Effects of rainfall exclusion on soil carbon gases and water relations in two boreal forest ecosystems

Estimates of the global cycles of methane, carbon dioxide, and water must account for fluxes from boreal forests, a biome second in total area only to Earth's tropical rainforests. In order to predict fluxes of CO{dollar}sb2{dollar}, CH{dollar}sb4{dollar}, and H{dollar}sb2{dollar}O, we need to examine their response to altered soil moisture regimes. At a floodplain and upland site in interior Alaska, a 0.10-ha rainout shelter limits precipitation from May through September, and soil fluxes of CO{dollar}sb2{dollar}, CH{dollar}sb4{dollar}, and water were observed. This study examined soil fluxes of CO{dollar}sb2{dollar} and CH{dollar}sb4{dollar} using soil gas chambers, soil moisture measured by time domain reflectometry, soil temperatures measured with thermistors; and tree water flux using sap probes. At each site, a 0.10-ha rainout shelter limits precipitation from May through September. Limiting precipitation at the upland site resulted in lower soil moisture and increased methane consumption. Average rates of methane consumption among upland plots were 0.29 {dollar}pm{dollar} 0.05 mg m{dollar}sp{lcub}-2{rcub}{dollar}d{dollar}sp{lcub}-1{rcub}{dollar} and 0.60 {dollar}pm{dollar} 0.06 mg {dollar}rm msp{lcub}-2{rcub}dsp{lcub}-1{rcub}{dollar} (s.e.) in unsheltered and sheltered soils, respectively. At the floodplain site, limiting precipitation decreased methane consumption from an average of 0.24 {dollar}pm{dollar} 0.03 mg {dollar}rm msp{lcub}-2{rcub}dsp{lcub}-1{rcub}{dollar} to 0.07 {dollar}pm{dollar} 0.03 mg {dollar}rm msp{lcub}-2{rcub}dsp{lcub}-1{rcub}{dollar} (s.e.). Limiting summer precipitation generally decreased CO{dollar}sb2{dollar} concentrations within the soil profile as well as soil efflux. At the upland site, efflux rates were 0.42 {dollar}pm{dollar} 0.06 g {dollar}rm msp{lcub}-2{rcub}hsp{lcub}-1{rcub}{dollar} from unsheltered and 0.31 {dollar}pm{dollar} 0.03 g {dollar}rm msp{lcub}-2{rcub}hsp{lcub}-1{rcub}{dollar} (s.e.) from sheltered soils. At the floodplain site, rates were reduced from 0.42 {dollar}pm{dollar} 0.03 g {dollar}rm msp{lcub}-2{rcub}hsp{lcub}-1{rcub}{dollar} to 0.30 {dollar}pm{dollar} 0.05 g {dollar}rm msp{lcub}-2{rcub}hsp{lcub}-1{rcub}{dollar} (s.e.). Temperature was positively related to soil respiration, particularly when sufficient moisture was available for microbial and root activity. Fick's Law could predict soil gas fluxes when the soil profile concentration gradient accurately represented the soil surface gradient and biological sources and sinks of the gas did not overwhelm flux calculations.; In spite of generally higher sap flux rates at the floodplain site, calculated stand water consumption was less at that site because of lower stand sapwood area. Unsheltered upland trees consumed an average of 0.82 {dollar}pm{dollar} 0.05 mm d{dollar}sp{lcub}-1{rcub}{dollar} over both summers; unsheltered floodplain trees consumed 0.70 {dollar}pm{dollar} 0.03 mm d{dollar}sp{lcub}-1{rcub}{dollar} (s.e.). Sheltered trees, especially at the drier upland site, had limited ability to respond to atmospheric driving variables. If soil water availability decreases with global climate change at boreal landscapes similar to these sites, we may see reduction in soil respiration, decreases in water uptake, and at drier sites, increases in soil methane consumption.