«（落葉樹の根圏動態に対する高CO2とO3及び高窒素負荷の影響に関する研究） Wang Xiaona 王 晓娜 Division of ...»
2.4 DISCUSSION I found the LRL (live root length or length-based standing crop) significantly influence by elevated CO2. In BF soil, LRL was higher under elevated CO2 than ambient, but this trend disappeared from the second year. Contrarily, in VA soil, elevated CO2 reduced the LRL for three observed years (Fig. 2.1). Generally, elevated CO2 stimulates plant growth (Norby and Zak 2011) and more carbon is allocated to the roots (Lukac et al. 2003), therefore, elevated CO2 was also assumed to increase the root/shoot ratio in earlier studies. However, new studies have revealed less pronounced effects (Bielenberg and Bassirirad 2005) or even negative responses of CO2 (Arnone et al. 2000; Higgins et al. 2002). Present results proved this point and
suggested that the effects of elevated CO2 diminished over time. On the other hand, photosynthesis down-regulation of birch seedlings occurred during the treatment years (unpublished data), this frequently observed in many seedling and sapling stage of trees (Tissue and Lewis 2010), thus, the unaffected LRL under elevated CO2 from 2012 in BF soil probably occurred.
The difference results between BF soil and VA soil, suggested the LRL strongly related with soil nutrient condition. My results found the soil N concentration in VA soil was relatively lower than BF soil as reported before (Eguchi et al. 2008).
Moreover, this down-regulation is expected to be clearly found in immature volcanic ash soil (Mao 2013), it limited photosyntate allocation to belowground. Importantly elevated CO2 accelerates plant growth, increases plant nutrient demand and uptake capacity (Bielenberg and Bassirirad 2005). As a result, with higher nutrient demand under elevated CO2, plant growth especially the belowground was likely restricted or even reduced in VA soil. Thus, a reduced LRL of white birch was found in VA soil.
Additionally, the unclear trend of elevated CO2 effect on LRL from second year in BF soil, and the negative effect of elevated CO2 on LRL in VA soil, it potentially derived from the changes of root production and mortality. For instance, changes of higher mortality or lower production under elevated CO2 can lead to a reduced LRL.
However, I did not find any clear trend for root production rate and mortality rate (Fig.
2.4, 2.5), the strongly correlation of ALRP and ALRM in all conditions suggested there was no significant change difference between them, even the correlation was slightly lower in VA soil than BF soil (Fig. 2.6). Therefore, I deduced this might depend on the root turnover and lifespan, following I discussed this point.
Elevated CO2 did not affect the fine root turnover of production and mortality (Table 2.2), the results indicate no effects of elevated CO2 on root turnover. It has
been estimated the effects of elevated CO2 that did not appear to alter the turnover of loblolly pine, despite an increased root length, production and mortality according to Pritchard et al. (2011). Moreover, an interactive effect with year was found that elevated CO2 reduced FRM turnover over time. It indicated a longer lifespan with CO2 enrichment as I detected (Table 2.2). Additionally, Eissenstat et al. (2000) concluded that elevated CO2 may be associated with longer root lifespan, by decreasing the root N concentration and reducing the root maintenance respiration, which was also reported by Arnone et al. (2000) that the longer lifespan was also found. Within year, FRM turnover increased by elevated CO2 in 2013, therefore there was lower LRL in the third year comparing with first two years during the observation, corresponding to a reduced root lifespan under elevated CO2 (Table 2.4).
Soil only had significant effect on FRP turnover, not affect the FRM turnover.
Overall, VA soil had low capacity for FRP turnover, this might be the reason that lifespan in VA soil was relative higher than BF soil in the same CO2 condition (Table 2.3). Another possibility is symbiotic effect of ectomycorrhiza (ECM), because the root with ECM symbiosis can live much longer or with lower production than non-colonized roots. This has been recently demonstrated by Bidartondo et al. (2001), who found ECM colonizing the roots (D = 0.3~0.6mm) of Bishop pine (Pinus muricata) prolonged the root longevity. Therefore, present result proved with limited nutrient in VA soil, lower turnover and longer lifespan suggested the slower root dynamic in VA soil than BF soil.
The median longevity was increased by elevated CO2 in both BF soil and VA soil, but the influence was not continuing from the second year in BF soil, and appeared a convergent effect of elevated CO2 in VA soil (Table 2.3). As it has been reported, plant under elevated CO2 usually increases water use efficiency and stimulate dramatic
aboveground growth (Qu et al. 2004; Koike et al. 2010). It has been proved that under elevated CO2, root uptake and provide nutrient resources for CO2-induced increases in aboveground with a more modest production in fine roots or longer lifespan roots (Housman et al. 2005). In VA soil, the root median longevity was increased consistently under elevated CO2. One possibility is the nutrient limitation resulted in a lower turnover of FRP and FRM, because the root longevity was inversely related to the duration of the resource supply (Pregitzer et al. 1993). Therefore, a longer root lifespan was found due to limited nutrient availability. Another reason is that plant is preferentially to enhance the growth of aboveground as I discussed above, this may readily occur in nutrient limited condition, because root lifespan would be increased if construction costs relative to maintenance costs are high, or if the nutrient availability is low (Eissenstat et al. 2000).
Root diameter in forest changed during the study with elevated CO2 (Pritchard et al. 2008), and highlights the importance of taking soil samples during the MR image acquisition, to get SRL and to be able to estimate biomass. This was not done in my study and I am therefore not able to convert root length to biomass. However, the root responses to the treatments of different diameter classes were estimated (see Table 2.4). The fine root median lifespan (D 2mm) significantly affect by different treatments, the thinnest roots (D 0.2 mm) were affected under elevated CO2 since 2013 after two growing seasons. The other order fine root lifespan was influenced by the beginning and unaffected under elevated CO2 with the root diameter larger than
0.4 mm since 2013. Talking about the fine root longevity, one thing must be taken into account is the mycorrhiza. Plant usually increases mycorrhizal colonization and decrease root N concentration under elevated CO2 (Pritchard and Rogers 2000;
Tingey et al. 2000). Given that root longevity is negatively correlated with tissue N
concentration (Pregitzer et al. 1998; Wells 1999), I concluded that the mycorrhiza symbiosis was strongly stimulated under elevated CO2 in the beginning. As a result, fine root performed a longer lifespan with no distinct effect by CO2 enrichment in present study. Moreover, mycorrhizal colonization under elevated CO2 is not consistently increasing, Shirano et al. (2007) found that elevated CO2, the ECM colonized Japanese larch (Larix kaempferi) with an increasing rate during the first year treatment, and later equilibrated to a stable lower rate. In my case, the shorter longevity of fine root was likely derived from the decreased ECM colonization during the third year. It was possible ECM assisted birch seedlings to survive in a new soil condition, and the shorter root lifespan revealed a completed necessary aboveground growth, and plants started to develop root system after that establishment (Eissenstat et al. 2000).
Additionally, regardless of the mycorrhiza symbiosis, the different response of different root diameter classes indicated root heterogeneity. The location of a root and branched system of root are potentially to influence the root lifespan (Guo et al. 2004).
All of these characters are required to deep understand under changing environment.
2.5 SUMMARY This study investigated the fine root dynamic responses to elevated CO2 of white birch regarding to root turnover as well as root longevity with different kinds of soil.
Changes in root production and mortality in response to elevated CO2 could be a key link between plant responses and long term changes in soil organic matter and ecosystem C balance (Norby and Jackson 2000).
In this study, elevated CO2 reduced fine root length standing crop in VA soil, and
a lower turnover of production and mortality were found comparing with BF soil.
This indicated a weak root dynamic of white birch in VA soil. Elevated CO2 increased root longevity, especially in VA soil during three observed growing seasons, suggested soil nutrient status affecting root longevity strongly. The shorter fine root longevity under elevated CO2 comparing with ambient in VA soil during the third growing season, suggested the root dynamic is getting higher, thus a C sequestration to soil may happen to increase. The result may due to the changes of mycorrhizal colonization, root specific character, and the position of a root on the branching root system. These factors can’t be ignored, thus more effort are required to contribute the root research, to thoroughly understand the response of root dynamic under changing environment.
Figure 2.1 Schematic of Minirhizotron (MR) techniques, (Image capture instrument with Digital Camera or Scanner MR) and different options to install the MR tubes (angled or vertical from the soil surface or horizontally from trenches) (Maeght et al.
Figure.2.2 Root images captured in one tube from the first session to the last one during one year observation. The accurate size of each image is 21.59 × 19.56 cm2, the number in each image denotes the sessions, 1-11 indicates the first to last session.
12 75 8 50 4 25 43 0 Figure 2.3 Relative length of live fine root (standing crop) of white birch seedlings growing under elevated (500 lmol mol-1) and control (370 lmol mol-1) [CO2] on volcanic ash (VA) and brown forest (BF) soil.
The maximum absolute vale of vertical axis of (a) and (b) is 16 mm/cm2 and 3.5 mm/cm2, respectively.
CHAPTER 2 CHAPTER 2
15 75 10 50 5 25 44 0 0 Figure 2.4 Fine root length production rate of birch seedlings growing under elevated and ambient [CO2] on volcanic ash (VA) and brown forest (BF) soil.
12 60 8 40 4 20
0 0 0 5 10 15 20 0 5 10 15 20
4 4 0 0 0 5 10 15 0 5 10 15 20
Table 2.4 Lifespan of different root diameter classes of birch seedlings growing in elevated and ambient [CO2] on volcanic ash (VA) and brown forest (BF) soil.
3.1 INTRODUCTION Concentrations of atmospheric CO2 and tropospheric or ground surface ozone (O3) have been increasing sharply since the Industrial Revolution. Both are predicted to continue rising during the next decades because of the over-burning of fossil fuels and deforestation (e.g., Cubasch et al. 2001; Koike et al. 2013a; Tans 2008). These changes in atmospheric air may alter the aboveground and belowground growth and development of trees, and therefore impact the CO2 sink of forest ecosystems (e.g., Larcher 2003; Qu et al. 2010).
The majority of belowground root systems in boreal forests are symbiotically colonized by ectomycorrhizal fungi (ECMF) (Taylor et al. 2000). In fact, larch (Larix sp.) is a typical ectomycorrhizal (ECM) species (Smith and Read 1997; Qu et al. 2004) that is widely planted as a dominant tree species for afforestation in the northeastern part of Eurasia (Koike et al. 2000; Qu et al. 2010) and parts of Europe (Matyssek and Schulze 1987). Recently, a new hybrid larch F1 (L. gmelinii var. japonica × L.
kaempferi; hereafter F1) was developed as a promising species. This hybrid has much better tolerance to cold climates, damage by red-back voles (Clethrionomys rutilus) grazing, shoot blight disease (Physalospora laricina), and strong winds (Ryu et al.
2009). It is already known that the growth of F1 is closely related to ubiquitous ECM association (Qu et al. 2004). However, studies of the relationship between F1 and ECM symbiosis remain limited (Qu et al. 2003； 2010). Up to 30% of total photo-assimilation products can be used in the growth and maintenance of ECM (Hampp and Nehls 2001). In turn, ECM usually acts as an efficient patronage to the root system of the host by absorbing water and essential nutrients such as phosphorous (P) and sometimes nitrogen (N) (e.g., Cairney 2011; Quoreshi et al.
The net primary production and the growth of trees are enhanced by elevated CO2, with a simultaneous increase in the carbon allocation to the belowground parts (Choi et al. 2005; McElrone et al. 2005; Nowak et al. 2004; Qu et al. 2004). For instance, Choi et al. (2005) reported that symbiosis with ECMF increased the growth of Japanese red pine (Pinus densiflora) seedlings under elevated CO2, because the photosynthetic activities of host plants were enhanced by an increase in root surface via widely ramified ECM hyphae. They also found an improvement in water use efficiency (WUE), and suggested that the colonization of pines with ECM leads to the allocation of more photosynthates to roots under elevated CO2 conditions. In larch species, colonization with ECM increased the growth of Japanese larch (L. kaempferi) and its hybrid F1 by a factor of 1.5–2.0 relative to uninfected larches in nutrient-poor soil in northern Japan and east Russia (Qu et al. 2004; 2010). Moreover, Buscot et al.
(2000) emphasized that a greater species richness of ECM communities assists the cycling of P from heterogeneous sources in forest soil ecosystems. Host spruce, larch, and pine trees grow more rapidly when they are infected by multiple ECM species rather than by a single species (e.g., Choi 2008; Qu et al. 2004; Qu et al. 2003).