23/12/2017 · Canopy Photosynthesis: ..

Canopy photosynthesis and water use efficiency in a deciduous forest ..

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Plant and soil/bedrock samples were collected simultaneously within each seral stage in April 2007. We selected six tree species representing contrasting leaf phenology (evergreen vs. drought-deciduous) for isotopic analyses (Table ). These species were chosen because they are abundant across the site and are found throughout the chronosequence. The reason that all three drought-deciduous species were from a single family is because the majority of the drought-deciduous species at our site belong to the Fabaceae family. The limitation to this sampling scheme is that physiological variation between species with evergreen versus drought-deciduous phenology could be the result of divergences deep in the phylogenetic tree. However, this would largely reflect the variation of phenological strategies at our site because of the preponderance of legumes. Within each seral stage, measurements were made on three representative trees for each species giving us a total of 18 trees per seral stage. The diameter at breast height (DBH) was measured for all trees. During the peak of the dry season, we measured midday (1200–1400 hours) bulk leaf water potentials on leafy twigs (ΨL) using a Scholander-type pressure chamber (model 1000; Plant Moisture Stress, Corvallis, OR, USA). Measurements of ΨL were made over three consecutive days, with measurements from trees in each seral stage taken each day to reduce any bias associated with sampling dates. For stable isotope analyses of xylem water, suberized twigs approximately 10 mm in diameter and 50–80 mm long were cut from individual trees. Stem samples were collected at 150 cm height or from the lower branches in taller trees to minimize any bias based on location within the canopy. Clipped twigs were immediately placed in a capped vial, wrapped in parafilm and stored in the freezer until water extraction for stable isotope analyses.

Photosynthetic and Water Use Efficiency Responses to Diffuse Radiation by an Aspen-Dominated Northern Hardwood Forest

Water, Nitrogen, Rising Atmospheric ..

Drought-deciduous trees in the early-seral stage had the greatest WUE, as indicated by the highest foliar δ13C values, and the lowest stomatal conductance, as indicated by the highest 18Ol values, among all seral stages (Fig. c, d). In contrast, drought-deciduous trees in the late-seral stage had the lowest WUE, as indicated by the lowest foliar δ13C, and the highest stomatal conductance, as indicated by the lowest 18Ol values (Fig. c, d). Drought-deciduous trees in the mid-seral stage had stomatal conductance and WUE values that fell inbetween those found in the early- and late-seral stages. Furthermore, when combining drought-deciduous trees for all seral stages, we found a positive correlation between foliar δ13C and 18Ol (Fig. ), suggesting that greater WUE was the result of reduced stomatal conductance. According to the conceptual model developed by Scheidegger et al. (), high foliar δ13C values for drought-deciduous trees, in combination with higher tissue δ18O values, can be interpreted as evidence for enhanced water stress and stomatal limitation to photosynthesis (Sternberg et al. ). It is important to note that several factors other than leaf level gas exchange, especially variation in mesophyll conductance and mean VPD, could contribute to such isotopic variability (Seibt et al. ). However, in this study, it appeared that for drought-deciduous trees stomatal regulation is an important strategy for minimizing evaporative water loss during the dry season.

Does day and night sampling reduce spurious …

As a result of decreased soil water availability, a number of trees in SDTF lose a large portion of their leaves during the dry season (drought-deciduous) (Murphy and Lugo ). Previous studies demonstrated that deciduous species minimize water loss during rainless periods and therefore dominate drier forests along precipitation gradients in the lowland tropics (Murphy and Lugo ; Santiago et al. ). Numerous studies have also shown a high degree of vertical partitioning of soil water resources among tropical forest tree species (Jackson et al. , ; Meinzer et al. ; Querejeta et al. ). The degree of soil water partitioning as well as variation in timing of leaf production has therefore been hypothesized to decrease competition and promote tree species coexistence in SDTF. However, the majority of these studies have focused on tree species in mature forests. Early- and late-seral trees experience different abiotic and biotic environments that can differentially alter resource acquisition, growth and survival (Brown and Lugo ; Allen et al. ). Consequently, there is a critical need to better understand how trees in different seral stages overcome seasonal water limitations in SDTF.

We hypothesized that spurious correlation among canopy photosynthesis ..


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The evaluated parameters were: foliar photosynthetic rate (A) and the water-use efficiency (A/gs). The measures were taken on a foliar area of 6cm², the chamber's air flow had a CO2 concentration of 380 µmol mol-1 . The air was collected through outside of the gully and then transported inside a protected container and then pumped to the chamber. A photosynthetic photon flux density (PFFD) of 800 µmol.m-2.s-1was used from an artificial light source (LI-6400-02B RedBlue LED, Li-Cor). The temperature was kept at 28ºC. The experimental designs were entirely randomized with 3 treatments (area of the edge, slope and the gully's bottom) with twelve repetitions. Averages and standart errors were done for data analysis.

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Analysis with mathematical models of crop physiology and biophysics have identified two strategies to increase WUE. First, by shifting a greater fraction of photosynthetic activity in the crop canopy from upper to lower leaves that operate in higher humidity air. This can be done by allowing greater radiation penetration into the canopy with upper canopy leaves that are lighter green or in a more erect position. Second, by decreasing the number of stomatal pores on the leaves, thus increasing resistance to water loss from the leaf to the atmosphere. Developing these new biotechnologies is timely because it leverages new technologies to assess and analyze the relationship between crop genes and physiological performance in a way that has not previously been possible.