Restoring structural features of old-growth forests, such as increased canopy gap sizes and coarse woody debris (CWD), is a common management goal in younger second-growth stands. Evidence from controlled experiments with pre-treatment assessment, however, has been limited. We experimentally manipulated forest structure through canopy gap creation and CWD additions in a second-growth northern hardwood forest in north-central Wisconsin following two years of pre-treatment monitoring. Gap sizes (50, 200, and 380 m²) were chosen to represent the typical range found in old-growth forests of the region. The short-term objectives were to quantify aboveground biomass and aboveground net primary productivity (ANPP) as influenced by canopy gap creation and gap size one year after treatment. We estimated dry biomass (Mg/ha) and ANPP (Mg/ha/yr) of the overstory trees (≥10 cm diameter at breast height [dbh]) and saplings (between 0.5 and 9.9 cm dbh) using regional allometric biomass equations, litterfall using 0.25 m² traps located throughout the plots, and the understory woody and herbaceous vegetation using clip quadrats and permanent vegetation monitoring quadrats for three years (2005-2007).

Results/Conclusions

In gap treatment plots, aboveground biomass decreased from an average of 232 Mg/ha pre-treatment to an average of 1.8 Mg/ha post-treatment as a direct result of harvest of overstory trees and subsequent damage to saplings. Aboveground biomass and ANPP of saplings decreased 78% as a result of harvest damage in gap treatment plots. However, aboveground biomass and ANPP of woody and herbaceous understory gap vegetation increased above pre-treatment levels in gap treatment plots due to reduced competition from overstory and sapling vegetation. Across the range of gap sizes, aboveground biomass and ANPP showed no clear trend, with 50 m² gaps having the highest biomass and ANPP and 200 m² gaps having the lowest biomass and ANPP. In addition, sapling biomass was highest in 50 m² gaps and lowest in 200 m² gaps both pre-treatment and post-treatment. We speculate the variability in aboveground biomass and ANPP across the range of gap sizes was attributed to differential damage to saplings and pre-treatment sapling biomass, not gap size. These results show that ANPP and biomass accumulation may be influenced by the stand conditions prior to treatment and potential damage to advanced regeneration (i.e. saplings), an important consideration for future forest management.