Ecological and Environmental Controls over Fifteen-Year Forest Net Ecosystem Production at the University of Michigan Biological Station

Abstract:

Forests in the US upper Midwest and northeast are broadly undergoing a gradual ecological transition in which short-lived tree species decline and give way to longer-lived species. Environmental changes are paralleling these ecological shifts with potential consequences for the region’s carbon (C) sink status. Long-term C flux measurements from the University of Michigan Biological Station Ameriflux core site meteorological tower (UMB) are demonstrating how changes in forest structure and composition, and the environment combine to constrain net ecosystem production (NEP) over decadal timescales.

Annual NEP of the UMB forest increased by nearly 1 Mg C ha^-1 yr^-1 from 1999 through 2013, while leaf area index (LAI) declined by a half unit. The UMB NEP time-series is characterized by several years of relative stability followed by a variable upward trend beginning in 2007. While growing season photosynthetic active radiation explains a majority of the interannual variation in NEP, the recent rise in production coincides with an abrupt decline in the LAI of early successional aspen (Populus spp.) and birch (Betula papyrifera), and a reciprocal rapid increase in the LAI of later successional and longer lived red oak (Quercus rubra) and red maple (Acer Rubrum). The product of apparent quantum yield, a metric of light-use efficiency, of the canopy and maximum NEP increased as LAI declined with the senescence of aspen and birch, suggesting that shifts in canopy composition improved how efficiently light is used to drive forest C uptake. We conclude that an upward trend in NEP at the UMB site is jointly caused by environmental and ecological change, the latter of which is progressively altering the physiology of the canopy.