Response Of Leaf Carbon Balance To Climate Change In Tree Species

Foliage carbon balance regulates the growth, development and reproduction of plant, and determines the carbon budget at individual and ecosystem level. The carbon balance of foliage depends on the dynamics of both photosynthesis (A) and respiration (R), and is strongly manipulated by environmental perturbation. Water availability and temperature, among others, are major determinant of leaf carbon balance. The underlying mechanisms of foliage carbon balance response to water availability and temperature, however, are not well understood currently. In the present study, several trees species native to contrasting climatic zones were chosen, and the impacts of water availability as well as seasonal temperature change on foliage A and R were investigated, in attempting to unravel the response and acclimation of foliage carbon balance upon natural environmental variations. In temperature desert climate zone, we choose Populus euphraticaas plant material and use the spatially changed depth to underground water table as gradient. The impacts of depth to underground water table on the leaf A and R of P. euphratica were studied; in temperature continental climate zone, three evergreen species(Chamaecyparis thyoides, Pinus strobus, Tsuga canadensis) were chosen, and the seasonality of leaf A and R were investigated. The main results are summarized as follows:In chapter 4, we reported the carbon balance and the corresponding regulatory mechanisms of three different types foliage (Borad ovate, Ovate and lanceolate, hereafter BO, O and L, respecitvely) in Populus euphratica. The results showed that:different types of foliage differed significantly in specific leaf area (SLA) but not mass-based nitrogen content (Nmass); area-based carbon assimilation rate (Aarea) was significantly higher in BO than in O and L, however, no statistically significant difference was found for mass-based carbon assimilation rate (Amass) and photosynthetic nitrogen use efficiency (PNUE); BO exhibited greater respiration rate than that of other 2 types foliage; foliage level carbon balance did not differ among leaf types. We propose that leaf with different carbon metabolisms may also differ functionally. The greater respiratory ability in BO would benefit the maintenance of metabolism, while the high carbon gain ability of L was of advantage to individual growth.In Ejina banner,Inner Mongolia, based on the different depth to underground water table, we established 2 sites (dry site and wet site). Leaf gas exchange characteristics and related leaf traits of P. euphratica were measured in 2 sites differing in the depth to the underground water table, the main source of water to support plant growth. Leaf relative water content (RWC) at the dry site was slightly lower than that of wetter site. No significant difference was found on leaf dry mass content (DMC) and specific leaf area (SLA). Leaf grown at dry site had a higher soluble sugar content and chlorophyll a:chlorophyll b (chl a:chl b) ratio but a lower area-based total chlorophyll content. Leaf photosynthetic capacity (Amax) was unaffected by decreased underground water table. Leaf stomatal conductance (gsmax) and apparent quantum yield (AQY) at dry site was increased by 22.5% and 47.1%, respectively. Rubisco maximum carboxylation rate (Vcmax) decreased in the site with the lower underground water table while maximum electron transport rate (Jmax) remained the same. We conclude that P. euphratica may increase water absorption through osmotic adjustment when subject to prolonged low soil water availability. Furthermore, photosynthetic capacity can be maintained mainly through decreased CO2 diffusive limitation, thereby compensating lowered Rubisco carboxylation activity under prolonged drought (Chapter 5).Water availability may affect leaf respiration (R) thereby influencing leaf, plant and ecosystem carbon balance. However, both the direction and the extent of variation are still poorly understood. In this chapter (Chapter 6) we investigated the impact of the depth to the underground water table on leaf respiration of Populus euphratica, with particular emphasis on the acclimation of respiration to soil water availability. Our results showed that leaf relative water content (RWC) and mass based nitrogen content (Nmass) was lower at the drier site than that of the wetter site. However, no significant difference was found between area based nitrogen content (Narea). R in darkness (Rn) and R in daylight (RL) were both higher at the site with the deeper water table, with the stimulatory effect being larger on RL. Leaf R was inhibited by light at both sites (e.g. RL<Rn), with the inhibitory effect ranging from 11% to 57% and was positively correlated with the rate of Rubisco oxygenation at saturated irradiance (vo). Water limitation induced upregulation of leaf R may result from the increased demand for energy and carbon skeleton for drought resistance, including osmotic adjustment as well anti-oxidant synthesis and protein turnover. This may also explain the less light inhibitory effect on R at the drier site.Respiratory metabolism is the determinant of plant growth, development as well as reproduction, and is also the key factor that regulates the carbon flux of terrestrial ecosystems. In this chapter (Chapter 7), we assessed the foliage respiratory characteristics (R) and related leaf traits of 5 evergreen tree species common to temperate coniferous-broadleaf forest of Northern America. Our results showed:(1) inter-species difference was found on leaf area-based nitrogen content (Narea), but not on mass-based nitrogen content (Nmass); likewise, area-based respiration rate (Rnarea) was species-specific and was also highly regulated by leaf morphological traits. (2) light inhibited leaf dark respiration in all species investigated, with the ratio of mitochondrial respiration under daylight to dark respiration rate (RL/Rn) spanning from 0.39 to 0.90; RL/Rn positively correlated with maximum oxygenation rate (v0) and carboxylation rate (vc) of Rubisco. (3) Qio of dark respiration ranged from 1.44～2.24 while activation energy (Eo) varied slightly among species; (4) leaf respiration rate could be well explained by its nitrogen content; moreover, positive relationship was also found among R, maximum carbon assimilation rate (A) and specific leaf area (SLA); (5) Modeled the sum of leaf level carbon assimilation rate (∑Anet) decreased by 3.45% when using a fixed Q10 (2.0) and assuming RL=Rn. However, the extent to which leaf carbon gain is under-or overestimated differs among species. Collectively, those results strongly demonstrated that leaf respiratory characteristics are species-specific. Simulating leaf level carbon flux without considering the effects of Q10 and light inhibition of dark respiration would therefore lead to incorrect outcomes in terms of leaf carbon gain. Apparently, such error will be undoubtedly magnified when scale up to canopy, ecosystem or global level.Thermal acclimation can alter the pattern of short-term physiological response, and therefore affects the carbon assimilation efficiency of plants, which in turn influences the carbon balance at individual and ecosystem level. Based on the results of previous study, three evergreen tree species were further selected and seasonality of leaf photosynthesis (A), dark respiration (Rn) and mitochondrial respiration under daylight (RL) were investigated, with particular highlight on the thermal acclamatory ability of their metabolic processes that regulate leaf carbon flux. Furthermore, a coupled model was used to estimate the seasonal variation of leaf carbon gain and to test the impacts of simplification on modeling results. We found that the maximum photosynthetic rate at set temperature (Amax) increased with rising growth temperature (T5); Rubisco maximum carboxylation rate (Vcmax) and electron transport rate (Jmax) tended to be higher in cold growth seasons that in warm growth season, while the ratio of Jmax to Vcmax (Jmax/Vcmax) displayed an opposite seasonal trend. Rn of Chamaecyparis thyoidesand Tsuga canadensis increased linearly with growth temperature, while RL showed a weak correlation with T5. By contrast, both Rn and RL of Pinus strobus declined with T5. Thermal sensitivity of respiration (Q10) and activation of respiration (E0) appeared to be higher in cold growth seasons than in warm growth seasons. Seasonal variation of leaf level carbon balance of three evergreen species exhibited a similar pattern, with the A/R ratio progressively increasing during experimental period. Furthermore, because the inhibitory effect of light on dark respiration, A/RL was consistently lower than A/Rn. Ignore the seasonality of Vcmax and Jmax slightly affected the estimated leaf level carbon gain, but the direction of influence was species-specific. On the other hand, assuming the same respiration under daylight to dark respiration (i.e. RL=Rn) and neglecting the thermal acclimation of respiration strongly underestimated and overestimated the leaf carbon gain, respectively. Collectively, our results demonstrated that the photosynthesis of three evergreen species have limited capacity to acclimate to temperature. Moreover, only P. strobus showed thermal acclimation of respiration, indicating the ability of thermal acclimation was species-specific. Ignore or simplify the seasonality of photosynthesis and respiration as well as the inhibitory effect of light on dark respiration will lead to incorrect estimation of leaf carbon gain (Chapter 8).