Warming affects leaf light use efficiency and functional traits in alpine plants: evidence from a 4-year in-situ field experiment.

Introduction: Light use efficiency (LUE) is a crucial determinant of plant productivity, while leaf functional traits directly affect ecosystem functions. However, it remains unclear how climate warming affects LUE and leaf functional traits of dominant species in alpine meadows. Methods: We conducted a 4-year in-situ field warming experiment to investigate the eco-physiological characteristics for a dominant species (Elymus nutans) and a common species (Potentilla anserina) on the Tibetan Plateau. The leaf traits, photosynthesis and fluorescence characteristics were measured, along with the soil physical-chemical properties associated with the two species. Results and discussions: Experimental warming increased the leaf LUE, maximum photochemical efficiency, non-photochemical quenching, relative water content and specific leaf area for both species. However, there was a decrease in leaf and soil element content. Different species exhibit varying adaptability to warming. Increasing temperature significantly increased the photosynthetic rate, stomatal conductance, transpiration rate, total water content, and specific leaf volume of E. nutans; however, all these traits exhibited an opposite trend in P. anserina. Warming has a direct negative impact on leaf LUE and an indirectly enhances LUE through its effects on leaf traits. The impact of warming on plant photosynthetic capacity is primarily mediated by soil nutrients and leaf traits. These results indicate that the two different species employ distinct adaptive strategies in response to climate change, which are related to their species-specific variations. Such changes can confer an adaptive advantage for plant to cope with environmental change and potentially lead to alterations to ecosystem structure and functioning.

Environmental filtering rather than dispersal limitation dominated plant community assembly in the Zoige Plateau.

Identifying the mechanisms that underlie the assembly of plant communities is critical to the conservation of terrestrial biodiversity. However, it is seldom measured or quantified how much deterministic versus stochastic processes contribute to community assembly in alpine meadows. Here, we measured the decay in community similarity with spatial and environmental distance in the Zoige Plateau. Furthermore, we used redundancy analysis (RDA) to divide the variations in the relative abundance of plant families into four components to assess the effects of environmental and spatial. Species assemblage similarity liner declined with geographical distance (p < .001, R 2 = .6388), and it decreased significantly with increasing distance of total phosphorus (TP), alkali-hydrolyzable nitrogen (AN), available potassium (AK), nitrate nitrogen (NO3 +-N), and ammonia nitrogen (NH4 +-N). Environmental and spatial variables jointly explained a large proportion (55.2%) of the variation in the relative abundance of plant families. Environmental variables accounted for 13.1% of the total variation, whereas spatial variables accounted for 11.4%, perhaps due to the pronounced abiotic gradients in the alpine areas. Our study highlights the mechanism of plant community assembly in the alpine ecosystem, where environmental filtering plays a more important role than dispersal limitation. In addition, a reasonably controlled abundance of Compositae (the family with the highest niche breadth and large niche overlap value with Gramineae and Cyperaceae) was expected to maintain sustainable development in pastoral production. These results suggest that management measures should be developed with the goal of improving or maintaining suitable local environmental conditions.

Leaf and canopy photosynthesis of four desert plants: considering different photosynthetic organs.

Desert plants evolve different photosynthetic organs to adapt to the extreme environment. We studied the leaf and canopy gas exchange, chlorophyll content, fluorescence parameters, and anatomical structure of different photosynthetic organs (leaf and assimilating stem) on four desert plants (Nitraria sphaerocarpa, Caragana korshinskii, Haloxylon ammodendron, and Calligonum mongolicum). The results showed a higher net photosynthetic rate (PN) in the assimilating stems of H. ammodendron and C. mongolicum, which also had a higher light saturation point and a lower light compensation point than leaves (N. sphaerocarpa and C. korshinskii), suggesting more efficient solar energy utilization in the former. Within each species, canopy apparent photosynthetic rate (CAP) was significantly lower than PN, and the daily average CAP of the assimilating stems was significantly higher than leaves. These findings indicated that the photosynthetic response of desert plants was specific to photosynthetic organs. We concluded that the assimilating stem was a superior adaption for desert plants to survive the arid environments.

Leaf and Community Photosynthetic Carbon Assimilation of Alpine Plants Under in-situ Warming.

The Tibetan Plateau is highly sensitive to elevated temperatures and has experienced significant climate warming in the last decades. While climate warming is known to greatly impact alpine ecosystems, the gas exchange responses at the leaf and community levels to climate warming in alpine meadow ecosystems remain unclear. In this study, the alpine grass, Elymus nutans, and forb, Potentilla anserina, were grown in open-top chambers (OTCs) for 3 consecutive years to evaluate their response to warming. Gas exchange measurements were used to assess the effects of in-situ warming on leaf- and community-level photosynthetic carbon assimilation based on leaf photosynthetic physiological parameters. We introduced a means of up-scaling photosynthetic measurements from the leaf level to the community level based on six easily measurable parameters, including leaf net photosynthetic rate, fresh leaf mass per unit leaf area, fresh weight of all plant leaves in the community, the percentage of healthy leaves, the percentage of received effective light by leaves in the community, and community coverage. The community-level photosynthetic carbon assimilation and productivity all increased with warming, and the net photosynthetic rate at the leaf level was significantly higher than at the community level. Under elevated temperature, the net photosynthetic rate of E. nutans decreased, while that of P. anserina increased. These results indicated that climate warming may significantly influence plant carbon assimilation, which could alter alpine meadow community composition in the future.

Responses of Plant Bud Bank Characteristics to the Enclosure in Different Desertified Grasslands on the Tibetan Plateau.

Asexual reproduction is the main mode of alpine plant reproduction, and buds play an important role in plant community succession. The purpose of this study is to explore whether the desertified grassland can recover itself through the existing bud bank. The bud bank composition, distribution and size of different desertified grasslands were studied using unit volume excavation on the Tibetan Plateau. The bud bank consisted of tiller, long and short rhizome buds, and more than 40% of buds were distributed in the 0-10 cm soil layer. Enclosure changed the bud density, distribution and composition. The bud densities were 4327 and 2681 No./m2 in light and middle desertified grasslands before enclosure, while that decreased to 3833 and 2567 No./m2 after enclosure. Tiller bud density and proportion of middle desertified grassland were the highest, increased from 2765 (31.26%, before enclosure) to 5556 No./m3 (62.67%, after enclosure). There were new grasses growing out in the extreme desertified grassland after enclosure. The meristem limitation index of moderate desertified grassland was the lowest (0.37), indicating that plant renewal was limited by bud bank. Plants constantly adjust the bud bank composition, distribution, and asexual reproduction strategy, and desertified grasslands can recover naturally, relying on their bud banks through an enclosure.

Drought conditions alter litter decomposition and nutrient release of litter types in an agroforestry system of China.

Evaluating how decomposition rates and litter nutrient release of different litter types respond to changes in water conditions is crucial for understanding global carbon and nutrient cycling. However, it is unclear how decreasing water affects litter mixture interactions for the maize-poplar system in arid regions. Here, the responses of the litter decomposition process and litter mixture interactions in the agroforestry system to changes in water conditions (control, light drought, and moderate drought) were tested. Moderate drought significantly decreased the decomposition rate for poplar leaf and mixed litters, and decomposition rate was significantly reduced for maize straw litter in light and moderate drought stress. The mass loss rates of maize straw and mixed litters were significantly higher than that of the poplar leaf litter under drought conditions, but there was no significant difference among the three litter types in the control. There was no interaction between mass loss of the mixed litter in the control and light drought conditions, and the litter mixture interaction showed nonadditive synergistic interactions under moderate drought. In terms of nutrient release, there was also no interaction between litter mixture with nitrogen and carbon, but there was antagonistic interaction with potassium release under the light drought condition. Our results demonstrate that drought conditions can lead to decreasing decomposition rate and strong changes in the litter mixture interactions from additive effects to nonadditive synergistic effects in moderate drought. Moreover, light drought changed the mixture interaction from an additive effect to an antagonistic interaction for potassium release.

Biophysical regulation of carbon fluxes over an alpine meadow ecosystem in the eastern Tibetan Plateau.

The eddy covariance method was used to measure net ecosystem CO2 exchange (NEE) between atmosphere and an alpine meadow ecosystem in the eastern Tibetan Plateau of China in 2010. Our results show that photosynthesis was reduced under low air temperature (T a), high vapor pressure deficit (VPD), and medium soil water content (SWC) conditions, when compared to that under other T a (i.e., medium and high), VPD (i.e., low and medium), and SWC (i.e., low and high) conditions. The apparent temperature sensitivity of ecosystem respiration (Q 10) declined with progressing phenology during the growing season and decreased with an increase of soil temperature (T s) during the non-growing season. Increased ecosystem respiration (R eco) was measured during spring soil thawing. By the path analysis, T a, T s, and VPD were the main control factors of CO2 exchange at 30-min scale in this alpine meadow. Integrated NEE, gross primary production (GPP), and R eco over the measured year were -156.4, 1164.3, and 1007.9 g C m(-2), respectively. Zoige alpine meadow was a medium carbon sink based on published data for grassland ecosystems.

[Research progress on carbon sink function of agroforestry system under climate change].

As a land comprehensive utilization system, agroforestry system can absorb and fix CO2 effectively to increase carbon storage, and also reduces greenhouse effect convincingly while reaching the aim of harvest. The regulatory role in CO2 makes humans realize that agroforestry systems have significant superiority compared with single cropping systems, therefore, understanding the carbon sinks of different components in an agroforestry system and its influencing factors play an important role in studying global carbon cycle and accurate evaluation of carbon budget. This paper reviewed the concept and classification of agroforestry system, and then the carbon sequestration potentials of different components in agroforestry systems and influencing factors. It was concluded that the carbon sequestration rate of plants from different agroforestry systems in different regions are highly variable, ranging from 0.59 to 11.08 t C · hm(-2) · a(-1), and it is mainly influenced by climatic factors and the characteristics of agroforestry systems (species composition, tree density and stand age). The soil C sequestration of any agroforestry system is influenced by the amount and quality of biomass input provided by tree and nontree components of the system and the soil properties such as soil texture and soil structure. Overall the amount of carbon storage in any agroforestry system depends on the structure and function of its each component. The future studies should focus on the carbon sink functions of structurally optimized agroforestry systems, the temporal variation and spatial distribution pattern of carbon storage in agroforestry system and its carbon sequestration mechanism in a long time.

[Photosynthetic rate, transpiration rate, and water use efficiency of cotton canopy in oasis edge of Linze].

The measurement system of Li-8100 carbon flux and the modified assimilation chamber were used to study the photosynthetic characteristics of cotton (Gossypium hirsutum L.) canopy in the oasis edge region in middle reach of Heihe River Basin, mid Hexi Corridor of Gansu. At the experimental site, soil respiration and evaporation rates were significantly higher in late June than in early August, and the diurnal variation of canopy photosynthetic rate showed single-peak type. The photosynthetic rate was significantly higher (P < 0.01) in late June than in early August, with the daily average value being (43.11 +/- 1.26) micromol CO2 x m(-2) x s(-1) and (24.53 +/- 0.60) micromol CO2 x m(-2) x s(-1), respectively. The diurnal variation of canopy transpiration rate also presented single-peak type, with the daily average value in late June and early August being (3.10 +/- 0.34) mmol H2O x m(-2) x s(-1) and (1.60 +/- 0.26) mmol H2O x m(-2) x s(-1), respectively, and differed significantly (P < 0.01). The daily average value of canopy water use efficiency in late June and early August was (15.67 +/- 1.77) mmol CO2 x mol(-1) H2O and (23.08 +/- 5.54) mmol CO2 x mol(-1) H2O, respectively, but the difference was not significant (P > 0.05). Both in late June and in early August, the canopy photosynthetic rate was positively correlated with air temperature, PAR, and soil moisture content, suggesting that there was no midday depression of photosynthesis in the two periods. In August, the canopy photosynthetic rate and transpiration rate decreased significantly, because of the lower soil moisture content and leaf senescence, but the canopy water use efficiency had no significant decrease.

Canopy and leaf gas exchange of Haloxylon ammodendron under different soil moisture regimes.

In order to reveal the drought resistance and adaptation of the C(4) desert plant Haloxylon ammodendron under artificially controlled soil moisture regimes, representative plants were selected to measure canopy photosynthesis using canopy photosynthetic measurement system. The results showed that appropriate soil moisture significantly enhances the canopy and leaf photosynthetic capacity, and extremely high soil moisture is not conducive to the photosynthesis of H. ammodendron.

[Effects of farmland shelterbelt on photosynthetic characteristics and grain yield of maize in Hexi corridor oasis].

Farmland shelterbelt is a main component of oasis agro-ecosystem in the Hexi corridor of Gansu Province, which plays an important role in maintaining local agricultural production and ecological balance. This paper studied the photosynthetic characteristics of maize at its grain-filling stage and the grain yield of maize under the protection of the shelterbelt. The results showed that at the same distances east and west of the shelterbelt, maize plants at the east side always intercepted more photosynthetic active radiation (PAR) than those at the west side. Within 20 m from the west side of the shelterbelt, the net photosynthetic rate (P(n)) of maize plants was lower because of the serious shading by trees; and within 15 m from the west side, the P(n) was lower than that from the east side. The grain yield of maize was decreased within 10 m from the east and 15-20 m from the west side, but increased or not affected in other locations.

Increased sensitivity to salt stress in an ascorbate-deficient Arabidopsis mutant.

The Arabidopsis thaliana ascorbate-deficient vtc-1 mutant has only 30% ascorbate contents of the wild type (WT). This ascorbate-deficient mutant was used here to study the physiological roles of ascorbate under salt stress in vivo. Salt stress resulted in a more significant decrease in CO2 assimilatory capacity in the vtc-1 mutant than in the WT. Photosystem II function in the Arabidopsis vtc-1 mutant also showed an increased sensitivity to salt stress. Oxidative stress, indicated by the hydrogen peroxide content, increased more dramatically in the vtc-1 mutant than in the WT under salt stress. To clarify the reason for the increased oxidative stress in the vtc-1 mutant, the contents of small antioxidant compounds and the activities of several antioxidant enzymes in the ascorbate-glutathione cycle were measured. Despite an elevated glutathione pool in the vtc-1 mutant, the ascorbate contents and the reduced form of ascorbate decreased very rapidly under salt stress. These results showed that the activities of MDAR and DHAR were lower in the vtc-1 mutant than in the WT under salt stress. Thus, low intrinsic ascorbate and an impaired ascorbate-glutathione cycle in the vtc-1 mutant under salt stress probably induced a dramatic decrease in the reduced form of ascorbate, which resulted in both enhanced ROS contents and decreased NPQ in the vtc-1 mutant.

[Functions of different cultivation modes in oasis agriculture on soil wind erosion control and soil moisture conservation].

During 2001-2002, the effects of different cultivation modes including winter irrigation and zero tillage, crop-grass intercropping, and early spring film mulching on sand entrainment, wind velocity gradient and soil moisture conservation were studied in the middle reaches of the Heihe River in the Hexi Corridor region. The results showed that all these modes could reduce soil wind erosion and halt sand entrainment to different degrees. Compared with the bare fields exposed by spring plowing, early spring film mulching could increase soil moisture storage by 35.6%. At present, spring plowing and sowing was a main factor responsible to the occurrence of sand storms and the increase in suspended dust content. Farmlands in the upper and middle reaches of the Heihe River generally produced a dust transport up to 4.8-6.0 million tons per year, which was higher than that of sandy desert in the same region. In the Hexi Corridor region, the suspended dust amount produced from 1 hm2 farmland was equivalent to that of 1.5 hm2 desert.