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[ACG39-04] Individual-based analyses of the carry-over effects from recent climatic conditions on the interannual variability of LAI of an evergreen conifer stand
Keywords:evergreen conifer forest, leaf area index, carry-over effects from recent climatic conditions, biomass growth, competition among individuals, interannual variability
The leaf area index (LAI) of a forest stand has been known to remain within a specific range after a peak at relatively young stand ages. For evergreen stands, however, little has been known about interannual variability of LAI and its effects on biomass increase. In this paper we introduce a series of studies ([1], [2]) in which interannual variability of the LAI and the growth rates of stem biomass were investigated with biological and climatic factors affecting them in an evergreen hinoki cypress (Chamaecyparis obtusa) stand between 21 and 40 years of stand age. In this long-term investigation, the stem diameter at the crown base (DCB) was recorded for all living trees each year. This allowed us to estimate the leaf area of a tree (tree LA) using an allometric equation, which is known to be little affected by the difference in stand ages. Given the tapered shape of a tree stem within the crown, a rapid rise of the crown base height can lead to a decrease in DCB, or a decrease in the tree LA. The other measurements also allowed us to estimate the stem dry mass for each tree in each year, and then the stand LAI and the stand stem biomass. The prediction intervals of LAI and stand stem biomass were evaluated for each year by propagating errors of those individual-based estimates. Monthly meteorological data were obtained using the climatic measurements recorded at the nearest weather station (AMeDAS).
The LAI varied between 7.1−8.8, with a relatively long fluctuation cycles over the observation period. This LAI range was maintained such that the gradual increase in the LA of the largest trees counterbalanced the gradual loss in LA of the smallest trees. The decrease in LA of the smallest trees resulted in their gradual death. Just before their death year, their tree LA had decreased to ~1% of the LAI of that year. Hence their death did not affect the LAI. These smallest trees were able to survive for many years even as their LA was decreasing, which would be important for LAI to be maintained without a sudden decrease. The total basal area decreased twice during the 20 years, whereas the stand stem biomass never decreased despite the death of trees. This was because the portion of the stem within the crown maintained a certain level of radial growth even in the trees with almost no stem radial growth at breast height.
There was a weak but significant positive relationship (r=0.40) between the LAI and the current-year mean summer temperature (July and August). This indicates that LAI tended to increase in the year with warm summer during which new leaves are produced. Further, we found that the moving average of the summer temperatures of 6 years (the current year and the previous 5 years) indicated a much stronger relationship (r=0.93) with the current-year LAI. According to a previous study in the same forest, the mean turnover time of the canopy leaves was estimated to be 4.3–6.3 years, suggesting that the summer temperature of the past years when the leaves in the current-year canopy accumulated was strongly influential in the LAI of the current year.
As canopy photosynthesis would generally be proportional to LAI, we had expected that the annual stem biomass increase rate (dBSTEM) would be greater in a year with a high LAI. However, the relationship between them was not significant (r=0.09). Instead, dBSTEM had a positive significant relationship with the sum of precipitation of early summer (May-July) of the current year. Preceding studies of tree physiology have pointed out that a considerable portion of non-structural carbohydrates (NSC) from photosynthesis are used for purposes other than growth (e.g., maintenance of physiological homeostasis), and that the priority of the NSC to be allotted to growth would be lower than any other uses. This partly explains why dBSTEM was not correlated with LAI.
[1] Sumida et al. (2013) Tree Physiology 33, 106–118; [2] Sumida et al. (2018) Scientific Reports 8, 13950.
The LAI varied between 7.1−8.8, with a relatively long fluctuation cycles over the observation period. This LAI range was maintained such that the gradual increase in the LA of the largest trees counterbalanced the gradual loss in LA of the smallest trees. The decrease in LA of the smallest trees resulted in their gradual death. Just before their death year, their tree LA had decreased to ~1% of the LAI of that year. Hence their death did not affect the LAI. These smallest trees were able to survive for many years even as their LA was decreasing, which would be important for LAI to be maintained without a sudden decrease. The total basal area decreased twice during the 20 years, whereas the stand stem biomass never decreased despite the death of trees. This was because the portion of the stem within the crown maintained a certain level of radial growth even in the trees with almost no stem radial growth at breast height.
There was a weak but significant positive relationship (r=0.40) between the LAI and the current-year mean summer temperature (July and August). This indicates that LAI tended to increase in the year with warm summer during which new leaves are produced. Further, we found that the moving average of the summer temperatures of 6 years (the current year and the previous 5 years) indicated a much stronger relationship (r=0.93) with the current-year LAI. According to a previous study in the same forest, the mean turnover time of the canopy leaves was estimated to be 4.3–6.3 years, suggesting that the summer temperature of the past years when the leaves in the current-year canopy accumulated was strongly influential in the LAI of the current year.
As canopy photosynthesis would generally be proportional to LAI, we had expected that the annual stem biomass increase rate (dBSTEM) would be greater in a year with a high LAI. However, the relationship between them was not significant (r=0.09). Instead, dBSTEM had a positive significant relationship with the sum of precipitation of early summer (May-July) of the current year. Preceding studies of tree physiology have pointed out that a considerable portion of non-structural carbohydrates (NSC) from photosynthesis are used for purposes other than growth (e.g., maintenance of physiological homeostasis), and that the priority of the NSC to be allotted to growth would be lower than any other uses. This partly explains why dBSTEM was not correlated with LAI.
[1] Sumida et al. (2013) Tree Physiology 33, 106–118; [2] Sumida et al. (2018) Scientific Reports 8, 13950.