Effects of a recalcitrant understory fern layer in an enclosed tropical restoration experiment

Abstract

Establishing mixed-species tree plantings and fencing them to protect seedlings from herbivory is a valuable strategy for reconnecting forest fragments separated by agropastoral lands. However, fencing may provide exotic plants with the escape from herbivory required to invade the understory of planted communities. Here we take advantage of such a situation to ask how the identity of planted species and the resulting canopy cover influenced invasion success by the Asian swordtail fern (Nephrolepis brownii Desv. Nephrolepidaceae) in a 13-year-old tropical restoration experiment. Through a seed addition experiment, we also evaluated the effects the ferns had on recruiting seedlings. We found that the invasion was most acute in the unplanted control plots where canopy cover was consistently scarce. Frond density correlated negatively with canopy cover, though most of the variance in the model is explained by the design of our experiment (r2m = .161, r2c = .460). Between planting treatments that differed in the dispersal mode of the planted trees, the wind-dispersed treatment had higher fern density and longer fronds than the animal-dispersed treatment. The animal-dispersed treatment had the highest recruiting species richness, which was negatively correlated with fern density (r2 = .748). The seed addition experiment confirmed that mortality rates increased where frond density was higher (F1,41 = 7.159, p = .011) and germination rates were lowered for the smaller-seeded species (F1,42 = 13.2, p = .002). To prevent recalcitrant understory layers from establishing in plantings in the future, we recommend: (1) establishing larger plantings or expanding existing ones to minimize edge effects (particularly light filtration), (2) supplementing young plantings with additional seedlings to prevent canopy gaps from forming, and (3) planting an assemblage of species that cover the full forest strata and have consistently full tree-canopies.

Return of forest structure and diversity in tropical restoration plantings

“…However, it is unlikely that natural succession would perform as well if they did not neighbor planted treatment plots, which likely enhances seed dispersal. Moreover, a benefit to being dispersed to natural succession plots is that there is more available colonizable space, a benefit available to seedlings capable of overcoming the invasive fern layer, potentially explaining the unexpectedly high non‐pioneer abundance (Beltrán et al, 2020).…”

“…Natural succession on the contrary had similar plant abundance as the animal‐plantings and forest (Appendix S2: Figure S1) but had lower tree basal area simply because these plots lacked planted trees. Comparing recruits only, the benefit of recruiting in the natural succession plots becomes clear: Seedlings that manage to grow past an invasive fern layer (Beltrán et al, 2020) benefit from growing under reduced tree‐tree competition. This is consistent with a previous study that used individual‐based modeling to show rapidly growing tree species in the natural succession plots had a 40% higher probability of reproduction than recruiting trees in the planted treatments (Caughlin et al, 2019).…”

“…Seedling predation by large mammals is prevented with barb wire fences; herbivory by small rodents is still possible, but the impact that rodents in the plots have on seedling survival and the mechanisms driving their foraging preferences remain to be fully evaluated (but see Beltrán, 2021; Guzman et al, 2021; Howe & Davlantes, 2017). Canopy cover does differ between planting treatments, with the wind‐plantings having more exposed and variable canopy covers ( x¯ = 82.4%, SE = 1.39%) than animal‐plantings ( x¯ = 88.89%, SE = 0.5%, p = 0.048) (supplemental material 1 in Beltrán et al, 2020). A more variable, though, generally full canopy could provide light conditions that favor a greater suite of plants (pioneers and non‐pioneers alike), a relationship described as the heterogeneity‐diversity hypothesis, potentially explaining the higher abundance of plants in wind‐plantings (Dupuy & Chazdon, 2008; Lundholm, 2009).…”

“…The restoration treatments (described in greater detail in Methods section) include the following: natural succession plots where nothing was planted, plots planted with wind‐dispersed species, and plots planted with animal‐dispersed species. Natural succession plots lack continuous canopy cover (Beltrán et al, 2020), which exacerbates dispersal and establishment limitations (Beltrán et al, 2020; de la Peña‐Domene & Martínez‐Garza, 2018). Plots with animal‐dispersed species have a fuller canopy and provide fleshy fruits that attract dispersal agents.…”

Siembra directa de árboles nativos para la restauración de la selva estacionalmente seca

“…Para ambas localidades se puso a prueba la hipótesis de que las especies tardías tienen mayor éxito en la emergencia y sobrevivencia de sus plántulas que las tempranas. Dado que la intervención mínima de restauración puede favorecer el establecimiento no solo de árboles sino también de pastos o helechos (Beltrán et al, 2020), la remoción del estrato herbáceo puede mejorar el desempeño de los árboles. Así, se espera que la remoción de cobertura vegetal favorezca la germinación y establecimiento de las plántulas, especialmente las de especies tardías que pueden ser más resistentes; también esperamos que la siembra directa sea más efectiva junto con las plantaciones de restauración donde podría haber una menor invasión de pastos y helechos debido a la disminución de luz por la presencia de un dosel arbóreo.…”