Reduced diurnal temperature range does not change warming impacts on ecosystem carbon balance of Mediterranean grassland mesocosms

Daily minimum temperature (T_min) has increased faster than daily maximum temperature (T_max) in many parts of the world, leading to decreases in diurnal temperature range (DTR). Projections suggest that these trends are likely to continue in many regions, particularly in northern latitudes and in arid regions. Despite wide speculation that asymmetric warming has different impacts on plant and ecosystem production than equal‐night‐and‐day warming, there has been little direct comparison of these scenarios. Reduced DTR has also been widely misinterpreted as a result of night‐only warming, when in fact T_min occurs near dawn, indicating higher morning as well as night temperatures. We report on the first experiment to examine ecosystem‐scale impacts of faster increases in T_min than in T_max, using precise temperature controls to create realistic diurnal temperature profiles with gradual day–night temperature transitions and elevated early morning as well as night temperatures. Studying a constructed grassland ecosystem containing species native to Oregon, USA, we found that the ecosystem lost more carbon at elevated than ambient temperatures, but remained unaffected by the 3 °C difference in DTR between symmetric warming (constantly ambient + 3.5 °C) and asymmetric warming (dawn T_min = ambient + 5 °C, afternoon T_max = ambient + 2 °C). Reducing DTR had no apparent effect on photosynthesis, probably because temperatures were most different in the morning and late afternoon when light was low. Respiration was also similar in both warming treatments, because respiration temperature sensitivity was not sufficient to respond to the limited temperature differences between asymmetric and symmetric warming. We concluded that changes in daily mean temperatures, rather than changes in T_min/T_max, were sufficient for predicting ecosystem carbon fluxes in this reconstructed Mediterranean grassland system.     

Effects of a warmer climate on seed germination in the subarctic

Background and Aims

In a future warmer subarctic climate, the soil temperatures experienced by dispersed seeds are likely to increase during summer but may decrease during winter due to expected changes in snow depth, duration and quality. Because little is known about the dormancy-breaking and germination requirements of subarctic species, how warming may influence the timing and level of germination in these species was examined.

Methods

Under controlled conditions, how colder winter and warmer summer soil temperatures influenced germination was tested in 23 subarctic species. The cold stratification and warm incubation temperatures were derived from real soil temperature measurements in subarctic tundra and the temperatures were gradually changed over time to simulate different months of the year.

Key Results

Moderate summer warming (+2·5 °C) substantially accelerated germination in all but four species but did not affect germination percentages. Optimum germination temperatures (20/10°C) further decreased germination time and increased germination percentages in three species. Colder winter soil temperatures delayed the germination in ten species and decreased the germination percentage in four species, whereas the opposite was found in Silene acaulis. In most species, the combined effect of a reduced snow cover and summer warming resulted in earlier germination and thus a longer first growing season, which improves the chance of seedling survival. In particular the recruitment of (dwarf) shrubs (Vaccinium myrtillus, V. vitis-idaea, Betula nana), trees (Alnus incana, Betula pubescens) and grasses (Calamagrostis lapponica, C. purpurea) is likely to benefit from a warmer subarctic climate.

Conclusions

Seedling establishment is expected to improve in a future warmer subarctic climate, mainly by considerably earlier germination. The magnitudes of the responses are species-specific, which should be taken into account when modelling population growth and migration of subarctic species.Key words: Climate change, cold stratification, dwarf shrubs, germination percentage, incubation temperature, mean germination time, seedling establishment, seeds, Silene, subarctic species, Vaccinium, warming     

Quo vadis C4? An ecophysiological perspective on global change and the future of C4 plants

C₄ plants are directly affected by all major global change parameters, often in a manner that is distinct from that of C₃ plants. Rising CO₂ generally stimulates C₃ photosynthesis more than C₄, but C₄ species still exhibit positive responses, particularly at elevated temperature and arid conditions where they are currently common. Acclimation of photosynthesis to high CO₂ occurs in both C₃ and C₄ plants, most notably in nutrient-limited situations. High CO₂ aggravates nitrogen limitations and in doing so may favor C₄ species, which have greater photosynthetic nitrogen use efficiency. C₄ photosynthesis is favored by high temperature, but global warming will not necessarily favor C₄ over C₃ plants because the timing of warming could be more critical than the warming itself. C₃ species will likely be favored where harsh winter climates are moderated, particularly where hot summers also become drier and less favorable to C₄ plant growth. Eutrophication of soils by nitrogen deposition generally favors C₃ species by offsetting the superior nitrogen use efficiency of C₄ species; this should allow C₃ species to expand at the expense of C₄ plants. Land-use change and biotic invasions are also important global change factors that affect the future of C₄ plants. Human exploitation of forested landscapes favors C₄ species at low latitude by removing woody competitors and opening gaps in which C₄ grasses can establish. Invasive C₄ grasses are causing widespread forest loss in Asia, the Americas and Oceania by accelerating fire cycles and reducing soil nutrient status. Once established, weedy C₄ grasses can prevent woodland establishment, and thus arrest ecological succession. In sum, in the future, certain C₄ plants will prosper at the expense of C₃ species, and should be able to adjust to the changes the future brings. This revised version was published online in August 2006 with corrections to the Cover Date.     

Control processes in the induction and relief of thermoinhibition of lettuce seed germination : actions of phytochrome and endogenous ethylene

Germination of lettuce seeds (Lactuca sativa L. cv Grand Rapids) in the dark was nearly 100% at 20°C but was inhibited at 27°C and higher temperatures (thermoinhibition). A single 5-minute exposure to red light completely overcame the inhibition at temperatures up to 28°C, above which the effectiveness of single light exposures gradually declined to reach a negligible level at 32°C. However, the promotive effect of light could be extended to 34°C by repeated irradiations. At any one temperature, increased frequency of irradiations increased germination percentage, and with each degree increase in temperature, increasingly frequent irradiations were necessary to elicit maximal germination. Loss of the effectiveness of single irradiations with increase in temperature may result either from acceleration of the thermal reversion of the far red-absorbing form of phytochrome or decrease in seed sensitivity toward a given percentage of the far red-absorbing form of phytochrome. Using continuous red light to induce germination, the role of endogenous C₂H₄ in germination at 32°C was studied. Ethylene evolution from irradiated seeds began to increase 2 hours prior to radicle protrusion, whereas the dark-incubated (nongerminating) seeds produced a low, constant amount of C₂H₄ throughout the 24 hour incubation period. Inhibition of C₂H₄ synthesis with 2-aminoethoxyvinyl glycine and/or inhibition of C₂H₄ action with 2,5-norbornadiene blocked the promotive effect of light. Exogenous C₂H₄ overcame these blockages. The results showed that participation by endogenous C₂H₄ was essential for the light-induced relief of thermoinhibition of lettuce seed germination. However, light did not act exclusively via C₂H₄ since exogenous C₂H₄ alone in darkness did not promote germination.     

Differential effects of diurnal asymmetric and symmetric warming on yield and water utilization of soybean

Aims Global warming does not mean similar warmer temperatures between daytime and nighttime. Soybean (Glycine max) is a widely planted legume crop around the world and an important food crop in China. The aim of this study was to understand the responses of soybean growth and water utilization to future asymmetric warming, which would provide scientific reference for evaluating the adaptation of soybean to the future climate scenarios.Methods This experiment was carried out in artificial climate chambers, using the method of potted plants, under three temperature conditions; contrast (CON, 26 °C during the day and 16 °C during night), symmetric warming (ETs, elevated temperature of 3 °C both during the day and night), asymmetric warming (ETa, elevated temperature of 2 °C during the day and elevated temperature of 4 °C during night). We investigated the differential effects of diurnal asymmetric and symmetric warming on the yield and water consumption of soybean. Important findings The results revealed that, under the background of 26 °C during the day and 16 °C during night: 1) the effect of ETs on soybean yields showed no significant function that mainly benefit from the increase in the amount of biomass to ease negative influence of decrease in the harvest index. ETa reduced yields of soybean by 38.9% (p < 0.05) due to both significant decrease in harvest index and yield components (pod number per plant, grain number per pod and 100-grain weight). 2) ETs showed no obvious effect on the whole growing stage evapotranspiration (ET) of soybean, while ETa reduced the whole growing stage ET by 14.8% (p < 0.05). 3) The effect of the two warming pattern on water consumption of soybean were not significant. The difference in water consumption was mainly derived from the difference in transpiration (T). ETs and ETa reduced total transpiration by 10.7% (p < 0.05) and 26.1% (p < 0.05), respectively. In conclusion, our results suggest that ETs will underestimate the detrimental effects of real climate warming (ETa) on the growth and yield of soybean, and overestimate the effects on water consumption of soybean.     

昼夜不对称性与对称性升温对大豆产量和水分利用的影响

全球气候变暖并不是白天和夜间的平均变暖, 而是呈现一定的不对称性。大豆(Glycine max)是世界范围内种植较广泛的豆科作物, 也是中国重要的粮食作物。研究大豆的生长与水分利用对不对称性气候变暖的响应, 可为预测未来气候变暖情景下大豆的适应提供科学的参考依据。该实验在人工气候箱中采用盆栽方式进行, 设立对照(CON, 昼26 ℃夜16 ℃)、对称性升温(ETs, 昼夜均升高3 ℃)和不对称性升温(ETa, 昼升高2 ℃, 夜升高4 ℃)三个温度情景, 研究了大豆产量和水分利用对昼夜不对称性与对称性升温的差异性响应。结果表明: 在昼/夜26 ℃/16 ℃的背景下, 1) ETs对大豆产量影响不显著, 主要是因为生物量的增加缓解了收获指数下降对大豆的不利影响; ETa使大豆产量减少38.9%, 是由于大豆的收获指数和产量构成要素(荚数、粒数、百粒重)均显著降低。2) ETs对大豆全生育期蒸散量(ET)的影响不显著, ETa使大豆整个生育期ET减少14.8%。3)两种升温模式对大豆耗水量中蒸发量的影响都不显著, 耗水量的差异主要来自蒸腾量的差异, 其中ETs和ETa分别使大豆全生育期蒸腾量降低10.7%和26.1%。综上所述, 只针对ETs进行研究, 而没有对ETa进行研究的实验会低估真正的气候变暖情景(ETa)对大豆生长和产量的不利影响, 高估其对大豆耗水量的影响。     

Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected C4crops and weeds

Plants of six weedy species (Amaranthus retroflexus, Echinochloa crus-galli, Panicum dichotomiflorum, Setaria faberi, Setaria viridis, Sorghum halapense) and 4 crop species (Amaranthus hypochondriacus, Saccharum officinarum, Sorghum bicolor and Zea mays) possessing the C₄type of photosynthesis were grown at ambient (38 Pa) and elevated (69 Pa) carbon dioxide during early development (i.e. up to 60 days after sowing) to determine: (a) if plants possessing the C₄photosynthetic pathway could respond photosynthetically or in biomass production to future increases in global carbon dioxide and (b) whether differences exist between weeds and crops in the degree of response. Based on observations in the response of photosynthesis (measured as A, CO₂assimilation rate) to the growth CO₂condition as well as to a range of internal CO₂(C_i) concentrations, eight of ten C₄species showed a significant increase in photosynthesis. The largest and smallest increases observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO₂concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C₄weeds (A. retroflexus, E. crus-galli, P. dichotomiflorum, S. viridis) relative to the ambient CO₂condition. Leaf water potentials for three selected species (A. retroflexus, A. hypochondriacus, Z. mays) indicated that differences in photosynthetic stimulation were not due solely to improved leaf water status. Data from this study indicate that C₄plants may respond directly to increasing CO₂concentration, and in the case of some C₄weeds (e.g. A. retroflexus) may show photosynthetic increases similar to those published for C₃species.     

Variation in Quantum Yield for CO(2) Uptake among C(3) and C(4) Plants

The quantum yield for CO₂ uptake was measured on a number of C₃ and C₄ monocot and dicot species. Under normal atmospheric conditions (330 microliters per liter CO₂, 21% O₂) and a leaf temperature of 30°C, the average quantum yields (moles CO₂ per einstein) were as follows: 0.052 for C₃ dicots, 0.053 for C₃ grasses, 0.053 for NAD-malic enzyme type C₄ dicots, 0.060 for NAD-malic enzyme type C₄ grasses, 0.064 for phosphoenolpyruvate carboxykinase type C₄ grasses, 0.061 for NADP-malic enzyme C₄ dicots, and 0.065 for NADP-malic enzyme type C₄ grasses. The quantum yield under normal atmospheric conditions was temperature dependent in C₃ species, but apparently not in C₄ species. Light and temperature conditions during growth appeared not to influence quantum yield. The significance of variation in the quantum yields of C₄ plants was discussed in terms of CO₂ leakage from the bundle sheath cells and suberization of apoplastic regions of the bundle sheath cells.     

Physiological and Growth Responses of C3 and C4 Plants to Reduced Temperature When Grown at Low CO2 of the Last Ice Age

During the last Ice age, CO2 concentration ([CO2]) was 180-200 μmol/mol compared with the modern value of 380 μmol/mol,and global temperatures were ～8 ℃ cooler. Relatively little is known about the responses of C3 and C4 species to longterm exposure to glacial conditions. Here Abutilon theophrasti Medik. (C3) and Amaranthus retroflexus L. (C4) were grown at 200 μmol/mol CO2 with current (30/24 ℃) and glacial (22/16 ℃) temperatures for 22 d. Overall, the C4 species exhibited a large growth advantage over the C3 species at low [CO2]. However, this advantage was reduced at low temperature, where the C4 species produced 5× the total mass of the C3 species versus 14× at the high temperature.This difference was due to a reduction In C4 growth at low temperature, since the C3 species exhibited similar growth between temperatures. Physiological differences between temperatures were not detected for either species, although photorespirationlnet photosynthesis was reduced in the C3 species grown at low temperature, suggesting evidence of improved carbon balance at this treatment. This system suggests that C4 species had a growth advantage over C3 species during low [CO2] of the last ice age, although concurrent reductions in temperatures may have reduced this advantage.     

Thermal thresholds as predictors of seed dormancy release and germination timing: altitude-related risks from climate warming for the wild grapevine Vitis vinifera subsp. sylvestris

Background and Aims

The importance of thermal thresholds for predicting seed dormancy release and germination timing under the present climate conditions and simulated climate change scenarios was investigated. In particular, Vitis vinifera subsp. sylvestris was investigated in four Sardinian populations over the full altitudinal range of the species (from approx. 100 to 800 m a.s.l).

Methods

Dried and fresh seeds from each population were incubated in the light at a range of temperatures (10–25 and 25/10 °C), without any pre-treatment and after a warm (3 months at 25 °C) or a cold (3 months at 5 °C) stratification. A thermal time approach was then applied to the germination results for dried seeds and the seed responses were modelled according to the present climate conditions and two simulated scenarios of the Intergovernmental Panel on Climate Change (IPCC): B1 (+1·8 °C) and A2 (+3·4 °C).

Key Results

Cold stratification released physiological dormancy, while very few seeds germinated without treatments or after warm stratification. Fresh, cold-stratified seeds germinated significantly better (>80 %) at temperatures ≥20 °C than at lower temperatures. A base temperature for germination (T_b) of 9·0–11·3 °C and a thermal time requirement for 50 % of germination (θ₅₀) ranging from 33·6 °Cd to 68·6 °Cd were identified for non-dormant cold-stratified seeds, depending on the populations. This complex combination of thermal requirements for dormancy release and germination allowed prediction of field emergence from March to May under the present climatic conditions for the investigated populations.

Conclusions

The thermal thresholds for seed germination identified in this study (T_b and θ₅₀) explained the differences in seed germination detected among populations. Under the two simulated IPCC scenarios, an altitude-related risk from climate warming is identified, with lowland populations being more threatened due to a compromised seed dormancy release and a narrowed seed germination window.     

Comparison of the effects of symmetric and asymmetric temperature elevation and CO2 enrichment on yield and evapotranspiration of winter wheat (Triticum aestivum L.)

Under the changing climate, asymmetric warming pattern would be more likely during day and night time, instead of symmetric one. Concurrently, the growth responses and water use of plants may be different compared with those estimated based on symmetric warming. In this work, it was compared with the effects of symmetric (ETs) and asymmetric (ETa) elevation of temperature alone, and in interaction with elevated carbon dioxide concentration (EC), on the grain yield (GY) and evapotranspiration in winter wheat (Triticum aestivum L.) based on pot experiment in the North China Plain (NCP). The experiment was carried out in six enclosed‐top chambers with following climate treatments: (1) ambient temperature and ambient CO₂ (CON), (2) ambient temperature and elevated CO₂ (EC), (3) elevated temperature and ambient CO₂ (ETs; ETa), and (4) elevated temperature and elevated CO₂ (ECETs, ECETa). In symmetric warming, temperature was increased by 3°C and in asymmetric one by 3.5°C during night and 2.5°C during daytime, respectively. As a result, GY was in ETa and ETs 15.6 (P < 0.05) and 10.3% (P < 0.05) lower than that in CON. In ECETs and ECETa treatments, GY was 14.9 (P < 0.05) and 9.1% (P < 0.05) higher than that in CON. Opposite to GY, evapotranspiration was 7.8 (P < 0.05) and 17.9% (P < 0.05) higher in ETa and ETs treatments and 7.2 (P < 0.05) and 2.1% (P > 0.05) lower in ECETs and ECETa treatments compared with CON. Thus, GY of wheat could be expected to increase under the changing climate with concurrent elevation of CO₂ and temperature as a result of increased WUE under the elevated CO₂. However, the gain would be lower under ETa than that estimated based on ETs due to higher evapotranspiration.     

Effects of Global Warming on Ancient Mammalian Communities and Their Environments

Background

Current global warming affects the composition and dynamics of mammalian communities and can increase extinction risk; however, long-term effects of warming on mammals are less understood. Dietary reconstructions inferred from stable isotopes of fossil herbivorous mammalian tooth enamel document environmental and climatic changes in ancient ecosystems, including C₃/C₄ transitions and relative seasonality.

Methodology/Principal Findings

Here, we use stable carbon and oxygen isotopes preserved in fossil teeth to document the magnitude of mammalian dietary shifts and ancient floral change during geologically documented glacial and interglacial periods during the Pliocene (∼1.9 million years ago) and Pleistocene (∼1.3 million years ago) in Florida. Stable isotope data demonstrate increased aridity, increased C₄ grass consumption, inter-faunal dietary partitioning, increased isotopic niche breadth of mixed feeders, niche partitioning of phylogenetically similar taxa, and differences in relative seasonality with warming.

Conclusion/Significance

Our data show that global warming resulted in dramatic vegetation and dietary changes even at lower latitudes (∼28°N). Our results also question the use of models that predict the long term decline and extinction of species based on the assumption that niches are conserved over time. These findings have immediate relevance to clarifying possible biotic responses to current global warming in modern ecosystems.     

Soil carbon storage under simulated climate change is mediated by plant functional type

The stability of soil organic matter (SOM) pools exposed to elevated CO₂ and warming has not been evaluated adequately in long‐term experiments and represents a substantial source of uncertainty in predicting ecosystem feedbacks to climate change. We conducted a 6‐year experiment combining free‐air CO₂ enrichment (FACE, 550 ppm) and warming (+2 °C) to evaluate changes in SOM accumulation in native Australian grassland. In this system, competitive interactions appear to favor C₄ over C₃ species under FACE and warming. We therefore investigated the role of plant functional type (FT) on biomass and SOM responses to the long‐term treatments by carefully sampling soil under patches of C₃‐ and C₄‐dominated vegetation. We used physical fractionation to quantify particulate organic matter (POM) and long‐term incubation to assess potential decomposition rates. Aboveground production of C₄ grasses increased in response to FACE, but total root biomass declined. Across treatments, C : N ratios were higher in leaves, roots and POM of C₄ vegetation. CO₂ and temperature treatments interacted with FT to influence SOM, and especially POM, such that soil carbon was increased by warming under C₄ vegetation, but not in combination with elevated CO₂. Potential decomposition rates increased in response to FACE and decreased with warming, possibly owing to treatment effects on soil moisture and microbial community composition. Decomposition was also inversely correlated with root N concentration, suggesting increased microbial demand for older, N‐rich SOM in treatments with low root N inputs. This research suggests that C₃–C₄ vegetation responses to future climate conditions will strongly influence SOM storage in temperate grasslands.     

Interactive Effects of Elevated CO2, Drought, and Warming on Plants

Adverse climate change attributed to elevated atmospheric carbon dioxide concentration (CO₂) and increased temperature components of global warming has been a central issue affecting economic and social development. Climate change, particularly global warming, imposes a severe impact on the terrestrial ecosystem. Elevated CO₂, drought, and high temperature have been extensively documented individually; however, relatively little is known about how plants respond to the interaction of these factors. To summarize current knowledge on the response of plants to global change factors, we focus on the interactive effects of CO₂ enrichment, warming, and drought on plant growth, carbon allocation, and photosynthesis. Stimulation due to elevated CO₂ might be suppressed under other negative climatic/environmental stresses such as drought, high temperature, and their combination. However, elevated CO₂ could alleviate deleterious effects of moderate drought via reducing stomatal conductance, altering leaf surface, and regulating gene expression. High CO₂ levels and rising temperatures may result in opposite responses in plant water use efficiency. Stimulation of plant growth due to elevated CO₂ for C₃ species occurs regardless of water conditions, but only under a water deficit for C₄ species. The positive effect of elevated CO₂ on C₄ species is derived mainly from the improved water status. Plant adaptive or maladaptive responses to multivariate environments are interactive; thus, researchers need to explore the ecological underpinnings involved in such responses to the multiple factors involved in climate change.     

Neotropical C3/C4 grass distributions – present,past and future

Changes in C₄ grass distribution and abundance are frequently observed in Quaternary, Holocene and future environmental‐change scenarios. However, the factors driving these dynamics are not fully understood, and conflicting theories have been reported. In this paper, we present a very large dataset of modern altitudinal distribution profiles of C₃ and C₄ grasses covering the entire Neotropical Andes, which was compared with actual climate data. The results of multivariate analysis demonstrate that, in the Neotropical Andes, mean annual temperature is the main factor governing the modern altitudinal distribution of C₃ and C₄ grass species. The C₃ and C₄ grass distributions were compared with simulations based on the Lund‐Potsdam‐Jena dynamic global vegetation model (LPJ‐DGVM), which allowed the present grass distribution to be estimated. Finally, the DGVM was employed to simulate past and future scenarios, using the IPCC's climate projections for 2100 and PMIP2 models for the Holocene Optimum (HO, 6000 years bp ) and the Last Glacial Maximum (LGM, 21 000 years bp ). The results were found to be significantly different from those obtained using a simple photosynthetic model. According to LPJ forced with the PMIP2 models for the LGM, during the LGM, the C₄ grasses would not have reached higher altitudes than found in the present day.     

Changes in phenological events in response to a global warming scenario reveal greater adaptability of winter annual compared with summer annual arabidopsis ecotypes

Background and AimsThe impact of global warming on life cycle timing is uncertain. We investigated changes in life cycle timing in a global warming scenario. We compared Arabidopsis thaliana ecotypes adapted to the warm/dry Cape Verdi Islands (Cvi), Macaronesia, and the cool/wet climate of the Burren (Bur), Ireland, Northern Europe. These are obligate winter and summer annuals, respectively.MethodsUsing a global warming scenario predicting a 4 °C temperature rise from 2011 to approx. 2080, we produced F₁ seeds at each end of a thermogradient tunnel. Each F₁ cohort (cool and warm) then produced F₂ seeds at both ends of the thermal gradient in winter and summer annual life cycles. F₂ seeds from the winter life cycle were buried at three positions along the gradient to determine the impact of temperature on seedling emergence in a simulated winter life cycle.Key ResultsIn a winter life cycle, increasing temperatures advanced flowering time by 10.1 d °C^–1 in the winter annual and 4.9 d °C^–1 in the summer annual. Plant size and seed yield responded positively to global warming in both ecotypes. In a winter life cycle, the impact of increasing temperature on seedling emergence timing was positive in the winter annual, but negative in the summer annual. Global warming reduced summer annual plant size and seed yield in a summer life cycle.ConclusionsSeedling emergence timing observed in the north European summer annual ecotype may exacerbate the negative impact of predicted increased spring and summer temperatures on their establishment and reproductive performance. In contrast, seedling establishment of the Macaronesian winter annual may benefit from higher soil temperatures that will delay emergence until autumn, but which also facilitates earlier spring flowering and consequent avoidance of high summer temperatures. Such plasticity gives winter annual arabidopsis ecotypes a distinct advantage over summer annuals in expected global warming scenarios. This highlights the importance of variation in the timing of seedling establishment in understanding plant species responses to anthropogenic climate change.     

Effects of elevated atmospheric CO2, cutting frequency, and differential day/night atmospheric warming on root growth and turnover of Phalaris swards

We investigated seasonal root production and root turnover of fertilized and well‐watered monocultures of Phalaris for 2 years using minirhizotrons installed in six newly designed temperature gradient tunnels, combined with sequential soil coring. Elevated atmospheric CO₂ treatments were combined with two cutting frequencies and three warming scenarios: no warming, +3.0/+3.0 and +2.2/+4.0°C (day/night) atmospheric warming. The elevated CO₂ treatment increased both new and net root length production primarily when combined with atmospheric warming, where the constant warming treatment had a greater positive effect than the increased night‐time warming treatment. Responses to elevated CO₂ were greater when the swards were cut more frequently and responsiveness varied with season. For Phalaris swards, 17% of total net primary productivity went belowground. On account of root turnover, only one‐third of the new roots produced in the year following establishment could be expected, on average, to be recovered from soil cores. The interaction between the effects of CO₂ and warming, combined with the differential effects of the two warming treatments, has important implications for modelling belowground responses to projected climate change.     

A Comparison of Dark Respiration between C(3) and C(4) Plants

Lower respiratory costs were hypothesized as providing an additional benefit in C₄ plants compared to C₃ plants due to less investment in proteins in C₄ leaves. Therefore, photosynthesis and dark respiration of mature leaves were compared between a number of C₄ and C₃ species. Although photosynthetic rates were generally greater in C₄ when compared to C₃ species, no differences were found in dark respiration rates of individual leaves at either the beginning or after 16 h of the dark period. The effects of nitrogen on photosynthesis and respiration of individual leaves and whole plants were also investigated in two species that occupy similar habitats, Amaranthus retroflexus (C₄) and Chenopodium album (C₃). For mature leaves of both species, there was no relationship between leaf nitrogen and leaf respiration, with leaves of both species exhibiting a similar rate of decline after 16 h of darkness. In contrast, leaf photosynthesis increased with increasing leaf nitrogen in both species, with the C₄ species displaying a greater photosynthetic response to leaf nitrogen. For whole plants of both species grown at different nitrogen levels, there was a clear linear relationship between net CO₂ uptake and CO₂ efflux in the dark. The dependence of nightly CO₂ efflux on CO₂ uptake was similar for both species, although the response of CO₂ uptake to leaf nitrogen was much steeper in the C₄ species, Amaranthus retroflexus. Rates of growth and maintenance respiration by whole plants of both species were similar, with both species displaying higher rates at higher leaf nitrogen. There were no significant differences in leaf or whole plant maintenance respiration between species at any temperature between 18 and 42°C. The data suggest no obvious differences in respiratory costs in C₄ and C₃ plants.     

Temperature response of photosynthesis in C3, C4, and CAM plants: temperature acclimation and temperature adaptation

Most plants show considerable capacity to adjust their photosynthetic characteristics to their growth temperatures (temperature acclimation). The most typical case is a shift in the optimum temperature for photosynthesis, which can maximize the photosynthetic rate at the growth temperature. These plastic adjustments can allow plants to photosynthesize more efficiently at their new growth temperatures. In this review article, we summarize the basic differences in photosynthetic reactions in C₃, C₄, and CAM plants. We review the current understanding of the temperature responses of C₃, C₄, and CAM photosynthesis, and then discuss the underlying physiological and biochemical mechanisms for temperature acclimation of photosynthesis in each photosynthetic type. Finally, we use the published data to evaluate the extent of photosynthetic temperature acclimation in higher plants, and analyze which plant groups (i.e., photosynthetic types and functional types) have a greater inherent ability for photosynthetic acclimation to temperature than others, since there have been reported interspecific variations in this ability. We found that the inherent ability for temperature acclimation of photosynthesis was different: (1) among C₃, C₄, and CAM species; and (2) among functional types within C₃ plants. C₃ plants generally had a greater ability for temperature acclimation of photosynthesis across a broad temperature range, CAM plants acclimated day and night photosynthetic process differentially to temperature, and C₄ plants was adapted to warm environments. Moreover, within C₃ species, evergreen woody plants and perennial herbaceous plants showed greater temperature homeostasis of photosynthesis (i.e., the photosynthetic rate at high-growth temperature divided by that at low-growth temperature was close to 1.0) than deciduous woody plants and annual herbaceous plants, indicating that photosynthetic acclimation would be particularly important in perennial, long-lived species that would experience a rise in growing season temperatures over their lifespan. Interestingly, across growth temperatures, the extent of temperature homeostasis of photosynthesis was maintained irrespective of the extent of the change in the optimum temperature for photosynthesis (T _opt), indicating that some plants achieve greater photosynthesis at the growth temperature by shifting T _opt, whereas others can also achieve greater photosynthesis at the growth temperature by changing the shape of the photosynthesis–temperature curve without shifting T _opt. It is considered that these differences in the inherent stability of temperature acclimation of photosynthesis would be reflected by differences in the limiting steps of photosynthetic rate.     

Improving our understanding of environmental controls on the distribution of C3 and C4 grasses

A number of studies have demonstrated the ecological sorting of C₃ and C₄ grasses along temperature and moisture gradients. However, previous studies of C₃ and C₄ grass biogeography have often inadvertently compared species in different and relatively unrelated lineages, which are associated with different environmental settings and distinct adaptive traits. Such confounded comparisons of C₃ and C₄ grasses may bias our understanding of ecological sorting imposed strictly by photosynthetic pathway. Here, we used MaxEnt species distribution modeling in combination with satellite data to understand the functional diversity of C₃ and C₄ grasses by comparing both large clades and closely related sister taxa. Similar to previous work, we found that C₄ grasses showed a preference for regions with higher temperatures and lower precipitation compared with grasses using the C₃ pathway. However, air temperature differences were smaller (2 °C vs. 4 °C) and precipitation and % tree cover differences were larger (1783 mm vs. 755 mm, 21.3% vs. 7.7%, respectively) when comparing C₃ and C₄ grasses within the same clade vs. comparing all C₄ and all C₃ grasses (i.e., ignoring phylogenetic structure). These results were due to important differences in the environmental preferences of C₃ BEP and PACMAD clades (the two main grass clades). Winter precipitation was found to be more important for understanding the distribution and environmental niche of C₃ PACMADs in comparison with both C₃ BEPs and C₄ taxa, for which temperature was much more important. Results comparing closely related C₃–C₄ sister taxa supported the patterns derived from our modeling of the larger clade groupings. Our findings, which are novel in comparing the distribution and niches of clades, demonstrate that the evolutionary history of taxa is important for understanding the functional diversity of C₃ and C₄ grasses, and should have implications for how grasslands will respond to global change.