De Zan, Carlo (2008) Analisi delle crescita intra-annuale in conifere e relazioni con il deficit idrico. [Ph.D. thesis]
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The goal of the PhD thesis is to verify the intra-annual radial growth in conifers, using the anatomical analysis of cells (microcores) and dendrometers data.
In the first chapter, using the microcores, the formation of ring wood is illustrated by the analysis of its different phases: cell enlargement, cell wall thickening and mature cell of xylem. In particular, the use of microcores allows to asses directly the ring formation, by counting the new cells produced every week. The production of new cells occurs by the cell divisions of the cambium which is regulated by internal physiological processes and by environmental factors, within limits established by genome. During their development, the cambial derivatives change in a morphological and physiological way, and they assume more or less specific characteristics and differentiate into the various elements that form stem tissues. The latter phenomenon is related to changes in the composition and in the organisation of cells walls. Because of the increase of the turgor pressure into vacuoles due to water absorption, cell walls become stretched, so that cell size increase. After the cellulose and lignin deposition and the protoplastes autolysis, cell maturation ends.
Two different study sites were selected, one facing north and one facing south; moreover, the sites have different altitude, in order to evaluate the differences in the radial growth. In each site, three species of treeline were sampled: Larix decidua, Picea abies and Pinus cembra.
Wood microcores were collected every week during the growing season 2002, 2003, 2004, and 2005 from 26 trees. Cross-sections were embedded in paraffin and cut at 8-12 ïm. Then they were observed with visible and polarized light to differentiate xylem cells of the growing tree-ring. The cells in the cambial zone, the enlargement cells, the secondary wall thickening cells and mature xylem cells were counted.
Similar patterns of tree-ring formation were observed among the three species studied. In facts, the growing season of the stem started at the end of April and continued until the beginning of October. The first specie reactivating the cambium was P. cembra followed by L. decidua and P. abies. In the site facing north, we found a delay in the beginning of the growing season and in the cell enlargement phase; furthermore, a smaller number of cells in the wood ring was observed at the end of the growing season.
The second chapter presents the second method used to study the intra-annual tree-ring development. Thanks to the use of automatic dendrometers, differently to the microcores, the radial fluctuations of the stem were continually monitored. Those instruments register data every five minutes, but the measurement is indirect because it does not allow to distinguish the different phases of tree-ring formation. Moreover, the method does not permit the discrimination between new xylem cells and phloem produced and is strongly influenced by water status. Particularly, during spring, the re-hydration of wood tissues causes a stem expansion that is not linked to a real growth. Instead, the re-hydration provokes a noise in the determination of the beginning of the growing season. For that reason, the volume variation measured by dendrometers was compared with the histological data, in order to verify if the results from the two methods are the same or not. Data collected with the two methods were both processed by the Gompertz equation and the function parameters were statistically compared. This comparison did not highlight significant differences in any cases. Nevertheless, microcores resulted essential in order to calibrate the volume variation measured by dendrometers and to precisely identify the beginning of the growing season.
In the third chapter, histological analysis were used to analyze the effects of a water stress on young trees of Abies balsamea which is a typical specie of boreal forest. The method utilized was the same described for the first chapter. 264 samplings were collected by taking whole young trees from which slides were realized. Young trees were submitted to two treatments: control (with no stress) and individuals with water stress. The dynamic of the ring formation did not show any differences between the two treatments. The dimension of the lumen and of the cell walls were also analyzed. The sampled cells were divided in three groups in accordance with the moment in which they were developed: pre-stress, stress and post-stress. When a water stress occurs, we found that the produced cells have smaller size, while after the treatment their size become the same of cells in control trees. The wall cells appeared more thin in treated trees than in control ones, showing that the water stress has an influence also after it was stopped.
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