Long‐term exclusion of invasive ungulates alters tree recruitment and functional traits but not total forest carbon
Kara Allen, Peter J. Bellingham, Sarah J. Richardson, Robert B. Allen, Larry Burrows, Bob Carswell, Sean W. Husheer, Mark G. St. John, Duane A. Peltzer
Abstract
Abstract Forests are major carbon (C) sinks, but their ability to sequester C and thus mitigate climate change, varies with the environment, disturbance regime, and biotic interactions. Herbivory by invasive, nonnative ungulates can have profound ecosystem effects, yet its consequences for forest C stocks remain poorly understood. We determined the impact of invasive ungulates on C pools, both above‐ and belowground (to 30 cm), and on forest structure and diversity using 26 paired long‐term (>20 years) ungulate exclosures and adjacent unfenced control plots located in native temperate rainforests across New Zealand, spanning 36–41° S. Total ecosystem C was similar between ungulate exclosure (299.93 ± 25.94 Mg C ha −1 ) and unfenced control (324.60 ± 38.39 Mg C ha −1 ) plots. Most (60%) variation in total ecosystem C was explained by the biomass of the largest tree (mean diameter at breast height [dbh]: 88 cm) within each plot. Ungulate exclusion increased the abundance and diversity of saplings and small trees (dbh ≥2.5, <10 cm) compared with unfenced controls, but these accounted for ~5% of total ecosystem C, demonstrating that a few, large trees dominate the total forest ecosystem C but are unaffected by invasive ungulates at a timescale of 20–50 years. However, changes in understory C pools, species composition, and functional diversity did occur following long‐term ungulate exclusion. Our findings suggest that, although the removal of invasive herbivores may not affect total forest C at the decadal scale, major shifts in the diversity and composition of regenerating species will have longer term consequences for ecosystem processes and forest C.