Carla A. Gunderson

Photosynthetic acclimation in trees to rising atmospheric CO2: A broader perspective.

Analysis of leaf-level photosynthetic responses of 39 tree species grown in elevated concentrations of atmospheric CO2 indicated an average photosynthetic enhancement of 44% when measured at the growth [CO2]. When photosynthesis was measured at a common ambient [CO2], photosynthesis of plants grown at elevated [CO2] was reduced, on average, 21% relative to ambient-grown trees, but variability was high. The evidence linking photosynthetic acclimation in trees with changes at the biochemical level is examined, along with anatomical and morphological changes in trees that impact leaf- and canopy-level photosynthetic response to CO2 enrichment. Nutrient limitations and variations in sink strength appear to influence photosynthetic acclimation, but the evidence in trees for one predominant factor controlling acclimation is lacking. Regardless of the mechanisms that underlie photosynthetic acclimation, it is doubtful that this response will be complete. A new focus on adjustments to rising [CO2] at canopy, stand, and forest scales is needed to predict ecosystem response to a changing environment.

NET PRIMARY PRODUCTIVITY OF A CO2‐ENRICHED DECIDUOUS FOREST AND THE IMPLICATIONS FOR CARBON STORAGE

A central question concerning the response of terrestrial ecosystems to a changing atmosphere is whether increased uptake of carbon in response to increasing at- mospheric carbon dioxide concentration results in greater plant biomass and carbon storage or, alternatively, faster cycling of C through the ecosystem. Net primary productivity (NPP) of a closed-canopy Liquidambar styraciflua (sweetgum) forest stand was assessed for three years in a free-air CO2-enrichment (FACE) experiment. NPP increased 21% in stands ex- posed to elevated CO2, and there was no loss of response over time. Wood increment increased significantly during the first year of exposure, but subsequently most of the extra C was allocated to production of leaves and fine roots. These pools turn over more rapidly than wood, thereby reducing the potential of the forest stand to sequester additional C in response to atmospheric CO2 enrichment. Hence, while this experiment provides the first evidence that CO2 enrichment can increase productivity in a closed-canopy deciduous forest, the implications of this result must be tempered because the increase in productivity resulted in faster cycling of C through the system rather than increased C storage in wood. The fate of the additional C entering the soil system and the environmental interactions that influence allocation need further investigation.

Empirical geographic modeling of switchgrass yields in the United States

Switchgrass (Panicum virgatum L.) is a perennial grass native to the United States that has been studied as a sustainable source of biomass fuel. Although many field‐scale studies have examined the potential of this grass as a bioenergy crop, these studies have not been integrated. In this study, we present an empirical model for switchgrass yield and use this model to predict yield for the conterminous United States. We added environmental covariates to assembled yield data from field trials based on geographic location. We developed empirical models based on these data. The resulting empirical models, which account for spatial autocorrelation in the field data, provide the ability to estimate yield from factors associated with climate, soils, and management for both lowland and upland varieties of switchgrass. Yields of both ecotypes showed quadratic responses to temperature, increased with precipitation and minimum winter temperature, and decreased with stand age. Only the upland ecotype showed a positive response to our index of soil wetness and only the lowland ecotype showed a positive response to fertilizer. We view this empirical modeling effort, not as an alternative to mechanistic plant‐growth modeling, but rather as a first step in the process of functional validation that will compare patterns produced by the models with those found in data. For the upland variety, the correlation between measured yields and yields predicted by empirical models was 0.62 for the training subset and 0.58 for the test subset. For the lowland variety, the correlation was 0.46 for the training subset and 0.19 for the test subset. Because considerable variation in yield remains unexplained, it will be important in the future to characterize spatial and local sources of uncertainty associated with empirical yield estimates.

Simulated Patterns of Forest Succession and Productivity as a Consequence of Altered Precipitation

Individual-based models of forest succession have long been used to address how productivity and species composition might respond to long-term changes in climate (Pastor and Post 1988; Solomon 1986; Urban et al. 1993; Bugmann et al. 2001). Because population dynamics are so strongly coupled with the forest water, carbon, and nitrogen cycles, gap models are well suited for the study of forest responses to environmental change. These models simulate a range of plant and ecosystem dynamics by considering interactions between physiological processes and individual tree growth, demographic processes and tree-population dynamics, microbial processes and nitrogen availability, and the roles played by climate and soils as they combine to influence site water balance (Shugart et al. 1992).

Effect of ethylene and related hydrocarbons on carbon assimilation and transpiration in herbaceous and woody species.

Although the hormonal effects of ethylene (C/sub 2/H/sub 4/) on plant growth and development are well documented, recent evidence suggests that carbon assimilation in some herbaceous species is highly sensitive to ethylene. Since ethylene is a common trace gas in many airsheds influenced by urban and industrial pollutant sources, elevated levels of ethylene may be affecting the productivity of some terrestrial vegetation. The objectives of this study were to investigate the responsiveness of carbon assimilation to ethylene and related low molecular weight hydrocarbons in plant species of dissimilar growth and physiological features, to address the physiological mechanism of photosynthetic inhibition, and to estimate minimum C/sub 2/H/sub 4/ concentrations causing incipient effects on carbon assimilation. Of the four hydrocarbons studied only ethylene influenced carbon assimilation in a variety of species. The level of ethylene needed to elicit a change in carbon assimilation differed markedly among species. Estimated 5-h concentrations of ethylene required to influence carbon assimilation in ethylene-sensitive species ranged from 0.60 to 19.5 ..mu..mol/m/sup 3/. The ethylene-induced inhibition of photosynthesis was correlated with a decline in stomatal conductance to H/sub 2/O vapor.

Photosynthesis, carbon allocation, and growth of sulfur dioxide ecotypes ofGeranium carolinianum L.

SummaryThis study investigated ways in which genetically determined differences in SO2 susceptibility resulting from ecotypic differentiation inGeranium carolinianum were expressed physiologically. The SO2-resistant and SO2-sensitive ecotypes were exposed to a combination of short- and long-term SO2 exposures to evaluate the responses of photosynthesis, H2S efflux from foliage (sulfur detoxification), photoassimilate retention, leaf-diffusive resistance to CO2, and growth. When exposed to SO2, both ecotypes re-emit sulfur in a volatile, reduced form, presumably as H2S. Because H2S efflux rates at various SO2 concentrations were comparable between ecotypes, genetic differences inG. carolinianum could not be attributed to a re-emission of excess sulfur as H2S. Incipient SO2 effects on photosynthesis were observed as cumulative SO2 flux into the leaf interior excecded 0.40 nmol·m−2 in the resistant ecotype and 0.26 nmol·m−2 in the sensitive ecotype. Although initial SO2-induced changes in photosynthesis in both ecotypes were mediated through an increase in stomatal resistance to CO2, the ecotype-specific patterns as a function of pollutant concentration and exposure time were associated with marked increases in residual resistance to CO2. Patterns in photosynthesis, photoassimilate retention, and growth following long-term SO2 exposures were also ecotype-specific. Although physiological accommodation of SO2 stress was observed in both ecotypes, it was more pronounced in the resistant ecotype. The physiological mechanisms underlying genetic differences inG. carolinianum in response to SO2 stress were concluded to be (1) dissimilar threshold levels of response to SO2 and/or its toxic derivatives and (2) differences in homeostatic processes governing the rate of repair or compensation for physiological injury.

Assessing the influence of exogenous ethylene on electron transport and fluorescence quenching in leaves of Glycine max

Abstract We conducted a series of modeling exercises designed to re-evaluate the light-response and CO 2 -response curves of Taylor and Gunderson ( Pl. Physiol. 86, 85–92, 1988) and to examine further their conclusion that ethylene-induced inhibition of electron transport may contribute to reduced CO 2 assimilation in leaves of Glycine max . By partitioning the response of CO 2 assimilation to either electron transport-limited or Rubisco-limited rates of carboxylation, we calculated that the electron transport capacity ( J max ) of ethylene-treated leaves decreased by over 30% following a 4-hr exposure to 10 μl/l ethylene and noted that ethylene-induced reductions in CO 2 assimilation could be explained without a decrease in Rubisco activity ( Vc max ). Measurements of in vivo Chl fluorescence supported these observations and indicated that the efficiency by which excitation energy was captured in PSII (i.e. ( F m − F o / F m ) was reduced from 0.80 to 0.73 after a 4-hr exposure to 10 μl/l ethylene. This reduction was also accompanied by a 12% decrease in steady-state photochemical quenching ( q p ), indicating that a lower proportion of open or oxidized PSII reaction centers were participating in light-dependent processes. Effects of ethylene on Chl fluorescence were amplified at increased irradiance, suggesting that photoinhibition may play a role in the ethylene-induced inhibition of CO 2 assimilation.