Exotic and Native Forbs: Plant Productivity, Soil Respiration, and Soil Nitrogen

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Exotic and Native Forbs: Plant Productivity, Soil Respiration, and Soil Nitrogen

January 18, 2015

Morgan McLeod, Kyle Jensen, Ylva Lekberg share results from an experiment that compared biomass production, respiration, and soil nitrogen in native and invasive forb monocultures after one year of growth.

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Invasive plants, such as spotted knapweed, outcompete native plants and reduce diversity at a local scale. Invaded areas often show increased plant growth and soil nitrogen. These differences may stem from comparing a grass to a forb, or a monoculture to a mixed plot, rather than direct effects of invasion. To look at direct and indirect effects of forb invasion, we established a grid of native and exotic plots in spring 2013. Plots contain monocultures of a single species, an even mixtures of species, or mixtures with one dominant species.

Based on evidence from field studies, we expected invaders to have higher biomass volume than natives. Two of the invaders, knapweed and leafy spurge, produced significantly greater biomass than native species (fig 1). Aboveground biomass production may be connected to belowground carbon allocation. We expected invaders to exhibit higher soil respiration than natives. Two invaders, knapweed and leafy spurge, exhibited higher soil respiration than native species (fig 2).

Based on previous field and experimental studies we expected higher amounts of soil nitrogen, measured here as ammonium (NH4 +) and nitrate (NO3 -), in invaded soil. We only detected higher soil NH4 + in a single invader, sulfur cinquefoil (fig 4). We measured lower soil NO3 - in a single native, sticky cinquefoil (fig 5).

Experimental plot aboveground standing biomass peaks during mid-June.