Have our Forests Stopped Growing?
Detecting changes in forest productivity through analyzing
150 years of aboveground biomass accumulation
in the White Mountains, New Hampshire

Nuengsigkapian, P.

Forest productivity within the northeastern United States is postulated to have suffered overall decline due to net effects of acid and nitrogen deposition, and other recent anthropogenic environmental effects. To determine if these forests have indeed suffered, we compared aboveground tree biomass data from five forested sites located in the White Mountains, NH. We analyzed patterns of long-term biomass accumulation in two unmanaged, even-aged forests using an 81-yr. chronosequence located in West Campton and a 150-yr. chronosequence in Bartlett Experimental Forest (BEF); both chronosequences include repeat measurements. The chronosequences were compared to biomass accumulation trends in Watershed 6 of Hubbard Brook Experimental Forest (HBEF) and to biomass accrued in uneven-aged stands of BEF and old-growth forests in Waterville Valley and Area I of the Bowl Research Natural Area. Aboveground tree productivity increased linearly for 70-80 years in both West Campton (1.79 Mg ha^-1 yr^-1; R² = 0.89) and in even-aged stands of BEF (1.47 Mg ha^-1 yr^-1; R² = 0.90). Aboveground biomass continued to increase in BEF, reached a mean peak biomass of 202 ± 5 Mg ha^-1 at 110 yr. and leveled off at 162 Mg ha^-1 at 150 yr. in 1991. Biomass accumulated in old-growth stands of BEF reached mean biomass of 207 ± 5 Mg ha^-1 in 1991 and the Bowl reached 224 Mg ha^-1 in 1994. Waterville Valley attained biomass value of 228 Mg ha^-1 in 1903. We conclude that forest growth in terms of aboveground tree biomass has not diminished significantly as a result of recent environmental effects.