Abstract
Plant ontogenetic stage and features of surrounding plant neighbourhoods can strongly influence herbivory and defences on focal plants. However, the effects of both factors have been assessed independently in previous studies. Here we tested for the independent and interactive effects of neighbourhood type (low vs. high frequency of our focal plant species in heterospecific stands) and ontogeny on leaf herbivory, physical traits and chemical defences of the English oak Quercus robur. We further tested whether plant traits were associated with neighbourhood and ontogenetic effects on herbivory. We found that leaf herbivory decreased in stands with a low frequency of Q. robur, and that saplings received less herbivory than adult trees. Interestingly, we also found interactive effects of these factors where a difference in damage between saplings and adult trees was only observed in stands with a high frequency of Q. robur. We also found strong ontogenetic differences in leaf traits where saplings had more defended leaves than adult trees, and this difference in turn explained ontogenetic differences in herbivory. Plant trait variation did not explain the neighbourhood effect on herbivory. This study builds towards a better understanding of the concurrent effects of plant individual- and community-level characteristics influencing plant-herbivore interactions.
Highlights
Plants grow in heterogeneous environments where the presence of conspecific or heterospecific neighbouring plants can greatly affect their growth, survival and reproduction[1, 2], as well as their interactions with antagonists[3,4,5] and mutualists[6]
Predictions for ontogenetic differences in herbivory and allocation to defences may vary depending on the neighbourhood context, where for example differences in apparency between adult and young plants of a focal plant species might be weaker in heterospecific relative to conspecific neighbourhoods, or in heterospecific neighbourhoods with a high vs. low frequency of that host plant
We selected two adjacent stands similar in size and total plant density: one with a high frequency of Q. robur individuals (>85% of the adult individuals were of the focal species) and another with a low frequency of this species (
Summary
Plants grow in heterogeneous environments where the presence of conspecific or heterospecific neighbouring plants can greatly affect their growth, survival and reproduction[1, 2], as well as their interactions with antagonists[3,4,5] and mutualists[6]. Plants that are easier to find (i.e., more apparent) either because they are larger in size, more abundant, or long-lived should invest more in defences than smaller, less abundant or ephemeral plants (Plant Apparency Theory[21]) Studies testing this prediction have mostly focused on inter-specific comparisons, we could extend this framework to make predictions about intra-specific variation, that occurring over ontogeny. Predictions for ontogenetic differences in herbivory and allocation to defences may vary depending on the neighbourhood context, where for example differences in apparency between adult and young plants of a focal plant species might be weaker in heterospecific relative to conspecific neighbourhoods, or in heterospecific neighbourhoods with a high vs low frequency of that host plant (i.e., resource concentration effects increasingly overrule ontogenetic differences with decreasing host plant frequency). This study builds towards a better understanding of the combined effects of plant neighbourhood and ontogeny on plant-herbivore interactions and plant traits associated with such effects
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