Effects of nutrient amendment and environment on growth and gas exchange for introduced Penniseturn setaceum in Hawaii

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Abstract
We examined the effects of a balanced soil nutrient amendment on photosynthesis, growth, and reproduction for the alien grass Penniseturn setaceum across an altitudinal gradient on the island of Hawaii. Nutrient amendment of plants enhanced aboveground growth, height, and numbers of reproductive tillers and inflorescences similarly across the altitudinal gradient. Nutrient amendment increased aboveground biomass 22 to 25% but had little effect on predawn water potential, leaf nitrogen concentration, or photosynthesis. Leaf nitrogen concentrations declined with altitude (22 ± 0.5 mg/g N at the coastal site; 16 ± 0.1 mg/g N at the subalpine site) and partially compensated for decreases in specific leaf area with altitude. Plants at the subalpine site had high dark respiration rates, low CO2 saturated photosynthetic rates, and low photosynthetic nitrogen use efficiencies compared with plants at lower altitudes. Chilling temperatures or high respiratory rates may limit net carbon uptake and growth for plants at higher altitude relative to plants at lower altitude. Seasonal patterns of net photosynthesis, stomatal conductance, and water potential suggest that drought colimited carbon uptake and growth at the low altitude site. Success of Penniseturn in Hawaii may stem, in part, from its ability to respond morphologically to compensate for heterogeneous nutrient and water availability across different habitats. Keywords: altitude, Hawaii, nutrient amendment, photosynthesis, Penniseturn setaceum, resource limitation.