Altered Soil Properties Inhibit Fruit Set but Increase Progeny Performance for a Foundation Tree in a Highly Fragmented Landscape

Tanya M. Llorens,Richard Fairman,Colin J. Yates,Bronwyn Macdonald,David J. Coates,Jane Sampson,Margaret Byrne,Carole P. Elliott

doi:10.3389/fevo.2018.00039

Tanya M. Llorens, Richard Fairman + Show 6 more

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https://doi.org/10.3389/fevo.2018.00039

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Abstract

Failing to test multiple or non-standard variables in studies that investigate the effects of habitat fragmentation on plant populations may limit the detection of unexpected causative relationships. Here, we investigated the impacts of habitat fragmentation on the pollination, reproduction, mating system and progeny performance of Eucalyptus wandoo, a foundation tree that is bird and insect pollinated with a mixed-mating system. We explored a range of possible causative mechanisms, including soil properties that are likely to be altered in the agricultural matrix of a landscape that has naturally nutrient-poor soils and secondary soil salinization caused by the removal of native vegetation. We found very strong negative relationships between soil salinity and fruit production, thus providing some of the first evidence for the effects of salinity on reproduction in remnant plant populations. Additionally, we found unexpectedly higher rates of seedling survival in linear populations, most likely driven by increased soil P content from adjacent cereal cropping. Higher rates of seed germination in small populations were related to both higher pollen immigration and greater nutrient availability. Trees in small populations had unexpectedly much higher levels of pollination than in large populations, but they produced fewer seeds per fruit and outcrossing rates did not vary consistently with fragmentation. These results are consistent with small populations having much higher insect abundances but also increased rates of self-pollination, combined with seed abortion mechanisms that are common in the Myrtaceae. This study highlights the need to better understand and mitigate sub-lethal effects of secondary soil salinity in plants growing in agricultural remnants, and indicates that soil properties may play an important role in influencing seed quality.

Highlights

In the past two decades many studies have investigated the ecological and genetic processes affecting plant population viability and species persistence in fragmented habitats
Our study shows that investigation of a broad range of factors can reveal unexpected and unconventional relationships regarding impacts of habitat fragmentation on plant species persistence
We found that Eucalyptus wandoo trees in small populations tended to produce fewer fruit and seeds, and population selfing rates did not vary consistently with fragmentation

Summary

Introduction

In the past two decades many studies have investigated the ecological and genetic processes affecting plant population viability and species persistence in fragmented habitats (for reviews see Hobbs and Yates, 2003; Aguilar et al, 2006, 2008). To address the need for a greater understanding of the influences of interacting landscape and biological variables, some recent studies have taken a broader approach to assessing plant responses to fragmentation These studies found important but unexpected causative relationships, such as the effect of remnant shape on pollen diversity, that may be overlooked by failing to test multiple or non-standard variables (Breed et al, 2012; Llorens et al, 2012, 2013). They emphasize the need to separate the consequences of different processes (Bunnell, 1999), to integrate abiotic and biotic interactions into the same study (Ouborg et al, 2006), and to treat different landscape parameters as independent variables (Fahrig, 2003; Llorens et al, 2012)

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