Research Assistant UC Santa Barbara, United States
Reforestation efforts in many regions of the world focus on planting of canopy trees and only rarely are understory species actively restored. While the influence of canopy trees on ecosystem processes is well studied, the influence of planted understory species is not. Here, we evaluate the influence of outplanted woody understory plants on invasive grass biomass and soil nutrient properties in 30+ year old planted stands of the native N-fixing tree Acacia koa used for reforestation in Hawaii. We ask whether current understory planting has a detectable influence in N cycling metrics and whether high density planting could have a strong rapid influence. We analyze soils from under plantation A. koa with and without planted woody understory and compare these to soils from under remnant trees of the pre-deforestation dominant, Metrosideros polymorpha where passive recruitment of woody understory plants is occurring. We also experimentally planted understory species at three times the density normally used by local managers to see if we could drive down invasive grass biomass and enhance soil organic matter ‘recovery’ in a relatively short (4 yr) time frame.
invasive grass biomass declined with understory planting in both surveyed sites and experimental plantings. Yet, contrary to predictions, woody understory planting under plantation A. koa had no effect on any metric of N cycling, either short or long term, or soil properties including organic matter. Short term N availability and nitrification potential were higher under A. koa compared to remnant M. polymorpha and metrics were not correlated with basal area of woody understory or canopy tree size. The only influence of understory on soil that we found was a positive correlation of soil organic matter with understory basal area under remnant M. polymorpha trees. Overall, our data demonstrate that to date, restoration of understory vegetation has not changed soil characteristics including N cycling beneath A. koa. Indeed, variation in sampled variables was low among A. koa trees despite wide variation in understory abundance suggesting that it is difficult to overcome the strong influence of these N-fixing trees on these soils even if invasive grass biomass declines with understory development.