Báo cáo lâm nghiệp: "Winter moisture content and frost-crack occurrence in oak trees (Quercus petraea Liebl. and Q. robur L.)"

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Nội dung Text: Báo cáo lâm nghiệp: "Winter moisture content and frost-crack occurrence in oak trees (Quercus petraea Liebl. and Q. robur L.)"

Winter moisture content and frost-crack occurrence in oak trees (Quercus petraea Liebl. and Q. robur L.) B. Cinotti Nancy, France la Qualite des Bois, INRA-CRF, Station de Recherches sur of differences in counteract the possible effects Introduction chosen. soil, were An increment borer was used to take diame- Our research investigates the relations tral cores in an approximate north-south direc- tion. Cores were then kept in plastic tubes between the frost-crack occurrence in whose diameters were just sufficient for the oaks and some physical and anatomical core to enter. This prevented water loss from parameters of solid wood. The starting the cores. point of our interest in water in trees is the In the laboratory, the increment core of each observation that frost-cracks occur with a tree was divided into 6 pieces: central heart- higher frequency in trees growing on soils wood (rings from pith to the 15th), external heartwood (rings from the 16th ring up to the with high winter moisture content (Schirp, and sapwood from sapwood-heartwood limit) 1968). Also, severe stem contractions with both the northern and southern sides of the freezing winter temperatures (greater than bole. Each piece was immediately weighed those arising from summer droughts, but while wet and than dried in an oven at 103°C equally dependent upon the moisture for at least one day. A second weighing gave the dry weight and, by subtraction, the initial content) have been commonly observed water weight. Moisture content (water weight (Hinckley et al., 1978). In this paper, we divided by the weight of dry wood) is expressed report our observations of winter moisture percentage. as a content variation in oak trees, using non- destructive sampling by increment cores, and relate it to frost-crack occurrence. Results Table I illustrates the variability of results Materials and Methods from one forest to the other (even on cores bored at proximate dates), and for Increment cores were taken from 90 oak trees the same plot and on the same trees from from 3 different forests in central and eastern one month to the next. However, the fol- France. Paired trees of similar diameter, as in lowing general observations could be our previous study (Cinotti, 1987), one frost- content in oak made: 1) winter moisture cracked and one sound in close proximity to
tree boles is from 60% to total Discussion saturation, i.e., in the range where wood freezing more or less frost- expansion counteracts and KObler, 1968; shrinkage (Schirp Our results correspond to those of Miller Kubler, 1983; Cinotti and Tahani, 1988); 2) (1987), which underlined the importance the sapwood moisture content is always of site conditions on frost-crack spatial higher than or equal to that of the middle distribution. Such differences could be heartwood, as was previously noted by site effect (either a climatic or seen as a Hinckley et aL (1978). edaphic one). Roosen (1956), while an Table II shows that frost-cracked trees studying the winter water status of always have a moisture content signifi- poplars, stated that the environment was cantly higher than that of sound trees and likely to influence tree moisture content: the more water in the soil, the higher the that north-south differences are only signi- moisture content. In our Bellary plot, for ficant for Q. petraea Liebl.
example, differences in the moisture Since a study of frost-shrinkage of small oakwood samples has shown that tangen- contents between sapwood, middle and central heartwoods and between north tial shrinkage (the main explanation of and south are no longer significant in frost-crack formation) depends mostly March; perhaps this is an effect of the upon moisture content exceeding a spring ascendant flow of sap which modi- threshold, then a difference in winter water status may be a first step towards an fies water distribution in the bole. explanation of such a defect. Our data suggest that high sapwood moisture content increases the risk of frost-crack. Site moisture content appears References to affect sapwood moisture content. Winter water status could be considered predisposing factor, discriminating be- Cinotti B. (1987) Influence de la structure du a (Quercus robur L. et Q. bois des chen