Proximity and abundance of mother trees affects recruitment patterns in a long‐term tropical forest restoration study

Rakan A Zahawi,Juan Abel Rosales,Leland K Werden,Karen D Holl,Jeffry Flores,Miriam San‐José

doi:10.1111/ecog.05907

Abstract

Remnant trees and forest fragments in agricultural landscapes can be important sources of propagules to facilitate forest recovery. However, many studies simply quantify forest cover in the surrounding landscape as a percentage, with little attention given to species composition, and subsequently fail to detect an effect on recruitment patterns. We assessed the relative importance of the spatial distribution and life‐history traits of 77 tree species on recruitment patterns at a landscape scale in a well‐replicated long‐term restoration study in southern Costa Rica. We censused and mapped potential mother trees in a 100‐m buffer surrounding eight replicate restoration plots and quantified respective tree recruits within each plot. We assessed how mother tree abundance, species life‐history characteristics (seed size, dispersal mode), tree size (DBH, height) and distance to restoration plot affected recruitment at coarse (plot: 50 × 50 m) and fine (quadrat: 3 × 3 m) spatial scales. The presence of a mother tree within 100 m of a restoration plot resulted in a 10‐fold increase in potential mean recruitment. Mother tree abundance was also an important driver of recruit density, and particularly so for large‐seeded (≥ 5 mm) zoochorous species with a fivefold increase in recruit density across the observed mother tree abundance range. An interaction between mother tree abundance and proximity demonstrated that the effect of mother tree abundance on recruit density was important but waned with increasing distance from restoration plots. At the fine spatial scale, proximity was uniformly important; height and DBH of the closest potential mother tree also affected recruit abundance but responses differed by seed size. Results highlight the importance of remnant vegetation composition to the recovery of adjacent degraded habitats, underscoring the outsized role nearby remnant forest and isolated trees can play for the persistence of localized biodiversity.

Highlights

Most tropical forests in the world have been heavily fragmented with the resulting landscape a patchwork mosaic of interconnected habitats that are less able to support remaining biodiversity (Haddad et al 2015, de Lima et al 2020)
We hypothesized that such results may be an artifact of the coarse resolution at which remnant forest cover is assessed, and predicted that distinct patterns would emerge if recruit dynamics were examined at the species level by mapping the presence of adult trees in remnant forest habitat surrounding targeted restoration plots
We found a strong effect of the presence of potential mother trees within 100 m of a restoration plot on recruitment, and mother tree proximity to restoration plots resulted in a three- to four-fold difference in average recruitment within the distance range covered

Summary

Introduction

Most tropical forests in the world have been heavily fragmented with the resulting landscape a patchwork mosaic of interconnected habitats that are less able to support remaining biodiversity (Haddad et al 2015, de Lima et al 2020). Many key tree genera are wind-dispersed (e.g. Cedrela, Ceiba), seed dispersal in wet tropical forests is overwhelmingly driven by animals (Gentry 1982). Animal movement patterns are impacted by forest fragmentation and loss, which can either increase or decrease their range depending on the species and study system in question (Nield et al 2020, Ramos et al 2020). Even though landscape-scale research has repeatedly shown that a minimum forest threshold is essential to maintaining a habitat’s functional integrity and diversity (Banks-Leite et al 2014, Arroyo-Rodríguez et al 2020), and many note that remnant forest patches and trees interspersed in agricultural landscapes are an essential source of tree propagules for recovering habitats (Laborde et al 2005, Cadavid-Florez et al 2019), localized site-specific studies often fail to note a direct effect of surrounding forest cover on patterns of recovery (Howe et al 2010, Zahawi et al 2013)

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Discussion

Conclusion