Effect of extraction pretreatment on radial variation of nitrogen concentration in tree rings

Past research in the paleoenvironmental subdiscipline of dendro-chemistry h as concluded that N concentration variation in tree rings cannot provide in formation on past conditions of environmental availability of N. The object ive of this study was to test wood extraction pretreatments to remove wood entractives and sap, both of which may obscure the environmental signal of N availability in tree rings. Three increment cores were collected from eac h of six trees (three ponderosa pines and three Douglas-firs). Within each tree, the first core was left untreated (referred to as CONTROL), the secon d core was extracted for several hours in organic solvents and distilled wa ter (referred to as EXTRACT), and the third core also was extracted but for a total time of 3 d (referred to as 3-DAY). A semimicro Kjeldahl method wa s used to determine total N on decadal groups of rings. Average N concentra tion of EXTRACT cores was significantly less than that of CONTROL, and the coefficient of variation of EXTRACT cores also was significantly less than that of CONTROL. Most CONTROL cores showed substantial temporal variation i n N concentration related to heartwood and sapwood and/or recently formed r ings. In contrast, most EXTRACT cores showed no substantial change in N con centration related to heartwood and sapwood and/or recently formed rings. T he 3-DAY cores confirmed, but did not improve upon, results obtained with e xtraction using the shorter time duration. Thus, pretreating wood by extrac tion appears to substantially reduce the variation in N concentration of tr ee rings, which is a necessary first step toward interpreting ring N as an indicator of past environmental N availability.