Maximum Leaf Temperature Research Articles

High heat tolerance and thermal safety margins in mangroves from the southwestern coast of India

Mangroves are key components of productive ecosystems that provide a multitude of ecosystem goods and services. How these species will respond to future climates with more frequent and severe extreme temperatures has not received much attention. To understand how vulnerable mangroves are to future warming, we quantified photosynthetic heat tolerance and estimated thermal safety margins for thirteen mangrove species from the southwestern Indian coast. We quantified heat tolerance as temperatures that resulted in a 5 % (T5) and 50 % (T50) decline in photosystem II function, and thermal safety margins (TSM) as the difference between T50 and maximum leaf temperatures. T50 ranged from 48.9 °C in Avicennia Marina to 55.3 °C in Bruguiera gymnorhiza, with a mean of 53.3 °C for the thirteen species. Heat tolerance was higher for species with bigger leaves which experience higher leaf temperatures, but was not related to the other leaf traits examined. Heat tolerance was exceptionally high in these mangroves compared to other woody species. With their high tolerance and large safety margins these mangroves may be relatively less vulnerable to future climates with higher temperatures.

Grapevine stem water potential estimation based on sensor fusion

Estimating vine water status is essential for achieving the desired balance between wine grape quality and yield in viticulture management and irrigation planning. The growing demands of smart farming for extracting meaningful information from field data to support irrigation decisions may be facilitated using field monitoring technology along with advanced modeling algorithms. This research took place in a Vitis Vinifera cv. “Sauvignon Blanc” experimental plot within a commercial vineyard. The main objective was to generate a water stress estimation model for wine grapevines based on data fusion from multiple sensors. Data were collected using sensors from five monitored grapevines, each treated with a different water stress regime. The sensors provided measurements of trunk and fruit growth, leaf temperature, and soil water content (SWC) at depths of 20 and 40 cm. Additionally, meteorological station in the vineyard recorded the temperature, relative humidity, wind speed, and solar radiation. A daily-scale multivariate time series was composed using the data derived from the sensors. Stem water potential (SWP) values were measured for the monitored vines and analyzed against the multivariate time series of factors to define their interrelations and interactions. Finally, a prediction model for SWP estimation was defined using boosted regression trees (BRT) algorithm, with an optimized set of factors. Validation was conducted using comparative statistics between a randomly selected test set and a predicted set of SWP values achieved using the trained BRT model. After processing the data and eliminating most of the factors due to multicollinearity, the model was composed of daily maximum leaf temperature (LT), SWC at noon at 40 cm, tree water deficit (TWD), SWC daily amplitude at 20 cm, water input, vapor pressure deficit, and phenological stages. The highest contributor to the BRT model was maximum LT (44.5%), followed by SWC at noon at 40 cm (16.9%) and TWD (16%). The model performance was estimated using various measures and resulted in a correlation of 0.9 between the estimated SWP values and the test set. No significant differences were found between their means (t = -0.31, p-value = 0.76) and distributions (D = 0.13, p-value = 0.77). The RMSE was 0.16 MPa which corresponds to a 12% error when normalized to the range of the test set. The suggested sensor fusion approach for estimating SWP is a promising technique that integrates representative factors of the soil–water-plant-atmosphere continuum; it should be further examined in different climate regimes and vine varieties.

Handling the heat - photosynthetic thermal stress in tropical trees.

Warming climate increases the risk for harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. The heat sensitivity may be particularly high in equatorial tropical tree species adapted to a thermally stable climate. Thermal thresholds of the photosynthetic system of sun-exposed leaves were investigated in three tropical montane tree species native to Rwanda with different growth and water use strategies (Harungana montana, Syzygium guineense and Entandrophragma exselsum). Measurements of chlorophyll fluorescence, leaf gas exchange, morphology, chemistry and temperature were made at three common gardens along an elevation/temperature gradient. Heat tolerance acclimated to maximum leaf temperature (Tleaf ) across the species. At the warmest sites, the thermal threshold for normal function of photosystem II was exceeded in the species with the highest Tleaf despite their higher heat tolerance. This was not the case in the species with the highest transpiration rates and lowest Tleaf . The results point to two differently effective strategies for managing thermal stress: tolerance through physiological adjustment of leaf osmolality and thylakoid membrane lipid composition, or avoidance through morphological adaptation and transpiratory cooling. More severe photosynthetic heat stress in low-transpiring montane climax species may result in a competitive disadvantage compared to high-transpiring pioneer species with more efficient leaf cooling.

Microclimate estimation under different coffee-based agroforestry systems using full-sun weather data and shade tree characteristics

In Central America, coffee is mainly grown in agroforestry systems. This practice modifies the microclimate, which, in turn, influences coffee growth and development. However, modeling these microclimate modifications is a challenge when trying to predict the development of a disease in the understory crop, based on variables usually monitored in weather stations exposed to full sunlight. Furthermore, critical variables for plant disease development, such as leaf wetness duration and leaf temperatures, are generally not measured by weather stations. In our study, we sought to build models explaining daily minimum and maximum coffee leaf temperatures, daily coffee leaf wetness duration, and minimum and maximum air temperatures in agroforestry systems with a single shade tree species, which are common in Central America, and which were characterized by shade tree height, canopy openness and light gap distribution. The modeled variables were mainly explained by one or more meteorological variables provided by reference weather stations exposed to full sunlight. The presence of shade trees resulted in a buffer effect, reducing daily maximum air and leaf temperatures, and increasing daily minimum air and leaf temperatures. Moreover, except for the daily minimum air temperature under shade, shade tree characteristics affected these microclimatic variables. Indeed, the buffer effect on the daily maximum air temperature increased with shade trees 7 m tall or over, whereas for extreme leaf temperatures, this effect seemed to be further intensified by a dense and homogeneous canopy. The tallest shade trees also tended to provide conditions that reduced coffee leaf wetness duration. The coffee leaf stratum affected the daily maximum leaf temperature, with a top layer intercepting radiation for the lower strata, but had no effect on the daily minimum leaf temperature, detected at night. The models developed were simple equations allowing interpretation of shade tree height, the effects of canopy characteristics on the microclimate and were therefore useful for designing and managing agroforestry system. The more accurate models could be incorporated into an early warning system for coffee pests and diseases in the region.

Thermal safety margins of plant leaves across biomes under a heatwave

Climate change has great impacts on forest ecosystems, especially with the increasing frequency of heatwaves. Thermal safety margin (TSM) calculated by the difference between body temperature and thermotolerance threshold is useful to predict thermal safety of organisms. It has been widely used for animals, whereas has rarely been reported for plants. Besides, most of the previous studies used only thermotolerance to estimate thermal safety or used thermotolerance and air temperature (Ta) to calculate TSM. However, leaf temperature (Tl) is the real “body” temperature of plant leaves. Tl decoupling from Ta might induce large error in TSM. Here, we investigated TSM of photosystem II (thermotolerance of PSII – the maximum Tl) of dominant canopy plants in four forests from tropical to temperate biomes during a heatwave, and compared the TSMs calculated by Tl (TSM.Tl) and Ta (TSM.Ta) respectively. Also, thermal related leaf traits were investigated. The results showed that both TSM. Tl and TSM.Ta decreased from the cool forests to the hot forests. TSM.Tl was highly correlated with the maximum leaf temperature (Tlmax), while had an opposite trend with thermotolerance across biomes. Thus, Tlmax instead of thermotolerance can be used to evaluate TSM. The maximum Ta (Tamax), Tlmax and leaf traits explained 68% of the variance of thermotolerance in a random forest model, where Tamax and Tlmax explained 62%. TSM.Ta could not distinguish thermal safety differences between co-occurring species. The overestimation of TSM by TSM.Ta increased from the tropical to the temperate forest, and increased with Tl within biome. Therefore, it is not recommended to use TSM.Ta in cold forests. The present study enriches the dataset of photosynthetic TSMs across biomes, proposes using Tlmax to estimate TSMs of leaves, and highlights the risk of hot dry forest during heatwaves.

Trees at the Amazonia-Cerrado transition are approaching high temperature thresholds

Land regions are warming rapidly. While in a warming world at extra-tropical latitudes vegetation adapted to higher temperatures may move in from lower latitudes this is not possible in the tropics. Thus, the limits of plant functioning will determine the nature and composition of future vegetation. The most temperature sensitive component of photosynthesis is photosystem II. Here we report the thermal safety margin (difference between photosystem II thermotolerance (T50) and maximum leaf temperature) during the beginning of the dry season for four tree species co-occurring across the forest-savanna transition zone in Brazil, a region which has warmed particularly rapidly over the recent decades. The species selected are evergreen in forests but deciduous in savannas. We find that thermotolerance declines with growth temperature >40 °C for individuals in the savannas. Current maximum leaf temperatures exceed T50 in some species and will exceed T50 in a 2.5 °C warmer world in most species evaluated. Despite plasticity in leaf thermal traits to increase leaf cooling in hotter environments, the results show this is not sufficient to maintain a safe thermal safety margin in hotter savannas. Overall, the results suggest that tropical forests may become increasingly deciduous and savanna-like in the future.

Variation in leaf temperatures of tropical and subtropical trees are related to leaf thermoregulatory traits and not geographic distributions

AbstractTo help predict the effects of global warming on plants, previous studies have investigated how the distributions and physiological performances of plants relate to environmental temperatures. This approach implicitly assumes that leaf temperatures are tightly linked to regional air temperatures. However, the thermoregulatory behaviors and physical properties of leaves can differ greatly between species, leading to different plants having different leaf temperatures even when they occur under similar conditions. It is important to understand this variation in leaf thermoregulatory traits and temperatures in order to predict how individual species will be impacted by global warming. We measured the thermal properties of leaves from >50 tropical and subtropical tree species grown at the Gifford Arboretum (Florida, USA). For each species, we measured maximum leaf temperature and rate of leaf warming using a new standardized protocol to control for both environmental variation and select plant thermoregulatory behaviors. We tested the relationships between the two thermal variables and several leaf functional traits. Even under laboratory‐controlled conditions, leaf temperatures varied by over 8.5°C among species. Maximum leaf temperature was positively correlated with leaf area, and rate of leaf warming was negatively correlated with water content per leaf area. This suggests that some species may be able to offset rising air temperatures through acclimation or adaptation of these leaf properties. Next, we tested the relationships between the species’ leaf thermal properties and their climatic niches/distributions, but we found no significant associations. These results call into question the use of regional air temperatures to model plant physiological and demographic performance.Abstract in Spanish is available with online material.

Photosynthetic heat tolerances and extreme leaf temperatures

Abstract Photosynthetic heat tolerances (PHTs) have several potential applications including predicting which species will be most vulnerable to climate change. Given that plants exhibit unique thermoregulatory traits that influence leaf temperatures and decouple them from ambient air temperatures, we hypothesized that PHTs should be correlated with extreme leaf temperatures as opposed to air temperatures. We measured leaf thermoregulatory traits, maximum leaf temperatures (TMO) and two metrics of PHT (Tcrit and T50) quantified using the quantum yield of photosystem II for 19 plant species growing in Fairchild Tropical Botanic Garden (Coral Gables, FL, USA). Thermoregulatory traits measured at the Garden and microenvironmental variables were used to parameterize a leaf energy balance model that estimated maximum in situ leaf temperatures (TMIS) across the geographic distributions of 13 species. TMO and TMIS were positively correlated with T50 but were not correlated with Tcrit. The breadth of species' thermal safety margins (the difference between T50 and TMO) was negatively correlated with T50. Our results provide observational and theoretical support based on a first principles approach for the hypothesis that PHTs may be adaptations to extreme leaf temperature, but refute the assumption that species with higher PHTs are less susceptible to thermal damage. Our study also introduces a novel method for studying plant ecophysiology by incorporating biophysical and species distribution models, and highlights how the use of air temperature versus leaf temperature can lead to conflicting conclusions about species vulnerability to thermal damage. A free Plain Language Summary can be found within the Supporting Information of this article.

High heat tolerance in plants from the Andean highlands: Implications for paramos in a warmer world.

Tropical plant species are expected to have high heat tolerance reflecting phenotypic adjustments to warm regions or their evolutionary adaptation history. However, tropical highland specialists adapted to the colder temperatures found in the highlands, where short and prostrated vegetation decouples plants from ambient conditions, could exhibit different upper thermal limits than those of their lowland counterparts. Here we evaluated leaf heat tolerance of 21 tropical alpine paramo species to determine: 1) whether species with restricted distribution (i.e., highland specialists) have lower heat tolerance and are more vulnerable to warming than species with widespread distribution; 2) whether different growth forms have different heat tolerance; and 3) whether species height (i.e., microhabitat) influences its heat tolerance. We quantified heat tolerance by evaluating T50, which is the temperature that causes a reduction in 50% of initial Fv/Fm values and reflects an irreversible damage to the photosynthetic apparatus. Additionally, we estimated the thermal safety margins as the difference between T50 and the maximum leaf temperature registered for the species. All species presented high T50 values ranging between 45.4°C and 53.9°C, similar to those found for tropical lowland species. Heat tolerance was not correlated with species distributions or plant height, but showed a strong relationship with growth form, with rosettes having the highest heat tolerance. Thermal safety margins ranged from 12.1 to 31.0°C. High heat tolerance and broad thermal safety margins suggest low vulnerability of paramo species to warming as long as plants are capable of regulating the leaf temperature within this threshold. Whether paramo plants would be able to regulate leaf temperature if drought episodes become more frequent and transpirational cooling is compromised is the next question that needs to be answered.

Spectral and thermal data as a proxy for leaf protective energy dissipation under kaolin application in grapevine cultivars

AbstractThe dynamic effects of kaolin clay particle film application on the temperature and spectral reflectance of leaves of two autochthonous cultivars (Touriga Nacional (TN, n=32) and Touriga Franca (TF, n=24)) were studied in the Douro wine region. The study was implemented in 2017, in conditions prone to multiple environmental stresses that include excessive light and temperature as well as water shortage. Light reflectance from kaolin-sprayed leaves was higher than the control (leaves without kaolin) on all dates. Kaolin’s protective effect over leaves’ temperatures was low on the 20 days after application and ceased about 60 days after its application. Differences between leaves with and without kaolin were explained by the normalized maximum leaf temperature (T_max_f_N), reflectance at 400 nm, 532 nm, and 737 nm, as assessed through TN data. The wavelengths of 532 nm and 737 nm are associated with plant physiological processes, which support the selection of these variables for assessing kaolin’s effects on leaves. The application of principal component analysis to the TF data, based on these four variables (T_max_f_N and reflectances: 400, 532, 737 nm) selected for TN, explained 83.56% of data variability (considering two principal components), obtaining a clear differentiation between leaves with and without kaolin. The T_max_f_N and the reflectance at 532 nm were the variables with a greater contribution for explaining data variability. The results improve the understanding of the vines’ response to kaolin throughout the grapevine cycle and support decisions about the re-application timing.

Drought affects the heat-hardening capacity of alpine plants as indicated by changes in xanthophyll cycle pigments, singlet oxygen scavenging, α-tocopherol and plant hormones

Alpine environments in Europe are increasingly affected by more erratic precipitation patterns, and more frequent drought and heat waves. Heat-hardening capacity is a key feature for survival of these abiotic stress factors, but it is poorly understood how heat and drought affect plant performance when combined. The main objectives of this study were (1) to determine maximum heat hardening capacity in 14 selected plant species and (2) to study how alpine plants respond to combined heat and drought stress compared to heat alone. (3) For risk assessment maximum leaf temperatures were measured in the field and (4) important methodological aspects of testing heat tolerance were evaluated. Heat hardening capacity was assessed by Tc, the heat threshold of photosystem II (PS II), and by heat tolerance tests based on visual inspection of leaf tissue damage or potential quantum efficiency of PS II (Fv/Fm). A purpose-built Heat Tolerance Testing System (HTTS) was used, which allows for controlled heat exposure of whole plants under nearly natural conditions. Additionally, in two species from contrasting habitats, Senecio incanus and Primula minima, the dynamics of heat hardening was studied during and after 8days exposure to heat (H), or to a combination of heat and severe drought (H+D) within a light-transmissive heat hardening chamber at the alpine field site. In both species, H treatment significantly increased heat tolerance (LT50), determined by the HTTS, to 58.0°C and 54.9°C, respectively, and was accompanied by elevated production of abscisic acid (ABA) and salicylic acid (SA), whereas jasmonic acid (JA) levels decreased. Under H+D the LT50 was only 56.5°C and 51.6°C, respectively, and levels of ABA were higher in S. incanus and SA lower in both species in comparison to H. Changes in xanthophyll cycle pigments, α-tocopherol and carotenoids:chlorophyll ratio were more pronounced in P. minima than in S. incanus. In P. minima both H and H+D significantly increased singlet oxygen (1O2) scavenging capacity, determined by electron paramagnetic resonance spectroscopy (EPR). In the field, the maximum half-hourly mean (HHM) leaf temperature of P. minima (32.2°C) was significantly lower than of S. incanus (46.5°C, a potentially harmful temperature). We conclude that the investigated species are well adapted to the prevailing temperature conditions in the field. They also possess an outstanding heat hardening capacity, but this can be curtailed when heat is combined with drought. As drought further increases leaf temperatures, the risk of suffering lethal heat damage of some species may increase in the future, particularly at south exposed, ruderal alpine sites with uncertain water supply.

A coupled model of leaf photosynthesis, stomatal conductance, and leaf energy balance for chrysanthemum (Dendranthema grandiflora)

While dynamic greenhouse climatic regimes are often applied to achieve energy efficiency, dynamic mechanistic models can assist in climate control decisions, and to elucidate plant stress under extreme microclimatic conditions. The present study developed a couple model with three integrated sub-models to predict net leaf photosynthesis (Pnl), stomatal conductance (gs), and leaf temperature under different microclimatic conditions: (1) a C3 photosynthesis biochemical model; (2) a stomatal conductance model; and (3) a leaf energy balance model. Leaf photochemical efficiency and maximum gross photosynthesis using a negative exponential light response curve were modelled with different leaf temperatures, light levels, and CO2 concentrations. The stomatal conductance and leaf energy balance models were calibrated independently. Pnl, gs, and leaf temperature model predictions were validated with independent measurements and climate input data. Model performance was evaluated by a linear regression of predicted values relative to observed values. The coupled model estimated Pnl with a 2–12% mean difference between the observed and the model, and a 1.82°C maximum leaf temperature difference between the observed and the model. An additional stomatal model was implemented for comparison, and tested against the model system. Our model showed a better fit to Pnl, leaf temperature, and stomatal conductance validation data. The coupled model was therefore a good predictor for crop growth and microclimate. We suggest a multi-model approach with self-selective sub-models to assist in decisions optimising light, temperature, and CO2 for maximum photosynthetic rates for climatic conditions applied in the model (i.e. high light, temperature, and CO2 concentration). Furthermore, the model leaf temperature prediction could be used for leaf temperature monitoring under unfavorable microclimatic conditions.

Estimation Model of the Change in Dairy Leaf Surface Temperature Using Scaling Technique

This study was conducted to develop a model to estimate crop leaf surface temperature. The results were as following; A definition for the daily time based on elapsed time from the midnight (00:00) as "E&E time" with the unit of Kmin. was suggested. The model to estimate the scaled temperature (<TEX>$T^*e$</TEX>) of crop leaf surface temperature by scale factor (<TEX>$T^*$</TEX>) according to the "E&E time : Kmin."(X) was developed as eq. (1) <TEX>$T^*e=0.5{\cdot}sin(X+780)+0.5$</TEX> (2) <TEX>$T^*=(Tx-Tn)/(Tm-Tn)$</TEX>, Tx : Daily leaf temperature, Tm : Daily maximum leaf temperature, Tn : Daily minimum leaf temperature. Relative sensitivity of the measured temperature compared to the estimated temperature of red pepper, soybean and persimmon was 1.078, 1.033 and 0.973, respectively.

Distribution of leaf characteristics in relation to orientation within the canopy of woody species

Over the last few decades considerable effort has been devoted to research of leaf adaptations to environmental conditions. Many studies have reported strong differences in leaf mass per unit area (LMA) within a single tree depending on the photosynthetic photon flux density (PPFD) incident on different locations in the crown. There are fewer studies, however, of the effects of differences in the timing of light incidence during the day on different crown orientations.Leaves from isolated trees of Quercus suber and Quercus ilex in a cold Mediterranean climate were sampled to analyze differences in LMA and other leaf traits among different crown orientations. Gas-exchange rates, leaf water potentials, leaf temperatures and PPFD incident on leaf surfaces in different crown orientations were also measured throughout one entire summer day for each species.Mean daily PPFD values were similar for the leaves from the eastern and western sides of the canopy. On the western side, PPFD reached maximum values during the afternoon. Maximum leaf temperatures were approximately 10–20% higher on the west side, whereas minimum leaf water potentials were between 10 and 24% higher on the east side. Maximum transpiration rates were approximately 22% greater on the west, because of the greater leaf-to-air vapor pressure deficits (LAVPD). Mean individual leaf area was around 10% larger on the east than on the west side of the trees. In contrast, there were no significant differences in LMA between east and west sides of the crown.Contrary to our expectations, more severe water stress on the west side did not result in increases in LMA, although it was associated with lower individual leaf area. We conclude that increases in LMA measured by other authors along gradients of water stress would be due to differences in light intensity between dry and humid sites.

Photosynthetic thermotolerance of woody savanna species in China is correlated with leaf life span.

Photosynthetic thermotolerance (PT) is important for plant survival in tropical and sub-tropical savannas. However, little is known about thermotolerance of tropical and sub-tropical wild plants and its association with leaf phenology and persistence. Longer-lived leaves of savanna plants may experience a higher risk of heat stress. Foliar Ca is related to cell integrity of leaves under stresses. In this study it is hypothesized that (1) species with leaf flushing in the hot-dry season have greater PT than those with leaf flushing in the rainy season; and (2) PT correlates positively with leaf life span, leaf mass per unit area (LMA) and foliar Ca concentration ([Ca]) across woody savanna species. The temperature-dependent increase in minimum fluorescence was measured to assess PT, together with leaf dynamics, LMA and [Ca] for a total of 24 woody species differing in leaf flushing time in a valley-type savanna in south-west China. The PT of the woody savanna species with leaf flushing in the hot-dry season was greater than that of those with leaf flushing in the rainy season. Thermotolerance was positively associated with leaf life span and [Ca] for all species irrespective of the time of flushing. The associations of PT with leaf life span and [Ca] were evolutionarily correlated. Thermotolerance was, however, independent of LMA. Chinese savanna woody species are adapted to hot-dry habitats. However, the current maximum leaf temperature during extreme heat stress (44·3 °C) is close to the critical temperature of photosystem II (45·2 °C); future global warming may increase the risk of heat damage to the photosynthetic apparatus of Chinese savanna species.

An experimental design applied to vineyards for identifying spatially and temporally variable crop parameters

Harvesting uniform batches of grapes is required to optimize must quality as one prerequisite for premium wine production. The definition of sub-units of vineyards based on within-field variation allows unitbased vineyard management during cultivation and harvest. Essential for such vineyard management is the definition of sub-units that correspond with uniform batches of quality parameters of the fruit (e.g. berry residual sugar, anthocyanin content) at harvest time or with physiological parameters measuring the vine during berry development until ripeness. The definition requires geo-referenced sampling and parameter analysis, usually in combination with interpolation and kriging methods employed to describe spatial vineyard variation. In an attempt to develop an assay for within-variation in vineyards physiological parameters assessed through chlorophyll fluorescence measurements and leaf temperature were assessed at bloom, veraison and post veraison in a randomized block design in two vineyards of Lower Austria. A statistical model based on a repeated measurement ANOVA was developed and showed suitability for the detection and monitoring of vineyard variability throughout the vegetation period based on the maximum quantum yield of photosystem II (Fv/Fm), the maximum leaf temperature (maxT leaf ) and malic acid. These parameters allow the prospective classification of sub-units according to the vine’s vitality and may be adopted for scientific experimentation and for practical viticulture.

Heat resistance and energy budget in different Scandinavian plants

Heat resistance and energy budget in different Scandinavian plants

CO2 assimilation by Dryas integrifolia on Devon Island, Northwest Territories

In situ measurements of CO2 assimilation by Dryas integrifolia at different stages of development and under different environmental conditions were made on Devon Island, Northwest Territories. Dryas can fix CO2 in excess of respiration over a 24-h period under conditions of clear nights and cloudy days. The maximum net assimilation rate measured was 4.2 mg g−1 dry weight h−1. The maximum amount of CO2 fixed in 24 h was 61.54 mg g−1 dry weight. Maximum net assimilation occurred at 8 to 10 °C leaf temperatures. Positive net assimilation occurred at 1 °C leaf temperature. Light compensation was shown to be less than 0.04 langley min−1. Leaf temperatures were always greater than ambient. The maximum leaf temperature measured was 39 °C. Net assimilation rates appear to decrease as the season progresses.

Bioclimate, Leaf Temperature, and Primary Production in Red Mangrove Canopies in South Florida

A model of primary production and transpiration of forest canopies was developed from the energy—budget equation of individual leaves to clarify some of the physical processes affecting primary production. The model calculates hourly vertical profiles of temperature, transpiration, respiration, and gross and net photosynthesis of both sunlit and shaded leaves; and calculates appropriately weighted totals for the hourly water loss and gross and net photosynthesis for each level in the canopy. The model includes variations in leaf resistances caused by changes in absorbed solar radiation and changes in leaf water deficit and also takes into account the interdependence of the infrared profiles and the leaf—temperature profiles within the canopy. The model was tested with data collected on red mangrove (Rhizophora mangle Roxb.) forests. Canopy structure and the daily courses of solar and infrared radiation, air temperature, humidity, and wind, and ground temperatures were measured and used as input data for the model. The model produced realistic leaf temperatures, leaf resistances, transpiration rates, and primary production rates and was used to indicate the relative importance of environmental variables in influencing leaf temperature transpiration, and primary production. Maximum leaf temperatures occurred at the top of the canopy in June and the overcast day in January, but at the bottom of the canopy on the clear day in January. The model presently calculates water uptake as a constant rate to leaves and does not include a redistribution of water within the plant. The calculated transpiration was about 20% of the total water loss of the stand, the remaining loss coming directly from the moist substrate under the canopy. The inclusion in the model of stomatal movements reduced daily transpiration under conditions of mild water stress by 0.04 cm day‐1. Total evapotranspiration was 0.67 cm day‐1, and transpirational water loss was 0.12 cm day‐1. Levels in the middle of the canopy had the highest transpiration rate because of their high leaf area, but leaves at the top had the highest transpiration rate per unit leaf area. Leaf water deficits great enough to initiate stomatal closure occurred early in the morning at the top of the canopy in June on clear days, later in the morning at the top of the canopy in June on cloudy days, early in the afternoon at the top of the canopy in January on clear days, and not at all at the top of the canopy in January on cloudy days.Leaf water deficits did not develop within the canopy in either June or January. Net photosynthesis calculated with the model was 5.6 g organic matter m‐2 day ‐1 for sunny days and 3.5 for cloudy days in June. Gross photosynthesis per unit leaf area was greater at the top of the canopy than at the bottom, but the middle levels of the canopy had the greatest production. The efficiency of water utilization increased from top to bottom of the canopy. A weighted monthly estimate for production was 3.4 for June and 2.2 for January, giving an average annual net production rate of 2.8 g organic matter m‐2 day‐1. The model predicts that the maximum photosynthesis for mangrove stands will occur with a leaf—area index of about 2.5 if no acclimation to shade within the canopy occurs. A leaf area greater than about 2.5 may decrease production. The environmental variables with the greatest influence on primary production were air temperature and humidity. Production was decreased by increasing air temperature and increasing humidity. Increasing total solar radiation increased production up to a point, then decreased it. Increasing the diffuse fraction of the total solar radiation increased production. Increasing infrared radiation decreased production. Production and transpiration increased with increasing leaf—area index with steeply inclined leaves and leaf absorptance. Production was decreased by increasing leaf—area index with nearly horizontal leaves, leaf width, and the ap value. The canopy distribution of red mangrove appears to be nearly optimum to maximize the efficiency of water utilization rather than production. This indicates that the canopy is adapted to maximize production under conditions of saturated water supply.