Multi-Decadal Carbon Cycle Measurements at the Howland Forest AmeriFlux Site

Abstract:

Multi-decadal measurements of carbon (C) cycle processes provide long-term estimates and needed perspectives on shorter-term apparent trends. We have conducted tower-based eddy covariance measurements at the Howland Forest of central Maine since 1996, supplemented by 20 years of soil respiration and litterfall measurements, and periodic inventories of aboveground biomass and woody increment that span 30 years. The Howland Forest supports a mature multi-aged forest dominated by red spruce and eastern hemlock with 10% deciduous tree species. Soils are Spodosols. Mean annual temperatures is 6.2°C and mean annual precipitation is 1148 mm. The forest was partially logged a century ago and has been unmanaged since. Here we present an integrated C budget for both aboveground and belowground processes. Despite large inter-annual variation and some apparent multi-year trends, we detected no significant long-term trends in mean annual woody increment (WI), fine litterfall (FL), and soil respiration (SR). Based on eddy covariance measurements initiated in 1996, net ecosystem exchange (NEE) increased from about 150 to over 250 g m^-2 yr^-1 today. However, biometric data from resampling of plots established in the late 1980s indicate a higher WI value in the early 1990s, so even the 25 year record of NEE measurements may be insufficient to demonstrate the true multi-decadal trend. The long-term average WI is 150 g C m^-2 yr^-1 in a 3-ha inventory plot several hundred m from the eddy covariance tower, although smaller plots nearer to the tower indicate rates as high as 200 g C m^-2 yr^-1. The soil sink is difficult to estimate, but our data suggest that it is unlikely to exceed 20 g m^-2 yr^-1. A trend of declining total ecosystem respiration (TER), from about 1000 g C m^-2 yr^-1 in the mid-1990s to about 860 g C m^-2 yr^-1 today, contrasts with the absence of a trend in SR, which has a long term average of 790 g C m^-2 yr^-1. Although there is significant inter-annual variation, this sub-boreal forest has remained a relatively consistent sink during recent decades of about 2 Mg C ha^-1 yr^-1.