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

Due to climate differences, an extreme range in productivity occurs along a 250-km, west-east transect at almost-equal-to 44-degrees north latitude in western Oregon, USA, where coniferous evergreen forests dominate. As part of the Oregon Transect Ecosystem Research (OTTER) project, our objective was to evaluate how climate constrains net primary production (NPP) by limiting the utilization of intercepted photosynthetically active radiation (IPAR). The forests measured along the transect intercepted from 22% to 99.5% of the incident PAR. With data collected from recording meteorological stations installed near each site, we defined the hourly conditions when photosynthesis was partly or completely limited by drought, extreme humidity deficits, or frost. From this analysis we calculated that the fraction of incident PAR that could be utilized throughout the year ranged from 92% in the coastal rainforests 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 harsher. 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.