Restoring ecosystem function: how understory species influence soil carbon and nitrogen cycling in a degraded and restored Hawaiian mesic forest

Background/Question/Methods

Feedbacks within degraded forests offer a lens to assess alternate stable states in the context of guiding native plant restoration and forest recovery. In largely cleared Hawaiian mesic forests, previously dominated by relatively slow-growing Meterosideros polymorpha (‘ōhi‘a), feedbacks between birds, plants and soil influence the persistence of a degraded ecosystem state. A forest canopy was “restored” in some areas by planting a fast-growing native nitrogen-fixing tree, Acacia koa (koa), but understory still has not recovered passively. Koa increases soil nitrogen (N), facilitating a verdant exotic grass understory hindering native understory recovery, while remnant ‘ōhi‘a facilitate some recovery via passive bird dispersal of native woody plants which form halos near the base of ‘ōhi‘a. We aim to understand how canopy and understory species influence soil C and N cycling. We hypothesize that high water-extractable N in koa litter increases soil N, exacerbated by volcanic soil minerals associating and stabilizing high-N koa organic matter. We sampled litter and soils beneath ‘ōhi‘a understories of halos and grass, and koa understories of outplanted ‘ōhi‘a and pilo (Coprosma rhynchocarpa), and grass. We measured water-extractable C and N, followed incubated soil C and N microbial cycling, and determined loss on ignition and mineral-associated %C and %N.

Results/Conclusions

Koa litter and green leaves leached significantly more N than ‘ōhi‘a litter, and had much lower DOC:DN ratios. Similarly, there was much greater soil nitrate (NO3-) beneath koa in all understory settings than beneath ‘ōhi‘a. Microbial biomass C:N was lowest beneath koa regardless of understory species. Soil beneath koa also had a greater proportion of heavy mineral fraction as opposed to light fraction than ‘ōhi‘a, which may result in a greater association, stabilization and accumulation of N (data processing still in progress). Soil extractable organic C and combustible C were greatest beneath ‘ōhi‘a halos and comparable beneath ‘ōhi‘a with grass understory and koa (regardless of understory). Soil beneath ‘ōhi‘a with halos also had greater microbial biomass C, microbial activity, and organic matter than ‘ōhi‘a with grass or koa with any understory species. This suggests that understory plant species, once well-established, can substantially influence soil C and N cycles, providing hope that outplantings beneath koa will eventually alter soil cycling and potentially mitigate the high N contributions of koa litter with more C-rich organic matter. These results also highlight the complex role of soil chemistry in ecological restoration and its importance for plant community recovery.