Long-term forest carbon storage and structural development as influenced by land-use history
Temperate forests are an important carbon sink, yet there is uncertainty regarding land-use history effects on biomass accumulation and carbon storage potential in secondary forests. Understanding long-term biomass dynamics is important for managing forests as carbon sinks and for co-benefits such as watershed protection and biodiversity. How have New England’s secondary forests recovered post nineteenth century agricultural abandonment? How has the region’s extensive land-use history influenced long-term structural development and aboveground carbon storage? To answer these questions, we employed a longitudinal study based on twelve years of empirical data (2001-2013) collected from 60 permanent monitoring plots within 16 reference stands at the Marsh-Billings-Rockefeller (MBR) National Historical Park in Woodstock, VT. We also used 150 years of documentary data from park management records. This research evaluates the effects of reforestation approaches (planting vs. natural regeneration), management regimes (varying levels of harvesting frequency and intensity), and stand development pathways and cover types on biomass outcomes. We generated biometrics indicative of stand structural complexity, including the H’ structural diversity index, and aboveground biomass (live trees, snags, and downed coarse woody debris pools) estimates based on allometric equations. Multivariate analyses evaluated the relative predictive strength of reforestation approach, management history, and site characteristics on carbon pools and structural complexity indicators.
Classification and Regression Tree (CART) analysis ranked forest age structure (even or uneven) as the strongest predictor of long-term aboveground carbon storage, while forest cover type, harvest frequency, and stand age were selected as secondary variables. Uneven-aged stands stored an average of 44.34 Mg ha-1 more carbon than even-aged stands. CART ranked forest cover type as the strongest predictor of H’ index, while harvest intensity, harvest frequency, and site class were selected as secondary variables. Due to recent increase in harvestings, MBR stands stored 30% less total aboveground carbon in 2013 than in 2003. There is a significant difference (P = 0.01) in total aboveground carbon among naturally regenerated and planted stands as of 2013. Our results suggest that a variety of long-term recovery pathways converge on high levels of aboveground carbon storage, including both conifer plantations and naturally regenerated hardwood stands, but choice of silvicultural management approach can dramatically alter those trajectories. In our data set there was a strong positive relationship between forest carbon storage and structural complexity, supporting the concept of multifunctional forestry emphasizing late-successional habitats.