Uppermost Canopy Research Articles

Nesting habitat selection and challenges of conservation of the vulnerable Lesser Adjutant Leptoptilos javanicus (Horsfield, 1821) in the Chitwan National Park, Nepal

Abstract The nesting ecology and conservation threats of the Lesser Adjutant (Leptoptilos javanicus) were studied in the Chitwan National Park, Nepal. We located nine nesting colonies during the nesting season. The number of nests was highly positively correlated with tree height, diameter at breast height and canopy cover. The uppermost canopy of the trees was the most preferred nesting place. Storks preferred to nest in compact colonies on large, widely branched trees with thin foliage cover, such as Bombax ceiba, and also nearby the foraging grounds such as wetlands and grasslands. Storks mostly preferred to nest in Bombax ceiba, but if this tree was not available, they nested in other trees, such as Shorea robusta, Ficus racemosa and Terminalia alata. During the breeding season, 180 adults, 76 nests and 88 chicks were recorded, where the highest number of chicks was recorded near the Sauraha area of the Chitwan National Park. Most of the colonies were far from human settlements, which suggest that human disturbance could be the major determinant of nesting habitat selection in this area. The wetlands nearby human settlements are either overexploited in terms of mass collection of the storks` prey species by people or disturbed highly due to presence of a large number of people. These empirical findings suggest that conservation of Lesser Adjutant mainly rely on the protection of mature Bombax ceiba trees and the reduction of human disturbance and of the collection of stork prey animals from foraging areas.

Only sun-lit leaves of the uppermost canopy exceed both air temperature and photosynthetic thermal optima in a wet tropical forest

Tropical forests have evolved under relatively narrow temperature regimes, and therefore may be more susceptible to climatic change than forests in higher latitudes. Recent evidence shows that lowland tropical forest canopies may already be exceeding thermal maxima for photosynthesis. Height can strongly influence both the microclimate and physiology of forest canopy foliage, yet vertical trends in canopy micrometeorology are rarely examined in tropical forests. To improve our understanding of how climatological and micrometeorological conditions affect tropical tree function, we assessed vertical gradients of photosynthetic photon flux density, vapor pressure deficit, air temperature, leaf temperature, and the difference between leaf and air temperature (ΔT) in a Puerto Rican tropical wet forest. Both air temperature and vapor pressure deficit increased linearly with height. Leaf temperature, however, did not significantly differ across the shaded foliage from 0-16 m, while the uppermost layer (20 m) was up to 4°C hotter than the rest of the foliage and up to 5°C hotter than air temperature at the highest radiation intensity. As a result, leaf temperatures in the shaded middle canopy and understory showed nearly poikilothermic behavior (i.e., leaf temperatures = air temperature), while the uppermost canopy strata showed megathermic behavior (i.e., leaf temperatures greater than air temperature), revealing different thermoregulation strategies for sun-lit versus shaded foliage. In addition, the uppermost canopy was the only stratum to exceed mean photosynthetic temperature optima for this site (Topt = 30.2 ± 1.1°C). Because the upper canopy plays a disproportionately large role in whole-forest photosynthesis, continued warming could potentially weaken the tropics’ carbon sink capacity. However, the shaded leaves may be able increase carbon uptake with further warming because they appear to be able to maintain temperatures below photosynthetic optima, possibly with the help of radiation shielding provided by the uppermost canopy layer.

The DeLeaves: a UAV device for efficient tree canopy sampling

Tree canopy sampling is critical in many forestry-related applications, including ecophysiology, foliar nutrient diagnostics, remote sensing model development, genetic analysis, and biodiversity monitoring and conservation. Many of these applications require foliage samples that have been exposed to full sunlight. Unfortunately, current sampling techniques are severely limited in cases where site topography (e.g., rivers, cliffs, canyons) or tree height (i.e., branches located above 10 m) make it time-consuming, expensive, and possibly hazardous to collect samples. This paper reviews the recent developments related to unmanned aerial vehicle (UAV) based tree sampling and presents the DeLeaves tool, a new device that can be installed under a small UAV to efficiently sample small branches in the uppermost canopy (i.e., <25 mm stem diameter, <500 g total weight, any orientation). Four different sampling campaigns using the DeLeaves tool are presented to illustrate its real-life use in various environments. So far, the DeLeaves tool has been able to collect more than 250 samples from over 20 different species with an average sampling time of 6 min. These results demonstrate the potential of UAV-based tree sampling to greatly enhance key tasks in forestry, botany, and ecology.

Regional Quantitative Cover Mapping of Tundra Plant Functional Types in Arctic Alaska

Ecosystem maps are foundational tools that support multi-disciplinary study design and applications including wildlife habitat assessment, monitoring and Earth-system modeling. Here, we present continuous-field cover maps for tundra plant functional types (PFTs) across ~125,000 km2 of Alaska’s North Slope at 30-m resolution. To develop maps, we collected a field-based training dataset using a point-intercept sampling method at 225 plots spanning bioclimatic and geomorphic gradients. We stratified vegetation by nine PFTs (e.g., low deciduous shrub, dwarf evergreen shrub, sedge, lichen) and summarized measurements of the PFTs, open water, bare ground and litter using the cover metrics total cover (areal cover including the understory) and top cover (uppermost canopy or ground cover). We then developed 73 spectral predictors derived from Landsat satellite observations (surface reflectance composites for ~15-day periods from May–August) and five gridded environmental predictors (e.g., summer temperature, climatological snow-free date) to model cover of PFTs using the random forest data-mining algorithm. Model performance tended to be best for canopy-forming PFTs, particularly deciduous shrubs. Our assessment of predictor importance indicated that models for low-statured PFTs were improved through the use of seasonal composites from early and late in the growing season, particularly when similar PFTs were aggregated together (e.g., total deciduous shrub, herbaceous). Continuous-field maps have many advantages over traditional thematic maps, and the methods described here are well-suited to support periodic map updates in tandem with future field and Landsat observations.

Community Ecological Revaluation of Acer pseudosieboldianum and Carpinus cordata in the Natural Deciduous Forest

Classified into sub-tree of the growth-form, Acer pseudosieboldianum and Carpinus cordata hardly reach the uppermost canopy with limited height growth potentiality in the natural deciduous forest. However, the two species usually dominate lower stories of the forest, playing an important role in overall ecological characteristics in the forest. Based on the vegetation data of 106 <TEX>$20m{\times}20m$</TEX> sample plots in Mt. Jumbong area, this study was carried out to evaluate ecological constitution by several quantitative analysis so as to understand the mechanism of the natural deciduous forest. The results indicated that individuals of A. pseudosieboldianum and C. cordata were absent or few in overstory, but emerged the most dominant species in midstory and understory, providing adequate proof of the ecological importance. The comparison of indices of succession between presented and predicted values in midstory did not make much difference, suggesting that the species composition would not change much and come close to steady state in midstory and understory. The pair combination of species association noted that A. pseudosieboldianum had significant positive association with C. cordata, Quercus mongolica, and Tilia amurensis had significant positive association with A. pseudosieboldianum, A. pictum subsp. mono and Fraxinus mandshurica but negative association with F. rhynchophylla. Being compared with other major canopy tree species in the study forest, the target species of A. pseudosieboldianum and C. cordata had strong regeneration strategies, partially characterized by large number of saplings and pole sized trees and high ratio of live crown, which indicated high shade tolerance to survive in the limited amount of light under the canopy. Even though A. pseudosieboldianum and C. cordata do not reach and occupy the canopy layer mainly due to the inherent growth form, they would have highest competitive potentiality to prosper and dominate in the midstory of the natural deciduous forest.

Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;–O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; triad above and within a natural grassland canopy

Abstract. The detailed understanding of surface–atmosphere exchange fluxes of reactive trace gases is a crucial precondition for reliable modelling of processes in atmospheric chemistry. Plant canopies significantly impact the atmospheric budget of trace gases. In the past, many studies focused on taller forest canopies or crops, where the bulk plant material is concentrated in the uppermost canopy layer. However, within grasslands, a land-cover class that globally covers vast terrestrial areas, the canopy structure is fundamentally different, as the main biomass is concentrated in the lowest part of the canopy. This has obvious implications for aerodynamic in-canopy transport, and consequently also impacts on global budgets of key species in atmospheric chemistry such as nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3). This study presents for the first time a comprehensive data set of directly measured in-canopy transport times and aerodynamic resistances, chemical timescales, Damköhler numbers, trace gas and micrometeorological measurements for a natural grassland canopy (canopy height = 0.6 m). Special attention is paid to the impact of contrasting meteorological and air chemical conditions on in-canopy transport and chemical flux divergence. Our results show that the grassland canopy is decoupled throughout the day. In the lowermost canopy layer, the measured transport times are fastest during nighttime, which is due to convection during nighttime and a stable stratification during daytime in this layer. The inverse was found in the layers above. During periods of low wind speed and high NOx (NO+NO2) levels, the effect of canopy decoupling on trace gas transport was found to be especially distinct. The aerodynamic resistance in the lowermost canopy layer (0.04–0.2 m) was around 1000 s m−1, which is as high as values determined previously for the lowest metre of an Amazonian rain forest canopy. The aerodynamic resistance representing the bulk canopy was found to be more than 3–4 times higher than in forests. Calculated Damköhler numbers (ratio of transport and chemical timescales) suggest a strong flux divergence for the NO–NO2–O3 triad within the canopy during daytime. During that time, the timescale of NO2 uptake by plants ranged from 90 to 160 s and was the fastest relevant timescale, i.e. faster than the reaction of NO and O3. Thus, our results reveal that grassland canopies of similar structure exhibit a strong potential to retain soil-emitted NO due to oxidation and subsequent uptake of NO2 by plants. Furthermore, photo-chemical O3 production was observed above the canopy, which was attributed to a deviation from the NO–NO2–O3 photostationary state by a surplus of NO2 due to oxidation of NO, by e.g. peroxy radicals. The O3 production was one order of magnitude higher during high NOx than during low NOx periods and resulted in an underestimation of the O3 deposition flux measured with the EC method.

Biochemical and molecular characteristics of leaf photosynthesis and relative seed yield of two contrasting rice cultivars in response to elevated [CO₂

Understanding the basis for intraspecific yield variability may be important in elucidating biological mechanisms that are associated with superior yield performance in response to projected increases in carbon dioxide concentration, [CO₂]. Using a free-air CO₂ enrichment (FACE) facility, two rice lines, S63 and W14, which differed consistently in their enhancement of seed yield when grown at elevated [CO₂] in multiple field trials, were examined. To determine if the different cultivar responses were linked to changes in photosynthetic characteristics at elevated [CO₂], spatial and temporal changes in photosynthetic stimulation and the occurrence of down-regulation, or acclimation, in relation to panicle sink development were quantified for the uppermost canopy leaves. Changes in photosynthetic capacity were determined by quantifying changes in the sink:source ratio, leaf nitrogen (N) content, the concentration and mRNA expression of the large Rubisco subunit, and changes in V c,max, the maximum ribulose bisphosphate (RuBP)-saturated rate of carboxylation. For the W14 cultivar, significant reductions in photosynthesis at the elevated, relative to ambient [CO₂], signalling photosynthetic acclimation, were observed following panicle initiation. The observance of photosynthetic acclimation was consistent with significant reductions in N, Rubisco content and expression, and V c,max. In contrast, for the cultivar S63, elevated [CO₂] resulted in increased spikelet number and grain weight, increased sink:source ratios, and continued stimulation of photosynthesis up to grain maturity. Overall, these data suggest that the greater response of the S63 line to elevated [CO₂] may be associated with enhanced carbon sinks relative to sources, and the ability to maintain photosynthetic capacity during grain development.

The effect of spectral and spatial degradation of hyperspectral imagery for the Sclerophyll tree species classification

Hyperspectral imaging can be a useful remote-sensing technology for classifying tree species. Prior to the image classification stage, effective mapping endeavours must first identify the optimal spectral and spatial resolutions for discriminating the species of interest. Such a procedure may contribute to improving the classification accuracy, as well as the image acquisition planning. In this work, we address the effect of degrading the original bandwidth and pixel size of a hyperspectral and hyperspatial image for the classification of Sclerophyll forest tree species. A HySpex-VNIR 1600 airborne-based hyperspectral image with submetric spatial resolution was acquired in December 2009 for a native forest located in the foothills of the Andes of central Chile. The main tree species of this forest were then sampled in the field between January and February 2010. The original image spectral and spatial resolutions (160 bands with a width of 3.7 nm and pixel sizes of 0.3 m) were systematically degraded by resampling using a Gaussian model and a nearest neighbour method, respectively (until reaching 39 bands with a width of 14.8 nm and pixel sizes of 2.4 m). As a result, 12 images with different spectral and spatial resolution combinations were created. Subsequently, these images were noise-reduced using the minimum noise fraction procedure and 12 additional images were created. Statistical class separabilities from the spectral divergence measure and an assessment of classification accuracy of two supervised hyperspectral classifiers (spectral angle mapper (SAM) and spectral information divergence (SID)) were applied for each of the 24 images. The best overall and per-class classification accuracies (>80%) were observed when the SAM classifier was applied on the noise-reduced reflectance image at its original spectral and spatial resolutions. This result indicates that pixels somewhat smaller than the tree canopy diameters were the most appropriate to represent the spatial variability of the tree species of interest. On the other hand, it suggests that noise-reduced bands derived from the full image spectral resolution rendered the best discrimination of the spectral properties of the tree species of interest. Meanwhile, the better performance of SAM over SID may result from the ability of the former to classify tree species regardless of the illumination differences in the image. This technical approach can be particularly useful in native forest environments, where the irregular surface of the uppermost canopy is subject to a differentiated illumination.

Separating foliar physiology from morphology reveals the relative roles of vertically structured transpiration factors within red maple crowns and limitations of larger scale models

A spatially explicit mechanistic model, MAESTRA, was used to separate key parameters affecting transpiration to provide insights into the most influential parameters for accurate predictions of within-crown and within-canopy transpiration. Once validated among Acer rubrum L. genotypes, model responses to different parameterization scenarios were scaled up to stand transpiration (expressed per unit leaf area) to assess how transpiration might be affected by the spatial distribution of foliage properties. For example, when physiological differences were accounted for, differences in leaf width among A. rubrum L. genotypes resulted in a 25% difference in transpiration. An in silico within-canopy sensitivity analysis was conducted over the range of genotype parameter variation observed and under different climate forcing conditions. The analysis revealed that seven of 16 leaf traits had a ≥5% impact on transpiration predictions. Under sparse foliage conditions, comparisons of the present findings with previous studies were in agreement that parameters such as the maximum Rubisco-limited rate of photosynthesis can explain ∼20% of the variability in predicted transpiration. However, the spatial analysis shows how such parameters can decrease or change in importance below the uppermost canopy layer. Alternatively, model sensitivity to leaf width and minimum stomatal conductance was continuous along a vertical canopy depth profile. Foremost, transpiration sensitivity to an observed range of morphological and physiological parameters is examined and the spatial sensitivity of transpiration model predictions to vertical variations in microclimate and foliage density is identified to reduce the uncertainty of current transpiration predictions.

Naturally regenerated English oak (Quercus robur L.) stands on abandoned agricultural lands in Rilate valley (Piedmont Region, NW Italy)

The present study was carried out in 14 sites located in Rilate valley, a hilly area in the south-east Piedmont (NW Italy) within the Monferrato district. Sites were selected where naturally regenerated forest communities are developing on abandoned crop lands. Altitude ranges from 190 to 240 m a.s.l., while the abandonment period varied from 1 to 50 years at the time of the surveys (year 2007). The presence of younger stands (i.e., stands with age < 12 yrs) demonstrates a rapid invasion of abandoned farmlands by forest communities dominated by English oak mixed with other hardwoods, such as wild cherry and common elm. Observations conducted in older stands (i.e., stands with age ranging between 20 and 50 yrs) confirm the ability of English oak to generate well structured woodlands in the study area. In these environmental conditions previous cropping and exposure seem to play a significant influence on stand characteristics. Other noble hardwoods, such as wild cherry and common elm, are frequently associated with the oak. These stands showed a mean annual increment varying from 4 to 6 m3 ha-1 year-1 at the age of 45-50 yrs. At the present, black locust is only sporadic in the uppermost canopy layer and generally confined in the understory. The rapid colonization by Q. robur is favoured by the presence of adult oak trees in the field edges, providing abundant acorns efficiently dispersed by small rodents and birds. Oak stands observed in this study are worthy of notice in the light of the current decline of many adult oak communities in northern Italy, as well as for their potential to produce lumber and veneer logs. Therefore, these stands should be preserved avoiding coppicing or other irrational cuts. Further studies would be needed to analyze the contribution of soil characteristics to the colonization processes.

Uncertainty within satellite LiDAR estimations of vegetation and topography

This paper demonstrates the ability to identify representative ground elevation and vegetation height estimates within the Ice, Cloud and land Elevation Satellite/Geoscience Laser Altimeter System (ICESat/GLAS) waveforms for an area of mixed vegetation and varied topography. Estimating vegetation height within large-footprint Light Detection and Ranging (LiDAR) waveforms relies on the ability to estimate the uppermost canopy surface (signal beginning) and an elevation representing the ground surface, both of which are influenced by vegetation properties and topographic slope. We examined sources of uncertainty for vegetation height estimation from ICESat/GLAS data using airborne LiDAR data, field measurements and the FLIGHT radiative transfer model. In comparison with an independent 10-m resolution digital terrain model (DTM), a method using Gaussian decomposition of the satellite waveform produced a mean bias of −0.10 m when estimating ground elevation. A second method of estimating vegetation height using waveform extent and a terrain index effectively removed slope as an error source but produced a greater ground surface offset (−0.83 m). The two methods of estimating vegetation height compared well with airborne LiDAR estimates (correlation coefficient (R 2) = 0.68, root mean square error (RMSE) = 4.4 m and R 2 = 0.61, RMSE = 4.9 m, respectively). However, the complex interplay of the structural and optical properties of the intercepted vegetation and slope requires further understanding. A tool such as FLIGHT provides a useful means to explore the sensitivity of the waveform to both vegetation properties and topographic slope.

Foliage profiles of individual trees determine competition, self-thinning, biomass and NPP of a Cryptomeria japonica forest stand: A simulation study based on a stand-scale process-based forest model

A simulation study was carried out to investigate simultaneously the effects of eco-physiological parameters on competitive asymmetry, self-thinning, stand biomass and NPP in a temperate forest using an atmosphere–vegetation dynamics interactive model (MINoSGI). In this study, we selected three eco-physiological relevant parameters as foliage profiles (i.e. vertical distribution of leaf area density) of individual trees (distribution pattern is described by the parameter η), biomass allocation pattern in individual tree growth ( χ) and the maximum carboxylation velocity ( V max). The position of the maximal leaf area density shifts upward in the canopy with increasing η. For scenarios with η < 4 (foliage concentrated in the lowest canopy layer) or η > 12 (foliage concentrated in the uppermost canopy layer), a low degree of competitive asymmetry was produced. These scenarios resulted in the survival of subordinate trees due to a brighter lower canopy environment when η < 4 or the generation of spatially separated foliage profiles between dominant and subordinate trees when η > 12. In contrast, competition between trees was most asymmetric when 4 ≤ η ≤ 12 (vertically widespread foliage profile in the canopy), especially when η = 8. In such cases, vertically widespread foliage of dominant trees lowered the opportunity of light acquisition for subordinate trees and reduced their carbon gain. The resulting reduction in carbon gain of subordinate trees yielded a higher degree of competitive asymmetry and ultimately higher mortality of subordinate trees. It was also shown that 4 ≤ η ≤ 12 generated higher self-thinning speed, smaller accumulated NPP, litter-fall and potential stand biomass as compared with the scenarios with η < 4 or η > 12. In contrast, our simulation revealed small effects of χ or V max on the above-mentioned variables as compared with those of η. In particular, it is notable that greater V max would not produce greater potential stand biomass and accumulated NPP although it has been thought that physiological parameters relevant to photosynthesis such as V max influence dynamic changes in forest stand biomass and NPP (e.g. the greater the V max, the greater the NPP). Overall, it is suggested that foliage profiles rather than biomass allocation or maximum carboxylation velocity greatly govern forest dynamics, stand biomass, NPP and litter-fall.

Vertical patterns and duration of surface wetness in an old-growth tropical montane forest, Indonesia

In tropical montane forests, the wetness of leaf surfaces is an important parameter which may influence gas exchange, growth and vitality of leaves, and forest productivity. Thirty surface wetness sensors were operated during May–August 2004 in a vertical profile inside an old-growth lower montane rain forest of Central Sulawesi, Indonesia, with the objective to analyse spatial and temporal patterns of surface wetness and to relate wetness duration to the microclimate inside the stand. The canopy was wet during 25–30% of time in this study period. In a dry period, however, surface wetness lasted for only 5% of the time, whereas the canopy was wet during 45–55% of the time in a rainy period. In the lower shade canopy, surface wetness continuously existed for periods of up to 22 h and more, although rainfall occurred only during afternoon thunderstorms of limited duration. The long duration of surface wetness has implications for forest interception models, which assume a complete drying of the canopy between subsequent rainfall events. In periods with rainfall, leaf wetness typically occurred in the afternoon, evening and first half of the night because intercepted water persisted on the leaves until about midnight. In dry periods, in contrast, surface wetness was mainly caused by dewfall in the second half of the night, and it occurred mainly in the uppermost canopy where radiative heat losses resulted in a substantial under-cooling of the leaves. Ecophysiological and hydrological importance is suggested by the long duration of surface wetting in this stand with possible implications for gas exchange, leaf growth, leaf colonization by epiphylls and the forest water balance.

Evaluation of SHAW Model in Simulating Energy Balance, Leaf Temperature, and Micrometeorological Variables within a Maize Canopy

Understanding and simulating plant canopy conditions can assist in better acknowledgment of plant microclimate characteristics, its effect on plant processes, and the influence of management and climate scenarios. The ability of the Simultaneous Heat and Water (SHAW) model to simulate the surface energy balance and profiles of leaf temperature and micrometeorological variables within a maize canopy and the underlying soil temperatures was tested using data collected during 1999 and 2003 at Yucheng, in the North China Plain. The SHAW model simulates the near‐surface heat and water movement driven by input meteorological variables and observed plant characteristic (leaf area index [LAI], height, and rooting depth). For 1999, the model accurately simulated air temperature and relative humidity in the upper one‐third of the canopy, but overpredicted midday temperature in the lower canopy. For 2003, although the surface energy balance was simulated quite well, radiometric canopy surface temperature and midday leaf temperature in the upper portion of the canopy were overpredicted, by approximately 5°C. Model efficiency (the fraction of variation in observed values explained by the model) for leaf temperature in the lower two‐thirds of the canopy ranged from 0.82 to 0.90, but fell to 0.38 for the uppermost canopy layer. Weaknesses in the model were identified and potentially include: the use of K‐theory to simulate turbulent transfer within the canopy; and simplifying assumptions with regard to long‐wave radiation transfer within the canopy. Model modifications are planned to address these weaknesses.

Seasonal changes in apparent hydraulic conductance and their implications for water use of European beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Matt.) Liebl] in South Europe

The water status of Fagus sylvatica L. and Quercus petraea (Matt) Liebl. was analysed during a cycle of progressive natural drought in southern Europe. Predawn (Ψpd) and midday water potential were measured in transpiring (Ψleaf) and non-transpiring leaves (Ψxyl). Furthermore, photosynthesis (A), stomatal conductance to water vapour (gs) and sap flow (Fd) were recorded on the same dates. Apparent leaf specific hydraulic conductance in the soil–plant–air continuum (Kh) and whole tree hydraulic conductance (Khsf) were calculated by using the simple analogy of the Ohm’s law. Kh was estimated at different points in the pathway as the ratio between transpiration (E) in the uppermost canopy leaves at midday and the gradient of water potential in the different compartments of the continuum soil–roots–stem–branches–leaves. There was a progressive decrease in water potential measured on non-transpiring leaves at the base of tree crown in both species (Ψlxyl) from the beginning of the growing season to the end of summer. A similar decrease was shown in shoot water potential (Ψuxyl) at the uppermost canopy. Predawn water potential (Ψpd) was high in both species until late July (28 July); afterwards, a significant decrease was registered in F. sylvatica and Q. petraea with minimum values of −0.81±0.03 and −0.75±0.06 MPa, respectively, by 15 September. In both species, leaf specific hydraulic conductance in the overall continuum soil–plant–air (Kh) decreased progressively as water stress increases. Minimum values of Kh and Khsf were recorded when Ψpd was lower. However, Q. petraea showed higher Kh than F. sylvatica for the same Ψpd. The decrease in Kh with water stress was mainly linked to its fall from the soil to the lowermost canopy (Ksrs). Nevertheless, a significant resistance in the petiole–leaf lamina (Kpl) was also recorded because significant differences in all dates were found on Ψ between transpiring and non-transpiring leaves from the same shoot. The decline in Kh was followed by an increase in stomatal control of daily water losses through the decrease of stomatal conductance to water vapour (gs) during the day. It promoted a seasonal increase in the stomatal limitation to carbon dioxide uptake for photosynthesis (A). These facts were more relevant in F. sylvatica, which had concurrently a higher decline in water use at the tree level than Q. petraea. The results showed a strong coupling in F. sylvatica and Q. petraea between processes at leaf and tree level. It may be hypothesised a role of specific hydraulic conductance not only in the regulation of water losses by transpiration but also of carbon uptake.

Dynamics of Norway spruce and silver fir natural regeneration in a mixed stand under uneven-aged management

In a 0.75-ha plot in a Norway spruce (Picea abies Karst.) – silver fir (Abies alba Miller) stand in Comelico (Italian eastern Alps), we analysed (i) the distribution and growth of natural regeneration of Norway spruce and silver fir as affected by stand structure and (ii) the age structure of all saplings between 0.2 and 10 m in height in a 30-year-old gap. In both species, most natural regeneration was clumped and located at the margin of the gaps; however, fir saplings were more represented in understorey environments and less represented in gaps as compared with spruce. Age structure of natural regeneration in the selected gap revealed that the majority (75%) of saplings appeared after the formation of the gap; however, for regeneration taller than 2 m (which has a better chance of reaching the uppermost canopy layer), saplings already present at gap formation predominated. We conclude that (i) gap edges represent a preferential regeneration niche in this forest and (ii) saplings established before gap formation can play an important role in gap refilling. These results provide useful information to ensure, through silvicultural practices, favourable conditions for the temporal and spatial continuity of the regeneration process.

Automated foliage chamber method for long-term measurement of CO2 flux in the uppermost canopy

Automated foliage chamber method for long-term measurement of CO2 flux in the uppermost canopy

Automated foliage chamber method for long-term measurement of CO2 flux in the uppermost canopy

Direct monitoring of CO2 exchange in forest ecosystems is necessary for evaluation of the annual net CO2 flux. We developed an automated foliage chamber system for long-term in situ measurement of the CO2 flux in foliage. The chamber closes automatically for 5 min in every 30. We found a high correlation between the photosynthesis rates measured by this system and by a portable H2O/CO2 analyzer (LCA-3, ADC). We used the new system to continuously monitor the CO2 flux in foliage of Quercus serrata. There was a positive correlation between CO2 flux and short-wave radiation. The daily photosynthesis rate varied widely depending on the environmental conditions. It increased from May to July and decreased temporarily in August. The seasonal pattern of photosynthesis was almost the same as that measured by the eddy covariance method. These results suggest that the new system is effective for tracking long-term seasonal change of the CO2 flux in foliage.

Leaf Gas Exchange Characteristics in Relation to Leaf Canopy Position of Myrica faya in its Native Environment (Tenerife, Canary Islands)

Abstract: Diurnal courses of gas exchange were measured throughout one year in fully expanded current‐year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) of Myrica faya Ait., a dominant component of the Canarian laurel forest in Tenerife, Canary Islands, Spain. M. faya showed large differences between sun and shade leaves in gas exchange characteristics (about 50 % of maximum carbon assimilation rate (Amax) reduction in shade leaves, but this reduction can be higher on specific days) that were modulated by strong light attenuation and high leaf area index (LAI) of the stand. This species presented low Amax, about 10 μmol m‐2 s‐1, high maximum transpiration (E, 8 mmol m‐2 s‐1) and stomatal conductance (gs, 750 mmol m‐2 s‐1) and very low instantaneous water use efficiency (WUE, mean maximum 1.1 mmol mol‐1) and A/gs (mean maximum 23.5 μmol mol‐1). M. faya responded to high air vapour pressure deficit (VPD), decreasing its gs but maintaining relatively high values of A and E during the studied period. Stomatal response to VPD showed a higher sensitivity than its congeners, M. cerifera, and Laurus azorica, tree species co‐occurring in the Canarian laurel forest. In general, all these gas exchange characteristics lead us to consider this species more similar to subtropical plants of humid regions than to species of the Mediterranean region.

Leaf gas exchange characteristics of a Canarian laurel forest tree species [ Persea indica (L.) K. Spreng.] under natural conditions

Persea indica (L.) K. Spreng. is a tree species of the Canarian laurel forest. Diurnal courses of gas exchange were measured during the period of one year in fully expanded current-year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) in order to understand the general behaviour of this tree species in relation with the environmental conditions of its habitat. Persea indica showed a great plasticity in gas exchange characteristics with large differences between sun and shade leaves (50 percnt; of maximum carbon assimilation rate (Amax) reduction in the shade) that were modulated by the strong light attenuation and high leaf area index (LAI) of the stand. This species presented low Amax, about 11 μmol m−2 s−1, high maximum transpiration (E, above 8 mmol m−2 s−1) and high stomatal conductance (gs, 800 mmol m−2 s−1) and therefore very low instantaneous water use efficiency (WUE, mean maximum 1.5 mmol mol−1) and A/gs (mean maximum between 20-35 μmol mol−1) and high mean minimum Ci value (around 237 μmol mol−1), measured in sun leaves. All these gas exchange characteristics fit well with a non-conservative strategy of water use efficiency, which is appropriate for species that are subject to mild or short duration droughts. Overall, this species is more similar to subtropical plants of humid regions than to sclerophyllous plants of Mediterranean-type climate. However, under strong sunlight and dry conditions, sun leaves of P. indica show midday stomatal closure and downregulation of photochemical efficiency, effective ways to avoid excess water loss and photodamage of photosynthetic apparatus.