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

Background/Question/Methods

We earlier presented calcium cycling budgets in northern hardwood stands in the White Mountains of New Hampshire indicating 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 subsequently established measurement plots in replicate stands of 3 ages (14-19 yr, 26-29 yr, and > 100 yr) at Bartlett Experimental Forest.  We characterized nutrients in soils in three quantitative soil pits in two stands of each age group, using a sequential extraction procedure (Nezat et al. 2007).  We sampled trees for allometric analysis (Fatemi et al. 2011) from the same six stands and analyzed leaf, branch, bark and wood tissues for nutrient concentrations.  Roots from the soil pits were analyzed for mass and concentration.  We hypothesized that the accumulation of Ca and other elements in biomass after forest harvesting would be reflected in depletion of soil available pools.

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 mineral soil in the old stands averaged 817 kg/ha, about as high as the living biomass pool.  In the stands regrowing post-harvest, apatite pools (142 kg/ha) were much lower (P = 0.01).  It seems plausible that this pool is responsible for some fraction of the Ca in the regrowing forest.  Finally, silicate pools of Ca were very high (22,883 kg/ha).  The sustainability of repeated forest harvest depends on the rate of mineral weathering, which may be somewhat responsive to biotic demand.