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Annual Growth Ring:
The layer of wood growth put on a tree during a single growing season. In the Temperate Zone the annual growth rings of many species (e.g., oaks, and pines) are readily distinguished because of differences in the cells formed during the early and late parts of the season. In some Temperate Zone species (black gum and sweetgum) and many tropical species, annual growth rings are not easily recognized.
Annual Growth Ring Analysis
1.Study Goal
Annual growth ring analysis gives insights into the developmental history of a tree and thus of a stand. Growth conditions during the previous decade can be more precisely determined through the variations in growth and the "indicator years", those years with extreme growth reduction or increase. The tests particulary serve the estimation of growth changes resulting from increasing pollution stresses. The separation of these effects from naturalfactors of influence can be accomplished with a detailed analysis of the climate-growth relationship. A particularly interesting question is whether an increase of pollutants has increased the sensitivity of trees toclimate.
Fig. 1: Trunk cross-section of a healty 35 year-old oak (Quercus robur)in the Grunewald district
2. MethodologyDendro-chronological tests were conducted on dominant pines in maturestands 90-155 years old, timber woods 45-60 old, and oaks 55-140 yearsold (V. LÜHRTE 1991, 1992). Two samples were taken of each tree at aheight of 1.3 m by a forest drill-core device. The dried and smoothed coresamples were measured with a growth ring measuring device. The next procedures were synchronic examination, the averaging of radius curves to tree curves, andanother procedure for stand mean curves. The negative and positive indicatoryears were determined by individual growth ring curves. Growth changes per treewere determined by a comparison with the previous year's values (reduction of 30-50% defined). The calculation of climate-growth relationships was conductedin the form of response functions on radius curves indexed and summarized intostand chronologies, taking into account temperature and precipitation values.
Fig. 2: Cumulative growth curves of pines (Pinus sylvestris) at GR63,GR91, GR151, and GR155. All are directly bordering mature and immaturestands.
3. ResultsThere were no indications of long-term changes in site conditions caused by airpollution in the mean curves of stands drawn from a total of 326 test trees(166 mature pine and 160 immature pines). The diameter growth of Berlin pinestands has mainly followed the normal course related to age sincethe beginning of the 20th century. In recent decades there has been neitherreduced growth of mature pines nor was there continuous increased growth ofimmature stands in comparison to the previous growth of mature stands. Fig. 2demonstrates this on the basis of cumulative growth curves of mature andimmature stands at two monitoring sites in Grunewald.
It was surprising that no correlation existed between diameter growth and degree of damage classifications of the forest damage survey, nor to the needle density grades determined by MEYER (1989) and KALHOFF (1993).Only trees with extreme needle loss showed reduced growth (see above). Two hypotheses were considered: growth reduction from production limitations causedby pollutant gasses (air vector); and root damage caused by acidity (soilvector). Neither hypothesis was supported by analyses of stand growth.
Emissions are not the only causes of growth reduction. Examinations were conducted to determine whether predispositions to other stress factors existed. Dendo-chronological studies analyze in particular the influence ofshort term stress factors such as extreme climatic conditions and pests and harmful organisms. Growth variations of Berlin pines up to 1900 can be explained mainly by climatic events. An analysis of negativeindicator years, years of extreme growth reduction, also made the outstandinginfluence of climate clear. The eight most prominent indicator years were hotand dry summers in 1911, 1954, 1976, and 1989; very cold winters in 1939/40,1941/42; and years when winter temperatures were strongly variable and followedby hot and dry summers, in 1947 and 1969. In order to test if there had been aclimate sensitivity increase caused by pollutants in recent decades, V.LÜHRTE divided the study span into two phases: 1905-1946, as a lessimpacted period; and 1946-1986, as a period of increasing air pollution.
Calculations of climate/growth functions (the so-called responsefunction) show that the amount of total growth variation in recent decadesexplainable by climate had increased. Values for 1905-1946 were 45-55%,depending on the location of the tested stands, but they rose to 60-75% after1946. But it cannot be immediately concluded that a greater sensitivity ofgrowth to climate was due to pollutants. The possibility must be consideredthat the increase of climate-related events is due to differences in theinteraction between such long-term climatic factors as temperature,precipitation and cloudiness. It is also possible that changes in siteconditions not due to pollution caused increases in sensitivity to climate.
The comprehensive analysis concludes that an increase in sensitivity ofdiameter growth to climatic events couldn't be proven. With the exception ofindividual trees with extreme canopy dieback (WSE damage classification >2), influence on growth was exerted primarily by climatic events and forestplantation management interventions (V. LÜHRTE 1992).
Dendro-chronological studies on 80 oaks (V. LÜHRTE 1991) show noreduction of trunk diameter growth in spite of greater crown defoliation. Arelation to the degree of defoliation (damage classification 0-2) could not beproven either. There were, however, clear differences between oaks and pines intheir sensitivity to climatic influences. Temperature was only a smallinfluence in the mean stand curves recorded from 1907 on. Growth reductionswere produced only by extremely cold winters. Dependency on precipitation wasmuch more strongly marked.
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