Invasive C₄ grasses that colonize tropical landscapes abandoned from use for intensive grazing and agriculture can inhibit natural regeneration of secondary forest. In Panama, dense stands of Saccharum spontaneum require active forest restoration to re-establish successional processes. In this region, restoration strategies typically involve clearing grass cover and applying fertilizer prior to planting tree seedlings. However, if fertilizers alleviate nutrient limitation in the grasses and enhance their competition with tree seedlings, it can add to costs for manual maintenance of the sites free of Saccharum. Here we evaluated how S. spontaneum responds to nitrogen and phosphorus addition in the field to determine whether S. spontaneum is nutrient limited in this system. S. spontaneum was both nitrogen and phosphorus as revealed through increased foliar nutrient concentrations. S. spontaneum biomass was significantly greater in both nitrogen and phosphorus addition plots after both the first growth period (early rainy season) and second growth period (late rainy season), with release from co-limitation of N and P, and the overall impact of N, greater during the second growth period. Nutrient limitation in S. spontaneum and seasonal shifts in resource allocation suggest caution when fertilizing areas under restoration that were previously dominated by exotic grasses.

This experiment consisted of twelve blocks, each with four nutrient treatments:  no nutrient addition (control; C), N addition (N), P addition (P), and N+P combination (N+P). Plots were 5 x 5 m², with a 2-m buffer between plots within a block, and a 3-m buffer between blocks. Dry fertilizer was added by hand after clearing S. spontaneum in July. Nitrogen was added as urea ((NH₂)₂CO₂), and phosphorus was added as triple super phosphate (Ca(H₂PO₄)₂•H₂O). Nutrient application corresponded to 125 kg ha^-1 N and 50 kg ha^-1 P, as recommended for this region.  Nutrients were applied after clearing the site in July and again in October, after the first biomass harvest. Soil cores were taken at the end of the study (December) from each plot to compare final soil nutrient availability across treatments. We assessed S. spontaneum performance, in terms of density and above-ground biomass, in September and December 2011. Additionally, in December we collected soil and leaf nutrient data to compare nutrient treatments. We randomly collected and homogenized 10 soil cores at 10-cm depth for each plot; a 20-g subsample was used to extract soil nitrogen and another for phosphorus. We followed the KCl and Mehlich standardized protocols to extract available nitrate (NO₃^-), ammonium (NH₄⁺), and phosphate  (PO₄^-2). Soil samples were placed directly into solution in the field and processed in the lab within 24 h of being collected. For plant tissue nutrient analysis, we collected the third mature leaf from the base of 15 randomly selected individuals in each plot. Leaves were dried for 3 days at 60^oC, and samples were processed at the University of California Santa Cruz. Five leaves were selected from each plot and leaf N and P were extracted following the Kjeldahl acid digestion protocol using a Lachat BD 46 block digester (Lachat Instruments, Milwaukee, WI USA).

Data are in a .csv file that can be opened with any spreadsheet or R.