Alteration of ecosystem nitrogen dynamics by exotic plants: A case study of C₄ grasses in Hawaii

Biological invaders can alter ecosystem processes via multiple pathways, yet few studies have compared the relative importance of these pathways. We assessed the impacts of exotic, invasive grasses on ecosystem nitrogen (N) cycling in the seasonal submontane woodlands of Hawaii Volcanoes National Park, where native grasses have been historically rare. Exotic grasses have become abundant over the past 30 yr and have altered two controls over N cycling: plant species composition and fire regime. Here we synthesize the results of a long-term investigation of species impacts in this system. To determine effects of grasses and fire on internal N cycling, we compared litterfall, decomposition, N mineralization from soil organic matter (SOM), and plant N uptake and production in invaded unburned forest, grass-removal plots within the forest, and woodland converted to grassland by fire. We measured ecosystem N loss via fire by comparing N pools among unburned, naturally burned, and experimentally burned sites. We also assessed the effects of fire on annual N fixation in the unburned forest vs. the grassland. Exotic grasses had relatively small effects on N cycling in the unburned woodland despite being abundant in the understory for 30 yr. Grasses contributed ∼30% of fine litterfall and primary-production mass and N in the unburned woodland. However, these contributions did not result in significantly increased totals because litterfall and production of Metrosideros polymorpha, the dominant native tree, was reduced in the invaded woodland relative to grass-removal plots, presumably due to competition with grasses. Although area-weighted decomposition was lower in the grass-removal treatment than in the control, net N mineralization from litter and SOM were similar between these treatments. Annual plant N uptake was similar to annual net N mineralization from SOM in both treatments. By contrast, the burned grassland exhibited much lower rates of litterfall and production mass and N, but higher rates of net N mineralization from SOM than the woodland. As a result, total annual plant N uptake was only 17% of annual net mineralization. This change was primarily due to the loss of native species. Aboveground N pools were significantly reduced with fire. Native species were largely eliminated by fire. However, across all burned and unburned sites there was no change in total ecosystem N because the N contained in biomass was relatively small compared to N in litter and soil. Soil contained >95% of ecosystem N in all sites. Only in the high-intensity experimental burn was there significant loss of N from the soil pool. Fire reduced N inputs through asymbiotic N fixation mainly due to the loss of M. polymorpha, whose litter is an important site of asymbiotic N fixation, and alteration of the soil O-layer. This reduction in N inputs makes it unlikely that fixation activity will replace N lost via combustion before the next fire. Fire and the ensuing loss of native species led to decreased N inputs, increased rates of N mineralization from litter and SOM, and dramatically reduced plant N uptake, potentially leading to a more leaky N cycle. It appears that the indirect effects of grasses on N cycling via the elimination of native species by fire is the most important pathway though which exotic grasses alter ecosystem N dynamics in this system.