Assisted restoration interventions drive functional recovery of tropical wet forest tree communities

Abstract

Choosing appropriate forest restoration interventions is challenging. Natural regeneration can rapidly facilitate forest recovery in many situations. However, barriers such as dispersal limitation and competition with non-native species can require assisted restoration approaches to facilitate plant community recovery. We used a study that has directly compared the outcomes of tropical wet forest restoration interventions across 11 replicate sites in southern Costa Rica. Within this framework, we examined the functional recovery trajectories of recruiting tree sapling communities across a gradient of restoration interventions including low (natural regeneration), intermediate (applied nucleation), and high (plantation) initial resource-investment, which we compared to remnant reference forest. We collated leaf and stem functional traits for tree species that comprised the bulk of recruiting saplings, then determined how community-weighted trait means and functional diversity metrics changed over a decade across treatments. Results show that assisted restoration approaches (applied nucleation, plantation) sped the development of more functionally diverse tree communities, more than tripling the functional richness (FRic) of recruiting communities when compared to natural regeneration. However, functional dispersion (i.e., the trait range of dominant species) was equivalent across interventions, and between 28 and 44% lower than remnant forest, indicating that increases in FRic under assisted restoration were driven by species recruiting in low abundances (<10 individuals across treatments). Recruits in assisted restoration treatments also had 10–15% tougher, less-palatable leaves, and leaves were even tougher in reference forest, which could be driven by increasing herbivory pressure along the gradient of interventions. Results show that tracking simple metrics such as species richness can mask a more mechanistic understanding of ecosystem recovery that is elucidated by taking a functional trait-driven approach toward evaluating outcomes. For example, our work identified a paucity of dense-wooded species recruiting across restoration interventions, wood density was 11–13% lower in restoration treatments than reference forests, underscoring such species as prime targets for enrichment planting. Overall, findings suggest that assisted restoration can catalyze the functional recovery of naturally recruiting tree communities in landscapes that are slow to recover naturally and highlight the importance of evaluating how different components of functional diversity shift over time to fully understand restoration outcomes.

Belowground differentiation among trees in a degraded tropical dry forest landscape: no evidence of a collaboration gradient

“…These results are consistent with other studies (Bergmann et al 2020; Arrieta-González et al 2021; Weemstra et al 2023). Surprisingly, conservative traits such as RTD and RDMC were negatively related to D (Werden et al 2022), challenging the traditional idea that D increases with RTD for carbon conservation (McCormack et al 2012; Weemstra et al 2016). This negative correlation may reflect that D increases through the root cortex, a parenchymatous tissue with low carbon content and dry weight, faster than the stele area, a tissue specialized in transporting nutrients and water through lignified cells (Kong et al 2014; Valverde-Barrantes et al 2017).…”

“…In Colombia, around 90% of its cover was replaced by the end of the 20th century (Etter et al 2008; García et al 2014), and only 8% of TDF’s original cover is left in land mosaics of early successional stages (González-M et al 2018). In these degraded landscapes, natural regeneration is challenging, and planting trees is necessary for re-establishing local biodiversity and ecosystem functioning (Rodrigues et al 2009; Garbowski et al 2020; Werden et al 2022). However, water limitation imposed by the pronounced dry season (3–6 dry months precipitation <100 mm⋅month −1 , Portillo-Quintero and Sánchez-Azofeifa, 2010; González-M. et al 2019), which is exacerbated by soil compaction due to livestock grazing (Batey 2009; Tracy et al 2011), represents a barrier to the successful restoration of degraded TDF.…”

“…Fine roots, which are the most distal orders of fine root systems (typically first- to third-order roots), are specialized in nutrient and water acquisition (McCormack et al 2015) and therefore critical for species performance in degraded ecosystems (Paz et al 2015; Garbowski et al 2020; Werden et al 2022). Recent studies have identified two main functional belowground gradients expressing globally (Bergmann et al 2020).…”

Mid‐ and long‐term ecological changes after enrichment planting with native tree species in Mexican tropical mountain forests

“…Méndez‐Dewar et al (2014) have argued on the need to further refine the position of tree species along the conventional gradient of shade‐intolerance to shade‐tolerance to better explain patterns of forest regeneration and restoration approaches. Distinction of smaller groups of species based on functional attributes like light response over different life stages, and the nature and amount of seed reserves (Valladares & Niinemets 2008; Williams‐Linera & Manrique‐Asencio 2020; Werden et al 2022), may help to better understand the relationship between oaks and shade‐tolerant trees in all possible habitats included in old‐growth stands.…”

Transplanting naturally regenerated tree seedlings for tropical forest restoration: a case study of Calophyllum brasiliense and Vochysia guatemalensis in Mexico

“…While passive restoration (natural regeneration) is the most affordable alternative, it is constrained by several barriers, such as dispersal limitation or competition with exotic species (Holl & Aide 2011). In highly degraded areas, or when enriching low‐diverse forests or plantations with slow‐growing species is desirable, assisted restoration approaches have to be applied (Werden et al 2022).…”