Seasonal Changes of Forest Canopy Properties 311 Leaf Area Index

Two-year measurements of LAI by using the hemispherical photography analysis showed remarkable changes in LAI over the growing season of deciduous canopy tree species (Fig. 1a). The low values of about 1.0 at the beginning and end of the season reflect the projected area of branches. LAI increased with increasing single leaf area expansion (data not shown) and reached the maximum in mid-June (DOY of about 170) in both 2003 and 2004. LAI remained constant by early October, and then decreased with leaf senescence and fall. Both increasing and decreasing rate of LAI were slightly different between the two years. These differences might be caused by the yearly variation of meteorological conditions including precipitation and air temperature. Further measurements for several years are necessary to analyze this fluctuation, and this would be one of the important ecological determinants of the seasonality of ecosystem carbon gain and its yearly variation (Yamamoto et al., 1999; Saigusa et al., 2002).

With these seasonal changes in LAI, light penetration into forest un-derstory, which was expressed as diffuse light transmittance ( — 1-FPAR (fraction of PAR absorbed by canopy)) estimated from the hemispherical canopy photographs, decreased during the period of leaf area expansion from May to June, remained constant until early October, and increased with leaf fall in autumn (Fig. 1b). The maximum diffuse light transmittance of about 0.5-0.6 was observed at the beginning and at the end of growing season, and the minimum of about 0.11-0.15 was observed in summer. The seasonal change of understory light environment affects considerably the photosynthetic rate of evergreen dwarf bamboo and deciduous shrub trees, whose leaf expansions begin earlier than the canopy trees (H. Muraoka, unpublished observation).

Figure 1: Seasonal changes of leaf area index (LAI) (a) and diffuse light transmittance to understory (b) for 2 years (2003, open circle; 2004, solid circle) in a cool-temperate deciduous broadleaved forest. Mean + SD for 14 locations within a 1-ha study plot are shown.

Figure 1: Seasonal changes of leaf area index (LAI) (a) and diffuse light transmittance to understory (b) for 2 years (2003, open circle; 2004, solid circle) in a cool-temperate deciduous broadleaved forest. Mean + SD for 14 locations within a 1-ha study plot are shown.

3.1.2 Leaf Physiological Characteristics

Photosynthetic rate at light saturation (Amax), dark respiration (R), and index of chlorophyll concentration (SPAD) showed remarkable seasonal changes in the two deciduous broadleaved tree species of Betula and Quercus (Fig. 2). Although the apparent patterns of leaf physiological characteristics were similar between the sunlit (canopy top) and shaded (inner canopy) leaves of Quercus, Amax and R were higher in sunlit leaves throughout the growing seasons, indicating that this deciduous broadleaf tree changes the photosynthetic characteristics along the gradient of local light environment in its canopy (see also Niinemets and Tenhunen, 1997; Niinemets and Valladares, 2004).

Quercus sunlit (canopy top)

Quercus shaded (inner canopy)

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Betula

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200 250 DOY

Quercus sunlit (canopy top)

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Figure 2: Seasonal changes of single leaf photosynthetic characteristics in Betula ermanii (left column), sunlit leaves (center column), and shaded leaves (right column) of Quercus crispula for 2 years (2003, open circle; 2004, solid circle). Light-saturated photosynthetic rate (Amax; a-c), dark respiration rate (R; d-f), and index of chlorophyll concentration (SPAD; g-i) for three to seven sample leaves (mean + SD) are shown.

Betula

At the beginning of the growing season, Amax and SPAD were the lowest and R was the highest. Then Amax and SPAD increased rapidly until their maximum in early summer and remained high values during the summer, and decreased in autumn. On the other hand, R decreased rapidly until the leaf area maturation at DOY of 170, and remained relatively constant until the leaf senescence. The increase of Amax and SPAD at the first phase of the phenology indicates the physiological growth of leaf biochemistry (Grassi and Magnani, 2005; Grassi et al., 2005), while the high R in this phase indicates the construction costs for leaf anatomy and expansion of leaf area (Miyazawa and Terashima, 2001).

The initiation of decline was different between Amax and SPAD, suggesting the different senescence schedule or responses to autumn climate between the components responsible for photochemistry and carbon fixation involved in the photosynthetic reaction. Our previous analyses for Vcmax and Jmax using the data obtained throughout the growing season showed that the ratio of Jmax to Vcmax changes seasonally; the ratio was higher at the beginning and the end of the season (Muraoka and Koizumi, 2005). Such tendency has also been reported in some studies (Hikosaka et al., 1999; Wilson et al., 2000, 2001; Xu and Baldocchi, 2003; Onoda et al., 2005), and it can be argued that the seasonal change of Jmax/Vcmax reflect the shift of nitrogen allocation within a leaf so as to have an efficient photosynthetic resource utilization under changing temperature conditions.

The 2-year measurements of these leaf physiological characteristics revealed the existence of year-to-year variation as was also observed in LAI. The most remarkable difference was found for Amax of Betula and sun leaves of Quercus in mid-summer (Fig. 2). In 2003, Amax of Betula and Quercus reached the maximum in late summer (late August to early September), while Amax of Betula reached the maximum in early summer (mid-June), but Amax of Quercus in mid-summer (mid-August). This seasonality in Amax and its yearly variation would no doubt affect the photosynthetic productivity of the forest ecosystem.

These differences in leaf physiological phenology would involve two different reasons: the leaf ontogenetic characteristics of the two tree species and the yearly variation of climate conditions. Betula is recognized to be a pioneer species characterized by its high photosynthetic activity under high light conditions (Koike, 1988). The rapid leaf morphological and physiological development of this species would highly contribute to the high growth rate. On the other hand, Quercus is recognized to be a mid-late successional species, which has tough leaves (Koike, 1988). The rates of leaf anatomical and physiological developments have been suggested to be related to the leaf thickness or leaf mass area ratio (LMA); the thicker or tougher the leaves, the slower the rate of physiological development (fulfilment of biochemical components) as compared to the anatomical development (Miyazawa et al., 1998). This perspective is thought to be adaptive in ecological sense since mid-late successional species have to grow in a resource-limited environment of forest understory during the juvenile stages. Therefore, the cost-benefit relationships, which are determined by costs for leaf construction and herbivory and benefit by photosynthesis for these species, should be more severe than the species in resource-rich environment (Mooney and Gulmon, 1982; Miyazawa et al., 1998).

The yearly difference of the developmental rate of Amax in both Betula and Quercus might reflect the difference of climate conditions in early to mid summer. In 2003, the rainy season was irregularly prolonged as compared to the other years, while the climate condition in 2004 was relatively regular. Since leaf physiological development and photosynthetic capacity (Amax) are constrained by light incidence (Niinemets et al., 2004), the shortage of light incidence by the long rainy season might have inhibited the growth of leaf biochemistry in 2003. If this is true, it gives us insight into the relationship between morphological (leaf area) and physiological (photosynthesis) developments and climate conditions, since such information would be important in modeling ecosystem function such as canopy photosynthesis.

The different phenology in leaf area and physiological development of trees composing the forest further lead to the different developmental rate of canopy LAI and photosynthetic capacity; the earlier development of LAI as compared to leaf photosynthetic capacity, and rapid decrease in respiration rate during leaf area development (Figs. 1 and 2). These results (Figs. 1 and 2) and other reports (Wilson et al., 2001; Morecroft et al., 2003; Xu and Baldocchi, 2003) strongly indicate that the photosynthetic productivity of the deciduous forest canopy should be evaluated by considering the different patterns of these plant characteristics; if one assumes that in the ecosystem-scale model the photosynthetic capacity (Amax or Vcmax) is constant throughout the growing season while LAI changes seasonally, the estimated photosynthetic productivity must be overestimated considerably (Ito et al., 2006).

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