Canopy Crane Access System

Biotic responses to a changing environment

1) Elevated CO2

Courtesy Klaus Winter

Mature trees represent most of forest carbon. Understanding how these mature trees respond to elevated CO2 with respect to their rate of photosynthetic carbon gain is therefore important. Nonetheless, few studies have attempted to investigate the effects of elevated CO2 in mature forest stands because of the difficulties involved in exposing whole trees to such concentrations and also accessing the canopy. Hence, the effects of atmospheric CO2 enrichment on mature trees in their natural environment are largely unknown. Körner and Würth (1996) experimented with a new and inexpensive technique which can be used in situ to address some key physiological questions related to the CO2 problem. Small, light-weight cups mounted on the lower side of rigid leaves at the top of tall trees at PNM were supplied with CO2-enriched air derived from a low-technology air mixing device utilizing forest floor CO2.

The accumulation of non-structural leaf carbohydrates is one of the most consistent plant responses to elevated CO2. It has been found in both fast-and slow-growing plants and is largely independent of the duration of exposure. Changes in leaf quality are thus to be expected, irrespective of other plant responses to atmospheric CO2 enrichment. However, there is no experimental evidence from tropical forests, the biome with the largest biomass carbon pool. We reported in situ responses of mature tropical trees to a doubling of CO2. Individually CO2-enriched leaves on 25 to 35m tall forest trees living at 26-35°C can be assumed to experience little sink limitation, and so, may be expected to exhibit no or very little carbohydrate accumulation. We tested this hypothesis using the leaf cup method on leaves accessible via the PNM crane. We also investigated the influence of the leaf-specific light regime, another possible environmental determinant of leaf carbon gain and mobile leaf carbohydrates. Total non-structural carbohydrates (TNC) reached a new steady state concentration after less than 4 days of exposure to twice ambient CO2 concentration. Against expectation, all four tree species investigated (Anacardium excelsum, Cecropia longipes, C. peltata, Ficus insipida) accumulated significant amounts of TNC (+ 41 to + 61%) under elevated CO2. The effect was stronger at the end of the daylight period (except for Ficus), but was still significant in all four species at the end of the dark period. In contrast, neither artificial nor natural shading affected leaf TNC. Taken together, these observations suggest that TNC accumulation reflects a tissue response specific to elevated CO2, presumably unrelated to sink limitations. Thus, leaves of tropical forests seem not to be an exception, and will most likely contain more non-structural carbohydrates in a CO2- rich world (Würth et al., 1998).

Further, small open-top chambers were used to enclose branchlets that were at a height of between 20 and 25 m in the canopy of the tree species Luehea seemannii at PNM. Elevated concentrations of CO2 increased the rate of photosynthetic carbon fixation and decreased stomatal conductance of leaves, but did not influence the growth of leaf area per chamber, the production of flower buds and fruit nor the concentration of non- structural carbohydrates within leaves. The production of flower buds was highly correlated with the leaf area produced in the second flush of leaves, indicating that the branchlets of mature trees of Luehea seemannii are autonomous to a considerable extent. Elevated levels of CO2 did increase the concentration of nonstructural carbohydrates in woody stem tissue. Elevated CO2 concentration also increased the ratio of leaf area to total biomass of branchlets, and tended to reduce individual fruit weight. These data suggest that the biomass allocation patterns of mature trees may change under future elevated levels of CO2. Although there were no effects on growth during the experiment, the possibility of increased growth in the season following CO2 enrichment due to increased carbohydrate concentrations in woody tissue cannot be excluded (Lovelock et al., 1999).

More info: C. Körner web site and the Swiss canopy crane

See also: Würth, M.K., Pelaez-Riedl, S., Wright, S.J. & Körner, C. 2005.
Non-structural carbohydrate pools in a tropical forest. Oecologia, 143, 11-24.

Pennisi, E. 2005. Sky-high experiments. Science, 309, 1314-1315.

Körner, C., Asshoff, R., Bignucolo, O., Hättenschwiler, S., Keel, S.G., Peláez-Riedl, S., Pepin, S., Siegwolf, R.T.V. & Zotz, G. Carbon Flux and Growth in Mature Deciduous Forest Trees Exposed to Elevated CO2. Science, 309, 1360-1362.

C. Lovelock & A. Virgo monitoring CO2 levels in small open-top chambers at Parque Natural Metropolitano

2) Cloud cover and photosynthesis

Courtesy Eric A. Graham

Global dimming refers to an observed reduction of 2.7% of solar radiation reaching the Earth’s surface recorded each decade since the 1950s (Stanhill & Cohen, 2001). Heavy tropical cloud cover can also reduce radiation reaching fully exposed canopy leaves by 90% or more and limit their carbon uptake through photosynthesis (Mulkey et al., 1996). The implications for global carbon uptake are significant if carbon uptake by tropical trees is limited by year-to-year variation and decadal scale changes in cloud cover and irradiance. As an experimental test of light limitation by cloud cover during tropical rainy seasons and by the unusually heavy cloud cover associated with the 1998-99 La Niña event, we installed high-intensity lamps above the PNM forest canopy during 1998-2000 (Graham et al., 2003). We supplemented light levels artificially whenever cloud cover reduced photosynthetic photon flux density (which refers to those wavelengths of light involved in photosynthesis or 400-700 nm, PPFD) for two replicate adult Luehea seemannii. Supplemental illumination only compensated partially for natural reductions in PPFD. We studied the response to augmented light of leaf-level photosynthesis, branch-level sap flow, leaf- and branch-level carbohydrate storage, branch extension growth, and fruit production.

During a representative cloudy day, photosynthesis increased with augmented light from the lamps for randomly chosen leaves. Daily net carbon gain increased by an average of 18.4% (from 270.6 to 320.4 mmol m-2 d-1) for exposed canopy leaves during the La Niña event. In addition, fully sun-exposed canopy leaves acclimated to the augmented illumination through an increase in photosynthetic potential. Branch extension growth, the number of new nodes, and the number of reproductive buds were all greatest on illuminated branches, intermediate for non-illuminated branches on illuminated trees, and least for control trees. The observed acclimation of photosynthetic potentials suggests that this widespread tropical tree species responds physiologically to seasonal and interannual variation of solar irradiance. Year-to-year climate variability may thus influence net CO2 uptake and growth through variation in solar irradiance caused by clouds and atmospheric particulates (Roderick et al., 2001) coupled with the photosynthetic flexibility of tropical canopy species.

See also: Gamon, J.A., Kitajima, K., Mulkey, S.S., Serrano, L. & Wright, S.J. (2005)
Diverse optical and photosynthetic properties in a Neotropical dry forest during the dry season: Implications for remote estimation of photosynthesis. Biotropica, 37, 547-560.

3) Isoprene emission

Courtesy Manuel T. Lerdau

Isoprene, a 5-carbon compound produced by plants during the daytime, is the single most abundant reactive hydrocarbon in the lower atmosphere (Lerdau et al., 1997). Unlike the reactive oxides of nitrogen, which are produced primarily through fossil fuel combustion and soil microbial activity, isoprene is produced only by plants and is a primary way in which biological processes influence the chemistry of the lower atmosphere. Isoprene may react with these oxides of nitrogen to produce ozone in the lower atmosphere, where ozone is an important pollutant. Isoprene may, alternatively, react with other oxidizing chemicals in the atmosphere to remove ozone. In addition, the oxidation of isoprene in the atmosphere can increase the atmospheric lifetime (and hence strength as a greenhouse gas) of methane. Finally, isoprene emission from forests can equal the net storage of carbon by those forests and thus determine whether or not forests act as net sources or sinks of carbon.

Global models of isoprene emissions are needed in order to understand both atmospheric chemistry and carbon storage in ecosystems. To date these models have been based on studies of plants in temperate zones. It has been known for over twenty years, however, that tropical forests are the source of over 70% of the world’s isoprene. Starting in 1995 researchers from the State University of New York at Stony Brook and the International Institute of Tropical Forestry in Puerto Rico have been studying isoprene emission at both the SL and PNM cranes. These studies have focused on both identifying and quantifying the crucial controls over isoprene emissions and on using these controls in improved global-scale models of emissions (Lerdau & Throop, 1999).

From detailed studies of the impacts of light and temperature upon emissions, we have demonstrated that the algorithms describing emission controls for temperate plants are not appropriate for tropical ones (Keller & Lerdau, 1999). Specifically, whereas temperate plants show isoprene emissions that saturate at approximately one half of full sun intensity, isoprene emission from tropical trees does not show light saturation. In addition, isoprene emission from tropical trees saturates at a higher temperature than does emission from temperate trees. These results mean that previous models of isoprene emissions from tropical forests may have underestimated emissions by 20%-50%. In other words, tropical forests play a much larger role in global atmospheric chemistry than previously supposed, especially in controlling the dynamics of ozone and methane in the lower atmosphere.

Studies of the physiological relationship between isoprene emission and photosynthesis have demonstrated that the enzymatic capacity of both processes are well correlated (Lerdau & Throop, 2000). This correlation allows one to predict isoprene emission based on photosynthetic capacity. Connecting photosynthetic capacity to isoprene emission allows one to apply satellite techniques developed for photosynthetic capacity estimations to predictions of isoprene emissions across large spatial scales. These satellite approaches are currently being used to develop and test new global models of emissions. These emissions models can, in turn, be used predict the influence of tropical regions on atmospheric chemistry and climate and how that influence might change in response to climatic and land-use changes.

More info: M.T. Lerdau web page

M. Lerdau & H. Throop measuring photosynthesis, transpiration and isoprene emission at Parque Natural Metropolitano

4) Plant uptake of reactive nitrogen

Courtesy Jed P. Sparks

Gaseous nitrogen oxides (principally NO, NO2, HNO3 and organic nitrates) in the lower atmosphere primarily result from microbial production of NO in the soil and from anthropogenic biomass and fossil fuel combustion. These compounds are important participants in atmospheric chemistry and their concentration in the atmosphere controls the production of tropospheric ozone (O3). Ozone is usually associated with the beneficial role it plays high in the atmosphere (the stratosphere) blocking ultraviolet radiation from the sun. However, low in the atmosphere (the troposphere) ozone is a highly reactive molecule that reacts with, and damages, the biological membranes found in both plants and animals. Additionally, chemical reactions involving nitrogen oxides in the atmosphere also lead to the acidification of precipitation and deposition of harmful nitrogen compounds to natural ecosystems worldwide.

Preliminary measurements have suggested that up to 60% of the soil emitted reactive nitrogen is assimilated by the overlying canopy. Current atmospheric models, however, tend to ignore the role of plant canopies, and instead focus on soil emission rates of reactive nitrogen as the primary biogenic input to tropospheric chemistry. We have studied intensely the factors influencing the capacity of leaves to assimilate reactive nitrogen at FTS in the Republic of Panama (Sparks et al., 2001). To date, we have demonstrated interspecific differences in the leaf level uptake rates of NO2 and found these rates were sensitive to stomatal conductance (i.e., how open or closed the leaf stomata are to the atmosphere), the amount of photosynthetic enzymes in the leaf, and the concentration of NO2 in the atmosphere. Interestingly, leaves appear to have the ability to both take up and emit reactive nitrogen from the leaves depending on the concentration of NO2 in the surrounding atmosphere. The ambient concentration outside the leaf above which uptake will occur is referred to as the compensation for NO2. When scaled to the entire canopy, soil NO2 emission rates to the atmosphere were estimated to be increased or decreased by ~19% by the overlying canopy depending on the ambient NO2 concentrations. These promising results have led to expanded studies of leaf assimilation of NO2 and other reactive nitrogen compounds in both tropical and temperate forest ecosystems. In the future as humans increase the level of reactive nitrogen in the atmosphere through pollution and biomass burning, understanding nitrogen cycling in forests will help us to predict the ultimate sustainability of natural, urban and agricultural ecosystems worldwide.

More info: J.P. Sparks web page

5) Tropical forest phenologies and remote sensing

Courtesy Stephanie A. Bohlman

Remote sensing of the world’s forests has become increasingly important for various ecological applications including inputs to biogeochemical cycling (Field et al., 1995). An important advantage to using remote sensing over point field studies is that it can provide continuous data on ecosystem variables that cannot readily be collected from the ground and it can monitor these parameters through time. Most field studies linking biophysical characteristics of the canopy to remotely sensed data have been performed in crop and temperate ecosystems (Gamon et al., 1995). Few studies have been conducted that test the current interpretations of remotely sensed data in tropical forest canopies because of the difficulty in accessing the canopy. Our work at the Panama canopy cranes has provided important insights into how remote sensing can be used to measure deciduousness (Condit et al., 2000), canopy light interception, and canopy structure and to identify tree canopy species (Cochrane, 2000) in seasonal tropical forests.

Remotely sensed images, which measure the amount of energy reflected from the forest canopy, track changes in leaf density between the wet and dry seasons, but only in the upper canopy, not the whole canopy profile. Remote sensing methods, such as the ubiquitous Normalized Difference Vegetation Index (NDVI) or spectral mixture modeling, can effectively quantify deciduousness in individual trees crowns, as well as the percentage of deciduous trees over the entire landscape of the Panama Canal watershed. Deciduousness in turns affects light absorption and carbon cycling in the canopy. Studies at the crane sites, where we can measure canopy light absorption in detail, show that dry season deciduousness affects the amount of light being absorbed, and thus available for photosynthesis, mostly in the upper strata of the canopy. Overall light absorption by the whole canopy remains high despite deciduousness in the overstorey. For carbon models that use remote sensing indices as a measure of canopy light absorption, the canopy should be represented by at least two layers, with only the upper strata receiving input remote sensing data. Finally, work at the PNM canopy crane shows the potential for mapping individual species. In the secondary forest of PNM, which has relatively low canopy species diversity and a large variety of phenological patterns in response to the severe dry season, we were able to map several tree species accurately in the dry season.

More info: S.A. Bohlman web page

See also: Bohlman, S. & O'Brien, S. (2006) Allometry, adult stature and regeneration requirement of 65 tree species on Barro Colorado Island, Panama. Journal of Tropical Ecology, 22, 123-136.

Dry season, enhanced true colour image of the PNM canopy crane, showing contrast between different species. Letters indicate different species: A=Anacardium excelsum, D=deciduous species, E=Enterolobium cyclocarpum, F=Ficus insipida, L=Luehea seemannii.

References cited

Cochrane, M. A. (2000)
Using vegetation indices variability for species level classification of hyperspectral data. International Journal of Remote Sensing, 21, 2075-2987.

Condit, R. C., Watts, K., Bohlman, S. A., Pérez, R., Foster, R. B. & Hubbell, S. P. (2000)
Quantifying the deciduousness of tropical forest canopies under varying climates. Journal of Vegetation Science, 11, 649-658.

Field, C. B., Randerson, J. T. & Malmstrom, C. M. (1995)
Global net primary production: combining ecology and remote sensing. Remote Sensing of Environment, 51, 74-88.

Gamon, J. A., Field, C. B., Goulden, M. L., Griffin, K. L., Hartley, A. E., Joel, G., Penuelas, J. & Valentini, R. (1995)
Relationships between NDVI, canopy structure, and photosynthesis in three Californian vegetation types. Ecological Applications, 5, 28-41.

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.

Keller, M. & Lerdau, M. (1999)
Isoprene emission from tropical forest canopy leaves. Global Biogeochemical Cycles, 13, 19-30.

Körner, C. & Würth, M. (1996)
A simple method for testing leaf responses of tall tropical forest trees to elevated CO2. Oecologia, 107, 421-425.

Lerdau, M. T., Guenther, A. & Monson, R. (1997)
Production and emission of volatile organic compounds by plants. BioScience, 47, 373-383.

Lerdau, M. T. & Throop, H. L. (1999)
Isoprene emission and photosynthesis in a tropical forest canopy: Implications for model development. Ecological Applications, 9, 1109-1117.

Lerdau, M. T. & Throop, H. (2000)
Sources of variability in isoprene emission and photosynthesis in two species of tropical wet forest trees. Biotropica, 32, 670-676.

Lovelock, C. E., Popp, M., Virgo, A. & Winter, K. (1999)
Effects of elevated CO2 on photosynthesis, growth and reproduction of branches of the tropical canopy tree species, Luehea seemannii (Tr. & Planch.). Plant Cell and Environment, 22, 49-59.

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

Roderick, M. L., Farquhar, G. D., Berry, S. L. & Noble, I. R. (2001)
On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation. Oecologia, 129, 21-30.

Sparks, J. P., Monson, R. K., Sparks, K. L. & Lerdau, M. (2001)
Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest: Implications for tropospheric chemistry. Oecologia, 127, 214-221.

Stanhill, G. & Cohen, S. (2001)
Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probably causes and possible agricultural consequences. Agricultural and Forest Meteorology, 107, 255-278.

Würth, M. K. R., Winter, K. & Körner, C. (1998)
Leaf carbohydrate responses to CO2 enrichment at the top of a tropical forest. Oecologia, 116, 18-25.

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