Pinus taeda is less efficient at nitrogen uptake in the presence of the invasive grass, Imperata cylindrica, compared to native understory

Background/Question/Methods

Several studies have examined the role of native understory species competition on the uptake and use of nitrogen (N) by young Pinus taeda.  To date, no work has shown how P. taeda N uptake is affected by the presence of Imperata cylindrica, an alien rhizomatous grass that is currently invading pine forests throughout the southeastern United States.  Using ¹⁵N stable isotope, we compared how P. taeda seedlings acquire and use N in the presence of I. cylindrica and native vegetation in an emerging pine forest in Florida.  With a vegetation free treatment used to determine the full potential of P. taeda to uptake and use N, 1-year-old P. taeda seedlings were planted in plots of one of three treatments: 1) vegetation free (VF) 2) native (NC) competition, and 3) I. cylindrica (IC) competition.  At planting, 5% ¹⁵N enriched Ammonium Sulfate was applied to the treatment plots at the rate of 55kg of N per ha.  At the end of the growing season, the ¹⁵N-fertilized P. taeda seedlings were harvested, dried, ground, and analyzed for total N and ¹⁵N concentration using an Isotope Ratio Mass Spectrometer.   

Results/Conclusions

After one full growing season, I. cylindrica competition led to smaller pine seedlings with significantly less N content in the pine foliage and roots than the native and vegetation free treatments (p<0.0001).  Competition from I. cylindrica for N contributed to the pine seedlings in the treatment taking up a greater percentage of the applied fertilizer than the seedlings competing with native vegetation. The %N derived from fertilizer (NDF) in pine seedlings in the VF treatment was significantly lower (p<0.0001) than the other treatments.  The total fertilizer N use efficiency (UFN) was significantly higher in the VF treatment than the other treatments (p<0.0001).  The pine seedlings in the other treatments were never half as efficient as the ones growing in the VF treatment.  The IC pine seedlings were least efficient at utilizing the fertilizer because of their reduced growth that resulted from the I. cylindrica competition.