Publications

The effect of elevated atmospheric CO₂ concentration (<em>C</em>_a) on the aboveground biomass of three oak species, <em>Quercus myrtifolia</em>, <em>Q. geminata</em>, and <em>Q. chapmanii</em>, was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire-regenerated scrub-oak ecosystem. After burning a stand of scrub-oak vegetation, re-growing plants were exposed to either current ambient (379 µL L⁻¹ CO₂) or elevated (704 µL L⁻¹ CO₂) <em>C</em>_a in 16 open-top chambers over a four-year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated <em>C</em>_a significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated <em>C</em>_a had no effect on shoot density. The relative effect of elevated <em>C</em>_a on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated <em>C</em>_a was species specific: elevated <em>C</em>_a significantly increased aboveground biomass of the dominant species, <em>Q. myrtifolia</em>, and tended to increase aboveground biomass of <em>Q. chapmanii</em>, but had no effect on aboveground biomass of the subdominant, <em>Q. geminata</em>. These results show that rising atmospheric CO₂ has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species-specific growth responses could, in the long term, alter the composition of the scrub-oak community.

To date, most research that has examined the effect of elevated atmospheric carbon dioxide concentration ([CO₂]) on litter decomposition has focused on changes in the leaf litter quality of individual species. Results from California grasslands indicate that other CO₂ responses may have greater consequences for decomposition rates. For instance, CO₂-driven changes in either species dominance or patterns of biomass allocation would alter both the quality and the position of grassland litter. We review the results from studies in California grasslands to identify the mechanisms that affect grassland litter decomposition. We use a simple calculation that integrates the results of two studies to identify three mechanisms that have the potential to substantially alter decomposition rates as the atmospheric [CO₂] rises.

Leaf conductance often decreases in response to elevated atmospheric CO₂ concentration (<em>C</em>_a) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated <em>C</em>_a during an eight-month period two years after <em>C</em>_a manipulation began. Whole-chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated <em>C</em>_a, despite an increase in leaf area index (LAI). Elevated <em>C</em>_a also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5–3 m), suggesting that the water savings in elevated <em>C</em>_a in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated <em>C</em>_a.

For two species of oak, we determined whether increasing atmospheric CO₂ concentration (<em>C</em>_a) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated <em>C</em>_a, or both. In particular, we tested whether acclimatory decreases in leaf-Rubisco content in elevated <em>C</em>_a would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of <em>Quercus myrtifolia</em> Willd. and <em>Quercus geminata</em> Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of <em>Q. myrtifolia</em>. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 µmol mol⁻¹) <em>C</em>_a, in open-top chambers (OTCs). For <em>Q. myrtifolia</em>, an increase in <em>C</em>_a from 360 to 710 µmol mol⁻¹ had no direct effect on R at any time during the year. In April 1999, R in young <em>Q. myrtifolia</em> leaves was significantly higher in elevated <em>C</em>_a—the only evidence for an indirect effect of growth in elevated <em>C</em>_a. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated <em>C</em>_a significantly reduced maximum RuBP-saturated carboxylation capacity (<em>V</em>_c max) for both the sun and shade leaves of only <em>Q. geminata</em>. However, we estimated that only 11–12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit leaf area or unit dry weight, and did not support the hypothesis that elevated <em>C</em>_a decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.

Portions of a regenerating scrub oak ecosystem were enclosed in open-top chambers and exposed to elevated CO₂. The distinct ¹³C signal of the supplemental CO₂ was used to trace the rate of C integration into various ecosystem components. Oak foliage, stems, roots and ectomycorrhizae were sampled over 3 years and were analyzed for ¹³C composition. The aboveground tissue ¹³C equilibrated to the novel ¹³C signal in the first season, while the belowground components displayed extremely slow integration of the new C. Roots taken from ingrowth cores showed that 33% of the C in newly formed roots originated from a source other than recent photosynthesis inside the chamber. In this highly fire-prone system, the oaks re-establish primarily by resprouting from large rhizomes. Remobilization from belowground C stores may support fine roots and mycorrhizae for several years into stand re-establishment and, therefore, may explain why belowground tissues contain less of the new photosynthate than expected. Though it has been shown that long-term cycles of C storage are theoretically advantageous for plants in systems with frequent and severe disturbances, such patterns have not been previously examined in wild systems.

The effects of grassland conversion to forest vegetation and of individual tree species on microbial activity in Siberia are largely unstudied. Here, we examined the effects of the six most commonly dominant tree species in Siberian forests (Scots pine, spruce, Arolla pine, larch, aspen and birch) on soil C and N mineralization, N₂O-reduction and N₂O production during denitrification 30 years after planting. We also documented the effect of grassland conversion to different tree species on microbial activities at different soil depths and their relationships to soil chemical properties. The effects of tree species and grassland conversion were more pronounced on N than on C transformations. Tree species and grassland conversion did significantly alter substrate-induced respiration (SIR) and basal respiration, but the differences were not as large as those observed for N transformations. Variances in SIR and basal respiration within species were markedly lower than those in N transformations. Net N mineralization, net nitrification, and denitrification potential were highest under Arolla pine and larch, intermediate under deciduous aspen and birch, and lowest beneath spruce and Scots pine. Tree species caused similar effects on denitrification potential, net N mineralization, and net nitrification, but effects on N₂O reduction rate were idiosyncratic, indicating a decoupling of N₂O production and reduction. We predict that deciduous species should produce more N₂O in the field than conifers, and that Siberian forests will produce more N₂O if global climate change alters tree species composition. Basal respiration and SIR showed inverse responses to tree species: when basal respiration increased in response to a given tree species, SIR declined. SIR may have been controlled by NH₄ ⁺ availability and related therefore to N mineralization, which was negatively affected by grassland conversion. Basal respiration appeared to be less limited by NH₄ ⁺ and controlled mostly by readily available organic C (DOC), which was higher in concentration under forests than in grassland and therefore basal respiration was higher in forested soils. We conclude that in the Siberian artificial afforestation experiment, soil C mineralization was not limited by N.

Increased levels of atmospheric carbon dioxide (CO₂) are likely to affect the trophic relationships that exist between plants, their herbivores and the herbivores' natural enemies. This study takes advantage of an open-top CO₂ fertilization experiment in a Florida scrub oak community at Kennedy Space Center, Florida, consisting of eight chambers supplied with ambient CO₂ (360 ppm) and eight chambers supplied with elevated CO₂ (710 ppm). We examined the effects of elevated CO₂ on herbivore densities and levels of leaf consumption, rates of herbivore attack by natural enemies and effects on leaf abscission. Cumulative levels of herbivores and herbivore damage were significantly lower in elevated CO₂ than in ambient CO₂. This may be because leaf nitrogen levels are lower in elevated CO₂. More herbivores die of host plant-induced death in elevated CO₂ than in ambient CO₂. Attack rates of herbivores by parasitoids are also higher in elevated CO₂, possibly because herbivores need to feed for a longer time in order to accrue sufficient nitrogen (N), thus exposing themselves longer to natural enemies. Insect herbivores cause an increase in abscission rates of leaves throughout the year. Because of the lower insect density in elevated CO₂, we thought, abscission rates would be lower in these chambers. However, abscission rates were significantly higher in elevated CO₂. Thus, the direct effects of elevated CO₂ on abscission are greater than the indirect effects on abscission mediated via lower insect densities. A consequence of increased leaf abscission in elevated CO₂ is that nutrient deposition rates to the soil surface are accelerated.