Reduced N cycling in response to elevated CO2, warming,and drought in a Danish heathland: Synthesizing results of the CLIMAITE project after two years of treatments

Field‐scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a seminatural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO₂), warming, and extended summer drought in all combinations. Here, we report on the short‐term responses of the nitrogen (N) cycle after 2 years of treatments. Elevated CO₂ significantly affected aboveground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co‐dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Belowground, elevated CO₂ had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH₄⁺ consumption, gross mineralization, potential nitrification, denitrification and N₂O emissions. Drought reduced belowground gross N mineralization and decreased fauna N mass and fauna N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue in response to elevated CO₂, as well as drought‐induced decreases in litter N production and fauna N mineralization prevailed in the full treatment combination. Overall, the simulated future climate scenario therefore lead to reduced N turnover, which could act to reduce the potential growth response of plants to elevated atmospheric CO₂ concentration.     

Differences in growth and water relations among Phaseolus vulgaris cultivars in response to induced drought stress

Relatively little ecophysiological research has been conducted to determine the responses to drought of Phaseolus vulgaris. Four bean cultivars (cvs.) from Brazil, A320, Carioca, Ouro Negro and Xodó were submitted to an imposed water deficit in order to evaluate the importance of some adaptive mechanisms of drought resistance through the analysis of growth parameters, water status, gas exchange and indicators of tolerance mechanisms at the cellular level. During the drought treatment, relative growth rates were more reduced for A320 and Xodó than Carioca and Ouro Negro. A320 closed its stomata very rapidly and complete stomatal closure was obtained at Psi(w)=-0.6 MPa, in contrast to the other cvs. where stomata were fully closed only at Psi(w)=-0.9 MPa. Net assimilation rates were closely related to stomatal conductances. Mechanisms at the cellular level appeared to be mostly important for higher tolerance. Carioca and Ouro Negro, when compared to A320 and Xodó, were characterized by having better drought tolerance mechanisms and higher tissue water retention capacity leading to a better growth under water deficits. The leaf dehydration rates of those cvs. were slow whereas those of the drought sensitive cvs. were rapid. The results were confirmed by the electrolyte leakage test and leaf osmotic potential measurements, which indicated higher membrane resistance and osmotic adjustment in the two tolerant cvs. Carioca and Ouro Negro. It appears from this study that despite being cultivated in the same geographical region, the four cvs. of P. vulgaris displayed somewhat different drought adaptive capacities for prolonged drought during the vegetative phase.     

Nocturnal feeding in the mouse--opiate and pineal influences

Mice displayed daily rhythms in their basal and morphine-induced food intake, consuming significantly greater amounts of food at night. Non-invasive inhibition of the activity of the pineal gland by either exposure to a bright pulse of light or treatment with the L-amino-acid decarboxylase inhibitor, benserazide, reduced the elevated night-time food intakes. These effects on feeding were most evident on the first night the activity of the pineal was reduced. On subsequent nights light pulses had a diminished effect on basal and morphine-induced food intake. These results suggest that although the enhanced nocturnal food intake of mice may be modulated by pineal and opioid sensitive mechanisms, pineal activity is not essential for the expression of opioid-mediated feeding.     

Carbon dioxide transport inxylem causes errors in estimation of rates of respiration in stemsand branches of trees

Complementary laboratory and field experiments showed that theinternal transport of carbon dioxide (CO₂) in the xylemof trees is an important pathway for carbon movement. Carbon dioxidereleased by respiration dissolves in sap and moves upward in thetranspirational stream. The concentration of CO₂ in xylemsap can be up to three orders of magnitude greater than that foundin the atmosphere. In the present experiments, diffusion outwardof a portion of xylem‐transported CO₂ caused a substantialoverestimation of the apparent rate of stem and branch respiration.Rates of CO₂ efflux were linearly related to sap CO₂ concentration.Direct manipulations of xylem sap CO₂ concentration producedrapid and reversible changes in CO₂ efflux from stemsand branches, in some cases quadrupling the rate of efflux. Theseresults demonstrated that apparent rates of stem and branch respirationof trees are in large part a by‐product of the rate of CO₂ diffusionfrom xylem sap.     

Stem CO2 efflux of ten species in temperate forests in Northeastern China

Stem CO₂ efflux (E _S) is an important component of forest ecosystem carbon budgets and net ecosystem CO₂ exchange, but little is known about E _S in temperate forests in Northeastern China, an area with a large extent of forest. We measured E _S along with stem temperature at 1?cm depth (Ts) over a 9?month period in 2007 on ten dominant tree species of secondary forests of the region. Other measurements included the autotrophic component of soil CO₂ efflux (E _A) and stem diameter at breast height (DBH). Our objectives were to (1) examine the seasonal patterns and species differences in E _S, and (2) determine the correlations between E _S and Ts, DBH and E _A. Mean E _S for the measurement period ranged from 1.09 to 1.74?μmol?CO₂?m^?2?s^?1 among the ten species. The sensitivity of E _S to Ts (Q ₁₀ ) ranged from 1.87 to 2.61. Across the ten species 57–89% of variation in E _S was explained by T _S and DBH. There was also a linear relationship between mean E _S and E _A. E _S was better predicted by Ts in the dormant season than the growing season, indicating that additional factors such as growth respiration and internal transport of CO₂ in the xylem became more important contributors to E _S during the growing season. Stem CO₂ efflux increased, and Q ₁₀ decreased, with increasing DBH in all species. Although temperature exerts strong control on the rate of cellular respiration, we conclude that in tree stems in situ, T _S, DBH and many other factors affect the relationship between CO₂ evolution by respiring cells and the diffusion of CO₂ to the stem surface.     

CO2 fluxes and respiration of branch segments of sycamore (Platanus occidentalis L.) examined at different sap velocities, branch diameters, and temperatures

Respiration of stems and branches of trees (R(S)) has typically been estimated by measuring radial CO(2) efflux from woody tissue (E(A)) and rates of efflux are often scaled temporally using a temperature relationship (Q(10)). High concentrations of CO(2) in xylem sap ([CO(2)*]) have been shown to affect E(A), and the transport of CO(2) in the xylem stream has been suggested as a mechanism to explain field observations of temperature-independent fluctuations in E(A). Sap velocity and temperature were manipulated in detached branch segments of sycamore (Platanus occidentalis L.) under controlled conditions to quantify these effects. Within individual branches of similar size, E(A) and [CO(2)*] were greater at low sap velocity, while the amount of respired CO(2) transported in sap (transport flux, F(T)) was greater at high sap velocity. E(A) was linearly correlated with [CO(2)*]. In branches of three diameter classes (1, 2, and 3 cm), volume-based E(A), F(T), and R(S) did not differ, but surface-area based CO(2) fluxes increased with diameter class. Regardless of diameter, E(A) accounted for only 30% of respired CO(2) at high sap velocity, while at low sap velocity, E(A) accounted for 71% of respired CO(2). E(A), F(T), and R(S) measured at 5, 20, and 35 degrees C at the same sap velocity showed a typical exponential response to temperature. However, at the lowest temperature, E(A) accounted for only 18% of the CO(2) released from respiring cells compared with 44% at the highest temperature, perhaps due to the effect of temperature on the solubility of CO(2) in water. These results directly demonstrate the transport of respired CO(2) in the xylem stream and may help to explain inconsistencies in stem and branch respiration measurements made in situ.     

Influence of temperature and water potential on root growth of white oak

Root growth of white oak ( Quercus alba L.) was observed under field conditions using a rhizotron. The effects of temperature, soil water potential, and leaf water potential were evaluated on three measures of root growth and development: root elongation rate, number of growing roots, and root growth intensity (sum of projected root area compared to the total root viewing area). Root elongation rate was linearly related to changes in soil temperature and soil water potential. At soil temperatures less than 17deg;C, temperature was the dominant factor affecting rate of growth, bat at temperatures greater than 17°C soil water potential became the important factor. Unlike root elongation rate, the number of growing roots and root growth intensity increased at cold soil temperatures (8°C) and at soil water potentials of-0.3 to -0.8 MPa. At high soil water potentials (-0.1 MPa) root elongation rate reached a maximum while the number of growing roots and root growth intensity were low. These differences showed that root growth and development were not exclusively affected by the soil environment. In addition, the relationship between root growth and predawn leaf water potential suggested that root growth was a contributing factor to the drought resistance of white oak.     

Net Photosynthesis and Early Growth Trends of a Dominant White Oak (Quercus alba L.)

Examination of the relationship between photosynthesis and growth of a dominant white oak (Quercus alba L.) tree has shown that most growth processes were either completed or well underway before the establishment of significant positive rates of net photosynthesis. Growth was initiated first in the root system (March 3), followed by stem cambial growth (March 26) and later by flower, leaf, and branch growth (April 10). During the period of rapid leaf and branch growth, root and cambial growth ceased and then resumed as the leaves approached maturity. The rapid rate of leaf maturation, the early appearance of positive rates of net photosynthesis in leaves (15% of final size) and the CO₂-refixing capability of elongating branch tissue reduced the period of time that this white oak tree was dependent on stored reserves. Lower temperature optima and compensation points in developing leaves and stems indicated that the growth-temperature response was optimized for the lower seasonal temperatures observed during the spring. This temperature adaptation further reduced the time that this tree was dependent on stored reserves.     

Increases in summer temperatures decrease the survival of an invasive forest insect

Higher temperatures projected under current climate change models are generally predicted to exert an overall positive effect on the success of invasive insects through increased survivability, developmental rates and fecundity, and by facilitating geographic range expansion. However, these effects have primarily focused on the shifts in winter temperatures with limited attention to the role that summer heat may play in shaping species ranges or fitness. We examined the thermal ecology of an ecologically important invasive forest insect, the hemlock woolly adelgid (Adelges tsugae), by determining survival during its summer dormancy phase under increasing temperature regimens. From laboratory and field experiments, we documented a positive association between increased temperatures and duration of exposure, and A. tsugae mortality. Adelges tsugae mortality was minimal (<20%) when exposed to summer temperatures characteristic to its native range (<25 °C), but markedly increased (up to 100%) when exposed to temperatures that occur occasionally or rarely in natural settings (>30 °C). At the warmest, southernmost edge of their range, field mortality of A. tsugae ranged from 8.5 to 81.9% and was strongly correlated with site temperature regimens. Further, we found no significant differences in A. tsugae survival between populations collected from Maine and Georgia, and over a 3-year period within Georgia, indicating that A. tsugae may not be acclimating to heat. These results highlight the importance of including summer temperatures in studies regarding increased temperatures on insect dynamics, and may alter historical predictions of climate change impacts on invasive insects and the conservation of forest ecosystems.