Josep Peñuelas

A Narrow-Waveband Spectral Index That Tracks Diurnal Changes in Photosynthetic Efficiency*

Abstract We present a new “physiological reflectance index” (PRI) isolated from narrow waveband spectral measurements of sunflower canopies. This index correlates with the epoxidation state of the xanthophyll cycle pigments and with the efficiency of photosynthesis in control and nitrogen stress canopies, but not in water stress canopies undergoing midday wilting. It is analogous in formulation to the broadband normalized difference vegetation index (NDVI) and uses reflectance at 531 nm and at a reference wavelength to minimize complications associated with diurnal sun angle changes. In conjunction with other methods, this index may lead to improved remote and ground-based estimates of canopy photosynthetic function.

The reflectance at the 950–970 nm region as an indicator of plant water status

Abstract We present new remote sensing indices of plant water status: the ratio between the reflectance at 970 nm, one of the water absorption bands, and the reflectance at a reference wavelength, 900 nm (R970/R9000; the first derivative minimum in this near-infrared region (dNIRminimum ) and the wavelength where this minimum is found ( λNIRminimum). In order to evaluate them, we carried out three experiments. Daily irrigated gerbera plants were allowed to dry until almost wilting and then daily irrigation was restarted; pepper and bean plants were grown for four months submitted to two different irrigation treatments; and bean detached leaves were submitted to progressive dehydration whereas pressure-volume curves were being carried out. In gerbera plants, the trough about 950–970 nm decreased as the drought was increasing. Therefore, the R970/R900 index and the dNIRminimum closely tracked the changes in relative water content (RWC), leaf water potential, stomatal conductance and the foliage-air temperat...

Reflectance indices associated with physiological changes in nitrogen- and water-limited sunflower leaves☆

Abstract We followed diurnal and seasonal changes in physiology and spectral reflectance of leaves throughout the canopies of sunflower plants grown in control, nitrogen (N)-limited, and water-stressed plots. Leaves from control sunflower plants had significantly higher levels of nitrogen, chlorophyll (chl), ribulose bis phosphate carboxylase / oxygenase (RuBPCase) activity and photosynthetic rates and lower starch content and leaf thickness than N-limited plants. Water-stressed plants had the highest N and chl contents (on an area basis). They also had the lowest water potential and photosynthetic rates, in spite of maintaining high RuBPCase activities. Leaves from stressed plants (especially N-limited) had significantly higher reflectances in the visible wavelengths and lower in the near IR than leaves from control plants. The only clear trend across canopy levels was the higher reflectance at all wavelengths but especially in the visible of the lower (oldest) leaves. NDVI-like parameters were useful in distinguishing stress and control leaves over the growing season. However, several narrow-band indices provided better physiological information than NDVI. The physiological reflectance index (PRI) (R550 − R530 / R550 + R530) followed diurnal changes in xanthophyll pigments and photosynthetic rates of control and N-limited leaves. The maximum of the first derivative of reflectance in the green (dG) was correlated with diurnal photosynthetic rate, and with seasonal chl and N changes. The normalized pigment chlorophyll ratio Index (NPCI) (R680 − R430 / R680 + R430) varied with total pigments / chl. The water band index (WBI) (R970 / R902) followed water status. The normalized ratio between the maxima of the first derivatives of reflectances at the red edge and green regions (EGFN) was correlated with chl and N content. Principal components analysis yielded several indicators of physiological status. The first principal component was higher in control leaves, the second was higher in N-limited leaves, and the third was higher in water-limited leaves. Discriminant analysis based on the combination of several narrow-band spectral indices clearly separated leaves into the three treatment groups. These results illustrate the promise of narrow-band spectroradiometry for assessing the physiological state of vegetation.

Visible and near-infrared reflectance techniques for diagnosing plant physiological status

Previously used reflectance indices, such as the normalized difference pigment index (NDPI), were empirically derived and might suffer from confounding effects introduced by leaf surface and structure. The NDPI is given by the following equation:Leaf reflectance, R, can be approximated by the following semi-empirical model[19xSee all References[19]:(Rs is the reflectance for a very high absorption coefficient; S is the structural effect on reflectance; and Ci is the concentration of absorbing compounds).In order to minimize the specular component dominant in the Rs term and the structural component dominant in S, a structure-independent pigment index (SIPI) that uses a near-infrared waveband (800 nm) as a subtracted and ratioed reference was recently defined in order to remove any additive and multiplicative factors[20xSemiempirical indices to assess carotenoids/chlorophyll a ratio from leaf spectral reflectance. Penuelas, J, Filella, I, and Baret, F. Photosynthetica. 1995; 31: 221–230See all References[20]:It provided a very good semi-empirical estimation of the carotenoids: chlorophyll a ratio for data from several species[20xSemiempirical indices to assess carotenoids/chlorophyll a ratio from leaf spectral reflectance. Penuelas, J, Filella, I, and Baret, F. Photosynthetica. 1995; 31: 221–230See all References[20].

The red edge position and shape as indicators of plant chlorophyll content, biomass and hydric status

Abstract Some red edge parameters in the first derivative reflectance curve (wavelength, amplitude and area of the red edge peak) were studied to evaluate plant chlorophyll content, biomassand RelativeWater Content (RWC).Plants of Capsicum annuum and Phaseolus vulgaris under different nitrogen and water availabilities, and plants of Gerbera jamesonii with different hydric status were studied. A high correlation was found between chlorophyll content and the wavelength of the red edge peak (λre ), and between LAI (leaf area index)and the amplitude of the red edge peak (drr e ), but the area of the red edge peak (σ680–780 nm) was the best estimator of LAI. Thus, red edge was found valuable for assessment of plant chlorophyll concentration and LAI, and therefore nutritional status. Water stress also affected drre, but only when the stress was well developed.

Estimation of plant water concentration by the reflectance Water Index WI (R900/R970)

Abstract Water Index WI (R900/R970) was used for the estimation of plant water concentration (PWC) by ground-based, reflectance measurements. Reflectance and PWC were measured for adult plants growing in the field throughout an annual cycle and in potted seedlings submitted to progressive desiccation. The species studied were characteristicly Mediterranean: Pinus halepensis, Quercus ilex, Quercus coccifera, Arbutus unedo, Cistus albidus, Cistus monspeliensis, Phillyrea angustifolia, Pistacia lentiscus and Brachypodium retusum . WI was significantly correlated with PWC when all the species were considered together, and with almost all the species considered individually, especially when a wider range of PWC was obtained by extreme dessication of experimental plants. The correlations increased when normalizing WI by NDVI. The wavelength of the trough corresponding to water absorption band tended to shift from 970-980 nm to lower wavelengths 930-950 nm with decreasing PWCs. Infrared measurement of plant temp...

Global patterns of foliar nitrogen isotopes and their relationships with climate, mycorrhizal fungi, foliar nutrient concentrations, and nitrogen availability.

Ratios of nitrogen (N) isotopes in leaves could elucidate underlying patterns of N cycling across ecological gradients. To better understand global-scale patterns of N cycling, we compiled data on foliar N isotope ratios (delta(15)N), foliar N concentrations, mycorrhizal type and climate for over 11,000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar delta(15)N by 2 per thousand, 3.2 per thousand, 5.9 per thousand, respectively, relative to nonmycorrhizal plants. Foliar delta(15)N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT >or= -0.5 degrees C, but was invariant with MAT across sites with MAT < -0.5 degrees C. In independent landscape-level to regional-level studies, foliar delta(15)N increased with increasing N availability; at the global scale, foliar delta(15)N increased with increasing foliar N concentrations and decreasing foliar phosphorus (P) concentrations. Together, these results suggest that warm, dry ecosystems have the highest N availability, while plants with high N concentrations, on average, occupy sites with higher N availability than plants with low N concentrations. Global-scale comparisons of other components of the N cycle are still required for better mechanistic understanding of the determinants of variation in foliar delta(15)N and ultimately global patterns in N cycling.

Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought

Climate change is progressively increasing severe drought events in the Northern Hemisphere, causing regional tree die-off events and contributing to the global reduction of the carbon sink efficiency of forests. There is a critical lack of integrated community-wide assessments of drought-induced responses in forests at the macroecological scale, including defoliation, mortality, and food web responses. Here we report a generalized increase in crown defoliation in southern European forests occurring during 1987–2007. Forest tree species have consistently and significantly altered their crown leaf structures, with increased percentages of defoliation in the drier parts of their distributions in response to increased water deficit. We assessed the demographic responses of trees associated with increased defoliation in southern European forests, specifically in the Iberian Peninsula region. We found that defoliation trends are paralleled by significant increases in tree mortality rates in drier areas that are related to tree density and temperature effects. Furthermore, we show that severe drought impacts are associated with sudden changes in insect and fungal defoliation dynamics, creating long-term disruptive effects of drought on food webs. Our results reveal a complex geographical mosaic of species-specific responses to climate change–driven drought pressures on the Iberian Peninsula, with an overwhelmingly predominant trend toward increased drought damage.

Phenology Feedbacks on Climate Change

A longer growing season as a result of climate change will in turn affect climate through biogeochemical and biophysical effects. Climate warming has advanced the biological spring and delayed the arrival of biological winter (1, 2). These changes in the annual cycle of plants and the lengthening of the green-cover season have many consequences for ecological processes, agriculture, forestry, human health, and the global economy (3). Studies on vegetation-atmosphere interactions (4) and particularly on the impact of leaf emergence on climate (5–9) suggest that the phenological shifts in turn affect climate. The magnitude and sign of this effect are unknown but depend on water availability and regional characteristics.

BVOCs and global change

Biogenic volatile organic compounds (BVOCs) produced by plants are involved in plant growth, reproduction and defense. They are emitted from vegetation into the atmosphere and have significant effects on other organisms and on atmospheric chemistry and physics. Here, we review current knowledge on the alteration of BVOC emission rates due to climate and global changes: warming, drought, land use changes, high atmospheric CO(2) concentrations, ozone and enhanced UV radiation. These alterations are very variable depending on the doses, timing, BVOC and species, but in overall terms are likely to increase BVOC emissions. These changed emissions can lead to unforeseeable consequences for the biosphere structure and functioning, and can disturb biosphere feedback on atmospheric chemistry and climate with a direction and intensity that warrants in-depth investigation.