Low N losses may drive pine invasion in central Great Plains

Woody encroachment into grasslands is a prevalent phenomenon worldwide and can drastically alter ecosystem functioning such as N cycling; however in the central US, woody species have had varying invasion success.  In the abandoned agricultural fields of central MN, two oak species, Quercus macrocarpa and Q. ellipsoidalis are slowly advancing, while eastern white pine (Pinus strobus) is invading at much higher rates.  In these strongly N limited systems, high productivity of P. strobus relative to its deciduous counterparts may be driving this invasion success.  In order to achieve these high levels of productivity, N is required by the plant.  P. strobus could have three strategies by which to get the required N: 1) increased N uptake by P. strobus; 2) reduced N losses associated with P. strobus; and 3) high N use efficiency (NUE).  In order to examine which of these hypotheses account for increased productivity in P. strobus, we calculated a total N budget for P. strobus, Q. macrocarpa, Q. ellipsoidalis, two dominant grasses, and two forb species using closed monoculture mesocosms at the Cedar Creek LTER in central MN.  We measured above and belowground productivity, photosynthetic rates, soil N levels, and NO₃ losses.  Using a one-way ANOVA we found no significant differences across all species and a bare soil control in total plot N (plants + soil); however P. strobus had the lowest NO₃ losses (23.1 ± 9.7 mg/m² for the entire growing season) from the plots along with the highest amount of N in leaf tissue (10.2 ± 1.8 g N/m²) compared to all other species (P < 0.05).  Our results indicate that the combination of high leaf longevity and low NO₃ leaching losses associated with P. strobus allows for more N available for photosynthesis which may influence its rate of woody encroachment into grasslands.