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Tree Physiology Advance Access originally published online on December 5, 2008
Tree Physiology 2009 29(1):137-145; doi:10.1093/treephys/tpn014
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Large-scale canopy opening causes decreased photosynthesis in the saplings of shade-tolerant conifer, Abies veitchii

Masako Mitamura1,2, Yasuo Yamamura1 and Takashi Nakano3

1 Laboratory of Ecology, Faculty of Science, Ibaraki University, 2-1-1, Bunkyo, Mito 310-8512, Japan
2 Corresponding author (rico3886ra{at}yahoo.co.jp)
3 Yamanashi Institute of Environmental Sciences, Yamanashi 403-0005, Japan


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
Although the environmental change by canopy gap formation in a forest improves the light availability for the saplings on the forest floor, it may result in stresses on the saplings due to high radiation and drought. In large-scale gaps, the photosynthesis of shade-tolerant species may be inhibited by high radiation and drought stress if they lack effective tolerance or avoidance mechanisms for the stresses. We investigated the photosynthetic traits and water relations of Abies veitchii Lindl. saplings in an open habitat created by an avalanche and in a nearby forest floor habitat undisturbed by the avalanche. We analyzed the influence of exposed conditions on sapling photosynthesis. The maximum photosynthetic rate of the saplings in the open habitat was lower than that in the forest habitat. The ratio of variable to maximum chlorophyll fluorescence (Fv/Fm) was lower in the open habitat than that in the forest habitat during the late growing season, indicating that the open habitat saplings suffer photoinhibition of photosystem II for a long period. A lower Rubisco concentration in needles in the open habitat indicated the breakdown of this photosynthetic protein because of excess solar energy resulting from serious photoinhibition. The shoot water potential of the saplings in the open habitat at daytime was higher than that of the saplings in the forest habitat because of less transpiration caused by the remarkable stomatal closure in the open habitat. Although these acclimations to high radiation improve the tolerance of A. veitchii saplings to high radiation and drought stress, they would result in low gain of daily carbon and a reduction in growth in the open habitat.

Keywords: avalanche, canopy gap, chlorophyll fluorescence, light acclimation, photoinhibition, Rubisco

Received July 3, 2008; Accepted September 15, 2008


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
Although canopy gap formation may provide improved availability of light for seedlings and saplings on the forest floor (Gray and Spies 1996), it may also cause severe stresses on the saplings due to high radiation and drought, particularly in large-scale gaps (Wayne and Bazzaz 1993, Naidu and DeLucia 1997, Houter and Pons 2005). Plant growth in a stressful environment has often been studied in relation to sensitivity and tolerance of stresses (cf. Larcher 1995, Schulze et al. 2005, Valladares et al. 2005). Therefore, for the seedlings and saplings on the forest floor, the responses of photosynthesis and growth to canopy gap formation must be associated with their stress response traits in the light regime in a gap (Canham 1988, 1989, Whitmore 1989, Mulkey and Pearcy 1992).

Excess light beyond the photosynthetic demand causes photoinhibition of photosystem II (PSII) (Long et al. 1994), and in severe cases this leads to photodamage; reactive oxygen species produced by the excess solar energy disrupt the photosynthetic apparatus (photo-oxidation; Demmig-Adams and Adams III 1992, Asada 1999). Therefore, leaves need to avoid photodamage to maintain high photosynthetic activity under high radiation (Mulkey and Pearcy 1992). Prolonged exposure to solar radiation often causes drought stress in plants. As increases in leaf temperature caused by solar radiation result in a large water vapor pressure deficit (VPD) on the leaf surface (Maherali et al. 1997), leaves exposed to high solar radiation have to transpire more and may consequently lose their water balance. Hence, plants must escape from lethal drought via dehydration tolerance (e.g., Nguyen-Queyrens and Bouchet-Lannat 2003), effective water supply/transport to the leaf (e.g., Maherali et al. 1997), or stomatal adjustment (e.g., Valladares and Pearcy 1997, 2002, Uemura et al. 2000). Stress tolerance or avoidance mechanisms are more important for plants exposed in large gaps than in small gaps because the extent of the stress depends on the gap size.

Snow avalanches, the major disturbances that affect subalpine vegetation, sometimes create large-scale canopy gaps in forests. When canopy trees are destroyed by a surface avalanche, seedlings and saplings that have been acclimated to shaded and moist conditions on the forest floor are exposed to strong sunlight and arid conditions. In a subalpine forest on the north-facing slope of Mt. Fuji, a surface avalanche destroyed the canopy trees in 1998 and dramatically changed the environment for Abies veitchii Lindl. saplings (cf. Nashimoto and Ishii 1999). Many reports have shown that the formation of a small-scale gap can be an opportunity for improved growth of A. veitchii saplings (e.g., Yamamoto 1993, 1995, Narukawa and Yamamoto 2001); however, in open sites created by avalanches, a decrease in the growth of A. veitchii saplings has been observed (Mitamura, personal observation).

The primary factor responsible for growth inhibition in A. veitchii saplings is the large-scale gap formation, and we hypothesized that the saplings cannot tolerate, or avoid, severe stress from high radiation and drought, and therefore, their photosynthetic systems cannot work efficiently in large canopy gaps. To test this hypothesis, we examined the photosynthetic traits and water relations of A. veitchii saplings both in an open habitat that was created by an avalanche and in a nearby forest floor habitat, and we analyzed the physiologic mechanisms that environmental stresses cause in terms of decreasing photosynthesis and growth in large-scale gaps.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
Site description and materials
On Mt. Fuji (138°44' N and 35°21' E; 3776 m altitude), a geologically young volcano in central Japan, Larix kaempferi (Lamb.) Carr. is the dominant vegetation at the forest limit being replaced by A. veitchii vegetation through succession (cf. Ohsawa 1984, Nakamura 1985). Our study was conducted in an L. kaempferi forest and the adjacent open avalanche site at an altitude of 2250 m on the north-facing slope of Mt. Fuji. The open site was created by a surface avalanche in 1998 that downed the canopy trees in the L. kaempferi forest over an area 160 m wide but did not disturb the forest floor vegetation containing the A. veitchii saplings. The maximum and minimum monthly mean air temperatures at 2250 m are 13.8 °C in August and –8.8 °C in January, estimated from climate data obtained at the base of Mt. Fuji (860 m altitude; Kawaguchiko Meteorological Observatory 1971–2001) using a mean lapse rate of –0.6 °C per 100-m increase in elevation. The mean annual precipitation is 1508 mm at the observatory. Both the open and the forest sites are covered with snow from December to May. The seasonal maximum depth of snow cover on the open site was from 38 cm (in 2005/2006) to 204 cm (in 2002/2003) in the past 8 years (Nashimoto, personal communication).

Abies veitchii is an evergreen conifer that dominates subalpine climax forest in Japan. On Mt. Fuji, A. veitchii saplings occur frequently in successional L. kaempferi forest, but rarely on bare ground (Ohsawa 1984, Nakamura 1985). The seedlings of A. veitchii cannot establish themselves on dry surfaces such as bare ground because of their short root system (Yura 1988, 1989), whereas they can establish and survive under the closed forest canopy because of their high-shade tolerance (Kohyama 1983). We compared the saplings between open and forest habitats. The saplings in the open site have shorter shoots and denser foliage than those in the forest habitat. We examined the upper, unshaded shoots of the saplings that were about 30–40 cm tall and 15–20 years old.

Light environment
Canopy openness of the open and forest habitats, determined by hemispherical photography at four points in each habitat on August 30, 2006, was 65.0 ± 0.6% (±SD) and 12.2 ± 1.3%, respectively. The photosynthetic photon flux density (PPFD) was measured 40 cm above the ground in both habitats using quantum sensors (IKS-27; Koito Industries, Japan), and the data were recorded every 10 min by a data logger (LP-97J; LP-Denshi, Japan) from September 5 to 11, 2006. In the open habitat, high PPFD > 1000 µmol m–2 s–1 was recorded for many hours on sunny days. The frequency of high PPFD (> 1000 µmol m–2 s–1) was ≥ 30% in daytime in the open site, whereas it was only ≤ 0.2% in the forest site, where the PPFD was generally low (< 100 µmol m–2 s–1; Figure 1).


Figure 1
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  		Figure 1. The PPFD in the open (open column) and forest (closed column) habitats over 7 days (September 5–11, 2006). The values were determined from the data measured every 10 min from sunrise to sunset. PPFD was divided into four classes.

 
Chlorophyll, nitrogen and Rubisco content of needles
Chlorophyll was extracted from 1-year-old needles collected in early September 2006 using 80% aqueous acetone, after the measurement of the projected area. The chlorophyll concentration was determined using a spectrophotometer (DU500; Beckman Coulter, Fullerton, CA) according to the methods of Porra et al. (1989).

Nitrogen and Rubisco concentrations in the needles collected in early October 2006 were measured. The nitrogen concentration was determined by gas chromatography using an N–C analyzer (Sumigraph NC-950; Sumika, Japan) after measuring the projected area and dry mass of the needles. The Rubisco concentration was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). For SDS-PAGE, the needles of known projected area were ground in a chilled pestle and mortar in a buffer containing 100 mM Tris–HCl (pH 8.0), 5 mM monoiodoacetic acid, 5 mM EDTA, 5% (w/v) SDS and 4 M urea. The homogenate was centrifuged at 10,000g for 15 min at 4 °C, and the supernatant was mixed with 5% (v/v) 2-mercaptoethanol and 0.01% (v/v) bromophenol blue and subjected to SDS-PAGE using 12.5% acrylamide gel. After SDS-PAGE, the gel was stained with Coomassie Brilliant Blue R-25, and the amount of Rubisco was measured spectrophotometrically by scanning the gel (GT-8700; EPSON, Japan) and by using the software provided by Densitograph (ATTO, Japan).

Chlorophyll fluorescence
To estimate the activity of PSII under strong light in 1-year-old needles, chlorophyll fluorescence was measured using a fluorometer (Mini-PAM; Heinz Walz, Germany). Before measurement, the needles were collected in the morning and placed in darkness for 30 min at room temperature (22 °C). Using the minimum fluorescence (Fo) and maximum fluorescence (Fm) in the dark-adapted state, the maximal photochemical efficiency of PSII was estimated as the ratio of variable to maximum fluorescence (Fv/Fm, where Fv = Fm – Fo). The needles were exposed to stepwise sequences of halogen and actinic lights. From these values, we determined the electron transport rate (ETR) around PSII as ETD = 0.5({Delta}F/Fm')PPFD{alpha}, where {Delta}F = Fm' – F, Fm' is the maximum fluorescence, F is the steady fluorescence in the light-adapted state and {alpha} is the needle absorptance calculated from the total chlorophyll concentration per unit needle area (Evans 1993). We also determined the dissipation of light energy in the photosystems: non-photochemical quenching (NPQ), a measure of thermal dissipation, as NPQ = Fm/Fm' – 1; and photochemical quenching (qP) as qP = (Fm' – F)/(Fm' – Fo'), where Fo' is the minimum fluorescence in darkened needles just after exposure to actinic light and Fo/(Fv/Fm + Fo/Fm') according to Oxborough and Baker (1997). The ratio Fv/Fm was measured at about 1-month intervals from May to October 2006, and ETR, NPQ and qP were measured in early September.

Water relations
To assess osmotic regulation, we determined the relationship between shoot water potential ({Psi}) and shoot water content (pressure–volume [P–V] curve analysis; Tyree and Hammel 1972, Maruyama and Morikawa 1983) in late August 2006. One-year-old shoots were collected on the previous day and kept in water in a humidified polyethylene bag in a dark room until measurement. The {Psi} was measured using a pressure chamber (PMS-600; Soil Moisture Stress, Corvallis, OR) just after the fresh mass was obtained. The measurement was carried out periodically while the shoot was drying at room temperature. A P–V curve was made in which {Psi} was plotted against relative water content (RWC), calculated from the difference between fresh and dry mass of the shoot. We calculated the osmotic potential at full turgor ({Psi}sat) and {Psi} and RWC at the turgor loss point ({Psi}tlp and RWCtlp) from the P–V curve.

Soil-to-shoot hydraulic conductance (Kp) indicates the water transport ability from the root to the leaf/needle surface and can be determined as the reciprocal of the slope of the regression of {Psi} and transpiration (Reich and Hinckley 1989, Saito et al. 2003). To determine Kp, we measured {Psi} in the field using the pressure chamber and the transpiration rate of needles (E) using an open gas exchange system (Li-6400; LiCor, Lincoln, NE) on August 22, 2007. The E for the shoots was measured in a LiCor chamber (2 x 3 cm; LiCor) just before the measurement of {Psi}, and two such measurements were taken several times during the morning. The projected area of the collected needles used in the measurements was then taken. The value of E and Kp may be overestimated in the measurement using Li-6400, because the boundary layer on the leaf surface is minimized in the chamber of Li-6400. The possible maximum error in E is about 20% at wind velocities between 0.1 and 1 m s–1, when stomatal conductance to water vapor (gs) is 0.2 mol m–2 s–1 and leaf length is 0.05 m (Ishida et al. 1996, 1999a).

Response of photosynthesis to light
To examine the response of photosynthesis to light, we measured the net photosynthetic rate (A) of 1-year-old needles at various light intensities using a Li-6400 in early September 2006. Shoots were collected in the early morning and were watered, and the needles on the shoot were placed in the LiCor chamber (Li-6400-02B; LiCor). The air temperature and VPD were controlled at 22 ± 3 °C and < 1.0 kPa, respectively. The CO2 concentration of the air introduced into the chamber was kept at 360 µmol mol–1, and PPFD was supplied in 11 steps between 0 and 1200 µmol m–2 s–1 using an LED light source. The projected area and dry weight of the needles were then measured.

Diurnal changes in gas exchange, ETR, shoot water potential and microclimate
On September 2, 2006, the diurnal changes in A, E, gs, and the chlorophyll fluorescence were measured in intact 1-year-old needles in a marked shoot of each of the four saplings in both habitats at 1- or 2-h intervals from about 07:00 to 14:00. The A, E and gs were measured using Li-6400, and the chlorophyll fluorescence was measured using the fluorometer to determine the ETR at the field PPFD. After the measurements, the projected areas and dry mass of the needles were obtained. At the same time, {Psi} was measured in some shoots near the saplings selected for gas exchange measurements using a pressure chamber.

During these measurements, the PPFD was determined using a quantum sensor, and air and needle temperatures were measured using copper–constantan thermocouples. The PPFD and air temperature were measured 40 cm above the ground, and needle temperature was measured on the abaxial side of needles. These data were recorded every 5 min by a data logger (Thermodac-E; Eto-Denki, Japan).


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
Dry matter and chemical content of needles
The leaf mass per unit area (LMA), a ratio of the dry mass to the projected area of the needles, was significantly greater in the open habitat than that in the forest habitat (Table 1; P < 0.05, t test). The total chlorophyll concentration (g m–2) in the open habitat was significantly lower than that in the forest habitat (P < 0.001, t test). The nitrogen concentration (g m–2) in needles did not differ significantly between habitats (P > 0.05, t test). The concentration of Rubisco that contains a large amount of nitrogen (Evans 1989) was lower in needles in the open habitat than in the forest habitat (P < 0.05, t test).


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  		Table 1. The LMA, and chlorophyll, nitrogen and Rubisco concentrations in the needles of A. veitchii sapling in open and forest habitats. The LMA was calculated from the needles used to measure photosynthesis. Values are the mean ± SD (n = 3–5). Significance differences between mean values from different habitats were evaluated by Student’s t test.

 
Chlorophyll fluorescence and photosynthetic capacity
The Fv/Fm of 1-year-old needles was significantly lower in the early spring than in the summer in both habitats (Figure 2; P < 0.01, t test). The Fv/Fm of the open habitat needles was significantly lower than that of the forest habitat needles on and after September 3 (P < 0.01, t test). Figure 3 shows the responses of ETR, NPQ and qP to PPFD for needles in the early September. The ETR of the open habitat needles was significantly lower than that of the forest habitat needles at PPFD < 1000 µmol m–2 s–1 (P < 0.001, t test), but it was not significantly different at higher PPFD (> 1000 µmol m–2 s–1; P > 0.05). The NPQ saturated at about 600 µmol m–2 s–1 PPFD in both habitats, and the saturated NPQ did not differ significantly between the habitats (P > 0.05). The qP of the open habitat needles was significantly higher than that of the forest habitat needles only at PPFD > 1000 µmol m–2 s–1 (P = 0.02).


Figure 2
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  		Figure 2. Seasonal changes in the Fv/Fm of 1-year-old needles in open (open circles) and forest (solid circles) habitats. Values are the mean ± SD (n = 4–7). Significant differences between habitats were evaluated by Student’s t test; **P < 0.01, ***P < 0.001.

 

Figure 3
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  		Figure 3. Dependence of the ETR in PSII, NPQ and qP on PPFD in 1-year-old needles from open (open circles) and forest (solid circles) habitats. Measurements were made on September 3, 2006. The mean ± SD is shown (n = 5). Significant differences between habitats were evaluated by Student’s t test; *P < 0.05, ***P < 0.001.

 
The net photosynthetic rate of 1-year-old needles was almost saturated at 1200 µmol m–2 s–1 PPFD in both habitats. If this saturated value is the maximum photosynthetic rate (Amax), the Amax of the open habitat needles was significantly lower than that of the forest habitat needles (Table 2; P < 0.001, t test).


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  		Table 2. Maximum photosynthetic rate (Amax), net photosynthetic rate (A10:00), transpiration rate (E10:00), stomatal conductance (gs10:00), and shoot water potential ({Psi}10:00) in the field at about 10:00, and predawn shoot water potential ({Psi}pd) in A. veitchii saplings from open and forest habitats. The Amax was saturated at 1200 µmol m–2 s–1 PPFD. The field values were measured on a clear morning on September 2. The ETR/A ratio was determined from ETR at about 10:00 and A10:00. The mean ± SD is shown (n = 4). Significant differences between habitats were evaluated by Student’s t test.

 
P–V analysis and hydraulic conductance
There were no significant differences in the {Psi}sat and {Psi}tlp of shoots between the open and the forest habitats (Table 3). The slope of the linear regression between {Psi} and E (open habitat: {Psi} = –0.505 – 0.264E, r2 = 0.608, P < 0.0001; forest habitat: {Psi} = –0.503 – 0.385E, r2 = 0.578, P < 0.0001) did not differ significantly between habitats (P > 0.05, t test), indicating that Kp, the reciprocal of the slope, did not differ between habitats (Table 3).


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  		Table 3. Osmotic adjustment parameters ({Psi}sat, {Psi}tlp and RWCtlp) and soil-to-shoot hydraulic conductance (Kp) of A. veitchii saplings from open and forest habitats, calculated from P–V curves. The mean ± SD is shown (n = 4). Significant differences between habitats were evaluated by Student’s t test. The Kp was determined as the reciprocal of the slope of the regression between {Psi} and the transpiration rate (n = 31).

 
Photosynthesis, water relations and ETR in the field
On September 2, 2006, when diurnal changes in photosynthesis and water relations were measured, there were no clouds until about 10:00, and then clouds often intercepted direct sunlight. The open habitat was exposed to high PPFD of > 500 µmol m–2 s–1 from 06:30 to 12:00, whereas the forest habitat was exposed to low PPFD of < 300 µmol m–2 s–1 during this time. The maximum air temperature was 18.7 °C in the open habitat (at 07:55) and 15.3 °C in the forest habitat (at 10:05). The needle temperature was lower than the air temperature in the forest habitat, but was similar to or higher than the air temperature in the open habitat from 09:00 to 17:00. The higher needle temperatures in the open habitat than in the forest habitat indicate that the needles in the open habitat were exposed to higher VPD. The A, E and gs of the open habitat needles tended to be lower than those of the forest habitat needles throughout the day, and these values were significantly lower at about 10:00 (Table 2; P < 0.001, t test). Figure 4 shows the relationship between VPD and gs that is obtained from diurnal gas exchange measurements. Regardless of the changes in the VPD, the gs remained low (< 0.05 mol m–2 s–1) in open habitat needles, whereas it was higher (0.05–0.12 mol m–2 s–1) throughout the day in the forest habitat needles with lower VPD. The predawn water potential in shoots ({Psi}pd) of open habitat saplings was significantly higher than that of the forest habitat saplings (Table 2; P < 0.05, t test). The difference in {Psi} between habitats increased until noon, and {Psi} in the open habitat saplings was significantly higher than that in the forest saplings at 10:00 (P < 0.001). The ETR in the open habitat increased in the morning from 26.4 ± 8.9 µmol m–2 s–1 (at 7:00, 180 µmol m–2 s–1 PPFD) to 237.7 ± 51.9 µmol m–2 s–1 (at 10:00, 1300 µmol m–2 s–1 PPFD), whereas ETR in the forest habitat changed less (3.84 ± 1.17 µmol m–2 s–1 at 7:00, 10 µmol m–2 s–1 PPFD and 6.80 ± 1.66 µmol m–2 s–1 at 10:00, 20 µmol m–2 s–1 PPFD). The change of ETR/A ratio in the morning was much greater in the open habitat than that in the forest habitat (Table 2); the ETR/A at 10:00 was 16 times in the open habitat and 0.8 times in the forest habitat as large as that at about 7:00.


Figure 4
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  		Figure 4. Relationship between VPD and gs of 1-year-old needles in open (open circles) and forest (solid circles) habitats. Measurements were made on September 2, 2006. PPFD during the measurements was 150–1880 µmol m–2 s–1 in the open habitat and 30–410 µmol m–2 s–1 in the forest habitat.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
Physiologic responses to environmental changes in the habitat
Our investigations show how A. veitchii saplings that survived an avalanche responded to the dramatic changes in light and moisture in a large canopy gap created by an avalanche. Saplings in the open habitat were exposed to strong solar radiation on clear days (Figure 1). When a leaf receives excess photons that cannot be used in photosynthesis, photoinhibition occurs (Long et al. 1994). For evergreen plants, the value of Fv/Fm is low (< 0.84, which is the value for undamaged leaves) during winter due to inactivated photosynthetic apparatus by low temperature (Öquist and Huner 2003). The low Fv/Fm in A. veitchii saplings in the early spring may be caused by low temperature (Figure 2). Moreover, the Fv/Fm of the open habitat needles from late summer to autumn was lower than that of the forest habitat needles and was also < 0.84. These indicate that the saplings in the open habitat suffer photoinhibition for a longer period than that in the forest habitat. As serious photoinhibition injures the photosynthetic apparatus (Asada 1999), there is a high risk of photodamage in the open habitat. Photodamage can be avoided by the prevention of excessive radiation absorption or the removal of excess energy from photosystem, such as NPQ (Demmig-Adams and Adams III 1992) and photorespiration (Kozaki and Tageba 1996) or both. The lower chlorophyll content in the needles in the open habitat led to the lower absorption of photons (Table 1) and thus a lower energy supply to PSII. Irrespective of the lower chlorophyll concentration in the open habitat needles (Table 1), the ETR was as high as that in the forest habitat needles at high PPFD (Figure 3). This may result because the open habitat needles had not developed the capacity for NPQ to dissipate excess energy despite having been exposed to high radiation for several years (Figure 3). On the other hand, the ETR/A ratio increased dramatically under high radiation in the open habitat needles (Table 2). This indicates that a part of electrons within PSII would be used not for CO2 assimilation but for another process such as photorespiration under high radiation.

Plants in canopy gaps not only experience high light stress, but also drought stress caused by the drying of soil and air because of the exposure to abundant radiation. Osmotic adjustment increases the dehydration tolerance (Ngugi et al. 2003, Nguyen-Queyrens and Bouchet-Lannat 2003); however, there were no differences in {Psi}sat, (and thus) {Psi}tlp, and RWCtlp (Table 3) that are the measures of osmotic adjustment between the open and the forest habitats. This indicates that the physiologic tolerance of drought did not increase in dry conditions in the open habitat. During the day, the open habitat saplings had higher {Psi} than the forest habitat saplings (Table 2), which may result from better water supply to shoots from the root system (Tani et al. 2001, Shimizu et al. 2005), the low transport resistance of the shoot system (Maherali et al. 1997), with or without the suppression of water release by stomatal closure (Bates and Hall 1982, Jones and Muthuri 1984, Muraoka et al. 1997, Valladares and Pearcy 1997, 2002). The Kp, an indicator of water transport within the whole plant, did not differ significantly between the open and the forest habitats (Table 3), whereas daytime gs was lower in the open habitat than that in the forest habitat (Table 2). These results suggest that the retention of water balance in A. veitchii saplings depends on the suppression of water release, rather than effective water supply. Stomatal closure often occurs under low light (Larcher 1995) or high VPD condition (Ishida et al. 1999a, Muraoka et al. 2000), or with low {Psi} (Comstock and Mencuccini 1998). The result of this study indicates that the open habitat saplings avoid water loss by increasing the sensitivity of stomata to the VPD, leading to rapid stomatal closure (Figure 4).

Influence of high radiation and drought on photosynthesis
Under high radiation and dry conditions, the Amax of the open habitat saplings was lower than that of the forest saplings (Table 2). The Amax is related to the Rubisco concentration; a high Rubisco concentration leads to more effective carboxylation (Hikosaka et al. 1998, Warren and Adams 2001, Takashima et al. 2004). Thus, the low Amax in open habitat needles may result from a low Rubisco concentration (Table 1). Excess solar energy in PSII leads to the breakdown of not only proteins in the photosystem, but also Rubisco (Mehta et al. 1992, Desimone et al. 1996, Ishida et al. 1999b). The lower qP at higher PPFD in the open habitat (Figure 3) indicates that the excess energy is accumulated in the open habitat needles more often than in the forest habitat needles. Therefore, the low Rubisco concentration of the open habitat needles may result from the breakdown of Rubisco by the excess energy supply.

The accumulation of excess energy in PSII also results from depression of energy use in subsequent photosynthetic processes. For example, in the needles of Abies mariesii Mast. exposed to winter solar radiation, active oxygen species are produced by the excess energy: i.e., not used in carbon metabolism because of the inactivation of the Calvin–Benson cycle enzymes by the low temperatures (Yamazaki et al. 2003). As stomatal closure under high light causes a CO2 deficiency due to limited uptake of CO2 through the stomata (e.g., Larcher 1995), light not used in carbon metabolism may remain as excess energy in the photosystem. The stomata of the open habitat needles close when exposed to high radiation; thus, the depression of energy use in carbon metabolism would promote severe photoinhibition and the breakdown of Rubisco. The breakdown of Rubisco might be the origin of a negative feedback that leads to the increased depression of photosynthesis in the open habitat needles.

Plants can respond to a change in the growth environment by morphologic or physiologic acclimations to compensate for the change (Chapin III et al. 2002). The saplings of A. veitchii showed physiologic acclimations under the large gap condition: the removal of excess energy (Table 2), probably by photorespiration, and the stomatal closure (Figure 4). However, these acclimations resulted in low photosynthesis (Table 2). This may be derived from the limitation of physiologic plasticity of A. veitchii as shown in Abies alba Mill. (Grassi and Bagnaresi 2001). The low photosynthesis leads to low daily carbon gain and a reduction in growth (Larcher 1995). These traits in A. veitchii saplings would make them less competitive with pioneer species that are adapted to such conditions, and thus reduce their contribution to the recovery of vegetation after canopy gap formation. In fact, in this large gap, L. kaempferi saplings that are established after the gap formation showed higher activity of photosynthesis and grew much better than the A. veitchii saplings (Mitamura et al. 2008). In the upper subalpine area on Mt. Fuji, successional change of vegetation has occurred from the pioneer species to the climax species (Ohsawa 1984), but it is often set back by avalanches (Tanaka et al. 2008). Avalanches that occur repeatedly on the same site would prevent the succession to Abies forest on the avalanche path. Our results proposed the ecologic significance of physiologic response of a climax species to the large-scale gap formation. To understand the growth pattern of the plant in the successional process at the large-scale gaps, further studies on the effect of the morphologic responses on the carbon balance in whole plant are required.


   Acknowledgements
 
The authors thank Jun-ya Yamazaki and Emiko Maruta for their technical support and valuable comments. They are grateful to Makoto Nashimoto for providing the data about snow. This study was supported by Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 19570013).


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

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