The structural complexity of old field vegetation and the recruitment of bird-dispersed plant species

Abstract

The input of bird-disseminated seeds into four old fields of different structural complexity was examined. Seed input was greatest along the edges of fields. Significantly more seeds were found in a 13 year old field that had structurally complex vegetation, than in a 3-year-old field with a single layer of vegetation. The lower input into the latter field was a function of both low fruit availability and low structural complexity of the field. Similarly, more seeds were found in a 2-year-old field which had artificial structures, simulating saplings, placed in it than in an adjacent control field of the same age. The shape of the structures was not a significant factor in the input of seeds. Timing of seed deposition was correlated with fruit ripening times, relative nutritional value of the fruit and the movements of frugivorous birds. The input of bird-disseminated seeds into fields appears to be directly related to the structural complexity of the vegetation. Woody plants increase the structural complexity of the old fields and serve as recruitment foci for bird-disseminated seeds. Thus, seed deposition by birds influences vegetation pattern, and conversely, the presence of recruitment foci in the vegetation may influence bird dispersal patterns of bird-disseminated seeds.

Nucleation in tropical ecological restoration

“…The perches serve, visually in the landscape, as beacons for resting, foraging and latrine sites, resulting in the nuclear deposition of high diversity seed rain, which also represents a feeding place for granivorous animals. Several authors have previously reported the effect of artificial perches on restoration (McDonnel and Stiles, 1983;Guevara et al, 1986;McClanahan and Wolfe, 1993;Whittaker and Jones, 1994;Holl, 1998Holl, , 1999GalindoGonzales et al, 2000;Shiels and Walker, 2003).…”

Isolated trees with high crown coverage and densities increase pasture seed rain

“…Additionally, the structural characteristics of plants that growing in pastures (such as their heights, diameters, crown coverage, and densities) will influence seed dispersal as tall, broad trees can provide shelter, nesting, and foraging sites (McDonnell 1986;Dean et al 1999). Trees with wide and more highly branched crowns promote greater seed abundance beneath them (McDonnell & Stiles 1983;Holl 1998;Cole et al 2010;Derroire et al 2016) and can serve as attractive environments for seed dispersers and will provide the immediate microclimate for seed germination and seedling growth (Belsky et al 1989;Callaway 2007;Derroire et al 2016). Trees with a larger diameters and hollows in their trunks are important for many bird species (Mazurek & Zielinski 2004), and trees with larger diameters have greater crown coverage and more branches, and will produce more flowers and seeds (Chapman et al 1992;Greene & Johnson 1994).…”

Influence of vegetation structure on spatial patterns of seed deposition by birds

“…This suggests that seed deposition in grassland is limited by a lack of suitable perch sites for dispersal vectors; this may lead to slow colonisation of grassland by forest species. By acting as "recruitment foci", isolated remnant trees (McDonnell & Stiles 1983;Guevara et al 1986) and artificial perches (McClanahan & Wolfe 1987,1993 can increase the input of seeds of late-sucessional, fleshy-fruited species to otherwise homogeneous herbaceous fields.…”

“…The distribution of bird-disseminated seed rain tends to deviate from the smooth decay curve characteristic of wind-dispersed seeds, and may instead be characterised by distinct peaks where perches such as isolated trees act as nuclei for deposition (McDonnell & Stiles 1983;Debussche & Isenmann 1994). Thus, high densities of bird-dispersed seeds have frequently been observed below post-foraging perch sites, with comparatively few seeds encountered in adjacent areas without perches (Debussche et al 1982;Kollmann & Pirl 1995).…”

“…The heterogeneous pattern of seed rain creates the potential for similar spatial heterogeneity to develop in the soil seed bank (Saulei & Swaine 1988;Kollmann & Pirl 1995). These spatial patterns may be particularly important in early-successional disturbed sites where isolated remnant trees can act as "recruitment foci" for dispersed seeds (McDonnell & Stiles 1983). Knowledge of seed rain and seed bank characteristics may be especially important in the context of ecological restoration projects aiming to promote recruitment of desirable native species while simultaneously minimising the spread of weeds, and may enable tentative predictions to be made of the outcome of disturbance events.…”