A method for experimental heating of intact soil profiles for application to climate change experiments

A new system for simulating future belowground temperature increases was conceived, simulated, constructed and tested in a temperate deciduous forest in Oak Ridge, TN, USA. The new system uses low‐wattage, 3 m deep heaters installed around the circumference of a defined soil volume. The heaters add the necessary energy to achieve a set soil temperature differential within the treatment area and add exterior energy inputs equal to those, which might be lost from lateral heat conduction. The method, which was designed to work in conjunction with aboveground heated chambers, requires only two control sensor positions one for aboveground air temperatures at 1 m and another for belowground temperatures at 0.8 m. The method is capable of achieving temperature differentials of at least +4.0±0.5 °C for soils to a measured depth of ?2 m. These +4 °C differential soil temperatures were sustained in situ throughout 2009, and both diurnal and seasonal cycles at all soil depths were retained using this simple heating approach. Measured mean energy inputs required to sustain the target heating level of +4 °C over the 7.1 m² target area were substantial for aboveground heating (21.1 kW h day^?1 m^?2), but 16 times lower for belowground heaters (1.3 kW h day^?1 m^?2). Observations of soil CO₂ efflux from the surface of the target soil volumes showed CO₂ losses throughout 2009 that were elevated above the temperature response curve that have been reported in previous near‐surface soil warming studies. Stimulation of biological activity within previously undisturbed deep‐soil carbon stocks is the hypothesized source. Long‐term research programs may be able to apply this new heating method that captures expected future warming and temperature dynamics throughout the soil profile to address uncertainties in process‐level responses of microbial, plant and animal communities in whole, intact ecosystems.     

Sensitivity of canopy transpiration to altered precipitation in an upland oak forest: evidence from a long-term field manipulation study

Climate‐induced changes in regional precipitation could have important implications for the carbon, water, and nutrient cycles of forest ecosystems. However, few studies have examined the response of deciduous forests to increases or decreases in precipitation. Therefore, the throughfall displacement experiment (TDE) was established in 1993 near Oak Ridge, Tennessee to examine the sensitivity of an upland oak (Quercus spp.) forest to ambient, wet (+33%), and dry (?33%) precipitation regimes. Sap flux measurements on co‐occurring tree species were scaled using species‐specific estimates of stand sapwood area to derive daily and seasonal rates of canopy transpiration (E_C) from 2000 to 2003. With the exception of 2003, which was an extremely wet year, daily E_C in the dry plot, and occasionally during extended droughts in the ambient and wet plots, declined as water potential in the upper 0.35 m soil profile approached ‐3.0 MPa. Seasonal patterns of soil water potential and treatment‐specific differences in E_C were dependent on precipitation frequency and intensity. Supplemental precipitation added to the wet plot increased seasonal E_C on average by 9% (range ?1% to 19%), whereas extended periods of drought on the dry plot in 2000, 2001, and 2002 were sufficient to reduce seasonal E_C by 26–30% compared with the ambient plot. There was a strong correlation between seasonal E_C and the water stress integral, a cumulative index of drought severity and duration. A polynomial fitted to these data indicated that reductions in seasonal E_C on the order of 40% were possible given TDE‐imposed reductions in soil water potential. Application of this equation to all years of the TDE (1994–2003) revealed considerable interannual and treatment‐specific variation in canopy transpiration. In general, a 33% removal of throughfall on the dry plot during 1995, 1998, and 2002 resulted in a 23–32% reduction in seasonal E_C compared with the ambient plot. While droughts in deciduous forests are often limited in duration and tend to occur late in the growing season, soil water deficits of the magnitude observed in this study have the potential to impact local and regional forest water budgets.