Use of thermal imaging to determine leaf conductance along a canopy gradient in European beech (Fagus sylvatica) (original) (raw)
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Journal of Geoscience and Environment Protection, 2018
Trees create microclimate under their crowns in comparison to the outside ambient atmospheric temperature. Sun is the pivotal source of radiant energy reaching the earth atmosphere of which heat is more important than light. The radiant energy reaches the ground without any barricade whereas the tree crown impedes it in reaching the earth's surface. During the day, when insolation impinges on tree crown, a portion of it is reflected back to the space, other portion is absorbed by the canopy increasing the temperature of leaves and the remaining part reaches the ground penetrating through the crown. Thus, a significant coolness is experienced under the shade of trees in comparison to open sunshine, with qualitative variations. The cooling produced by trees under their shades varies with species to species due to variation in several anatomical, structural and physiological attributes of the species. Climate is changing more rapidly prominently due to human activities especially indiscriminate felling of trees and it is feared that it will create problems on availability of energy, water and food security. Economic value takes over ecological benefits in selection of species in plantation programmes and this might have been due to the lack of scientific data about varying effectiveness of ecological services bestowed by different species. In the present study, an endeavor has been made to understand as to how a tree is integrated to the effects on atmosphere and responses to changing conditions with respect to differential cooling produced by five selected forestry tree species belonging to different categories. Analysis of data has come out with gradation of the sample species in respect to their cooling effect in the atmosphere in terms of yearly, quarterly, monthly and diurnal basis.
Drought-sensitivity ranking of deciduous tree species based on thermal imaging of forest canopies
Agricultural and Forest Meteorology, 2011
Most climate change projections for Central Europe predict higher mean summer temperatures and prolonged summer drought periods. However, in diverse mixed forest stands we expect tree species specific responses to water shortage, as tree species are highly variable in rooting depth and physiological traits related to the water balance. Here, we assessed the drought sensitivity of the water relations of six deciduous forest tree species at four sites with contrasting water availability by airborne thermal imagery of canopy foliage temperature, sap flow and soil water potential. Canopy architecture had a consistent influence on canopy foliage temperature with 'dense canopy' species (Acer pseudoplatanus, Fagus sylvatica and Tilia platyphyllos) being warmer (0.5-1.5 K) than 'open canopy' species (Fraxinus excelsior, Prunus avium and Quercus petraea). While the canopy foliage was close to air temperature at the beginning of the drought period (T C−A = −0.1 to 0.7 K) it strongly warmed up with ongoing drought, especially at the two 'dry' sites with a T C−A of 3.5-5 K. The pronounced canopy foliage warming at the 'dry' sites after 22 days of drought was associated with reduced transpiration rates as sap flow was curtailed by 20-35% in all species except F. excelsior and Q. petraea. Based on canopy foliage temperature and sap flow data, we considered A. pseudoplatanus the most drought sensitive species followed by F. sylvatica, T. platyphyllos and P. avium and the two ring-porous species F. excelsior and Q. petraea being clearly the least sensitive ones. At drier sites, increasing summer temperatures and drought might change the competitive abilities of tree species in favour of those that are able to maintain transpirational fluxes and cooler canopies such as F. excelsior and Q. petraea.
Tree Physiology, 2007
Response of whole-leaf hydraulic conductance (G L) in little-leaf linden (Tilia cordata Mill.) to temperature and photosynthetic photon flux (Q P) was estimated by the evaporative flux method under natural conditions in a mixed forest canopy. Mean midday G L in the lower-and upper-crown foliage was 1.14 and 3.06 mmol m-2 s-1 MPa-1 , respectively. Over the study period, leaf temperature (T L) explained about 67% of the variation in G L , and Q P explained about 10%. Leaf water potential and crown position also affected G L significantly. About a third of the temperature effect was attributable to changes in the viscosity of water, and two thirds to changes in protoplast permeability (i.e., symplastic conductance). Leaf hydraulic conductance was highly sensitive to changes in Q P when Q P was less than 200 µmol m-2 s-1 , and G L sensitivity decreased with increasing irradiance. Sensitivity of G L to variation in T L increased consistently with increasing temperature in the range of 16 to 29°C. There were positive interactions between temperature and light in their effects on G L : the light response was more pronounced at higher leaf temperatures. Because of frequent rains during the study period, the trees experienced no soil water deficit, and, within the range experienced, soil water potential had no effect on G L. Leaf hydraulic conductance exhibited a seasonal pattern that could be explained primarily by temporal variability in mean air temperature and irradiance, in addition to which an age-related trend (P < 0.001) of increasing G L from the end of June to the beginning of August was observed.
Forest Ecology and Management, 2005
Responses of leaf conductance (g L) to variation in photosynthetic photon flux density (Q P), leaf-to-air vapour pressure difference (VPD), bulk leaf water potential (C x) and soil to leaf hydraulic conductance (G T) were studied in silver birch (Betula pendula Roth) foliage with respect to leaf position within the canopy. The upper canopy leaves demonstrated 2.0-2.4 times higher (P < 0.001) daily maxima of g L as compared to the lower-canopy leaves growing in the shadow of upper branches. Functional acclimation of the shade foliage occurred in the form of both a steeper initial slope of the light-response curve and a lower light-saturation point of g L. Leaf conductance decreased if C x fell below certain values after the noon, while the sun foliage experienced greater negative water potentials than the shade foliage. In a diurnal scale the influence of bulk leaf water potential on g L altogether was rather weak. The lower-canopy foliage exhibited more conservative water-use behaviour, having lower maximum stomatal conductance and greater sensitivity to C x bringing about a smaller responsiveness to VPD than the upper canopy foliage. The mean G T was 1.7-1.8 times bigger (P < 0.001) for the upper canopy, compared to the lower canopy; the shade foliage responded more sensitively to changes in G T ; there were steeper water potential gradients between the soil and lower-canopy leaves; g L in the lower-canopy foliage was more strictly controlled by leaf water status-the evidence suggesting that the shade foliage may be hydraulically more constrained. We set up a hypothesis that stomatal conductance at the base of the live crown is limited not only by low light availability but also by plant's inner hydraulic constraints.
Canopy Temperature-Based Water Stress Indices: Potential and Limitations 14
Canopy Temperature-Based Water Stress Indices: Potential and Limitations, 2018
Water stress in plant is associated with reduced availability of soil moisture under higher ambient temperature and wider vapour pressure deficit for a considerable period of time. Instruments like pressure chambers and porometers are being used to quantify crop water stress under field conditions, but their use is limited because of the numerous time-consuming measurements that must be made. The application of thermal indices involving canopy temperature for monitoring crop water stress and irrigation scheduling has been demonstrated by several researchers in the last five decades since the evolution of portable infrared thermometers in the 1960s. As the temperature of plant canopy is a manifestation of canopy energy balance, a water-stressed canopy is hotter than a well-watered one under the same environmental conditions. Infrared thermometer integrates the thermal radiation from all exposed surfaces in the field of view of the instrument that included the plant surface and exposed soil surfaces into a single measurement and converts it into temperature unit applying the principle of Stefan-Boltzmann law. However, different plant physiological as well as micro-climatic factors like solar radiation, turbulence, air temperature and humidity must influence the canopy temperature at the time of observation. Hence, stoma-tal conductance and transpiration rates cannot be estimated by canopy temperature alone. In other words, canopy temperature alone is not enough to make estimates of plant water status. For this reason many researchers have attempted to normalize the canopy temperature to account for the influence of other variable
Tree Physiology
As the global climate warms, a key question is how increased leaf temperatures will affect tree physiology and the coupling between leaf and air temperatures in forests. To explore the impact of increasing temperatures on plant performance in open air, we warmed leaves in the canopy of two mature evergreen forests, a temperate Eucalyptus woodland and a tropical rainforest. The leaf heaters consistently maintained leaves at a target of 4 °C above ambient leaf temperatures. Ambient leaf temperatures (Tleaf) were mostly coupled to air temperatures (Tair), but at times, leaves could be 8–10 °C warmer than ambient air temperatures, especially in full sun. At both sites, Tleaf was warmer at higher air temperatures (Tair > 25 °C), but was cooler at lower Tair, contrary to the ‘leaf homeothermy hypothesis’. Warmed leaves showed significantly lower stomatal conductance (−0.05 mol m−2 s−1 or −43% across species) and net photosynthesis (−3.91 μmol m−2 s−1 or −39%), with similar rates in lea...
THE EFFECT OF TREES ON TEMPERATURE
The objective of this study is to quantify the effects of trees in a Midwest urban area on air temperature and humidity to determine if the effects are significantly different for: different species of trees, trees of the same species in different environments, and whether the effects can be explained by physical characteristics of the individual trees. Replicate trees in each of five categories were studied: sugar maple, pin oak and walnut individuals overgrass, sugar maple individuals along streets over concrete, and sugar maple clumps over grass. All the trees show a consistent effect: temperatures are reduced and humidities are elevated under the canopies. The greatest cooling effect (0.7 -1.3°C) occurs in the early afternoon. The difference between species is insignificant, but street trees are significantly less effective in reducing temperature than either individual trees or clumps planted over grass. The clumps had no greater effect than the individual trees. The amount of cooling observed in this study was considerably less than that documented in many previous studies. No consistent linear relationships were determined between physical characteristics of the trees, such as leaf area index, and temperature reductions or humidity increases.
The Thermal Tolerances, Distributions, and Performances of Tropical Montane Tree Species
Frontiers in Forests and Global Change, 2020
Due to global warming, many species will face greater risks of thermal stress, which can lead to changes in performance, abundance, and/or geographic distributions. In plants, high temperatures above a species-specific critical thermal maximum will permanently damage photosystem II, leading to decreased electron transport rates, photosynthetic failure, and eventual leaf and plant death. Previous studies have shown that plant thermal tolerances vary with latitude, but little is known about how they change across smaller-scale thermal gradients (i.e., with elevation) or about how these thermal tolerances relate to species' local performances and geographic distributions. In this study, we assess the maximum photosynthetic thermal tolerances (T 50) of nearly 200 tropical tree species growing in 10 forest plots distributed across a >2,500 m elevation gradient (corresponding to a 17 • C temperature gradient) in the northern Andes Mountains of Colombia. Using these data, we test the relationships between species' thermal tolerances and (1) plot elevations and temperatures, (2) species' largescale geographic distributions, and (3) changes in species' abundances through time within the plots. We found that species' T 50 do in fact decrease with plot elevation but significantly slower than the corresponding adiabatic lapse rate (−0.4 vs. −5.7 • C km −1) and that there remains a large amount of unexplained variation in the thermal tolerances of co-occurring tree species. There was only a very weak association between species' thermal tolerances and their large-scale geographic distributions and no significant relationships between species' thermal tolerances and their changes in relative abundance through time. A potential explanation for these results is that thermal tolerances are adaptations to extreme leaf temperatures that can be decoupled from regional air temperatures due to microclimatic variations and differences in the species' leaf thermoregulatory properties.
Comparative Study of Transpiration in Cooling Effect of Tree Species in the Atmosphere
Journal of Geoscience and Environment Protection, 2018
Trees create microclimate under their crowns in comparison to the outside ambient atmosphere, which is a result of physical as well as physiological functions of the tree. The cooling produced by trees varies with species due to variation in several anatomical, structural and physiological attributes of the species. Transpiration is one of the most significant physiological functions performed by plants, which affects cooling produced by a tree under its shade. When solar energy impinges on the leaf, water emerges from its surface through transpiration taking the latent heat to convert it into water vapour. This leads to a rise in humidity of the atmosphere and reduction in temperature of the leaf. To remain leaf in equilibrium, it takes heat from the surrounding atmosphere resulting in reduction in temperature of surroundings. Since, transpiration takes place through stomata which are normally located on the ventral side of the leaf, this reduction in temperature is more experienced beneath the crown of the tree. Therefore, the present study was carried out to analyze the role of transpiration in cooling effect of five forestry tree species. The cooling produced by tree species under their shades has been found positively correlated to the transpiration rate whereas the rate of transpiration has responded positively to the ambient temperature and water conductance. However, no definite relationship has been found between frequency of open stomata and the rate of transpiration.
Trees, 2012
This study assessed the variation of leaf anatomy, chlorophyll content index (CCI), maximal stomatal conductance (g s max ) and leaf wettability within the canopy of an adult European beech tree (Fagus sylvatica L.) and for beech saplings placed along the vertical gradient in the canopy. At the top canopy level (CL 28m ) of the adult beech, CCI and leaf anatomy reflected higher light stress, while g s max increased with height, reflecting the importance of gas exchange in the upper canopy layer. Leaf wettability, measured as drop contact angle, decreased from 85.5°± 1.6°(summer) to 57.5°± 2.8°(autumn) at CL 28m of the adult tree. At CL 22m , adult beech leaves seemed to be better optimized for photosynthesis than the CL 28m leaves because of a large leaf thickness with less protective and impregnated substances, and a higher CCI. The beech saplings, in contrast, did not adapt their stomatal characteristics and leaf anatomy according to the same strategy as the adult beech leaves. Consequently, care is needed when scaling up experimental results from seedlings to adult trees.