No abstract

Remote sensing of sun-induced chlorophyll fluorescence (SIF) has been suggested as a promising approach for probing changes in global terrestrial gross primary productivity (GPP). To date, however, most studies were conducted in situations when/where changes in both SIF and GPP were driven by large changes in the absorbed photosynthetically active radiation (APAR) and phenology. Here we quantified SIF and GPP during a short-term intense heat wave at a Mediterranean pine forest, during which changes in APAR were negligible. GPP decreased linearly during the course of the heat wave, while SIF declined slightly initially and then dropped dramatically during the peak of the heat wave, temporally coinciding with a biochemical impairment of photosynthesis inferred from the increase in the uptake ratio of carbonyl sulfide to carbon dioxide. SIF thus accounted for less than 35% of the variability in GPP and, even though it responded to the impairment of photosynthesis, appears to offer limited potential for quantitatively monitoring GPP during heat waves in the absence of large changes in APAR.

Summary The drier climates predicted for many regions will result in reduced evaporative cooling, leading to leaf heat stress and enhanced mortality. The extent to which nonevaporative cooling can contribute to plant resilience under these increasingly stressful conditions is not well known at present. Using a novel, high accuracy infrared system for the continuous measurement of leaf temperature in mature trees under field conditions, we assessed leaf‐to‐air temperature differences (ΔTleaf–air) of pine needles during drought. On mid‐summer days, ΔTleaf–air remained < 3°C, both in trees exposed to summer drought and in those provided with supplemental irrigation, which had a more than 10‐fold higher transpiration rate. The nonevaporative cooling in the drought‐exposed trees must be facilitated by low resistance to heat transfer, generating a large sensible heat flux, H. ΔTleaf–air was weakly related to variations in the radiation load and mean wind speed in the lower part of the canopy, but was dependent on canopy structure and within‐canopy turbulence that enhanced the H. Nonevaporative cooling is demonstrated as an effective cooling mechanism in needle‐leaf trees which can be a critical factor in forest resistance to drying climates. The generation of a large H at the leaf scale provides a basis for the development of the previously identified canopy‐scale ‘convector effect’.

Summary Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high‐precision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remote‐sensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c. 2°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation Lbg. A novel ‘dual‐reference’ method was developed to increase the accuracy of infrared needle‐leaf surface temperature measurements in the field. It accounts for variations in Lbg and corrects for the systematic camera offset using two reference plates. We accurately captured surface temperature and leaf‐to‐air temperature differences of needle‐leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ± 0.23°C and ± 0.28°C, respectively. Routine high‐precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf‐scale processes and their dynamics.

No abstract

Drier climates predicted for many regions can result in reduced evaporative cooling leading to overheating of leaves and enhanced mortality. To what extent non-evaporative cooling can contribute to plant resilience to the increasingly stressful conditions is poorly known at present. Using a novel, high accuracy infrared system for continuous measurements of leaf temperature in mature trees under field conditions, we assessed leaf-to-air temperature differences ΔTleaf-air of pine needles during drought. ΔTleaf-air was weakly related to variations in the radiation load and wind speed, but highly dependent on canopy structure and within-canopy turbulence that enhance the sensible heat flux H. On mid-summer days, ΔTleaf-air remained around 2°C, both in trees exposed to summer drought, and in those provided with a supplement irrigation having a 10x higher transpiration rate. The non-evaporative cooling in the drought-exposed trees must be facilitated by low resistance to heat transfer generating large H. Non-evaporative cooling is demonstrated as an effective cooling mechanism in needle-leaf trees, which can be a critical factor in forest resistance to drying climates. The generation of large H at the leaf scale provides a basis for the development of the previously identified canopy-scale heat dissipation through the `convector effect'.

Questions: Understanding how trees affect their understorey plants and soils is crucial to understand savanna ecosystems. Most studies focus on the differences between canopy and open microsites, but how do different positions within large tree canopies influence soils and plants? Are these potential differences likely to change depending on environmental conditions (i.e. annual rainfall and grazing)?Location: One hundred sites across a rainfall gradient (220-1400 mm) in NSW, Australia.Methods: We measured the cover, richness and community composition of understorey plants and 12 soil attributes related to infiltration, erodibility and fertility across three positions within the canopy of large eucalypts (trunk, mid-canopy and edge) and in open areas. We also estimated the percentage similarity in plant communities across microsites, and the percentage species within the landscape occurring solely in one of the four microsites assayed. We tested the interactions between the effect of environmental conditions (rainfall and grazing) and canopy position on all these soil and vegetation attributes.Results: Soil attributes explained~50% of the effect of trees on understorey plants, and soil attributes improved with proximity to the trunk and increasing rainfall. The effect of canopy position 9 rainfall interactions depended on the response variable considered. These interactions did not affect soil attributes, the percentage of facilitation-obligate species or species richness, and weakly affected plant composition. However, we found a strong reduction in similarity among plant communities within edge and mid-canopy compared with open sites towards drier environments, and canopy position 9 rainfall interactions also significantly affected plant cover. We attribute these weak interactions between canopy position and environmental conditions to richness or the frequency of facilitation to the high turnover of facilitated species across microsites and across different environmental conditions. Conclusions: Our study can be used to better understand community dynamics in ecosystems with scattered trees by showing the differential effects of trees on their understorey. Our results also contribute to the body of research on the relationships between plant-plant interactions and the environment by illustrating the importance of gradient length and the number of different microsites considered.

Remote sensing (RS) for vegetation monitoring can involve mixed pixels with contributions from vegetation and background surfaces, causing biases in signals and their interpretations, especially in low-density forests. In a case study in the semi-arid Yatir forest in Israel, we observed a mismatch between satellite (Landsat 8 surface product) and tower-based (Skye sensor) multispectral data and contrasting seasonal cycles in near-infrared (NIR) reflectance. We tested the hypothesis that this mismatch was due to the different fractional contributions of the various surface components and their unique reflectance. Employing an unmanned aerial vehicle (UAV), we obtained high-resolution multispectral images over selected forest plots and estimated the fraction, reflectance, and seasonal cycle of the three main surface components (canopy, shade, and sunlit soil). We determined that the Landsat 8 data were dominated by soil signals (70%), while the tower-based data were dominated by canopy signals (95%). We then developed a procedure to resolve the canopy (i.e., tree foliage) normalized difference vegetation index (NDVI) from the mixed satellite data. The retrieved and corrected canopy-only data resolved the original mismatch and indicated that the spatial variations in Landsat 8 NDVI were due to differences in stand density, while the canopy-only NDVI was spatially uniform, providing confidence in the local flux tower measurements.