Clumped planting arrangements improve seed production in a revegetated eucalypt woodland

Abstract

The arrangement of plants within revegetated sites is rarely considered an important characteristic of these communities. However, in natural systems, plant spatial arrangements can influence a range of ecological processes, including pollination and seed set. Pollinators tend to preferentially visit larger and/or more closely spaced populations, with plants in these populations generally receiving more outcrossed pollen, resulting in increased seed set and better quality seed. Similar trends may occur in revegetated populations, but little is known about the influence of planting arrangement on seed production in restored systems. Here, we quantified the effect of plant abundance (number of conspecifics within 100 m) and distance to nearest reproductive conspecific on the level of seed set for six eucalypt species (n = 422 trees in total) in 1 year and for one of these species (Eucalyptus leucoxylon), across three additional years. Seed number per fruit was highly variable both between individuals and within individuals across years. Despite this variability, there was a consistent trend of higher seed production (seed number per fruit) when another reproductive conspecific was within 20 m. In contrast, plant abundance had little influence on seed production. Further investigation of nearest neighbor arrangements found the distance to either the first, second, third, or fourth reproductive neighbors were the key predictors of seed production. Therefore, revegetation designs that consider plant spacing and aggregation, rather than only planting to overall density criteria (i.e. trees/ha), at least for the eucalypts studied here, has the potential to improve seed production in revegetated populations. Implications for Practice• Using a standard number of plants per hectare to guide revegetation, without consideration of the spatial arrangement of those plants, may limit the reproductive output of restored populations. • Planting designs that consider the spacing between conspecifics (avoiding large distances between plants) and conspecific aggregation, have the potential to increase seed production and seed quality in revegetated populations. • Aggregated arrangements are common in natural systems, so planting designs that mimic natural patterns of conspecific spacing and aggregation may improve seed production and the self-sustaining nature of revegetated populations.

Plants, position and pollination: Planting arrangement and pollination limitation in a revegetated eucalypt woodland

“…() supports this hypothesis, indicating the distance between trees is an important predictor of seed production in revegetated eucalypts (McCallum et al . ). However, it is unclear if planting arrangements, specifically conspecific aggregation (planting individuals of the same species in clumps), influence other aspects of plant reproduction, such as fruit set, germination and the degree of pollination limitation.…”

“…A study by McCallum et al . () supports this hypothesis, indicating the distance between trees is an important predictor of seed production in revegetated eucalypts (McCallum et al . ).…”

“…There is no evidence of spatial genetic structuring (i.e. genetically similar individuals more likely to be closer together spatially) in the revegetated population (McCallum ).…”

“…Donor pollen was sourced from revegetated Blue Gum individuals, with trees treated with pollen from three different donors. Donor trees were sourced at least 50 m from experimental trees, but it was not possible to ensure that source trees were not related to experimental trees due to the absence of spatial genetic structure in the revegetated population (McCallum ). However, efforts were made to collect donor flowers from trees that had physical differences to the experimental trees in order to maximize pollen diversity (e.g.…”

“…It can be hypothesized that similar trends will occur in revegetated communities if planting activities create populations that are smaller and/or more spatially dispersed than those that occur naturally . A study by McCallum et al (2019) supports this hypothesis, indicating the distance between trees is an important predictor of seed production in revegetated eucalypts (McCallum et al 2019). However, it is unclear if planting arrangements, specifically conspecific aggregation (planting individuals of the same species in clumps), influence other aspects of plant reproduction, such as fruit set, germination and the degree of pollination limitation.…”