanding of the mechanism of xylem growth and the process of cell production. The determination of onset, cessation and year-to-year variability of cambial activity will also allow a more detailed interpretation of growth-climate relationships, which have already been determined for this species within this area and the susceptibility of stands to impacts of drought stress can be elucidated more precisely. Furthermore, based on results of the proposed study an optimisation of the climate-growth model used for spring rainfall reconstruction within this inner Alpine dry valley will be made possible.

Within a dry inner Alpine valley in the Eastern Central Alps (750 m a.s.l., Tyrol, Austria) the influence of climate variables (precipitation, air humidity, temperature) and soil water content on intra-annual dynamics of tree-ring development was determined in Scots pine (Pinus sylvestris L.) at two sites differing in soil water availability (xeric and dry-mesic site). Radial stem development was continuously followed by band dendrometers and repeated micro-sampling of the developing tree ring of mature trees. Determination of non-structural carbohydrates (starch and soluble sugars) was performed using specific enzymatic assays. Onset of xylem growth during 2007- 2010 ranged from mid-April to early May, but was not significantly different between study sites. Daily and seasonal fluctuations of the stem radius, which reached almost half of total annual increment, primarily reflected changes in tree water status and masked radial stem growth especially during drought periods in spring. Temporal dynamics of intra-annual radial growth determined by both methods were found to be quite similar, when onset of radial growth in dendrometer traces was defined by the occurrence of first enlarging xylem cells. Wood formation stopped c. 4 weeks earlier at the xeric compared with the dry-mesic site in 2007-2008, indicating a strong influence of drought stress on cell differentiation. This was supported by radial widths of earlywood cells, which were found to be significantly narrower at the xeric than at the dry-mesic site. Radial stem increments during the growing period showed statistically significant relationships with precipitation and relative air humidity, while soil water content and air temperature were not significantly correlated with growth. Early culmination of radial stem growth was detected at both study plots around mid-May, prior to occurrence of more favourable climatic conditions, i.e. an increase in precipitation during summer. We also found that in 2009, when dry conditions prevailed during the growing season, there was no particular shortage in starch and soluble carbohydrates, which would cause P. sylvestris to draw on stem reserves more heavily during drought. Results suggest that the early decrease in radial growth rate is due to a high belowground demand for carbohydrates to ensure adequate resource acquisition on the drought prone substrate. Furthermore, although spring precipitation is known to limit radial growth in P. sylvestris exposed to dry inner Alpine climate, results of this study revealed that (i) a daily minimum air temperature threshold for onset of xylem growth in the range of 5-6 C exists and (ii) air temperature sum rather than precipitation or soil temperature triggers start of xylem growth. In conclusion, this study revealed that spatial variability in the dynamics of tree-ring development in P. sylvestris exposed to drought is strongly influenced by water availability, while the onset of xylem growth (cambial acitivity and cell differentiation) is controlled by air temperature.