Most terrestrial carbon sequestration at mid-latitudes in the Northern Hemisphere occurs in seasonal, montane forest ecosystems. Winter respiratory carbon dioxide losses from these ecosystems are high, and over half of the carbon assimilated by photosynthesis in the summer can be lost the following winter. The amount of winter carbon dioxide loss is potentially susceptible to changes in the depth of the snowpack; a shallower snowpack has less insulation potential, causing colder soil temperatures and potentially lower soil respiration rates. Recent climate analyses have shown widespread declines in the winter snowpack of mountain ecosystems in the western USA and Europe that are coupled to positive temperature anomalies. Here we study the effect of changes in snow cover on soil carbon cycling within the context of natural climate variation. We use a six-year record of net ecosystem carbon dioxide exchange in a subalpine forest to show that years with a reduced winter snowpack are accompanied by significantly lower rates of soil respiration. Furthermore, we show that the cause of the high sensitivity of soil respiration rate to changes in snow depth is a unique soil microbial community that exhibits exponential growth and high rates of substrate utilization at the cold temperatures that exist beneath the snow. Our observations suggest that a warmer climate may change soil carbon sequestration rates in forest ecosystems owing to changes in the depth of the insulating snow cover.

We took advantage of the distinctive system-level measurement capabilities of the Biosphere 2 Laboratory (B2L) to examine the effects of prolonged exposure to elevated [CO 2 ] on carbon flux dynamics, above-and belowground biomass changes, and soil carbon and nutrient capital in plantation forest stands over 4 years. Annually coppiced stands of eastern cottonwoods (Populus deltoides) were grown under ambient (400 ppm) and two levels of elevated (800 and 1200 ppm) atmospheric [CO 2 ] in carbon and N-replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO 2 exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO 2 exchange under different scales in space and time. Elevated [CO 2 ] significantly stimulated whole-system maximum net CO 2 influx by an average of 21% and 83% in years 3 and 4 of the experiment. Over the 4-year experiment, cumulative belowground, foliar, and total aboveground biomass increased in both elevated [CO 2 ] treatments. After 2 years of growth at elevated [CO 2 ], early season stand respiration was decoupled from CO 2 influx aboveground, presumably because of accelerated fine root production from stored carbohydrates in the coppiced system prior to canopy development and to the increased soil carbohydrate status under elevated [CO 2 ] treatments. Soil respiration was stimulated by elevated [CO 2 ] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate-induced respiration in vitro. Elevated [CO 2 ] accelerated depletion of soil nutrients, phosphorus, calcium and potassium, after 3 years of growth, litter removal, and coppicing, especially in the upper soil profile, although total N showed no change. Enhancement of aboveand belowground biomass production by elevated [CO 2 ] accelerated carbon cycling through the coppiced system and did not sequester additional carbon in the soil.

No abstract

Many invasions, like the wide-spread establishment of annual grasses and forbs in semi-arid shrublands, are associated with climate change. In order to predict ecosystem carbon (C) storage it is critical that we understand how invasion affects soil respiration (Rt). Because plants and microbes have different seasonal dynamics, determining the relative contribution of autotrophic (Ra) and heterotrophic (Rh) respiration provides critical insight into soil C processes. Using automated soil respiration measurements and root exclusion cores we evaluated the moisture and temperature sensitivity of Rt and Rh and calculated the contribution of Ra in native shrub and invaded areas.

Invasion increased cumulative Rt by 40% from 695 (±51) g C m⁻² under shrubs to 1050 g C m⁻² (±44) in invaded areas. Cumulative Rh did not change but invasion altered the seasonal pattern of Rh. Throughout the season Rt and Rh responded positively to temperature increases when soils were wet and negatively when soils were dry. Invasion increased temperature sensitivity of Rt and Rh in wet soils and decreased temperature sensitivity in dry soils. The altered temperature sensitivity of invasives was attributed largely to differences in phenology. Early phenology of invasive grasses caused rapid Ra increases early in the season; late phenology of invasive forbs resulted in the surprising maintenance of diurnal Ra and Rh signals despite high temperatures and low soil moisture.

Invasion extended the respiration season of the system. Ability of the invasive community to withstand high temperatures and drought could confer greater resilience if temperature and precipitation patterns in the region change. The high contribution of Ra by invasive annuals means ecosystem C storage will depend heavily on seasonal rainfall dynamics and productivity of invasive annuals. In semi-arid ecosystems even small scale changes in plant community composition alter Rt, Ra and Rh and should be considered when attempting to predict Rt

No abstract