The interactive effects of heat wave regimes, elevated CO₂ concentration, and drought on tree physiology and growth

Our global climate is changing. In addition to rising CO2 concentrations and increasing average temperatures, our weather is also becoming more extreme. A key factor of this growing extremity are heat waves, which have been predicted to become more severe but also more frequent. Since heat waves are very often accompanied by drought, it is easy to imagine the devastating impact this can have, and already has had, on crop and forest ecosystems. Heat waves can impact a wide variety of tree functions such as photosynthesis, stomatal regulation, and carbon allocation. At the leaf level, photosynthesis is reduced, photo-oxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. Stomatal conductance has been found to increase at high temperatures in some species, which may be a mechanism for heat stress avoidance. At the whole tree level, heat stress has been shown to decrease growth and shift carbon allocation. We investigated this impact on Quercus rubra L. and Pinus taeda L. seedlings by applying repeated weeklong heat waves, during which temperatures could go up to 53°C and water availability was limited. We observed that occurrence of a +12°C heat wave significantly reduced net photosynthesis of Quercus rubra L. seedlings in comparison with a constant +3°C increase in temperature, but only in the afternoon. Morning net photosynthesis was observed to increase at the beginning of the heat wave. Stomatal conductance was higher at the beginning of the heat wave than before, but continuously decreased as the heat wave progressed. After the heat wave, stomatal conductance was even lower than before the heat wave. Since transpiration followed the same trend as stomatal conductance, we suggest that this was a heat stress avoidance mechanism of the seedlings. We furthermore observed that elevated [CO2] completely eliminated the negative effect of heat waves on Q. rubra biomass accumulation. In fact, Q. rubra in the heat wave treatment had increased stem height and diameter in the elevated [CO2] treatment, compared with all other treatments. The impact of drought was more dubious in our observations. While the combination of heat waves and drought clearly had a greater negative impact on gas exchange and biomass than either heat waves or drought alone, the difference (both relative and absolute) between seedlings exposed to a +12°C heat wave and seedlings exposed to a constant +3°C temperature elevation was smaller under drought conditions than under well-watered conditions. When we measured chlorophyll a fluorescence parameters, we even found that low soil moisture had no additional effect on Fv'/Fm' or PSII. We have found one putative example of acclimation to these heat waves, which was the reduced stomatal conductance and transpiration outside of the heat wave, and subsequent the increase in these parameters, supplemented by an increase in net photosynthesis, during the heat wave. These data suggest that the seedlings with previous heat wave exposure activated a stomatal regulation mechanism which increased stem water storage outside of the heat wave, to allow greater evaporative cooling during the heat wave, reflecting a mechanism to conserve water that prioritises survival over carbon acquisition

High-resolution in vivo imaging of xylem-transported CO2 in leaves based on real-time 11C-tracing

Plant studies using the short-lived isotope C-11 to label photosynthate via atmospheric carbon dioxide (CO2), have greatly advanced our knowledge about the allocation of recent photosynthate from leaves to sinks. However, a second source for photosynthesis is CO2 in the transpiration stream, coming from respiration in plant tissues. Here, we use in vivo tracing of xylem-transported (CO2)-C-11 to increase our knowledge on whole plant carbon cycling.We developed a newmethod for in vivo tracing of xylem-transported CO2 in excised poplar leaves using C-11 in combination with positron emission tomography (PET) and autoradiography. To show the applicability of both measurement techniques in visualizing and quantifying CO2 transport dynamics, we administered the tracer via the cut petiole and manipulated the transport by excluding light or preventing transpiration. Irrespective of manipulation, some tracer was found in main and secondary veins, little of it was fixed in minor veins or mesophyll, while most of it diffused out the leaf. Transpiration, phloem loading and CO2 recycling were identified as mechanisms that could be responsible for the transport of internal CO2. Both C-11-PET and autoradiography can be successfully applied to study xylem-transported CO2, toward better understanding of leaf and plant carbon cycling, and its importance in different growing conditions

Phloem transport: a review of mechanisms and controls

It is generally believed that an osmotically generated pressure gradient drives the phloem mass flow. So far, this widely accepted Münch theory has required remarkably few adaptations, but the debate on alternative and additional hypotheses is still ongoing. Recently, a possible shortcoming of the Münch theory has been pointed out, suggesting that the Münch pressure flow is more suitable for herbs than for trees. Estimation of the phloem resistance indicates that a point might be reached in long sieve tubes where the pressure required to drive the Münch flow cannot be generated. Therefore, the relay hypothesis regained belief as it implies that the sieve tubes are shorter then the plant’s axial axis. In the source phloem, three different loading strategies exist which probably result from evolutionary advantages. Passive diffusion seems to be the most primitive one, whereas active loading strategies substantially increase the growth potential. Along the transport phloem, a leakage-retrieval mechanism is observed. Appreciable amounts of carbohydrates are lost from the sieve tubes to feed the lateral sinks, while a part of these lost carbohydrates is subsequently reloaded into the sieve tubes. This mechanism is probably involved to buffer short-term irregularities in phloem turgor and gradient. In the long term, the mechanism controls the replenishment and remobilization of lateral stem storage tissues. As phloem of higher plants has multiple functions in plant development, reproduction, signalling, and growth, the fundamental understanding of the mechanisms behind phloem transport should be elucidated to increase our ability to influence plant growth and development

Impact of heat waves, drought stress and elevated CO2 on northern red oak seedlings

Background/Question/Methods: The frequency and intensity of heat waves are predicted to increase in the future. We investigated whether heat waves of different severities would have the same impact as a constant increase in temperature with the same heat sum, and whether elevated [CO2] would mitigate, and soil drought would exacerbate, plant response to severe heat. We measured gas exchange and biomass yield of Quercus rubra L. seedlings grown in pots in climate-controlled chambers at a field site near Athens GA. Treatments consisted of ambient or elevated [CO2] (380 or 700 μmol CO2 mol-1) and four temperature treatments: ambient (+0°C), ambient +3°C, moderate heat wave (+6°C), or severe heat wave (+12°C). Heat wave treatments were applied as a week-long heat wave every other week of +6°C, or every fourth week of +12°C throughout the growing season. This schedule yielded the same heat sum and average temperature for all three elevated temperature treatments for each four week period, and for the entire growing season. Half of the seedlings in each treatment combination were well-watered and half were grown under low soil moisture. Results/Conclusions: In ambient [CO2], well-watered plants had significantly less biomass when exposed to either the +6oC or +12oC heat waves compared to a constant +3oC increase, or ambient, temperatures. The trend was the same for plants in the low water treatment, but was not statistically significant. In all temperature x [CO2] treatment combinations the low-water treatment substantially reduced biomass compared to the well-watered treatment. Under well-watered conditions biomass accumulation in elevated [CO2] was generally higher than under ambient [CO2], and significant differences among temperature treatments were absent. Biomass accumulation under elevated [CO2] was lower in the low-water treatment compared to the well-watered treatment, and also higher than in those treatments under ambient [CO2]. Heat waves significantly reduced afternoon net photosynthesis (Anet), but not morning Anet. Elevated [CO2] increased Anet but did not completely mitigate effects of heat and drought stress on instantaneous performance. Low soil moisture significantly reduced Anet outside of the heat wave treatments, and diminished the heat wave-induced Anet decrease in both [CO2] treatments. We conclude that a) heat waves produced more stress than the same amount of heat applied uniformly and b) elevated [CO2] mitigated many of the negative effects of the heat waves

Effect of repeated heat waves, elevated [CO2] and low water availability on growth of Quercus rubra and Pinus taeda seedlings

Background/Question/Methods: The frequency and intensity of heat waves are predicted to increase. We investigated whether repeated heat waves of different severities would have the same impact as an equivalent constant increase in temperature and considered atmospheric [CO2] and soil moisture as potential interacting factors. We measured growth, biomass accumulation and partitioning of Quercus rubra and Pinus taeda seedlings grown under ambient or elevated [CO2] (380/700 mmol CO2 mol-1) and three heat treatments: ambient +3°C, a moderate heat wave every second week (+6°C) or a severe heat wave every fourth week (+12°C). All treatments had the same average temperature across the five month growing season. Half the seedlings were watered to a soil water content near field capacity, half to 50% of that amount. Height and diameter were measured monthly. Biomass accumulation was measured after 5 months. Leaf gas exchange was also measured before, during and after a mid-summer heat wave event. Results/Conclusions: In ambient [CO2], seedlings had significantly less total biomass when exposed to +12oC heat waves compared to a constant +3oC increase. These differences were eliminated under elevated [CO2] in Q. rubra but not in P. taeda. Total biomass accumulation in seedlings subjected to +6°C heat waves was not significantly different than in the constant +3oC control treatment. The +12oC heat waves also affected stem, root and leaf biomass partitioning, but to a different degree in the two species. Generally, the Q. rubra seedlings were more sensitive to the heat wave treatments than the P. taeda seedlings. All measures of growth, including height, diameter and leaf, stem, root and total biomass accumulation were significantly reduced in the low water treatment. Water availability had a much greater effect on growth than either [CO2] or temperature. Reduced net photosynthesis during a heat wave event and in the low water treatment was correlated with reduced growth. We conclude that a) heat waves can produce more stress than the same amount of heat applied uniformly; b) elevated [CO2] can partially mitigate negative heat wave effects and c) drought was a more severe stress than repeated heat waves

Elevated [CO2] and growth temperature have a small positive effect on photosynthetic thermotolerance of Pinus taeda seedlings

Growth temperature had little effect on the response of net photosynthesis to high temperatures (up to 47 A degrees C). On the other hand, elevated [CO (2) ] increased net photosynthesis at high temperatures. We investigated whether Pinus taeda seedlings grown under elevated CO2-concentration ([CO2]) and temperature would be able to maintain positive net photosynthesis (A (net)) longer than seedlings grown under ambient conditions when exposed to temperatures up to 47 A degrees C. Additionally, we investigated whether a locally applied temperature increase would yield the same short-term gas exchange response to temperatures up to 47 A degrees C as a naturally occurring latitudinal temperature increase of equal magnitude. Growth conditions were applied for 7 months (February to August) in treatment chambers constructed at two sites in the native range of P. taeda in the southern US. The sites were located 300 km apart along a north-south axis with a natural temperature difference of 2.1 A degrees C. Seedlings were grown under ambient temperature and [CO2] conditions at both sites. At the northern site, we also applied a temperature increase of 2 A degrees C (T (E)), ensuring that this treatment equalled the mean temperature at the southern site. Additionally, at the northern site, we applied a treatment of elevated [CO2] (C (E)). Gas exchange was measured on all plants in walk-in environmentally controlled chambers. Under C (E), there was no difference in A (net) of seedlings grown in ambient or ambient +2 A degrees C temperatures at any measurement temperature, while differences were present under ambient [CO2]. Furthermore, A (net) was higher under C (E) than under ambient [CO2]. At 47 A degrees C, A (net) was negative in all seedlings except those in the C (E) and ambient temperature treatment combination. Seedlings at the northern site in the T (E) treatment showed no significant differences in A (net) compared with seedlings grown at ambient temperature at the southern site, indicating that the plants responded equally to a manipulated temperature increase and a latitudinal increase in temperature. Our results suggest that elevated [CO2] increases photosynthetic thermotolerance at high temperature (> 41 A degrees C), but this effect diminishes as temperature increases further. Temperature manipulations could provide accurate information on the effect of latitudinal differences in temperature on leaf gas exchange of P. taeda

The effect of heat waves, elevated [CO₂] and low soil water availability on northern red oak (Quercus rubra L.) seedlings

The frequency and intensity of heat waves are predicted to increase. This study investigates whether heat waves would have the same impact as a constant increase in temperature with the same heat sum, and whether there would be any interactive effects of elevated [CO2] and soil moisture content. We grew Quercus rubra seedlings in treatment chambers maintained at either ambient or elevated [CO2] (380 or 700 mu mol CO2 mol(-1)) with temperature treatments of ambient, ambient +3 degrees C, moderate heat wave (+6 degrees C every other week) or severe heat wave (+12 degrees C every fourth week) temperatures. Averaged over a 4-week period, and the entire growing season, the three elevated temperature treatments had the same average temperature and heat sum. Half the seedlings were watered to a soil water content near field capacity, half to about 50% of this value. Foliar gas exchange measurements were performed morning and afternoon (9:00 and 15:00 hours) before, during and after an applied heat wave in August 2010. Biomass accumulation was measured after five heat wave cycles. Under ambient [CO2] and well-watered conditions, biomass accumulation was highest in the +3 degrees C treatment, intermediate in the +6 degrees C heat wave and lowest in the +12 degrees C heat wave treatment. This response was mitigated by elevated [CO2]. Low soil moisture significantly decreased net photosynthesis (A(net)) and biomass in all [CO2] and temperature treatments. The +12 degrees C heat wave reduced afternoon A(net) by 23% in ambient [CO2]. Although this reduction was relatively greater under elevated [CO2], A(net) values during this heat wave were still 34% higher than under ambient [CO2]. We concluded that heat waves affected biomass growth differently than the same amount of heat applied uniformly over the growing season, and that the plant response to heat waves also depends on [CO2] and soil moisture conditions

The effect of induced heat waves on Pinus taeda and Quercus rubra seedlings in ambient and elevated CO₂ atmospheres

Here, we investigated the effect of different heat-wave intensities applied at two atmospheric CO2 concentrations ([CO2]) on seedlings of two tree species, loblolly pine (Pinus taeda) and northern red oak (Quercus rubra). Seedlings were assigned to treatment combinations of two levels of [CO2] (380 or 700 mu mol mol-1) and four levels of air temperature (ambient, ambient +3 degrees C, or 7-d heat waves consisting of a biweekly +6 degrees C heat wave, or a monthly +12 degrees C heat wave). Treatments were maintained throughout the growing season, thus receiving equal heat sums. We measured gas exchange and fluorescence parameters before, during and after a mid-summer heat wave. The +12 degrees C heat wave, significantly reduced net photosynthesis (Anet) in both species and [CO2] treatments but this effect was diminished in elevated [CO2]. The decrease in Anet was accompanied by a decrease in Fv'/Fm' in P. taeda and FPSII in Q. rubra. Our findings suggest that, if soil moisture is adequate, trees will experience negative effects in photosynthetic performance only with the occurrence of extreme heat waves. As elevated [CO2] diminished these negative effects, the future climate may not be as detrimental to plant communities as previously assumed

Acclimation effects of heat waves and elevated [CO2] on gas exchange and chlorophyll fluorescence of northern red oak (Quercus rubra L.) seedlings

Heat wave frequency and intensity are predicted to increase. We investigated whether repeated exposure to heat waves would induce acclimation in Quercus rubra seedlings and considered [CO2] as an interacting factor. We measured gas exchange and chlorophyll fluorescence of seedlings grown in 380 (C A) or 700 (C E) μmol CO2 mol−1, and three temperature treatments (ambient, ambient +3 °C, and an ambient +12 °C heat wave every fourth week). Measurements were performed during the third and fourth +12 °C heat waves (July and August 2010) at Whitehall Forest, GA, USA. Additionally, previously unexposed seedlings were subjected to the August heat wave to serve as a control to determine acclimation of seedlings which were previously exposed. Seedlings with a history of heat wave exposure showed lower net photosynthesis (A net) and stomatal conductance (on average −47 and −38 %, respectively) than seedlings with no such history, when both were subjected to the same +12 °C heat wave. During both heat waves, A net significantly declined in the +12 °C treatment compared with the other treatments. Additionally, the A net decline during the August compared with the July heat wave was stronger in C E than in C A, suggesting that elevated [CO2] might have had a negative effect on acclimation capacity. We conclude that seedlings subjected to consecutive heat waves will moderate stomatal conductance outside the heat wave, to reduce water usage at lower temperatures, increasing survival at the expense of carbon assimilation