Soil freezing alters fine root dynamics in a northern hardwood forest

The retention of nutrients within an ecosystem depends on temporal and spat ial synchrony between nutrient availability and nutrient uptake, and disrup tion of fine root processes can have dramatic impacts on nutrient retention within forest ecosystems. There is increasing evidence that overwinter cli mate can influence biogeochemical cycling belowground, perhaps by disruptin g this synchrony. In this study, we experimentally reduced snow accumulatio n in northern hardwood forest plots to examine the effects of soil freezing on the dynamics of fine roots (< 1 mm diameter) measured using minirhizotr ons. Snow removal treatment during the relatively mild winters of 1997-1998 and 1998-1999 induced mild freezing temperatures (to -4 degreesC) lasting approximately three months at shallow soil depths (to -30 cm) in sugar mapl e and yellow birch stands. This treatment resulted in elevated overwinter f ine root mortality in treated compared to reference plots of both species, and led to an earlier peak in fine root production during the subsequent gr owing season. These shifts in fine root dynamics increased fine root turnov er but were not large enough to significantly alter fine root biomass. No d ifferences in morality response were found between species. Laboratory test s on potted tree seedlings exposed to controlled freezing regimes confirmed that mild freezing temperatures (to -5 degreesC) were insufficient to dire ctly injure winter-hardened fine roots of these species, suggesting that th e marked response recorded in our forest plots was caused indirectly by mec hanical damage to roots in frozen soil. Elevated fine root necromass in tre ated plots decomposed quickly, and may have contributed an excess flux of a bout 0.5 g N/m(2).yr, which is substantial relative to measurements of N fl uxes from these plots. Our results suggest elevated overwinter mortality te mporarily reduced fine root length in treatment plots and reduced plant upt ake, thereby disrupting the temporal synchrony between nutrient availabilit y and uptake and enhancing rates of nitrification. Increased frequency of s oil freezing events, as may occur with global change, could alter fine root dynamics within the northern hardwood forest disrupting the normally tight coupling between nutrient mineralization and uptake.