Assessing the interplay between canopy energy balance and photosynthesis with cellulose δ18O: large-scale patterns and independent ground-truthing

Isotopic clumping in wood as a proxy for photorespiration in trees

Photorespiration can limit gross primary productivity in terrestrial plants. The rate of photorespiration relative to carbon fixation increases with temperature and decreases with atmospheric [CO2]. However, the extent to which this rate varies in the environment is unclear. Here, we introduce a proxy for relative photorespiration rate based on the clumped isotopic composition of methoxyl groups (R-O-CH3) in wood. Most methoxyl C-H bonds are formed either during photorespiration or the Calvin cycle and thus their isotopic composition may be sensitive to the mixing ratio of these pathways. In water-replete growing conditions, we find that the abundance of the clumped isotopologue 13CH2D correlates with temperature (18-28 °C) and atmospheric [CO2] (280-1000 ppm), consistent with a common dependence on relative photorespiration rate. When applied to a global dataset of wood, we observe global trends of isotopic clumping with climate and water availability. Clumped isotopic compositions are similar across environments with temperatures below ~18 °C. Above ~18 °C, clumped isotopic compositions in water-limited and water-replete trees increasingly diverge. We propose that trees from hotter climates photorespire substantially more than trees from cooler climates. How increased photorespiration is managed depends on water availability: water-replete trees export more photorespiratory metabolites to lignin whereas water-limited trees either export fewer overall or direct more to other sinks that mitigate water stress. These disparate trends indicate contrasting responses of photorespiration rate (and thus gross primary productivity) to a future high-[CO2] world. This work enables reconstructing photorespiration rates in the geologic past using fossil wood.

Evaluation of the MOD11A2 product for canopy temperature monitoring in the Brazilian Atlantic Forest

Forest canopies have an important influence on the global climate balance. Through the analysis of the temperature of the canopy, it is possible to infer about the physiological aspects of the plants, helping to understand the behavior of the vegetation and, consequently, in the environmental monitoring and management of green areas. This study aims to validate the MOD11A2 V006 product from canopy surface temperature data obtained by an infrared radiation sensor. For the validation of the MOD11A2 product, a comparative analysis was performed between the land surface temperature (LST) data, obtained by the MODIS sensor, and the canopy temperature data, obtained by the SI-111 infrared radiation sensor coupled to the Itatiaia National Park (PNI) micrometeorological tower. Meteorological variables and land surface temperature collected from January to December 2018 in the PNI were also analyzed. The results reveal that the MOD11A2 product overestimates the canopy temperature in the daytime (MB ranging from 1.56 to 3.57 °C) and underestimates in the night time (MB ranging from - 0.18 to - 4.22 °C). During daytime, the months corresponding to the dry season presented a very high correlation (r = 0.74 and 0.86) and the highest values for the Willmott index (d = 0.70 and 0.64). At nighttime, the MOD11A2 product did not present a good performance for the LST estimation, especially in the rainy season. Therefore, we observed that the MOD11A2 product has limitations to estimate the land surface temperature and that possible changes in the algorithm of this product can be performed for high atmospheric humidity conditions.

No evidence of homeostatic regulation of leaf temperature in Eucalyptus parramattensis trees: integration of CO2 flux and oxygen isotope methodologies

Thermoregulation of leaf temperature (T_leaf ) may foster metabolic homeostasis in plants, but the degree to which T_leaf is moderated, and under what environmental contexts, is a topic of debate. Isotopic studies inferred the temperature of photosynthetic carbon assimilation to be a constant value of c. 20°C; by contrast, leaf biophysical theory suggests a strong dependence of T_leaf on environmental drivers. Can this apparent disparity be reconciled? We continuously measured T_leaf and whole-crown net CO₂ uptake for Eucalyptus parramattensis trees growing in field conditions in whole-tree chambers under ambient and +3°C warming conditions, and calculated assimilation-weighted leaf temperature (T_L-AW ) across 265 d, varying in air temperature (T_air ) from -1 to 45°C. We compared these data to T_L-AW derived from wood cellulose δ¹⁸ O. T_leaf exhibited substantial variation driven by T_air , light intensity, and vapor pressure deficit, and T_leaf was strongly linearly correlated with T_air with a slope of c. 1.0. T_L-AW values calculated from cellulose δ¹⁸ O vs crown fluxes were remarkably consistent; both varied seasonally and in response to the warming treatment, tracking variation in T_air . The leaves studied here were nearly poikilothermic, with no evidence of thermoregulation of T_leaf towards a homeostatic value. Importantly, this work supports the use of cellulose δ¹⁸ O to infer T_L-AW , but does not support the concept of strong homeothermic regulation of T_leaf.

THEMS: an automated thermal and hyperspectral proximal sensing system for canopy reflectance, radiance and temperature

BACKGROUND

Earth Observation 'EO' remote sensing technology development enables original insights into vegetation function and health at ever finer temporal, spectral and spatial resolution. Research sites equipped with monitoring infrastructure such as flux towers operate at a key bridging scale between satellite platform measurements and on-the-ground leaf-level processes.

RESULTS

This paper presents the technical details of the design and operation of a proximal observation system 'THEMS' that generates unattended long-term high quality thermal and hyperspectral images of a forest canopy on a short (sub-daily) timescale. The primary purpose of the system is to measure canopy temperature, spectral reflectance and radiance coincident with a highly instrumented flux tower site for benchmarking purposes. Basic system capability is demonstrated through low level data product descriptions of the high-resolution multi-angular imagery and ancillary data streams. The system has been successfully operational for more than 2 years with little to no intervention.

CONCLUSIONS

These data can then be used to derive remotely sensed proxies of canopy and ecosystem function to study temporal forest dynamics over a wide range of wavelengths, spatial scales (individual trees to canopy), and temporal scales (minutes to multiple years). The multi-purpose system is intended to provide unprecedented spatio-temporal ecophysiological insight and to underpin upscaling of remotely sensed dynamic ecosystem water, CO₂, and energy exchange processes.

Plant's-eye view of temperature governs elevational distributions

Explaining species geographic distributions by macroclimate variables is the most common approach for getting mechanistic insights into large-scale diversity patterns and range shifts. However, species' traits influencing biophysical processes can produce a large decoupling from ambient air temperature, which can seriously undermine biogeographical inference. We combined stable oxygen isotope theory with a trait-based approach to assess leaf temperature during carbon assimilation (T_L ) and its departure (ΔT) from daytime free air temperature during the growing season (T_gs ) for 158 plant species occurring from 3,400 to 6,150 m a.s.l. in Western Himalayas. We uncovered a general extent of temperature decoupling in the region. The interspecific variation in ΔT was best explained by the combination of plant height and δ¹³ C, and leaf dry matter content partly captured the variation in T_L . The combination of T_L and ΔT, with ΔT contributing most, explained the interspecific difference in elevational distributions. Stable oxygen isotope theory appears promising for investigating how plants perceive temperatures, a pivotal information to species biogeographic distributions.

Reduction in lumen area is associated with the δ18 O exchange between sugars and source water during cellulose synthesis

High temporal resolution measurements of wood anatomy and the isotopic composition in tree-rings have the potential to enhance our interpretation of climate variability, but the sources of variation within the growing season are still not well understood. Here we test the response of wood anatomical features in Pinus ponderosa and Pseudotsuga menziesii, including cell-wall thickness (CWT) and lumen area (LA), along with the oxygen isotopic composition of α-cellulose (δ¹⁸ O_cell ) to shifts in relative humidity (RH) in two treatments, one from high-low RH and the second one form low-high RH. We observed a significant decrease in LA and a small increase in CWT within the experimental growing season in both treatments. The measured δ¹⁸ O_cell along the ring was responsive to RH variations in both treatments. However, estimated δ¹⁸ O_cell did not agree with measured δ¹⁸ O_cell when the proportion of exchangeable oxygen during cellulose synthesis (P_ex ) was kept constant. We found that P_ex increased throughout the ring as LA decreased. Based on this varying P_ex within an annual ring, we propose a targeted sampling strategy for different hydroclimate signals: earlier season cellulose is a better recorder of RH while late-season cellulose is a better recorder of the source water.