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1.
Conducting manipulative climate change experiments in complex vegetation is challenging, given considerable temporal and spatial heterogeneity. One specific challenge involves warming of both plants and soils to depth. We describe the design and performance of an open‐air warming experiment called Boreal Forest Warming at an Ecotone in Danger (B4WarmED) that addresses the potential for projected climate warming to alter tree function, species composition, and ecosystem processes at the boreal‐temperate ecotone. The experiment includes two forested sites in northern Minnesota, USA, with plots in both open (recently clear‐cut) and closed canopy habitats, where seedlings of 11 tree species were planted into native ground vegetation. Treatments include three target levels of plant canopy and soil warming (ambient, +1.7 °C, +3.4 °C). Warming was achieved by independent feedback control of voltage input to aboveground infrared heaters and belowground buried resistance heating cables in each of 72‐7.0 m2 plots. The treatments emulated patterns of observed diurnal, seasonal, and annual temperatures but with superimposed warming. For the 2009 to 2011 field seasons, we achieved temperature elevations near our targets with growing season overall mean differences (?Tbelow) of +1.84 °C and +3.66 °C at 10 cm soil depth and (?Tabove) of +1.82 °C and +3.45 °C for the plant canopies. We also achieved measured soil warming to at least 1 m depth. Aboveground treatment stability and control were better during nighttime than daytime and in closed vs. open canopy sites in part due to calmer conditions. Heating efficacy in open canopy areas was reduced with increasing canopy complexity and size. Results of this study suggest the warming approach is scalable: it should work well in small‐statured vegetation such as grasslands, desert, agricultural crops, and tree saplings (<5 m tall).  相似文献   

2.
Gas exchange and water relations were evaluated under full‐season in situ infrared (IR) warming for hard red spring wheat (Triticum aestivum L. cv. Yecora Rojo) grown in an open field in a semiarid desert region of the southwest USA. A temperature free‐air controlled enhancement (T‐FACE) apparatus utilizing IR heaters maintained canopy air temperature above 3.0 m Heated plots of wheat by 1.3 and 2.7 °C (0.2 and 0.3 °C below the targeted set‐points of Reference plots with dummy heaters) during daytime and nighttime, respectively. Control plots had no apparatus. Every 6 weeks during 2007–2009 wheat was sown under the three warming treatments (i.e., Control, Heated, Reference) in three replicates in a 3 × 3 Latin square (LSQ) design on six plantings during 4 months (i.e., January, March, September, December), or in a natural temperature variation treatment (i.e., Control) in three replicates in a randomized complete block (RCB) design on nine plantings during 7 months (i.e., January, February, April, June, July, August, October). Soil temperature (Ts) and volumetric soil‐water content (θs) were 1.3 °C warmer and 14% lower in Heated compared with Reference plots, respectively. Other than a 1% shading effect, no artifacts on gas exchange or water relations were associated with the IR warming apparatus. IR warming increased carbon gain characteristic of an increase in metabolic rates to higher temperature that may have been attributed to the well‐watered wheat crop and the supplemental irrigation that minimized plant‐to‐air water vapor pressure differences between IR‐warmed and nonwarmed plots. Nevertheless, seasonal oscillations in the IR warming response on carbon gain occurred. IR warming decreased leaf water status and provided thermal protection during freeze events. IR warming is an effective experimental methodology to investigate the impact of global climate change on agronomic cropping and natural ecosystems to a wide range of natural and artificially imposed air temperatures.  相似文献   

3.
A new system for simulating future belowground temperature increases was conceived, simulated, constructed and tested in a temperate deciduous forest in Oak Ridge, TN, USA. The new system uses low‐wattage, 3 m deep heaters installed around the circumference of a defined soil volume. The heaters add the necessary energy to achieve a set soil temperature differential within the treatment area and add exterior energy inputs equal to those, which might be lost from lateral heat conduction. The method, which was designed to work in conjunction with aboveground heated chambers, requires only two control sensor positions one for aboveground air temperatures at 1 m and another for belowground temperatures at 0.8 m. The method is capable of achieving temperature differentials of at least +4.0±0.5 °C for soils to a measured depth of ?2 m. These +4 °C differential soil temperatures were sustained in situ throughout 2009, and both diurnal and seasonal cycles at all soil depths were retained using this simple heating approach. Measured mean energy inputs required to sustain the target heating level of +4 °C over the 7.1 m2 target area were substantial for aboveground heating (21.1 kW h day?1 m?2), but 16 times lower for belowground heaters (1.3 kW h day?1 m?2). Observations of soil CO2 efflux from the surface of the target soil volumes showed CO2 losses throughout 2009 that were elevated above the temperature response curve that have been reported in previous near‐surface soil warming studies. Stimulation of biological activity within previously undisturbed deep‐soil carbon stocks is the hypothesized source. Long‐term research programs may be able to apply this new heating method that captures expected future warming and temperature dynamics throughout the soil profile to address uncertainties in process‐level responses of microbial, plant and animal communities in whole, intact ecosystems.  相似文献   

4.
Theory and performance of an infrared heater for ecosystem warming   总被引:5,自引:0,他引:5  
In order to study the likely effects of global warming on future ecosystems, a method for applying a heating treatment to open-field plant canopies (i.e. a temperature free-air controlled enhancement (T-FACE) system) is needed which will warm vegetation as expected by the future climate. One method which shows promise is infrared heating, but a theory of operation is needed for predicting the performance of infrared heaters. Therefore, a theoretical equation was derived to predict the thermal radiation power required to warm a plant canopy per degree rise in temperature per unit of heated land area. Another equation was derived to predict the thermal radiation efficiency of an incoloy rod infrared heater as a function of wind speed. An actual infrared heater system was also assembled which utilized two infrared thermometers to measure the temperature of a heated plot and that of an adjacent reference plot and which used proportional–integrative–derivative control of the heater to maintain a constant temperature difference between the two plots. Provided that it was not operated too high above the canopy, the heater system was able to maintain a constant set-point difference very well. Furthermore, there was good agreement between the measured and theoretical unit thermal radiation power requirements when tested on a Sudan grass (Sorghum vulgare) canopy. One problem that has been identified for infrared heating of experimental plots is that the vapor pressure gradients (VPGs) from inside the leaves to the air outside would not be the same as would be expected if the warming were performed by heating the air everywhere (i.e. by global warming). Therefore, a theoretical equation was derived to compute how much water an infrared-warmed plant would lose in normal air compared with what it would have lost in air which had been warmed at constant relative humidity, as is predicted with global warming. On an hourly or daily basis, it proposed that this amount of water could be added back to plants using a drip irrigation system as a first-order correction to this VPG problem.  相似文献   

5.
In order to facilitate interpretation and comparison of warming effects on ecosystems across various habitats, it is imperative to quantify changes in microclimate induced by warming facilities. This paper reports observed changes in air temperature, soil temperature and soil‐moisture content under experimental warming and clipping in a tallgrass prairie in the Great Plains, USA. We used a factorial design with warming as the primary factor nested with clipping as the secondary factor. Infrared heater was used in order to simulate climatic warming and clipping to mimic mowing for hay or grazing. The warming treatment significantly increased daily mean and minimum air temperatures by 1.1 and 2.3 °C, respectively, but had no effect on daily maximum air temperature, resulting in reduced diurnal air‐temperature range. Infrared heaters substantially increased daily maximum (2.5 and 3.5 °C), mean (2.0 and 2.6 °C) and minimum (1.8 and 2.1 °C) soil temperatures in both the unclipped and clipped subplots. Clipping also significantly increased daily maximum (3.4 and 4.3 °C) and mean (0.6 and 1.2 °C) soil temperatures, but decreased daily minimum soil temperature (1.0 and 0.6 °C in the control and warmed plots, respectively). Daily maximum, mean and minimum soil temperatures in the clipped, warmed subplots were 6.8, 3.2 and 1.1 °C higher than those in the unclipped, control subplots. Infrared heaters caused a reduction of 11.0% in soil moisture in the clipped subplots, but not in the unclipped subplots. Clipping reduced soil‐moisture content by 17.7 and 22.7% in the control and warmed plots, respectively. Experimental warming and clipping interacted to exacerbate soil‐moisture loss (26.7%). Overall, infrared heaters simulated climate warming well by enhancing downward infrared radiation and by reducing the diurnal air‐temperature range.  相似文献   

6.
Knowledge about the role of litter and dung decomposition in nutrient cycling and response to climate change and grazing in alpine ecosystems is still rudimentary. We conducted two separate studies to assess the relative role of warming and grazing on litter mass loss and on the temperature sensitivity of litter and dung mass loss. Experiments were conducted for 1–2 years under a controlled warming–grazing system and along an elevation gradient from 3200 to 3800 m. A free‐air temperature enhancement system (FATE) using infrared heaters and grazing significantly increased soil temperatures (average 0.5–1.6 °C) from 0 to 40 cm depth, but neither warming nor grazing affected soil moisture except early in the growing seasons at 30 cm soil depth. Heaters caused greater soil warming at night‐time compared with daytime, but grazing resulted in greater soil warming during daytime compared with night‐time. Annual average values of the soil temperature at 5 cm were 3.2, 2.4 and 0.3 °C at 3200, 3600 and 3800 m, respectively. Neither warming nor grazing caused changes of litter quality for the first year of the controlled warming–grazing experiment. The effects of warming and grazing on litter mass losses were additive, increasing litter mass losses by about 19.3% and 8.3%, respectively, for the 2‐year decomposition periods. The temperature sensitivity of litter mass losses was approximately 11% °C?1 based on the controlled warming–grazing experiment. The annual cumulative litter mass loss was approximately 2.5 times that of dung along the elevation gradient. However, the temperature sensitivity (about 18% °C?1) of the dung mass loss was about three times that of the litter mass loss. These results suggest greater warming at night‐time compared with daytime may accelerate litter mass loss, and grazing will enhance carbon loss to atmosphere in the region through a decrease of litter biomass and an increase of dung production with an increase of stocking rate in future warmer conditions.  相似文献   

7.
In order to investigate the relative impacts of increases in day and night temperature on tree carbon relations, we measured night‐time respiration and daytime photosynthesis of leaves in canopies of 4‐m‐tall cottonwood (Populus deltoides Bartr. ex Marsh) trees experiencing three daytime temperatures (25, 28 or 31 °C) and either (i) a constant nocturnal temperature of 20 °C or (ii) increasing nocturnal temperatures (15, 20 or 25 °C). In the first (day warming only) experiment, rates of night‐time leaf dark respiration (Rdark) remained constant and leaves displayed a modest increase (11%) in light‐saturated photosynthetic capacity (Amax) during the day (1000–1300 h) over the 6 °C range. In the second (dual night and day warming) experiment, Rdark increased by 77% when nocturnal temperatures were increased from 15 °C (0·36 µmol m?2 s?1) to 25 °C (0·64 µmol m?2 s?1). Amax responded positively to the additional nocturnal warming, and increased by 38 and 64% in the 20/28 and 25/31 °C treatments, respectively, compared with the 15/25 °C treatment. These increases in photosynthetic capacity were associated with strong increases in the maximum carboxylation rate of rubisco (Vcmax) and ribulose‐1,5‐bisphosphate (RuBP) regeneration capacity mediated by maximum electron transport rate (Jmax). Leaf soluble sugar and starch concentration, measured at sunrise, declined significantly as nocturnal temperature increased. The nocturnal temperature manipulation resulted in a significant inverse relationship between Amax and pre‐dawn leaf carbohydrate status. Independent measurements of the temperature response of photosynthesis indicated that the optimum temperature (Topt) acclimated fully to the 6 °C range of temperature imposed in the daytime warming. Our findings are consistent with the hypothesis that elevated night‐time temperature increases photosynthetic capacity during the following light period through a respiratory‐driven reduction in leaf carbohydrate concentration. These responses indicate that predicted increases in night‐time minimum temperatures may have a significant influence on net plant carbon uptake.  相似文献   

8.
A new technique, called Free Air Temperature Increase (FATI), was developed to artificially induce increased canopy temperature in field conditions without the use of enclosures. This acronym was chosen in analogy with FACE (Free Air CO2 Enrichment), a technique which produces elevated CO2 concentrations [CO2] in open field conditions. The FATI system simulates global warming in small ecosystems of limited height, using infrared heaters from which all radiation below 800 nm is removed by selective cut-off filters to avoid undesirable photomorpho-genetic effects. An electronic control circuit tracks the ambient canopy temperature in an unheated reference plot with thermocouples, and modulates the radiant energy from the lamps to produce a 2.5°C increment in the canopy temperature of an associated heated plot (continuously day and night). This pre-set target differential is relatively-constant over time due to the fast response of the lamps and the use of a proportional action controller (the standard deviation of this increment was <1°C in a 3 week field study with 1007 measurements). Furthermore, the increase in leaf temperature does not depend on the vertical position within the canopy or on the height of the stand. Possible applications and alternative designs are discussed.  相似文献   

9.
Leaves and branches of mature trees, lianas, and gap species were warmed in an Amazonian forest for 4 mo to observe the effect of warming on photosynthesis, stomatal conductance, and transpiration. Electric resistance heaters increased air temperatures near the leaves by approximately 2°C. Sunlit leaf temperatures increased by 2–3°C on average, but during some periods leaf temperatures increased by >5°C. Maximum photosynthesis (Amax) decreased significantly in the warmed leaves vs. the control leaves over the 13‐wk study period with an average decrease in Amax of 1.4 μmol/m2s (19% decrease from a mean Amax of 7.2 μmol/m2s) when measured at 30°C and there were no signs of acclimation to higher temperatures within existing leaves. The decline in Amax was likely due to irreversible temperature damage caused by very high leaf temperatures and not due to Ci limitation of carboxylation. Warming had a larger negative impact on Amax in canopy level tree species than other tested functional groups such as lianas or gap species. Transpiration did not significantly increase in the warmed leaves compared with the control group. This study indicates that increased temperatures due to global warming could potentially decrease future tropical forest carbon uptake by a significant amount. Abstract in Portuguese is available at http://www.blackwell‐synergy.com/loi/btp .  相似文献   

10.
Rising atmospheric CO2 concentration ([CO2]) and attendant increases in growing season temperature are expected to be the most important global change factors impacting production agriculture. Although maize is the most highly produced crop worldwide, few studies have evaluated the interactive effects of elevated [CO2] and temperature on its photosynthetic physiology, agronomic traits or biomass, and seed yield under open field conditions. This study investigates the effects of rising [CO2] and warmer temperature, independently and in combination, on maize grown in the field throughout a full growing season. Free‐air CO2 enrichment (FACE) technology was used to target atmospheric [CO2] to 200 μmol mol?1 above ambient [CO2] and infrared heaters to target a plant canopy increase of 3.5 °C, with actual season mean heating of ~2.7 °C, mimicking conditions predicted by the second half of this century. Photosynthetic gas‐exchange parameters, leaf nitrogen and carbon content, leaf water potential components, and developmental measurements were collected throughout the season, and biomass and yield were measured at the end of the growing season. As predicted for a C4 plant, elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Canopy warming caused a large shift in aboveground allocation by stimulating season‐long vegetative biomass and decreasing reproductive biomass accumulation at both CO2 concentrations, resulting in decreased harvest index. Warming caused a reduction in photosynthesis due to down‐regulation of photosynthetic biochemical parameters and the decrease in the electron transport rate. The reduction in seed yield with warming was driven by reduced photosynthetic capacity and by a shift in aboveground carbon allocation away from reproduction. This field study portends that future warming will reduce yield in maize, and this will not be mitigated by higher atmospheric [CO2] unless appropriate adaptation traits can be introduced into future cultivars.  相似文献   

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