Laurel J. Anderson

Nonlinear grassland responses to past and future atmospheric CO2

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO2 concentrations (Ca) as Ca increases to more than 500 µmol mol-1 this century from interglacial levels of less than 200 µmol mol-1 (refs 1–6). However, such carbon storage depends on feedbacks between plant responses to Ca and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past Ca than to those forecast for the coming century. Along a continuous gradient of 200 to 550 µmol mol-1 (refs 9, 10), increased Ca promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient Ca, but was unchanged at elevated Ca where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in Ca there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future Ca and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.

POTENTIAL NITROGEN CONSTRAINTS ON SOIL CARBON SEQUESTRATION UNDER LOW AND ELEVATED ATMOSPHERIC CO2

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.

WATER AND TREE-UNDERSTORY INTERACTIONS: A NATURAL EXPERIMENT IN A SAVANNA WITH OAK WILT

Savanna trees influence water, light, and nutrient availability under their canopies, but the relative importance of these resources to understory plants is not well understood. In a three-year study in a Texas savanna, trees infected with the disease oak wilt were used in a natural experiment to isolate the effects of light and soil resources, particularly water, in oak–understory interactions. Herbaceous biomass and survival of transplanted Prosopis glandulosa (mesquite) seedlings were monitored in plots under healthy and symptomatic Quercus fusiformis (live oak) trees, and in open sites. Shade cloth maintained similar midday light levels in plots under symptomatic and healthy trees. Plant physiological attributes, soil parameters, and woody plant densities were also compared across habitats. Water availability was significantly lower near healthy trees than near symptomatic trees or in the open. Shade-cloth plots under symptomatic trees had over twice the herbaceous biomass of ambient-light plots under...

Measuring Water Availability and Uptake in Ecosystem Studies

Terrestrial productivity depends strongly on the availability of water in the environment (Lieth 1972). Measuring the availability and movement of water among soil, plants, and the atmosphere requires methods from ecosystem studies, plant physiology, soil science, and biogeochemistry (Casper and Jackson 1997). Techniques for estimating the water status of soils have been reviewed recently (Rundel and Jarrell 1989; Boyer 1995) and those for measuring plant water status at cellular and whole-plant levels are thoroughly described in physiological ecology and plant physiology texts (e.g., Koide et al. 1989; Kramer and Boyer 1995). Rather than reviewing all methods for estimating plant and soil water, we emphasize techniques that are increasing in importance or changing rapidly for ecosystem studies. We include methods for determining the water content of soil, the availability of that water for plant uptake, and the transport of water through the plant (Fig. 13.1). Since many of these methods are also used in physiological ecology, our chapter is designed to bridge the gap between the scales of physiological and ecosystem ecology, as a foundation for other contributions in this book.

Estimating nitrogen uptake of individual roots in container- and field-grown plants using a 15N-depletion approach

We only have a limited understanding of the nutrient uptake physiology of individual roots as they age. Despite this shortcoming, the importance of nutrient uptake processes to our understanding of plant nutrition and nutrient cycling cannot be underestimated. In this study, we used a 15N depletion method that allowed for the measurement of nitrate-N uptake rates on intact, individual, fine roots of known age. We expected that N uptake would decline rapidly as fine roots aged, regardless of the environmental conditions and species used. We compared age dependent uptake patterns of young grape cuttings with those of mature vines and with those of tomato. Although patterns of declining uptake with increasing root age were similar for all species and conditions tested, large differences in maximum N uptake rates existed between young cuttings and mature vines, and between woody and herbaceous species. Maximum rates were 10-fold higher for tomato and 3-fold higher for the grape cuttings, when compared with uptake rates of fine roots of mature vines. Coefficients of variation ranged from 43 to 122% within root age groups. The large variability in physiological characteristics of fine roots of the same age, diameter and order suggests that there is a functional diversity within fine roots that is still poorly understood.

Transforming Ecological Science at Primarily Undergraduate Institutions through Collaborative Networks

Ecologists at primarily undergraduate institutions (PUIs) are well positioned to form collaborative networks and make transformative contributions to the study and teaching of ecology. The spatial and temporal complexity of ecological phenomena rewards a collaborative research approach. A network of PUI ecologists can incorporate closely supervised data collection into undergraduate courses, thereby generating data across spatial gradients to answer crucial questions. These data can offer unprecedented insight into fine- and large-scale spatial processes for publications, resource management, and policy decisions. Undergraduate students benefit from the collaborative research experience as they gain experiential learning in team building, project design, implementation, data collection, and analysis. With appropriate funding, collaborative networks make excellent use of the intellectual and experiential capital of PUI faculty for the benefit of science, pedagogy, and society.

Effects of urbanization on the population structure of freshwater turtles across the United States: Urbanization Effects on Turtles

Landscape-scale alterations that accompany urbanization may negatively affect the population structure of wildlife species such as freshwater turtles. Changes to nesting sites and higher mortality rates due to vehicular collisions and increased predator populations may particularly affect immature turtles and mature female turtles. We hypothesized that the proportions of adult female and immature turtles in a population will negatively correlate with landscape urbanization. As a collaborative effort of the Ecological Research as Education Network (EREN), we sampled freshwater turtle populations in 11 states across the central and eastern United States. Contrary to expectations, we found a significant positive relationship between proportions of mature female painted turtles (Chrysemys picta) and urbanization. We did not detect a relationship between urbanization and proportions of immature turtles. Urbanization may alter the thermal environment of nesting sites such that more females are produced as urbanization increases. Our approach of creating a collaborative network of scientists and students at undergraduate institutions proved valuable in terms of testing our hypothesis over a large spatial scale while also allowing students to gain hands-on experience in conservation science.