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Plant respiration can acclimate to changing environmental conditions and vary between species as well as biome types, although belowground respiration responses to ongoing climate warming are not well understood. Understanding the thermal acclimation capacity of root respiration (R root) in relation to increasing temperatures is therefore critical in elucidating a key uncertainty in plant function in response to warming. However, the degree of temperature acclimation of R root in rainforest trees and how root chemical and morphological traits are related to acclimation is unknown. Here we investigated the extent to which respiration of fine roots (≤2 mm) of four tropical and four warm-temperate rainforest tree seedlings differed in response to warmer growth temperatures (control and +6 °C), including temperature sensitivity (Q 10) and the degree of acclimation of R root. Regardless of biome type, we found no consistent pattern in the short-term temperature responses of R root to elevated growth temperature: a significant reduction in the temperature response of R root to +6 °C treatment was only observed for a tropical species, Cryptocarya mackinnoniana , whereas the other seven species had either some stimulation or no alteration. Across species, R root was positively correlated with root tissue nitrogen concentration (mg g−1), while Q 10 was positively correlated with root tissue density (g cm−3). Warming increased root tissue density by 20.8% but did not alter root nitrogen across species. We conclude that thermal acclimation capacity of R root to warming is species-specific and suggest that root tissue density is a useful predictor of R root and its thermal responses in rainforest tree seedlings. [ABSTRACT FROM AUTHOR]

Leaf-level photosynthetic processes and their environmental dependencies are critical for estimating CO2 uptake from the atmosphere. These estimates use biochemical-based models of photosynthesis that require accurate Rubisco kinetics. We investigated the effects of canopy position, elevated atmospheric CO2 [eC; ambient CO2 (aC)+240 ppm] and elevated air temperature (eT; ambient temperature (aT)+3 °C) on Rubisco content and activity together with the relationship between leaf N and Vcmax (maximal Rubisco carboxylation rate) of 7 m tall, soil-grown Eucalyptus globulus trees. The kinetics of E. globulus and tobacco Rubisco at 25 °C were similar. In vitro estimates of Vcmax derived from measures of E. globulus Rubisco content and kinetics were consistent, although slightly lower, than the in vivo rates extrapolated from gas exchange. In E. globulus, the fraction of N invested in Rubisco was substantially lower than for crop species and varied with treatments. Photosynthetic acclimation of E. globulus leaves to eC was underpinned by reduced leaf N and Rubisco contents; the opposite occurred in response to eT coinciding with growth resumption in spring. Our findings highlight the adaptive capacity of this key forest species to allocate leaf N flexibly to Rubisco and other photosynthetic proteins across differing canopy positions in response to future, warmer and elevated [CO2] climates. [ABSTRACT FROM AUTHOR]

This study highlights the dynamic nature of the temperature dependence of leaf respiration in plants experiencing future climate change scenarios, particularly with respect to drought and elevated [CO2].Climate change is resulting in increasing atmospheric [CO2], rising growth temperature (T), and greater frequency/severity of drought, with each factor having the potential to alter the respiratory metabolism of leaves. Here, the effects of elevated atmospheric [CO2], sustained warming, and drought on leaf dark respiration (Rdark), and the short-term T response of Rdark were examined in Eucalyptus globulus. Comparisons were made using seedlings grown under different [CO2], T, and drought treatments. Using high resolution T–response curves of Rdark measured over the 15–65 °C range, it was found that elevated [CO2], elevated growth T, and drought had little effect on rates of Rdark measured at T <35 °C and that there was no interactive effect of [CO2], growth T, and drought on T response of Rdark. However, drought increased Rdark at high leaf T typical of heatwave events (35–45 °C), and increased the measuring T at which maximal rates of Rdark occurred (Tmax) by 8 °C (from 52 °C in well-watered plants to 60 °C in drought-treated plants). Leaf starch and soluble sugars decreased under drought and elevated growth T, respectively, but no effect was found under elevated [CO2]. Elevated [CO2] increased the Q10 of Rdark (i.e. proportional rise in Rdark per 10 °C) over the 15–35 °C range, while drought increased Q10 values between 35 °C and 45 °C. Collectively, the study highlights the dynamic nature of the T dependence of Rdark in plants experiencing future climate change scenarios, particularly with respect to drought and elevated [CO2]. [ABSTRACT FROM AUTHOR]

We explored the impact of canopy position on leaf respiration (R) and associated traits in tree and shrub species growing in a lowland tropical rainforest in Far North Queensland, Australia. The range of traits quantified included: leaf R in darkness (RD) and in the light (RL; estimated using the Kok method); the temperature (T)-sensitivity of RD; light-saturated photosynthesis (Asat); leaf dry mass per unit area (LMA); and concentrations of leaf nitrogen (N), phosphorus (P), soluble sugars and starch. We found that LMA, and area-based N, P, sugars and starch concentrations were all higher in sun-exposed/upper canopy leaves, compared with their shaded/lower canopy and deep-shade/understory counterparts; similarly, area-based rates of RD, RL and Asat (at 28 °C) were all higher in the upper canopy leaves, indicating higher metabolic capacity in the upper canopy. The extent to which light inhibited R did not differ significantly between upper and lower canopy leaves, with the overall average inhibition being 32% across both canopy levels. Log–log RD–Asat relationships differed between upper and lower canopy leaves, with upper canopy leaves exhibiting higher rates of RD for a given Asat (both on an area and mass basis), as well as higher mass-based rates of RD for a given [N] and [P]. Over the 25–45 °C range, the T-sensitivity of RD was similar in upper and lower canopy leaves, with both canopy positions exhibiting Q10 values near 2.0 (i.e., doubling for every 10 °C rise in T) and Tmax values near 60 °C (i.e., T where RD reached maximal values). Thus, while rates of RD at 28 °C decreased with increasing depth in the canopy, the T-dependence of RD remained constant; these findings have important implications for vegetation-climate models that seek to predict carbon fluxes between tropical lowland rainforests and the atmosphere. [ABSTRACT FROM PUBLISHER]

To investigate whether long-term elevated carbon dioxide concentration ([CO2]) causes declines in photosynthetic enhancement and leaf nitrogen (N) owing to limited soil fertility, we measured photosynthesis, carboxylation capacity and area-based leaf nitrogen concentration (Na) in Pinus taeda L. growing in a long-term free-air CO2 enrichment (FACE) facility at an N-limited site. We also determined how maximum rates of carboxylation (Vcmax) and electron transport (Jmax) varied with Na under elevated [CO2]. In trees exposed to elevated [CO2] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity (Anet-Ca) and Na was significantly reduced by 37% in 1-year-old needles, whereas it was unaffected in current-year needles. The slope of the relationships of both Vcmax and Jmax with Na decreased in 1-year-old needles after up to 9 years of growth in elevated [CO2], which was accompanied by a 15% reduction in N allocation to the carboxylating enzyme. Nitrogen fertilization (110 kg N ha-1) in the ninth year of exposure to elevated [CO2] restored the slopes of the relationships of Vcmax and Jmax with Na to those of control trees (i.e., in ambient [CO2]). The Jmax: Vcmax ratio was unaffected by either [CO2] or N fertilization. Changes in the apparent allocation of N to photosynthetic components may be an important adjustment in pines exposed to elevated [CO2] on low-fertility sites. We conclude that fundamental relationships between photosynthesis or its component processes with Na may be altered in aging pine needles after more than 5 years of exposure to elevated atmospheric [CO2]. [ABSTRACT FROM AUTHOR]