The individual growth of tree diameter at breast height (dbh) is analyzed in an even-aged plantation of Cryptomeria japonica from stand age of 45 to 94 years, to examine how the growth of individual trees has been affected by the changes in spacing resulting from thinning operations. At any age, a significant proportion (0.37-0.46) of the variation in dbh growth during a 5-11-year period was explained by dbh at the beginning of the period, probably due to greater leaf mass of larger trees. Next, either one-sided or two-sided competition was added to the model, by calculating the basal area (BA) of neighboring trees around each tree within a given radius or BA for trees having larger dbh than the focal tree within the radius. After preliminary analyses, a radius of 8 m was selected as the critical range for tree competition. Although both types of competition explained a significant proportion (0.09-0.43) of growth variation, one-sided competition was not significant at ages greater than 54 years. Based on the model at 45 years of age, the initial deviation of growth rate for each tree from the predicted rate was calculated and added to the models as a third variable. This raised the coefficient of determination up to 0.50-0.74. These findings have practical significance for forest plantation management, particularly for controlling the growth of standing trees via thinning, to produce high-quality timber in the future.
1.One of the major problems in understanding growth trends in long-lived trees is the difficulty of separately quantifying the effects of tree size and age. Careful statistical control of the axiomatic age×size covariation is therefore required to identify long-term trends in tree growth and their drivers, and to predict forests' responses to environmental changes reliably.
2.To address this issue, we present a novel tree growth model: a 'two-dimensional lognormal growth model'. This is an extension of the one-dimensional lognormal growth model, in which tree growth is modelled primarily as a function of size. Our model assesses the trend in tree growth over time by explicitly partitioning the effects of age and size, controlling the covariation. The model is then extended to incorporate the effects of neighbourhood crowding and individual tree variation.
3.To demonstrate our model, we apply it to long-term monitoring data from a mature (104-yearold) plantation of Japanese cedar. Thinning operations of various intensities have been applied to this plantation, and the diameter of each individual tree has been measured repeatedly.
4.We observed a pronounced age-related decline in diameter growth. However, at each age, greater tree size was associated with a higher growth rate. The growth-size curve predicted from the model became flatter with tree age, and the curve's peak shifted rightwards as tree age increased. The model reveals that the sensitivity of a target tree to neighbourhood crowding depends strongly on neighbours' size, and also provides an estimate of among-tree variation in
Accepted ArticleThis article is protected by copyright. All rights reserved. growth performance.
5.Although the relationships between growth, size and age in long-lived trees are very complex, our growth model supports the conclusion that it is possible to predict long-term trends in tree growth reliably with respect to both age and size. In addition, the model's flexibility will facilitate more robust testing of species-specific responses to long-term environmental changes.
We have studied the photosynthetic production and growth of Thujopsis dolabrata var. hondai (hiba) seedlings under typical light conditions found in mixed forests, including constant shade, phenological gaps under broad-leaved trees with different lengths of foliation period, and in an open plot. Leaves sampled from the open plot had significantly higher rates of light-saturated gross photosynthesis and dark respiration, and a significantly lower specific leaf area than leaves from the other plots. The relative growth rate of whole plant biomass was significantly higher in plots that received larger amounts of light. The variations in these properties under the various light conditions are considered to reflect the ability of hiba seedlings to adapt to available light. In the late autumn, leaves sampled from the open and from the gap under deciduous trees indicated photoinhibition. However, the estimated value of monthly net photosynthesis of hiba seedlings was greater in these plots, indicating that the benefit of a greater light intensity for photosynthesis seems to be larger than the detriment of photoinhibition. The seasonal pattern of photosynthesis by understory hiba seedlings was affected by the phenology of canopy trees. Light availability under the canopy of deciduous trees associated with phenological gaps helped hiba seedlings to tolerate the relatively dark conditions during the subsequent foliation period. These results for reaction to the light regime and for the phenology of hiba seedlings are practicable for hiba forest management.
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