Craig V. M. Barton

Remote sensing of canopy light use efficiency using the photochemical reflectance index Model and sensitivity analysis

A growing number of studies have shown that reflectance changes at 531 nm, associated with the xanthophyll cycle and the related thylakoid energisation are widespread among plant species. The photochemical reflectance index (PRI), derived from narrow band reflectance at 531 and 570 nm has been related with some success to photosynthetic light use efficiency (LUE). Such a relationship would enable the estimation of stand photosynthesis from remotely sensed data. However, canopy PRI is an integral of the component leaf response weighted by the strength of the signal from each leaf to the sensor. This analysis investigates the extent to which canopy structure, view, and illumination angles are likely to influence the measured canopy PRI. A one-dimensional ray tracing radiative transfer model was used to estimate light distribution within a canopy and the dynamic response of individual foliar elements, based on a published relationship between PRI and LUE and a simple photosynthetic light response function. The model estimated the LUE of the canopy, based on both incident and absorbed light, and reflectance of the canopy at the desired wavelengths and hence the canopy PRI. A range of solar zenith, leaf area index (LAI), leaf angle distributions (LAD), and soil types were used to determine the likely influence on measured canopy PRI. The results show a positive correlation between PRI and LUE variation at canopy scale. However, the index shows a greater variation of view angle than most vegetation indices. The index is strongly influenced by varying soil background for LAI 30°) the index is also sensitive to LAD. Correction for Rayleigh scattering is necessary to relate the index to ground measured PRI. Results show that the PRI value is most sensitive to changes in LAI. Utilisation of the relationship to predict or improve estimates of canopy LUE based on either absorbed or incident light will require an independent estimate of LAI change between dates/locations of in situ measurements and of remote sensing observations.

Why is plant-growth response to elevated CO2 amplified when water is limiting, but reduced when nitrogen is limiting? A growth-optimisation hypothesis

Experimental evidence indicates that the stomatal conductance and nitrogen concentration ([N]) of foliage decline under CO2 enrichment, and that the percentage growth response to elevated CO2 is amplified under water limitation, but reduced under nitrogen limitation. We advance simple explanations for these responses based on an optimisation hypothesis applied to a simple model of the annual carbon-nitrogen-water economy of trees growing at a CO2-enrichment experiment at Oak Ridge, Tennessee, USA. The model is shown to have an optimum for leaf [N], stomatal conductance and leaf area index (LAI), where annual plant productivity is maximised. The optimisation is represented in terms of a trade-off between LAI and stomatal conductance, constrained by water supply, and between LAI and leaf [N], constrained by N supply. At elevated CO2 the optimum shifts to reduced stomatal conductance and leaf [N] and enhanced LAI. The model is applied to years with contrasting rainfall and N uptake. The predicted growth response to elevated CO2 is greatest in a dry, high-N year and is reduced in a wet, low-N year. The underlying physiological explanation for this contrast in the effects of water versus nitrogen limitation is that leaf photosynthesis is more sensitive to CO2 concentration ([CO2]) at lower stomatal conductance and is less sensitive to [CO2] at lower leaf [N].

Seasonal responses of xylem sap velocity to VPD and solar radiation during drought in a stand of native trees in temperate Australia

Xylem sap velocity of two dominant tree species, Eucalyptus crebra F. Muell. and Callitris glaucophylla J. Thompson & L.A.S. Johnson, in a native remnant forest of eastern Australia was measured in winter and summer during a prolonged (> 12 months) and extensive drought. The influence of vapour pressure deficit (VPD) and solar radiation levels on the velocity of sap was determined. Pronounced hysteresis in sap velocity was observed in both species as a function of VPD and solar radiation. However, the rotation of the hysteresis curve was clockwise for the response of sap velocity to VPD but anti-clockwise in the response of sap velocity to radiation levels. A possible reason for this difference is discussed.The degree of hysteresis (area bounded by the curve) was larger for the VPD response than the response to solar radiation and also varied with season. A simple linear model was able to predict sap velocity from knowledge of VPD and solar radiation in winter and summer. The consistent presence of hysteresis in the response to sap velocity to VPD and solar radiation suggests that large temporal and spatial models of vegetation water use may require some provision for the different responses of sap velocity, and hence water use, to VPD and solar radiation, between morning and afternoon and between seasons.

Photosynthesis of temperate Eucalyptus globulus trees outside their native range has limited adjustment to elevated CO2 and climate warming

Eucalyptus species are grown widely outside of their native ranges in plantations on all vegetated continents of the world. We predicted that such a plantation species would show high potential for acclimation of photosynthetic traits across a wide range of growth conditions, including elevated [CO2] and climate warming. To test this prediction, we planted temperate Eucalyptus globulus Labill. seedlings in climate-controlled chambers in the field located >700 km closer to the equator than the nearest natural occurrence of this species. Trees were grown in a complete factorial combination of elevated CO2 concentration (eC; ambient [CO2] +240 ppm) and air warming treatments (eT; ambient +3 °C) for 15 months until they reached ca. 10 m height. There was little acclimation of photosynthetic capacity to eC and hence the CO2-induced photosynthetic enhancement was large (ca. 50%) in this treatment during summer. The warming treatment significantly increased rates of both carboxylation capacity (V(cmax)) and electron transport (Jmax) (measured at a common temperature of 25 °C) during winter, but decreased them significantly by 20-30% in summer. The photosynthetic CO2 compensation point in the absence of dark respiration (Γ*) was relatively less sensitive to temperature in this temperate eucalypt species than for warm-season tobacco. The temperature optima for photosynthesis and Jmax significantly changed by about 6 °C between winter and summer, but without further adjustment from early to late summer. These results suggest that there is an upper limit for the photosynthetic capacity of E. globulus ssp. globulus outside its native range to acclimate to growth temperatures above 25 °C. Limitations to temperature acclimation of photosynthesis in summer may be one factor that defines climate zones where E. globulus plantation productivity can be sustained under anticipated global environmental change.

Rooting depth explains [CO2] × drought interaction in Eucalyptus saligna

Elevated atmospheric [CO(2)] (eC(a)) often decreases stomatal conductance, which may delay the start of drought, as well as alleviate the effect of dry soil on plant water use and carbon uptake. We studied the interaction between drought and eC(a) in a whole-tree chamber experiment with Eucalyptus saligna. Trees were grown for 18 months in their C(a) treatments before a 4-month dry-down. Trees grown in eC(a) were smaller than those grown in ambient C(a) (aC(a)) due to an early growth setback that was maintained throughout the duration of the experiment. Pre-dawn leaf water potentials were not different between C(a) treatments, but were lower in the drought treatment than the irrigated control. Counter to expectations, the drought treatment caused a larger reduction in canopy-average transpiration rates for trees in the eC(a) treatment compared with aC(a). Total tree transpiration over the dry-down was positively correlated with the decrease in soil water storage, measured in the top 1.5 m, over the drying cycle; however, we could not close the water budget especially for the larger trees, suggesting soil water uptake below 1.5 m depth. Using neutron probe soil water measurements, we estimated fractional water uptake to a depth of 4.5 m and found that larger trees were able to extract more water from deep soil layers. These results highlight the interaction between rooting depth and response of tree water use to drought. The responses of tree water use to eC(a) involve interactions between tree size, root distribution and soil moisture availability that may override the expected direct effects of eC(a). It is essential that these interactions be considered when interpreting experimental results.

Soil (N) modulates soil C cycling in CO2-fumigated tree stands: a meta-analysis

Under elevated atmospheric CO(2) concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO(2) effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta-analysis to test the hypotheses that: (1) elevated atmospheric CO(2) stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO(2) induces a C allocation shift towards below-ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO(2). Soil N concentration strongly interacted with CO(2) fumigation: the effect of elevated CO(2) on fine root biomass and -production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO(2) are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.

Woody clockworks: circadian regulation of night‐time water use in Eucalyptus globulus

The role of the circadian clock in controlling the metabolism of entire trees has seldom been considered. We tested whether the clock influences nocturnal whole-tree water use. Whole-tree chambers allowed the control of environmental variables (temperature, relative humidity). Night-time stomatal conductance (gs ) and sap flow (Q) were monitored in 6- to 8-m-tall Eucalyptus globulus trees during nights when environmental variables were kept constant, and also when conditions varied with time. Artificial neural networks were used to quantify the relative importance of circadian regulation of gs and Q. Under a constant environment, gs and Q declined from 0 to 6 h after dusk, but increased from 6 to 12 h after dusk. While the initial decline could be attributed to multiple processes, the subsequent increase is most consistent with circadian regulation of gs and Q. We conclude that endogenous regulation of gs is an important driver of night-time Q under natural environmental variability. The proportion of nocturnal Q variation associated with circadian regulation (23-56%) was comparable to that attributed to vapor pressure deficit variation (25-58%). This study contributes to our understanding of the linkages between molecular and cellular processes related to circadian regulation, and whole-tree processes related to ecosystem gas exchange in the field.

Advances in remote sensing of plant stress

Since we first began to actively cultivate plants, we have been using remote sensing to assess the health and vigour of our crops and ornamentals. By looking at plants and observing changes in the angle of the leaves over time we can detect water stress, the colour of the leaves has informed us of nutrient limitations and imbalances, the patchiness of leaf colour and form often relates to pest and disease attack. Our ability to assess the health of plants and vegetation quickly and accurately simply by “looking” at them is being raised to new levels with the advent of new sensors and instruments that can “see” across a wider range of wavelengths than our eyes, and improved understanding of the physics and biochemistry underlying the relationships between vegetation status and its “appearance”. When light strikes a leaf, part of the light spectrum is reflected towards the observer. This reflectance is governed by leaf surface properties, internal structure and the concentration and distribution of biochemical components within the leaf (e.g. nitrogen, lignin, cellulose). Thus there is information in the reflected light that relates to the physical and biochemical properties of the leaf. At the canopy scale, factors such as leaf angle distribution, leaf area index, litter and soil properties and the view and illumination geometry all influence the reflectance properties of the scene. The interpretation of this complex radiation pattern is the major challenge of remote sensing. Vegetation reflectance can be detected using narrow-bandwidth spectroradiometers that measure in the visible and near-infrared parts of the spectrum. In the visible spectrum (400–700 nm), leaf reflectance is low because of absorption by photosynthetic pigments (mainly chlorophyll and carotenoids). In the near-infrared (700–1,300 nm), on the other hand, the reflectance is influenced by structural properties in the leaf. Variation in reflectance in the middle infrared region (1,300–3,000 nm) is related to absorption characteristics of water and other compounds. Both absolute and relative differences in the reflectance among these various wavebands have been used to derive indices that correlate with vegetation condition. For example, a change in the chlorophyll content will influence the reflectance in the red part of the spectrum but not the near-infra red; an index based on the ratio of these two wave bands (the simple ratio, NIR/R) has been shown to relate to green biomass Plant Soil (2012) 354:41–44 DOI 10.1007/s11104-011-1051-0

Does physiological acclimation to climate warming stabilize the ratio of canopy respiration to photosynthesis

Given the contrasting short-term temperature dependences of gross primary production (GPP) and autotrophic respiration, the fraction of GPP respired by trees is predicted to increase with warming, providing a positive feedback to climate change. However, physiological acclimation may dampen or eliminate this response. We measured the fluxes of aboveground respiration (Ra ), GPP and their ratio (Ra /GPP) in large, field-grown Eucalyptus tereticornis trees exposed to ambient or warmed air temperatures (+3°C). We report continuous measurements of whole-canopy CO2 exchange, direct temperature response curves of leaf and canopy respiration, leaf and branch wood respiration, and diurnal photosynthetic measurements. Warming reduced photosynthesis, whereas physiological acclimation prevented a coincident increase in Ra . Ambient and warmed trees had a common nonlinear relationship between the fraction of GPP that was respired above ground (Ra /GPP) and the mean daily temperature. Thus, warming significantly increased Ra /GPP by moving plants to higher positions on the shared Ra /GPP vs daily temperature relationship, but this effect was modest and only notable during hot conditions. Despite the physiological acclimation of autotrophic respiration to warming, increases in temperature and the frequency of heat waves may modestly increase tree Ra /GPP, contributing to a positive feedback between climate warming and atmospheric CO2 accumulation.

Growth and Carbon Sequestration Rates at Age Ten Years of Some Eucalypt Species in the Low- to Medium-rainfall Areas of New South Wales, Australia

Summary This paper presents the results of a study of tree growth in farm forestry eucalypt plantations in the low—to medium-rainfall (450–700 mm y−1) regions of New South Wales, Australia, in an attempt to estimate the productivity of the plantations. The species measured include Eucalyptus camaldulensis, E. botryoides, E. globulus, E. albens, E. polyanthemos, E. microcarpa, E. melliodora, E. sideroxylon, E. crebra and Corymbia maculata. At age 10 y, mean dominant height (100 tallest trees per hectare) ranged from 7.5 to 18.8 m, mean top basal area (thickest 100 stems ha−1) from 1.5 to 9.2 m2ha−1, volume from 9.5 to 125.9 m3 ha−1, total above-ground biomass from 12.5 to 105.8 t ha−1, and mean carbon density (above ground) from 11.2 to 35.2 t ha−1.