Field-Based Radiometry to Estimate Tidal Marsh Plant Growth in Response to Elevated CO<SUB>2</SUB> and Nitrogen Addition

Abstract

Plant growth is one of the most important variables to measure in long-term research plots, but the negative effects of labor-intensive and destructive sampling can restrict frequent assessment of plant biomass. Here, we used field-based, active radiometry to assess plant biomass in an ongoing, experimental manipulation of atmospheric CO 2 and soil nitrogen availability in a tidal wetland. We compared the ability of several radiometric vegetation indices (VIs) to predict total plant biomass and that of two plant functional groups, sedges and grasses. All VIs estimated total biomass better in July than in October, when senescence had begun. All VIs correlated strongly and positively to grass biomass (average r = 0.83) and weakly or negatively to sedge biomass ( r = -0.30). Modified soil-adjusted vegetation index (MSAVI2) performed well through space (average July total biomass r = 0.83) and time (across four sampling times r = 0.83) and predicted CO 2 and nitrogen treatment effect sizes. In conjunction with conventional biomass measurements field-based, active radiometry provides (1) a frequent estimate of biomass that can reveal plant responses to environmental stimuli that would otherwise escape detection, and (2) a viable alternative to frequent destructive sampling for assessing growth of fine-stemmed species such as Spartina patens and Distichlis spicata .