Tree seed rain and seed removal, but not the seed bank, impede forest recovery in bracken (Pteridium aquilinum (L.) Kuhn)‐dominated clearings in the African highlands

Abstract

Considerable areas dominated by bracken Pteridium aquilinum (L.) Kuhn occur worldwide and are associated with arrested forest recovery. How forest recovery is impeded in these areas remains poorly understood, especially in the African highlands. The component processes that can lead to recruitment limitation—including low seed arrival, availability and persistence—are important determinants of plant communities and offer a potential explanation for bracken persistence. We investigated key processes that can contribute to recruitment limitation in bracken‐dominated clearings in the Bwindi Impenetrable National Park, Uganda. We examined if differences in seed rain (dispersal limitation), soil seed bank, or seed removal (seed viability and persistence) can, individually or in combination, explain the differences in tree regeneration found between bracken‐dominated areas and the neighboring forest. These processes were assessed along ten 50‐m transects crossing the forest–bracken boundary. When compared to the neighboring forest, bracken clearings had fewer seedlings (bracken 11,557 ± 5482 vs. forest 34,515 ± 6066 seedlings/ha), lower seed rain (949 ± 582 vs. 1605 ± 335 tree seeds m−2 year−1), comparable but sparse soil seed bank (304 ± 236 vs. 264 ± 99 viable tree seeds/m2), higher seed removal (70.1% ± 2.4% vs. 40.6% ± 2.4% over a 3‐day interval), and markedly higher rodent densities (25.7 ± 5.4 vs. 5.0 ± 1.6 rodents per 100 trapping sessions). Camera traps revealed that rodents were the dominant animals visiting the seeds in our seed removal study. Synthesis: Recruitment limitation contributes to both the slow recovery of forest in bracken‐dominated areas, and to the composition of the tree species that occur. Low seed arrival and low persistence of unburied seeds can both explain the reduced density of seedlings found in bracken versus neighboring forest. Seed removal, likely due to rodents, in particular appears sufficient to constrain forest recovery and impacts some species more severely than others.

Post-Dispersal Seed Removal in a Large-Seeded Palm by Frugivore Mammals in Western Ecuador

“…Identifying animal seed removers using camera traps is a common practice in ecological research on seed dispersal (Blackham & Corlett, 2015; Campos et al., 2018; Christianini & Galetti, 2007; Cramer et al., 2007; da Silva et al., 2011; Dai et al., 2018; Galetti et al., 2015; Iob & Vieira, 2008; Razafindratsima, 2017; Seufert et al., 2010; Ssali et al., 2018). Several studies have used camera traps to attribute single events of seed removal to particular animal species (Brown et al., 2016; Jansen et al., 2012; White et al., 2017).…”

Conceptualising the Global Forest Response to Liana Proliferation

“…For example, the disturbance-favouring niche can be occupied by ferns (Kessler, 2010), bamboo (Griscom and Ashton, 2006), shrubs (Jiménez-Castillo et al, 2007), or grasses (Van Nes et al, 2014), particularly at high elevations and latitudes, or in degraded forests. These plant forms can be common and widespread, e.g., bracken fern (Pteridium aquilinum) is found on every continent except Antarctica and inhibits forest succession through soil deterioration and shading, and by facilitating seed predation (Ssali et al, 2018(Ssali et al, , 2019. Grass is widely implicated in thresholds between forest and savanna through fuelling wildfires (Van Nes et al, 2014).…”

Disturbance and the elevation ranges of woody plant species in the mountains of Costa Rica

“…Furthermore, disturbance could accentuate climatic extremes, promoting the persistence of those species more tolerant to drought or frost. There may also be undesirable persistent vegetation states that are favored by certain disturbance processes (Ssali, Moe, & Sheil, ). In any case, the use of disturbance would need to be guided by the best available information and would need to be reevaluated locally.…”