Effects of forest disturbances and an introduced ungulate on carbon storage in boreal forests

Abstract

Selective browsing by ungulates alters forest structure and composition, with research suggesting that these effects may be amplified in recently disturbed forests and in novel environments (i.e., introduced ungulates). Though the net effects of herbivory following disturbances on carbon storage in boreal forests are not well understood, they are likely important to inform climate-smart ecosystem management. We conducted observational and experimental (i.e., 24- to 27-year-old paired exclosure–control plots) field studies to determine the effects of non-native moose and common boreal forest disturbances (i.e., fires and insect outbreaks) on total, aboveground, and belowground carbon storage in Newfoundland, Canada. We measured carbon stocks at 46 plots in the field and used published allometric equations to estimate the carbon stored in trees, deadwood, woody shrubs, saplings, herbaceous plants, ground vegetation, and roots. We also collected samples of forest litter and organic soil layers for analysis of carbon content. We then fit a suite of generalized linear models to assess the effects of disturbances and moose herbivory on carbon storage. Using our observational data, we found that total carbon stored was highest in mature forests (198.18 ± 136.77 kg C/9 m²; mean ± SD), compared to insect-disturbed (57.47 ± 30.72 kg C/9 m²) and previously burned (42.57 ± 34.28 kg C/9 m²) areas. We found further evidence of this using our experimental data, as aboveground carbon storage was reduced in disturbed forests. Although, on average, exclosures stored more carbon (133.54 ± 134.69 kg C/9 m²; mean ± SD) than their paired controls (95.23 ± 66.94 kg C/9 m²), we did not detect a statistically significant effect of moose presence on carbon storage after 24–27 years of exclusion. We also did not find a relationship between moose density and carbon storage in our observational data. Overall, we demonstrated that forest disturbances are a key driver of carbon storage and that moose effects on carbon storage are highly variable across the landscape. We recommend that future research be done in areas having consistent ungulate management and long-term population data and that detailed plant community data be collected to better understand the roles of ungulate density and plant palatability on carbon storage.