Diurnal changes in leaf gas exchange characteristics in the uppermost canopy of a rain forest tree, Dryobalanops aromatica Gaertn. f

Photosynthetic gas exchange responses of Swietenia macrophylla King and Melia azedarach L. plantations under drought conditions

BACKGROUND

The environmental stresses caused by climate change have become more severe in recent decades, affecting tree growth and physiology. Tropical forests have great potential for global carbon sequestration. However, they suffer from heavy rainfall and prolonged dry periods due to climate change. Swietenia macrophylla King and Melia azedarach L. are economically valuable trees that are widely planted in southern Taiwan. Plantations are exposed to either prolonged dry periods or heavy rainfall within the seasons of tropical monsoon areas. Photo-physiological comparisons may provide information that can improve management of S. macrophylla and M. azedarach plantations in tropical regions.

RESULTS

Both species exhibited a midday depression in leaf photosynthesis regardless of the season. The net photosynthetic rate (P _N), stomatal conductance (g _s), and transpiration rate (E) in the dry season all significantly decreased in both tree species. In addition, M. azedarach used water more efficiently than did S. macrophylla during the dry season, but S. macrophylla had higher P _N compared with that in M. azedarach during the wet season. Temperature and vapor pressure deficit influenced P _N variation in S. macrophylla and M. azedarach, respectively. Our data suggested that the P _N and g _s of M. azedarach, but not of S. macrophylla, were linearly correlated during the dry season. The reduction of the leaf area was more sever in M. azedarach than in S. macrophylla, thus preventing water loss more efficiently.

CONCLUSIONS

M. azedarach adapted to drought by reducing total leaf area and maintaining higher P _N, g _s, E, and WUE compared with those measured in S. macrophylla during the dry season. M. azedarach is more drought adaptation and more suitable for both humid and semi-humid areas than S. macrophylla, whereas the latter should be limited to more humid areas.

Ecological distribution of leaf stomata and trichomes among tree species in a Malaysian lowland tropical rain forest

The vertical structure of a tropical rain forest is complex and multilayered, with strong variation of micro-environment with height up to the canopy. We investigated the relation between morphological traits of leaf surfaces and tree ecological characteristics in a Malaysian tropical rain forest. The shapes and densities of stomata and trichomes on the abaxial leaf surfaces and their relation with leaf characteristics such as leaf area and leaf mass per area (LMA) were studied in 136 tree species in 35 families with different growth forms in the tropical moist forest. Leaf physiological properties were also measured in 50 canopy and emergent species. Most tree species had flat type (40.4 %) or mound type (39.7 %) stomata. In addition, 84 species (61.76 %) in 22 families had trichomes, including those with glandular (17.65 %) and non-glandular trichomes (44.11 %). Most leaf characteristics significantly varied among the growth form types: species in canopy and emergent layers and canopy gap conditions had higher stomatal density, stomatal pore index (SPI), trichome density and LMA than species in understory and subcanopy layers, though the relation of phylogenetically independent contrasts to each characteristic was not statistically significant, except for leaf stomatal density, SPI and LMA. Intrinsic water use efficiency in canopy and emergent tree species with higher trichome densities was greater than in species with lower trichome densities. These results suggest that tree species in tropical rain forests adapt to a spatial difference in their growth forms, which are considerably affected by phylogenetic context, by having different stomatal and trichome shapes and/or densities.

Height-related changes in leaf photosynthetic traits in diverse Bornean tropical rain forest trees

Knowledge of variations in morphophysiological leaf traits with forest height is essential for quantifying carbon and water fluxes from forest ecosystems. Here, we examined changes in leaf traits with forest height in diverse tree species and their role in environmental acclimation in a tropical rain forest in Borneo that does not experience dry spells. Height-related changes in leaf physiological and morphological traits [e.g., maximum photosynthetic rate (Amax), stomatal conductance (gs), dark respiration rate (Rd), carbon isotope ratio (δ(13)C), nitrogen (N) content, and leaf mass per area (LMA)] from understory to emergent trees were investigated in 104 species in 29 families. We found that many leaf area-based physiological traits (e.g., A(max-area), Rd, gs), N, δ(13)C, and LMA increased linearly with tree height, while leaf mass-based physiological traits (e.g., A(max-mass)) only increased slightly. These patterns differed from other biomes such as temperate and tropical dry forests, where trees usually show decreased photosynthetic capacity (e.g., A(max-area), A(max-mass)) with height. Increases in photosynthetic capacity, LMA, and δ(13)C are favored under bright and dry upper canopy conditions with higher photosynthetic productivity and drought tolerance, whereas lower R d and LMA may improve shade tolerance in lower canopy trees. Rapid recovery of leaf midday water potential to theoretical gravity potential during the night supports the idea that the majority of trees do not suffer from strong drought stress. Overall, leaf area-based photosynthetic traits were associated with tree height and the degree of leaf drought stress, even in diverse tropical rain forest trees.

Leaf gas exchange in the frankincense tree (Boswellia papyrifera) of African dry woodlands

A conceptual model was tested for explaining environmental and physiological effects on leaf gas exchange in the deciduous dry tropical woodland tree Boswellia papyrifera (Del.) Hochst. For this species we aimed at (i) understanding diurnal patterns in leaf gas exchange, (ii) exploring cause-effect relationships among external environment, internal physiology and leaf gas exchange, and (iii) exploring site differences in leaf gas exchange in response to environmental variables. Diurnal courses in gas exchange, underlying physiological traits and environmental variables were measured for 90 trees on consecutive days at two contrasting areas, one at high and the other at low altitude. Assimilation was highest in the morning and slightly decreased during the day. In contrast, transpiration increased from early morning to midday, mainly in response to an increasing vapor pressure deficit (VPD) and gradual stomatal closure. The leaf water potential varied relatively little and did not influence gas exchange during the measurement period. Our results suggest that the same cause-effect relationships function at contrasting areas. However, leaves at the higher altitude had higher photosynthetic capacity, reflecting acclimation to higher light levels. Trees at both areas nevertheless achieved similar leaf assimilation rates since assimilation was down-regulated by stomatal closure due to the higher VPD at the higher altitude, while it became more light limited at the lower altitude. Gas exchange was thus limited by a high VPD or low light levels during the wet season, despite the ability of the species to acclimate to different conditions.

Midday depression of leaf CO2 exchange within the crown of Dipterocarpus sublamellatus in a lowland dipterocarp forest in Peninsular Malaysia

We observed diurnal and seasonal patterns of leaf-scale gas exchange within the crown of a Dipterocarpus sublamellatus Foxw. tree growing in a lowland dipterocarp forest at Pasoh, Peninsular Malaysia. Observations were carried out nine times over 6 years, from September 2002 to December 2007. Observation periods included both wet and mild-dry periods, and natural and saturated photosynthetic photon flux density (PPFD) light conditions. In situ measurements of the diurnal change in net photosynthetic rate and in stomatal conductance were carried out on canopy leaves of a 40-m-tall D. sublamellatus tree, which was accessed from a canopy corridor. A diurnal change in electron transport rate was observed under saturated PPFD conditions. The maximum net assimilation rate was approximately 10 micromol m(-2) s(-1). There was a clear inhibition of the net assimilation rate coupled with stomatal closure after late morning and this inhibition occurred year-round. Although the electron transport rate decreased alongside this inhibition, it sometimes followed on. Numerical analysis showed that the main factor in the inhibition of the net assimilation rate was patchy bimodal stomatal closure, which occurred in both mild-dry and wet periods. The midday depression occurred year-round, though there are fluctuations in soil moisture during the mild-dry and wet periods. The magnitude of the inhibition was not related to soil water content but was related to vapor pressure deficit (VPD): that is, whether the days were sunny and hot or cloudy and cool. On cloudy, cool days in the wet period, the net photosynthesis was only moderately inhibited, but it still decreased in the afternoon and was coupled with patchy stomatal closure, even in quite moderate VPD, leaf temperature and PPFD conditions. Our results suggest that patchy stomatal closure signaled by the increase in VPD, in transpiration and by circadian rhythms, was the key factor in constraining midday leaf gas exchange of the D. sublamellatus canopy leaves.

Large-scale canopy opening causes decreased photosynthesis in the saplings of shade-tolerant conifer, Abies veitchii

Although the environmental change by canopy gap formation in a forest improves the light availability for the saplings on the forest floor, it may result in stresses on the saplings due to high radiation and drought. In large-scale gaps, the photosynthesis of shade-tolerant species may be inhibited by high radiation and drought stress if they lack effective tolerance or avoidance mechanisms for the stresses. We investigated the photosynthetic traits and water relations of Abies veitchii Lindl. saplings in an open habitat created by an avalanche and in a nearby forest floor habitat undisturbed by the avalanche. We analyzed the influence of exposed conditions on sapling photosynthesis. The maximum photosynthetic rate of the saplings in the open habitat was lower than that in the forest habitat. The ratio of variable to maximum chlorophyll fluorescence (F(v)/F(m)) was lower in the open habitat than that in the forest habitat during the late growing season, indicating that the open habitat saplings suffer photoinhibition of photosystem II for a long period. A lower Rubisco concentration in needles in the open habitat indicated the breakdown of this photosynthetic protein because of excess solar energy resulting from serious photoinhibition. The shoot water potential of the saplings in the open habitat at daytime was higher than that of the saplings in the forest habitat because of less transpiration caused by the remarkable stomatal closure in the open habitat. Although these acclimations to high radiation improve the tolerance of A. veitchii saplings to high radiation and drought stress, they would result in low gain of daily carbon and a reduction in growth in the open habitat.

Seasonal pattern of leaf production and its effects on assimilation in giant summer-green herbs in deciduous forests in northern Japan

Understory vegetation of northern deciduous forests in far eastern Asia is characterized by giant summer-green herbs. We examined the patterns of height growth, leaf accumulation, photosynthetic characteristics, daily net assimilation, and dry matter allocation within aboveground parts of six giant summer-green herbs with reference to light conditions in deciduous forests. Plant height, leaf number, and total leaf area per plant increased with progressing tree-canopy closure in five species ( Cacalia hastata L. subsp. orientalis Kitam., Cirsium kamtschaticum Ledeb., Filipendula kamtschatica (Pall.) Maxim. f. kamtschatica, Senecio cannabifolius Less., and Urtica platyphylla Wedd.) that had continuous leaf production throughout the growing season, whereas one species ( Veratrum album L. subsp. oxysepalum Hulten) with early leaf production, completed leaf production mostly before the beginning of tree-canopy closure. Maximum photosynthetic and dark respiration rates decreased seasonally in all species. Species with continuous leaf emergence accumulated leaves acclimatized to shade conditions, which offset the decreasing photosynthesis of individual leaves with progressing tree-canopy closure, resulting in stable carbon gain even under decreasing light availability. In contrast, V. album assimilated vigorously during the short period of high irradiance before tree-canopy closure, and decreased its assimilation rate continuously thereafter.

Sources or sinks? The responses of tropical forests to current and future climate and atmospheric composition

How tropical rainforests are responding to the ongoing global changes in atmospheric composition and climate is little studied and poorly understood. Although rising atmospheric carbon dioxide (CO2) could enhance forest productivity, increased temperatures and drought are likely to diminish it. The limited field data have produced conflicting views of the net impacts of these changes so far. One set of studies has seemed to point to enhanced carbon uptake; however, questions have arisen about these findings, and recent experiments with tropical forest trees indicate carbon saturation of canopy leaves and no biomass increase under enhanced CO2. Other field observations indicate decreased forest productivity and increased tree mortality in recent years of peak temperatures and drought (strong El Niño episodes). To determine current climatic responses of forests around the world tropics will require careful annual monitoring of ecosystem performance in representative forests. To develop the necessary process-level understanding of these responses will require intensified experimentation at the whole-tree and stand levels. Finally, a more complete understanding of tropical rainforest carbon cycling is needed for determining whether these ecosystems are carbon sinks or sources now, and how this status might change during the next century.

Macadamia physiology review: a canopy light response study and literature review

The management of mature macadamia orchards has evolved largely through the need to control inter-row crowding to maintain machinery access. The current study was undertaken to identify physiological constraints to production and priorities for future research and development. The two components of the study were a preliminary field study to determine the impact of current pruning and hedging management strategies on canopy photosynthetic performance and a literature review to identify physiological issues affecting orchard productivity. The field photosynthesis study demonstrated that emerging flush leaves have a negative light saturation net assimilation rate (Amax) that increases to that of mature leaves (Amax 8–10 µmol CO2/m2.s) over 28 days. Leaf age has no effect on Amax of light-adapted leaves. Shade-adapted macadamia leaves cannot attain the photosynthetic capacity of light-adapted leaves. This means that a late hedging strategy to remove around 1 m of canopy from the side of trees to improve orchard access reduces the photosynthetic capacity of the orchard until sufficient flushing occurs to restore the canopy. The literature review focussed on light interception and distribution, photosynthesis, carbohydrate and nitrogen cycling, environmental response, flowering and fruit set, nut abscission, canopy management, and nutrition and disease control. Light interception modelling work, which has been widely adopted for deciduous temperate fruit crops, has immediate application to macadamia production and can explain many of the yield responses recorded by macadamias to canopy management. Macadamia yields appear to increase up to about 96% light interception. Variation in light distribution within canopies that affects the yield and quality of temperate crops is also a present problem with macadamias. It leads to uneven distribution of leaf and fruit throughout the canopy and to a heavily shaded void developing in the middle of trees. The relationship between irradiance, leaf photosynthetic capacity and longevity, flushing, flowering, and fruit set is poorly understood. No information is available on the training of young trees to improve light distribution and canopy photosynthetic efficiency. The current practice of light annual hedging of mature macadamias appears to restrict the production of fruiting wood. Earlier nut abscission can be achieved by the use of ethephon, which offers greater flexibility in the timing of hedging. Research work is required on the effects of timing, frequency and severity of hedging on fruiting wood production, flowering, and fruit set of macadamias. Information on the need to supplement mechanical hedging with manual pruning to improve light distribution throughout canopies is also required. The cyclical and highly variable nature of macadamia yields needs to be further analysed to determine whether a consistent pattern exists and whether environmental factors are an influence. A simple measure of the storage levels of the major assimilates, carbohydrates and nitrogen compounds, is unlikely to predict cyclical yield patterns because both are continually cycled within the tree and it appears that, from comprehensive studies on another evergreen fruit crop, avocado, the major supply of carbohydrate during the fruit-filling stage is from current photosynthesis. Several studies have failed to demonstrate irrigation responses by field-grown macadamias. The cyclical and variable yield of macadamias demonstrates that yield responses from field experiments cannot be expected in less than 5 years, even assuming adequate plot size, buffering, and replication. On the drier sites with light-textured soils in south-eastern Queensland, yield responses could be expected. Following the success of temperate fruit tree crop breeding programs and trends with other evergreen tree crops, macadamia breeding needs to focus on dwarfing clonal rootstocks that provide uniformity, and vigour control to improve cropping efficiency, to reduce canopy management costs and to minimise the reduction in yield following tree topping.