ENVIRONMENTAL LIMITS ON NET PRIMARY PRODUCTION AND LIGHT-USE EFFICIENCY ACROSS THE OREGON TRANSECT

Due to climate differences, an extreme range in productivity occurs al ong a 250-km, west-east transect at almost-equal-to 44-degrees north l atitude in western Oregon, USA, where coniferous evergreen forests dom inate. As part of the Oregon Transect Ecosystem Research (OTTER) proje ct, our objective was to evaluate how climate constrains net primary p roduction (NPP) by limiting the utilization of intercepted photosynthe tically active radiation (IPAR). The forests measured along the transe ct intercepted from 22% to 99.5% of the incident PAR. With data collec ted from recording meteorological stations installed near each site, w e defined the hourly conditions when photosynthesis was partly or comp letely limited by drought, extreme humidity deficits, or frost. From t his analysis we calculated that the fraction of incident PAR that coul d be utilized throughout the year ranged from 99% in the coastal rainf orests to <25% in the juniper woodland. NPP varied from 3 to 26 Mg.ha- 1.yr-1 with the fraction of belowground NPP, estimated from litterfall , increasing from 20% to 60% of the total as the environment becomes h arsher. Light-use efficiency (epsilon(u)), calculated under conditions when the environment did not constrain photosynthesis, averaged 0.8 g /MJ for aboveground NPP and 1.3 g/MJ for total NPP.