論文の公開元へPhotosynthesis Properties of Pioneer Species on Volcanically Devastated Sites in Miyake-jima Island, Japan
概要
1.1. Background of the research
Volcanic eruptions are powerful agents of large infrequent disturbances and create exemplary living laboratories that can be used to investigate both initial disturbance effects on ecosystems and longer-term successional processes (del Moral and Grishin 1999). Primary succession often occurs following the complete destruction of a biosystem where the ground surface is covered by rocks and/or inorganic materials (del Moral and Bliss 1993; Tsuyuzaki 1995; Vitousek and Walker 1987; Vitousek et al. 1989). Researchers have conducted a series of studies to explore what controls, constrains, or promotes the vegetation development in such harsh habitats (Walker and del Moral 2003) by exploring the importance of microsites (Wood and del Moral 1987), mycorrhizae (Titus and del Moral 1998), biotic interactions (del Moral and Rozzell 2005), and dispersal (Fuller and del Moral 2003; del Moral and Eckert 2005). Many ecological studies of successional changes in species composition, biomass, growth form, leaf size, leaf shape, and canopy arrangement have been conducted following volcanic eruption (e.g., the volcano Mount St. Helens in the USA (del Moral Wood 1993; Wright et al. 2001; Weber et al. 2006) and Mt. Usu (Haruki and Tsuyuzaki 2001); and Miyake-jima volcanic island (Kamijo et al. 2002; Kamijo and Hashiba 2003) in Japan). However, most of the studies were conducted using the insight of the population and community (Mooney and Godron 1983; White 1985; Huston 1994). Few studies have focused on the physiological ecology of species living in such a harsh habitat, which is important for revealing the mechanism of succession and is important for the restoration of the local ecosystem.
1.2. Environmental feedback
The initiation of any succession depends on the initiating disturbance, after which environmental and landscape factors come into play. Stress is the major factor that governs successional rates. Grime (1977) defined stress as any external constraint that limits the rate of dry matter production of all or part of the vegetation. Once plant species commence their invasion, a process strongly affected by landscape factors, conditions normally start to ameliorate, and interaction with the local environment commences. Any environmental factor that affects the rate of biomass accumulation will affect the rate of succession.
The environment of a plant may vary daily, seasonally, vertically, and horizontally. The level of variability is determined by many factors, including climate, geographical location, geomorphological features, the nature of site disturbances, and the number and type of species present. The influence of the environment on the plant depends not only on the level of environmental variability and the predictability of that variation but also on the change in physiology over time. Changes in plant species composition during primary succession are interpreted as reflecting gradients from the high-light and low-nutrient conditions occurring early in the succession to the low-light and high- nutrient conditions occurring later on (Tilman 1982, 1986; Vitousek et al. 1989). The environmental variability in open, early successional habitats is generally thought to be higher than that in closed, late-successional habitats. In the early stage of succession, stresses including infertility and drought (Kardol et al. 2010) severely constrain the vegetation recovery process.
1.2.1. Changes in soil nutrient content
Nitrogen (N) and phosphorus (P) are the nutrients that most frequently limit primary productivity in terrestrial ecosystems (Vitousek and Howarth 1991; Elser et al. 2007). Overall, plant growth in ecosystems with young soils might generally be expected to be N-limited, whereas growth on older soils should be P-limited; in intermediate-aged sites N and P equilibrate at higher supply levels (Walker and Syers 1976).
In the early stages of volcanic primary succession, nitrogen is often the most limiting resource (Vitousek and Howarth 1991; Elser et al. 2007). Unlike other nutrients that can be released through weathering of the underlying rock, nitrogen must either be transported to primary successions through leaching and deposition or fixed in situ. The soil nitrogen content varies greatly at different stages of succession. Plant growth during this stage is severely limited by soil N content. The limitation of N is more serious to early pioneer species than it is to later species (Huante et al. 1995; Fetcher et al. 1996). In general, the difference in nutrient content among soils at the same site is often due to the different types of disturbance, the disturbance severity and the initial conditions (Vitousek and Sanford 1986; Uhl and Jordan 1984; Uhl 1987).
1.2.2. Changes in light conditions
During the succession, light is also an important ecological factor that affects plant growth, survival and distribution (Messier and Puttonen 1995; Veneklaas and Oudena 2005; Wyka et al. 2007; Jensen et al. 2012). The variability of the physical environment is related primarily to the amount of energy that reaches the soil surface and the way in which it is dispersed from the surface. In the early-stage successional habitat, due to the lack of vegetation coverage, plants grow in an open environment with full sunlight. The total amount of radiation above the community varies only minimally. Energy exchange occurs at or near the soil surface, where maximum temperature fluctuations occur, and light energy reaches the surface unaltered. In the later successional stage habitat, the surface of energy exchange is the upper layers of the canopy. The presence of plant canopies results in a low quantity and quality of light, thereby severely limiting the growth and survival of understory plants (Givnish 1988; Messier et al. 1999; Robakowski et al. 2003; Zhang et al. 2003; Veneklaas and Oudena 2005). Temperature fluctuations below the canopy are buffered by the vegetation itself, and progressively less energy penetrates toward the forest floor; the light that does reach the floor is markedly depleted of photosynthetically active wavelengths. Thus, plant growth in the understory during the late successional stage, except in areas of large light gaps, experience a less variable and less extreme environment in the forest with respect to temperature, humidity, and wind. Sunflecks under a canopy result in extremely variable light intensity and possible rapid fluctuation of leaf temperature.
Although the relationships between species and habitat environment have been well clarified for plants grown in constant or controlled environments (Vitousek et al. 1993; Ellsworth et al. 1996; Sun et al. 2016), we know considerably less about such relationships in natural volcanic environments.
1.3. Ecophysiological characteristics of pioneer species after volcanic disturbance
After volcanic eruptions, the damaged vegetation ecosystem begins to enter the recovery phase. The different disturbance severity, and the high heterogeneity of habitats require the vegetation to have distinctive physiological characteristics to adapt to these different habitats. For example, species that successfully colonize bare land generally have physiological or ecological characteristics that are well-adapted to harsh environments and are more tolerant to drought and nutrient stress (del Moral and Wood 1993). Among the countless plant species, pioneer species are undoubtedly the best choice for such conditions. Several studies have shown that pioneer plant species can have a large effect on the entire local ecosystem by influencing the input of nitrogen by biological fixation (Vitousek and Walker 1987; Hughes and Denslow 2005), rates of biogeochemical processes (Ehrenfeld 2003), decomposition and mineralization rates of plant litter (Ehrenfeld et al. 2001; Standish et al. 2004), and stock of nutrients in the soil (Hughes and Denslow 2005). Thus, these effects of pioneer species should be most pronounced when they differ in N-fixing ability and photosynthetic pathway (e.g., C3, C4).
Leaf nitrogen (N) content (Narea), dark respiration rate (Rd), and photosynthetic capacity (Amax, photosynthesis under saturating light) are the core leaf economics traits (Wright et al. 2004). Efficient use of these N is believed to contribute to plant fitness (Aerts and Chapin 1999) and is thought to influence the N cycling and productivity of local ecosystems (Vitousek 2004). The net photosynthesis rate accomplished per unit N, termed photosynthetic N-use efficiency (PNUE), has been considered an important plant functional trait to characterize species in relation to their leaf economics, physiology, and strategy (see Hikosaka 2004 for review). These leaf traits and trait relationships have been well studied on a global scale. However, these growth strategy- related leaf traits for pioneer species in volcanically devastated sites are not well understood. At the leaf level, there is a very strong correlation between Amax and Narea (e.g., Field and Mooney 1986; Sage and Pearcy 1987; Anten et al. 1995). However, the Amax achieved at a given Narea (PNUE) is not the same for all plants (Field and Mooney 1986). In particular, more complete knowledge of PNUE for pioneer species differing in N-fixing ability and photosynthesis pathway, is critical to ecological explanations of succession mechanisms and functions of the volcanic ecosystem.
Colonization of new soils by N-fixing plant species results in greatly improved ecosystem process rates, soil fertility, and nutrition of the co-occurring non-N-fixing plant species (Vitousek et al.1987; Hooper and Vitousek 1998). Furthermore, N-fixing species often have higher leaf nutrient concentrations (Cornelissen et al. 1997; Peltzer et al. 2009) and low LMA (Craine et al. 2002; Wright et al. 2004; Tjoelker et al. 2005) when compared to those of non-N-fixing species. These traits can be associated with higher rates of litter decomposition (Knops et al. 2002; Tateno et al. 2007). As for non- N-fixing species differing in photosynthetic pathway, species possessing the C4 photosynthetic pathway are generally found to have higher PNUE than that of those with the C3 photosynthetic pathway (e.g., Sage and Pearcy 1987; Anten et al. 1995). These trait differences, in turn, have functional consequences. If these key traits of pioneer species do indeed differ in N-fixing ability and photosynthetic pathway, then the impacts of these pioneer species on ecological processes could potentially be predicted from their N-fixing ability and photosynthetic pathway.
1.4. Restoration using pioneer species
Successful ecosystem restoration requires a fundamental understanding of the ecological characteristics of the component species, combined with knowledge of how they assemble, interact and function as communities (Pywell and Putwain 1996). There is increasing interest in using species traits, as well as the grouping of species into functional types using their traits, to both predict plant community responses to environmental change and to address the mechanisms underlying these responses (Lavorel et al. 1997; Weiher et al. 1999). The importance of pioneer species in restoring local ecology has long attracted the attention of many researchers. Owing to their respective advantages in persisting the extreme local environmental conditions, native pioneer species are important candidates for restoring local ecology. Furthermore, in volcanically devastated sites, combined with some revegetation technology, local environment restoration using native pioneer species has resulted in rapid development (Vitousek and Walker 1989; del Moral and Grishin 1999). The advantages of using native pioneer species as candidates for ecological restoration have been well clarified. However, the fundamental knowledge of the advantages of leaf physiological traits remains unclear.
1.5. Purpose of research
The Miyake-jima Island is an active volcano in Japan. Historically, there have been many eruptions, the most recent of which occurred in 2000 (Kamijo and Hashiba 2003). After this eruption, the vegetation was severely damaged by the substaintial amount of volcanic ash, and extensive bare land was formed around the volcanic crater. As the distance from the crater increased, the extent of the damage was progressively reduced. On the volcanically devastated sites of Miyake-jima Island, representative pioneer species, such as Miscanthus condensatus, Alnus sieboldiana and Fallopia. japonica var. hachidyoensis, have successfully invaded and established, with M. condensatus as the most dominant species. These three pioneer species differed in their N-fixing ability and photosynthetic pathway.
The current research on the photosynthetic properties of pioneer species in volcanically devastated sites with poor nutrient condition has been conducted in Hawaii (Cordell et al. 2001), Mt. Fuji (Sakata et al. 2006), and Miyake-jima Island (Choi et al. 2014). Choi et al. (2014) focuses on Alnus sieboldiana, an N-fixing pioneer tree species, and examined the response of photosynthetic properties to the concentration of volcanic gas. However, only a few studies have suggested the effects of different habitat conditions (mainly light and low-nutrient soil conditions) on the photosynthetic characteristics, leaf structure parameters, and biochemical parameters (leaf nitrogen) of the pioneer species in volcanic succession. In addition, there have been no physiological studies of M.condensatus in volcanically devastated sites, despite its ecological importance in volcanic succession.
In this study, I concentrate on the physiological adaptations of pioneer species to environmental variability. To evaluate the photosynthetic properties of pioneer species in volcanically devastated sites, I conducted a series of studies as follows:
1) As M. condensatus on Miyake-jima Island has been the absolutely dominant species following the eruption in 2000, to conserve the native ecosystem, the Tokyo Metropolitan Government plans to use native species in the revegetation program. M. condensatus is a candidate plant for the program because of its ability to adapt to the special environment of volcanically devastated sites and the fact that its seed is easy to collect (Iwata et al. 2005). Thus, I first focused on its physiological advantages in an extremely volcanically devastated site (Chapter 3). In this chapter, I comprehensively studied the photosynthesis-related leaf traits of M. condensatus in response to different habitat conditions. This chapter was published by Zhang et al. (2020).
2) In order to fully understand the physiological and ecological characteristics of all pioneer species in Miyake-jima volcanically devastated site. I measured the leaf photosynthesis-related traits of M. condensatus, A. sieboldiana and F. japonica var. hachidyoensis to determine their growth advantages in physiological and ecological characteristics (Chapter 4).
In the final chapter, I provide a general discussion regarding the physiological strategies of pioneer species in volcanically devastated sites.
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