Jeffrey S. Pippen

Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO2

The earths future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO₂. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO₂ stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO₂ as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.

Community composition and photosynthesis by photoautotrophs under quartz pebbles, Southern Mojave Desert

We used 16s rDNA sequences to identify novel species of cyanobacteria beneath translucent quartz pebbles in the desert pavement on an alluvial piedmont of the Coxcomb Mountains in the southern Mojave Desert, California, USA. Transmission of light, as measured with an integrating sphere, was about 0.08% beneath the thickest pieces of quartz (25 mm) harboring these hypolithic autotrophs. The photosynthetic rate ranged from 0.1 to 1.0 μmol·m−2·s−1 in the linear range of its response to light (PAR of 0–50 μmol·m−2·s−1), over which the apparent quantum-use efficiency was 0.019. Light-saturated rates of 1.7–2.7 μmol·m−2·s−1 were recorded at light intensities of 200–400 μmol·m−2·s−1. The hypolithic community had an upper thermal tolerance of >90°C in laboratory conditions. The quartz pebbles confer a modest greenhouse effect that may be important for photosynthetic activity during cool, wet, wintertime periods that prevail in the Mojave Desert.

The Duke Forest FACE Experiment: CO2 Enrichment of a Loblolly Pine Forest

Free-air CO2 enrichment (FACE) in the Duke Forest provides a whole-ecosystem arena in which to examine the response of a temperate coniferous forest to high, future levels of atmospheric CO2. At the end of 8 years of the experiment, we conclude: Photosynthetic rates by canopy foliage have increased up to 50 % over controls. Basal area increment has been stimulated 13–27 % versus that in control plots, with interannual variation due to variations in temperature and moisture during the growing season. Biomass increment has increased by 108 g C m-2 year-1 (27 %) over that in control plots. Growth and respiration of roots are higher in CO2 fumigated plots. Litterfall is greater in high CO2 plots and forest floor accumulation has increased. There has been little or no change in the total amount of soil organic matter as a result of CO2 fumigations. While the stimulation of growth by high CO2 persists after 8 years of fumigation, there is evidence of nitrogen limitation in the fumigated plots.