Sustainable forest harvest requires calcium supply from soil pools: Ecosystem budgets for second-growth northern hardwoods in New Hampshire

Background/Question/Methods

Calcium cycling budgets in northern hardwood stands in the White Mountains of New Hampshire indicated that young stands were accumulating Ca in the forest floor as well as the vegetation and that leaf litter was more Ca-rich in young (<30 yr old) than in older stands (Hamburg et al. 2003, Yanai et al. 2005).  Stream concentrations of Ca at Hubbard Brook also remain elevated for decades in young stands.  We suggested that accelerated apatite weathering could explain the high rate of Ca mobilization in young stands.  We established measurement plots in replicate stands of 3 ages (14-19 yr, 26-29 yr, and > 100 yr) at Bartlett Experimental Forest.

Results/Conclusions

Calcium in aboveground and belowground biomass averaged 1102 kg/ha in mature stands.  The amount of Ca in exchangeable form in soils was only 287 kg/ha in the mature stands, clearly not enough to supply Ca to forest regrowth after harvest, even for the first few decades.  The young and mid-aged forests contained 375 and 540 kg Ca/ha in living biomass.  Exchangeable Ca in the young and mid-aged stands was 230 and 184 kg/ha, not significantly different from the pre-harvest (old) condition.  Thus the exchangeable pool is neither sufficient to explain forest growth nor is it depleted by forest growth.  Apatite pools in the stands regrowing post-harvest averaged only (219 kg/ha). Finally, silicate pools were very high (11,000 - 26,000 kg/ha).  The sustainability of repeated forest harvest depends on weathering rates of Ca and other elements.