Understory Response to Selective Tree Harvesting in East Kalimantan, Indonesia

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Non-native plant invasions frequently occur following disturbances in forest ecosystems. Canopy openings and disturbed soil conditions may facilitate such invasions and reduce successful tree regeneration of desired species. The forest cover of post-harvest natural forest concessions remain dense following selective harvesting and reduced impact logging methods can minimize impacts to the forest canopy. The remaining dense forest cover may not provide enough light for non-native plant invasions. However, there is a lack of research on invasive understory vegetation and its relation to natural tree regeneration in logged over forests. We examined the biomass of non-native understory plants and the density of native tree regeneration in post-harvest compartments including 1-year, 3-years and 5-years post-harvest in East Kalimantan, Indonesia. We installed plots in different sampling strata including skid trails, harvesting gaps, and adjacent non-harvested areas (non-gaps) in each post-harvest forest to record understory plant biomass and characterize the light environment using leaf area index (LAI). We found that LAI was significantly higher 5-year post-harvest (4.14) than 1-year post-harvest (2.36). We also found that LAI was significantly higher in non-gaps (4.31) than skid trails (2.03). Skid trails generally had the lowest LAI values across the post-harvest sites and the LAI increased as post-harvest year increased. We found three non-native plant species across post-harvest years and sampling strata, except in non-gap and gap of 5-year post-harvest, with the average biomass of non-native plants contributed less than 1% of the overall plant biomass. We did not find a significant correlation between invasive non-native plant biomass with LAI (p-value >0.05). We identified desired tree seedlings based on commercially valuable trees harvested by the forest manager. We found five species of desired tree seedlings in our plots out of 21 harvestable trees; Shorea spp., Litsea macrophylla, Vatica rassak, Diospyros borneensis, and Hopea driobalanoides. Only Litsea macrophylla and Hopea driobalanoides were significantly different throughout post-harvest years: year-1 post-harvest was dominated by Litsea macrophylla, while year-5 post-harvest was dominated by Hopea driobalanoides. Our results did not show that small gaps created by skid trails and tree harvesting facilitate the invasion of non-native plants from logging roads and other heavily disturbed areas into the residual forest stands. We found that five years post-harvest in selective harvesting and RIL forests appears to be enough time for LAI recovery as LAI increased significantly in 5-year post-harvest. Our results showed that % N significantly increased in 3-year and 5-year post-harvest compared to 1-year post-harvest and we would expect shade tolerant species more abundant as time post-harvest increases. However, we found Litsea macrophylla, a shade tolerant species most abundant in 1-year post-harvest areas as compared to 3- or 5-year post harvest. We did not find a correlation between desired tree seedling densities and soil components, thus we do not believe the soils are limiting seedling recruitment in Dipterocarp forests.

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