Net Carbon Exchange of an Old-Growth Hemlock Forest at Harvard Forest HEM Tower since 2000

This project estimates carbon exchange rates of multiple forest types at Harvard
Forest (see HF072) and compares them to long-term ongoing carbon exchange
measurements at the EMS, which is located in a mesic, 60-90 year old red oak and red
maple dominated forest on abandoned farmland (HF004). Measurements in each forest
type are used to investigate climatic influences on carbon exchange. This mesic hemlock-dominated forest with most trees 100-200 years old on
undisturbed soils stored only about 3 Mg/ha of carbon in 2001, compared to over 4
Mg/ha in the 60-90 year old oak/maple stand. However, both sites stored more carbon
in 2001 than was measured in the oak/maple stand in any previous year since 1991 (see
HF004). The hemlock forest behaved very differently from the oak-maple stand in that
the highest rates of carbon storage occurred in spring, while there was very little
carbon storage in mid to late summer. Statistical models of carbon exchange in the
hemlock forest showed that carbon storage was positively related to daily minimum air
temperature in spring, but negatively correlated with soil temperature in the summer.
The first effect was attributable to a positive influence of above-freezing minimum
temperatures on photosynthesis by hemlock foliage. The negative relationship of soil
temperature to carbon storage by hemlock forest in summer was due to exponentially
increasing soil and ecosystem respiration, accompanied by a neutral or negative
effect of high air temperature on photosynthesis by hemlock trees (see HF063). These
effects indicate that carbon storage in the hemlock forest could be strongly affected
by climate warming, but the effects will probably be in opposite directions in spring
and summer.