Canopy Crane Access System

Plant ecophysiology and phenology

All plants face a fundamental tradeoff between the uptake of carbon dioxide during photosynthesis and the concomitant loss of water vapour through transpiration. Gases move into and out of leaves passively through small pores in the leaf surface called stomates. Plants control the opening and closing of stomates; however, stomates must be open for carbon dioxide uptake from the atmosphere. The very low concentration of carbon dioxide in the atmosphere and the very high concentration of water vapour internal to leaves insure that several hundred molecules of water vapour are lost for every molecule of carbon dioxide that enters a stomatal pore. Leaves desiccate and are damaged when roots and xylem are unable to replenish the water vapour lost to transpiration. This is a real danger when rainfall and water availability are seasonal or whenever the evaporative demand of the atmosphere is excessive (typical near mid-day on sunny days for the uppermost canopy leaves). Thus, water availability controls carbon uptake in tropical forests, which in turn sets the stage for biodiversity, carbon flux, and forest-atmosphere gas exchange.

1) Water movement in tropical trees: physiological integration from leaf to canopy

Courtesy Frederick C. Meinzer & Guillermo Goldstein

Because of their large size and logistical difficulties in gaining access to their crowns, the physiology of trees has traditionally been studied over a narrow range of scale, usually limited to individual leaves or branches. However, because trees are large, integrated organisms rather than mere collections of leaves, the entire individual is often the appropriate scale for characterizing physiological behavior that determines their utilization of water. The availability of the STRI canopy cranes over the last decade has presented exciting opportunities for studying large trees as whole, integrated organisms. This approach has led to considerable progress in understanding how the fundamental process of transpiration is regulated and integrated from the leaf to the whole tree and has highlighted the often dominant role that tree size, architecture and allometry play in governing physiological behavior. Taken together, our findings have pointed to substantial functional convergence in regulation of water use among taxonomically, phylogenetically and architecturally diverse tree species (Meinzer, 2003).

Among co-occurring Panamanian canopy tree species, tree size, rather than species, is the major determinant of total daily water utilization per individual (Meinzer et al., 2001; James et al., 2002; Meinzer, 2003). On a leaf area basis, tree hydraulic architecture was found to be the major determinant of differences in stomatal regulation of transpiration among species. When stomatal conductance of four co-occurring species was normalized by the branch leaf area/sapwood area ratio, an index of potential transpirational demand in relation to water transport capacity, contrasting stomatal responses to humidity coalesced into a single relationship between conductance and evaporative demand (Meinzer et al., 1997). Consistent with this finding, a subsequent study revealed a common relationship between stomatal conductance and the leaf area-specific total hydraulic conductance of the soil/leaf pathway among five co-occurring species (Andrade et al., 1998). Moreover, divergent leaf area-based transpiration rates converged when they were normalized by the branch leaf area/sapwood area ratio. In yet another study conducted at the PNM crane, a common relationship between sapwood specific hydraulic conductivity and rates of water movement through sapwood was observed (James et al., 2003).

Utilization of water stored in stems and other organs was found to lower the effective resistance of the hydraulic pathway by transiently uncoupling canopy transpiration from water absorption by roots (Andrade et al., 1998; Goldstein et al., 1998). Total internal water storage capacity increased sharply with tree size and trees with greater storage capacity maintained maximum rates of transpiration for a substantially longer fraction of the day than trees with smaller water storage capacity (Goldstein et al., 1998). Reliance on stored water to temporarily replace transpirational losses thus appears to be an important homeostatic mechanism for maintaining photosynthetic gas exchange as hydraulic path length and potential resistance increases with tree size and canopy height. In a more recent study, several whole-tree water transport properties, including minimum branch water potential, hydraulic conductance on a sapwood area basis, and movement of a tracer injected into the sapwood were found to show species-independent scaling with sapwood capacitance, a measure of intrinsic sapwood water storage capacity (Meinzer et al., 2003).

Differences in water movement among tree species consistently disappear when re-expressed in terms of tree size or tree sapwood area. This functional convergence promises to simplify hydrological models of tropical forests. Water loss by the hundreds to thousands of tree species that often co-exist in a single tropical forest can be captured by a single relationship with tree size or sapwood area.

More info: G. Goldstein web page

See also: Meinzer, F.C., Clearwater, M.J. & Goldstein, G. 2001.
Water transport in trees: current perspectives, new insights and some controversies. Environmental and Experimental Botany 45, 239–262.

Meinzer, F.C., James, S.A. & Goldstein, G. (2004)
Dynamics of transpiration, sap flow and use of stored water in tropical forest canopy trees. Tree Physiology, 24, 901–909.

Santiago, L.S., Goldstein, G., Meinzer, F.C., Fisher, J.B., Machado, K., Woodruff, D.R. & Jones, T.H. (2004) Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees. Oecologia, 140, 543–550.

Members of F.C. Meinzer’s group using a chlorophyll fluorescence instrument to estimate photosynthetic capacity at Parque Natural Metropolitano

2) Carbon uptake: the canopy microclimate and plant growth

Courtesy Kaoru Kitajima

Light availability is the major determinant of photosynthetic productivity in the canopy. Many factors, such as leaf optical properties, arrangement and density of leaves in canopy branches, patterns of leaf production and self-shading affect the amount of light reaching lower layers of the forest. Light at the canopy surface can exceed the solar constant due to reflectance by banks of clouds. As much as 99% of the light received by the uppermost canopy surface can be absorbed or reflected over just the first 5 m of the upper canopy (Mulkey et al., 1996). Liana leaves often represent 20-40% of the canopy leaf surface (Avalos & Mulkey, 1999), and significantly modify light heterogeneity. Although the leaf optical properties of lianas and trees differ little (Avalos et al., 1999), large differences in branch architecture and seasonal leaf production affect competitive interactions for light between trees and lianas.

When water supply becomes limited, stomatal conductance (the degree of stomatal openness) and photosynthetic rates go down. Consequently, hydraulic characteristics, leaf arrangements, boundary layer characteristics and stomatal behavior affect both transpiration rates (see section on water use, above) and photosynthetic productivity on both daily and seasonal time scales (Hogan et al., 1995; Zotz & Winter, 1996). Intrinsic water use efficiency, the ratio of photosynthetic rates to stomatal conductance, is often estimated from ratios of stable carbon isotopes, but Terwillliger et al. (2001) found that leaf age can significantly bias this relationship.

Leaf photosynthetic productivity of canopy leaves, measured as in-situ rates of CO2 uptake, varies greatly among species, and among seasons within a species. Zotz et al. (1995) used the PNM crane to measure CO2 uptake rates for the tree Ficus insipida, which proved to have the highest rates of photosynthesis yet known, both in terms of maximum values under the full sun light (Amax) and total daily net values, yet observed for a wild plant. Along with their previous data from other canopy epiphytes and trees, they have also demonstrated that Amax predicts daily net photosynthetic production by individual leaves. Differences in Amax among tree species are strongly linked to their life history traits, such as successional status and leaf longevity. Mulkey et al. (1995) showed that a general negative relationship exists between photosynthetic capacity and mean leaf longevity. Further, age-related declines in Amax are also related to leaf longevity in a manner predicted by a cost-benefit theory of leaf longevity (Kitajima et al., 1997, 2002).

Seasonal changes in light and water availability significantly alter canopy photosynthetic productivity. During the rainy season, water is generally not limiting, although photosynthetic rates are often depressed in the afternoon due to stomatal closure after trees lose large amounts of water in the morning (Zotz et al. 1995). However, lower light availability caused by heavy cloud cover appears to be a greater constraint on canopy photosynthetic productivity during the rainy season (Graham et al., 2003). In contrast, the dry season is a time with high light availability and low water availability. Many tree species at PNM exhibit seasonal leaf phenotypes in a manner adaptive to these contrasting seasonal patterns of water and light availability (Kitajima et al., 1997). Leaves produced in the early rainy season have lower Amax than leaves produced immediately before the dry season. Early-wet season leaves have low photosynthetic rates, which are appropriate to the lower light availability encountered during the cloudy wet season. Pre-dry season leaves experience much greater light availability, and exploit the window of opportunity in the early dry season for high productivity when light is more abundant due to less cloud cover before soils become progressively dry. An experimental study by Graham et al. (2003) clearly demonstrated that cloud cover depresses photosynthetic productivity during the rainy season (see below). In this study, light was augmented with high intensity lamps during the rainy season to approximate light intensity expected under clear skies during the dry season. In response to this high light treatment, Luehea seemannii produced leaves with high Amax similar to the dry season phenotype, and increased vegetative growth and reproduction.

More info: K. Kitajima web page

More info: S. Mulkey web page

More info: K. Winter web page

More info: G. Zotz web page

See also: Kitajima, K., Mulkey, S.S. & Wright, S.J. (2005)
Variation in crown light utilization characteristics among tropical canopy trees. Annals of Botany, 95, 535-547.

Santiago, L.S., Goldstein, G., Meinzer, F.C., Fisher, J.B., Machado, K., Woodruff, D.R. & Jones, T.H. (2004)
Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees. Oecologia, 140, 543–550.

3) Plant phenology and leaf area seasonality

Courtesy S. Joseph Wright & Mirna Samaniego

Phenology, or the seasonal timing of growth and reproduction, remains one of the most effective mechanisms available to plants to cope with the seasonally contrasting availability of light and water that characterizes many tropical forests. The timing of leaf production by tropical trees and lianas can be predicted with great confidence from seasonal patterns of rainfall and solar irradiance and mechanisms of drought resistance. If water is readily available, most tropical tree species will produce leaves and reproduce in the season of greatest irradiance. This situation characterizes evergreen rain forests and also the rare seasonal forest where the dry season is consistently cloudy so that peak irradiance occurs in the wet season. In most seasonal forests, however, the dry season is less cloudy than the wet season and peak irradiance occurs in the dry season. Under these circumstances, disproportionately large numbers of species with adaptations such as deep roots that maintain dry-season water uptake still produce leaves and reproduce in the drier, sunnier season, whereas growth by species that lack these adaptations is largely limited to the wetter, cloudier season (Wright, 1996).

Canopy leaf area is generally assumed to be maximal throughout the rainy season in tropical forests. The PNM crane made possible the first quantitative counts of seasonal changes in leaf number for a tropical forest canopy. Counts were performed for 100 randomly chosen branches from the uppermost canopies of the seven abundant tree and liana species. Leaf area seasonality was much more complex than anticipated. Each species examined had a unique seasonal pattern of canopy leaf numbers, and leaf numbers varied two-fold during the eight-month wet season for each species. This unexpected seasonality alters individual allocation strategies, interactions between species, and forest carbon fixation. Species with similar seasonal leaf dynamics will have a greater impact on one another than will species with dissimilar seasonal leaf dynamics. To the extent that this reduces growth and increases the risk of mortality, ecological associations may arise between species pairs with dissimilar seasonal leaf dynamics. Seasonal leaf dynamics will also affect annual carbon gain and water vapour loss at the level of forest stands. Current models of forest carbon gain assume that the photosynthetic capacity of tropical deciduous forests increases seasonally with actual evapotranspiration. To the extent that seasonal changes in leaf numbers alter forest-level carbon gain, these models are wrong. Accurate models of the contribution of tropical forest to global carbon balances will incorporate accurate estimates of leaf area seasonality.

More info: S.J. Wright web page

Monitoring leaf phenology in the San Lorenzo canopy

References cited

Andrade, J. L., Meinzer, F. C., Goldstein, G., Holbrook, N. M., Cavelier, J., Jackson, P. & Silvera, K. (1998)
Regulation of water flux through trunks, branches and leaves in trees of a lowland tropical forest. Oecologia, 115, 463-471.

Avalos, G. & Mulkey, S. S. (1999)
Seasonal changes in liana cover in the upper canopy of a Neotropical dry forest. Biotropica, 31, 186-192.

Avalos, G., Mulkey, S. S. & Kitajima, K. (1999)
Leaf optical properties of trees and lianas in the outer canopy of a tropical dry forest. Biotropica, 31, 517-520.

Goldstein, G., Andrade, J. L., Meinzer, F. C., Holbrook, N. M., Cavelier, J., Jackson, P. & Celis, A. (1998)
Stem water storage and diurnal patterns of water use in tropical forest canopy trees. Plant, Cell and Environment, 21, 397-406.

Graham, E. A., Mulkey, S. S., Kitajima, K., Phillips, N. G. & Wright, S. J. (2003)
Cloud cover limits net CO2 uptake and growth of a rainforest tree during tropical rainy seasons. Proceedings of the National Academy of Sciences, 100, 572-576.

Hogan, K. P., Smith, A. P. & Samaniego, M. (1995)
Gas exchange in six tropical semi-deciduous forest canopy tree species during wet and dry seasons. Biotropica, 27, 324-333.

James, S. A., Clearwater, M. J., Meinzer, F. C. & Goldstein, G. (2002)
Heat dissipation sensors of variable length for the measurement of sap flow in trees with deep sapwood. Tree Physiology, 22, 277-283.

James, S. A., Meinzer, F. C., Goldstein, G., Woodruff, D., Jones, T., Restom, T., Mejia, M., Clearwater, M. & Campanello, P. (2003)
Axial and radial water transport and internal water storage in tropical forest canopy trees. Oecologia, 134, 37-45.

Kitajima, K., Mulkey, S. S., Samaniego, M. & Wright, S. J. (2002)
Decline of photosynthetic capacity with leaf age and position in two tropical pioneer tree species. American Journal of Botany, 89, 1925-1932.

Kitajima, K., Mulkey, S. S. & Wright, S. J. (1997)
Seasonal leaf phenotypes in the canopy of a tropical dry forest: photosynthetic characteristics and associated traits. Oecologia, 109, 490-498.

Meinzer, F. C. (2003)
Functional convergence in plant responses to the environment. Oecologia, 134, 1-11.

Meinzer, F. C., Andrade, J. L., Goldstein, G., Holbrook, N. M., Cavelier, J. & Jackson, P. (1997)
Control of transpiration from the upper canopy of a tropical forest: the role of stomatal, boundary layer and hydraulic architecture components. Plant, Cell and Environment, 20, 1242-1253.

Meinzer, F. C., Goldstein, G. & Andrade, J. L. (2001)
Regulation of water flux through tropical forest canopy trees: Do universal rules apply? Tree Physiology, 21, 19-26.

Meinzer, F. C., James, S. A., Goldstein, G. & Woodruff, D. (2003)
Whole-tree water transport scales with sapwood capacitance in tropical forest canopy trees. Plant, Cell and Environment, 26, 1147-1155.

Mulkey, S. S., Kitajima, K. & Wright, S. J. (1995)
Photosynthetic capacity and leaf longevity in the canopy of a dry tropical forest. Selbyana, 16, 169-173.

Mulkey, S. S., Kitajima, K. & Wright, S. J. (1996)
Plant physiological ecology of tropical forest canopies. Trends in Ecology and Evolution, 11, 408-412.

Terwilliger, V. J., Kitajima, K., Le Roux-Swarthout, D. J., Mulkey, S. S. & Wright, S. J. (2001)
Intrinsic water-use efficiency and heterotrophic investment in tropical leaf growth of two Neotropical pioneer tree species as estimated from alpha13C values. New Phytologist, 152, 267-281.

Wright, S. J. (1996).
Phenological responses to seasonality in tropical forest plants. Tropical Forest Plant Ecophysiology. S. S. Mulkey, R. L. Chazdon and A. P. Smith. New York, Chapman and Hall: 440-460.

Zotz, G., Königer, M., Harris, G. & Winter, K. (1995)
High rates of photosynthesis in the tropical pioneer tree, Ficus insipida, Willd. Flora, 190, 265-272.

Zotz, G. & Winter, K. (1996).
Diel patterns of carbon dioxide exchange in rainforest canopy plants. Tropical Forest Plant Ecophysiology. S. S. Mulkey, R. L. Chazdon and A. P. Smith. New York, Chapman & Hall: 89-113.

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