Weed species radiation-use efficiency as affected by competitive environment

Plant canopy radiation-use efficiency (RUE), defined as unit of dry biomass produced per unit of absorbed photosynthetically active radiation (APAR), has been widely studied both in experimental and theoretical contexts because the use of this relationship greatly simplifies estimating biomass production in plant growth models. Previous studies have indicated that RUE may be sensitive to changes in the fractions of diffuse and direct radiation; RUE has been shown experimentally to increase under conditions of increased diffuse light caused either by atmospheric conditions or by shading from other plants in intercrops. Therefore, we hypothesized that weed species RUE would be greater for weeds grown in mixed weed-crop communities than for weeds grown in more uniform and less dense monotypic communities. To address this question, field experiments were conducted during 2001 and 2002 to determine the vegetative-stage RUE of giant ragweed, velvetleaf, woolly cupgrass, and wildproso millet grown in monorypic communities or in corn. ANOVA indicated that the effect of community type on RUE was significant (P < 0.0001) and that the interaction between species and year effects was significant (P = 0.0152). Paired comparisons showed that giant ragweed RUE differed from velvetleaf RUE in 2001 (P = 0.0381) and that giant ragweed RUE differed between years (P = 0.0455). Pooled across species types and years, RUE was approximately 50% greater for weeds grown in weed-corn communities than for weeds grown in monorypic communities. These results indicate that more complex canopy architecture in mixed-species communities (i.e., greater total leaf area index [LAI] and heterogeneity of height among individuals) was associated with greater weed RUE. Including weed RUE response to the competitive environment may be one approach to improving the predictive accuracy of process-based growth models for weed biomass accumulation in mixedspecies communities.