Restoration interventions mediate tropical tree recruitment dynamics over time

Abstract

Forest restoration is increasingly heralded as a global strategy to conserve biodiversity and mitigate climate change, yet long-term studies that compare the effects of different restoration strategies on tree recruit demographics are lacking. We measured tree recruit survival and growth annually in three restoration treatments—natural regeneration, applied nucleation and tree plantations—replicated at 13 sites in southern Costa Rica—and evaluated the changes over a decade. Early-successional seedlings had 14% higher survival probability in the applied nucleation than natural regeneration treatments. Early-successional sapling growth rates were initially 227% faster in natural regeneration and 127% faster in applied nucleation than plantation plots but converged across restoration treatments over time. Later-successional seedling and sapling survival were similar across treatments but later-successional sapling growth rates were 39% faster in applied nucleation than in plantation treatments. Results indicate that applied nucleation was equally or more effective in enhancing survival and growth of naturally recruited trees than the more resource-intensive plantation treatment, highlighting its promise as a restoration strategy. Finally, tree recruit dynamics changed quickly over the 10-year period, underscoring the importance of multi-year studies to compare restoration interventions and guide ambitious forest restoration efforts planned for the coming decades. This article is part of the theme issue ‘Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration’.

Assessing the use of natural colonization to create new forests within temperate agriculturally dominated landscapes

“…There is increasing evidence to suggest that a degree of spatially targeted tree planting, such as establishing small patches of trees, that is “woodland islets” and “applied nucleation” (Benayas et al 2008), can actually assist rather than hinder natural processes. For example by enhancing seed dispersal, providing habitats and future seed sources, and ameliorating the local environment (Holl et al 2020; Kulikowski et al 2023).…”

Fifteen essential science advances needed for effective restoration of the world's forest landscapes

“…planting monocultures [ 123 ], exotic/invasive species [ 124 ] or ecologically inappropriate species [ 125 ], inappropriate locations [ 126 ] and inadequate local input/collaboration [ 49 ]. Decisions regarding where to plant trees remain challenging, in terms of both where to focus restoration in a landscape (Advance 2), and what species and spatial arrangements to use for planting to maximize potential for natural regeneration and to minimize costs [ 127 ]. Consequently, restoration plantings often have high rates of mortality, inadequate species composition and hence also low functional or socio-economic value [ 25 ].…”

“…If potential impacts of competitive plants can be adequately understood and predicted, restoration methods can be selected that are more appropriate and cost-effective (Advance 16). Competitive plant removal is a standard treatment alongside tree-planting, as used successfully in Costa Rica in this theme issue [ 127 ]. Thresholds of forest biomass are also used in the theme issue to demonstrate widespread potential for competitive grass, bracken and liana management across a region of high biodiversity value in Tanzania [ 32 ].…”

“…Assessing recovery rates using chronosequences does not offer the same level of information provided by long-term studies and can be misleading [ 164 ]. Long-term monitoring within individual restoration sites provides critical information on demographic rates of seedlings, saplings and trees which inform the mechanisms of treatment effects [ 127 ]. Large, long-term datasets are also crucial for proving and recognizing tipping points, which can require very large amounts of data [ 165 ].…”

Low predictability in aboveground biomass accumulation in Brazilian semi‐deciduous seasonal Atlantic Forest restoration sites

“…One key issue related to this is the longevity of planted tree species (Holl & Zahawi 2014; Körner 2017). The trees planted in many forest restoration projects are mostly short‐lived, fast‐growing, low‐wood density species, and as might be expected, early sapling recruitment into new trees can largely contribute to maintain these characteristics for a long time (Holl & Zahawi 2014; Kulikowski et al 2022). Thus, these strong, early gains in carbon storage following faster tree growth can be transitory (Körner 2003).…”