Tree biomass and net increment in an old aspen forest in New Mexico

Previous work indicated that net ecosystem production (NEP) was negative in a 90-year-old aspen (Populus tremuloides Michx.) forest in the Tesuque watershed of northern New Mexico. The loss of nitrogen (N) in streamwater was very low for this forest, which would be surprising if NEP were negative for a substantial period. We remeasured live tree biomass of this case-study forest after an additional 27 years of growth, as a long period with negative NEP would likely demonstrate declining live tree biomass. Contrary to expectation, live tree biomass increased substantially through this period (averaging 1.37 Mg ha^-1 yr^-1). The net increment for this period was relatively high (within 10% of the mean annual increment) well into the second century of stand development in this high-elevation aspen forest. We conclude that the high retention of N was not anomalous given the later increase in live tree biomass, and we expect that NEP had not remained negative for a long period.