Long-term impact of a stand-replacing fire on ecosystem CO2 exchange of a ponderosa pine forest

Ponderosa pine (Pinus ponderosa) forests of the southwestern United States are a mosaic of stands where undisturbed forests are carbon sinks, and stands recovering from wildfires may be sources of carbon to the atmosphere for decades after the fire. However, the relative magnitude of these sinks and sources has never been directly measured in this region, limiting our understanding of the role of fire in regional and US carbon budgets. We used the eddy covariance technique to measure the CO₂ exchange of two forest sites, one burned by fire in 1996, and an unburned forest. The fire was a high‐intensity stand‐replacing burn that killed all trees. Ten years after the fire, the burned site was still a source of CO₂ to the atmosphere [109±6 (SEM) g C m^?2 yr^?1], whereas the unburned site was a sink (?164±23 g C m^?2 yr^?1). The fire reduced total carbon storage and shifted ecosystem carbon allocation from the forest floor and living biomass to necromass. Annual ecosystem respiration was lower at the burned site (480±5 g C m^?2 yr^?1) than at the unburned site (710±54 g C m^?2 yr^?1), but the difference in gross primary production was even larger (372±13 g C m^?2 yr^?1 at the burned site and 858±37 g C m^?2 yr^?1at the unburned site). Water availability controlled carbon flux in the warm season at both sites, and the burned site was a source of carbon in all months, even during the summer, when wet and warm conditions favored respiration more than photosynthesis. Our study shows that carbon losses following stand‐replacing fires in ponderosa pine forests can persist for decades due to slow recovery of the gross primary production. Because fire exclusion is becoming increasingly difficult in dry western forests, a large US forest carbon sink could shift to a decadal‐scale carbon source.     

The burrow structure of the European rabbit (Oryctolagus cuniculus L.)

Samples of rabbit burrows were excavated and measured at four areas around Edinburgh. Of these, 31 burrows over 2 m long were analysed for internal structural relationships and the effect of soil composition and slope of the ground on their form. Seven variables were measured for each burrow—total length, number of entrance holes, number of junctions, number of ends, average length of sections between holes, junctions and ends, average depth and maximum depth. There are three main trends in burrow variation—size, an inverse relationship between relative number of holes and average depth, and an inverse relationship between average section length and relative number of junctions. Burrows in sand have relatively fewer holes and junctions, longer element lengths, a higher average depth and three times the enclosed volume per hole. Burrows dug into slopes have a higher average depth. Areas differed significantly in the relative number of holes and junctions, and average depth. It was concluded that an apparently complex structure could be summarized in terms of relatively few components. The chief of these, size, was largely independent of the soil and site characteristics, whereas the remaining two were dependent on the soil in which the burrow was dug. This may have implications for the ecology and behaviour of rabbits in different areas, and is relevant to the success of some rabbit control procedures such as burrow fumigation and warren ripping.