Tree taxon effects on the phenology of caterpillar abundance and biomass

Pied Flycatcher, by John Harding/BTO

Author(s): Macphie, K.H., Samplonius, J.M., Hadfield, J.D., Pearce-Higgins, J.W. & Phillimore, A.B.

Published: December 2024  

Journal: Oikos

Digital Identifier No. (DOI): 10.1111/oik.10972

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Changes to the timing of spring events are some of the most obvious and highly documented responses to climate change. Differences in the response of timing between interacting groups, such as predators and prey, have the potential to leave some ‘mismatched’, meaning predators lack access to resources on which they rely.

The reliance of many woodland breeding birds on caterpillars to feed nestlings in spring has received considerable research attention, as birds often respond less to temperature change than caterpillars, a resource that is only present at high abundance for a matter of weeks. However, most work on mismatch between caterpillars and breeding passerines has focused on dynamics within oak-dominated woodlands. To date, little is understood about how the caterpillar peak varies among woodlands composed of different deciduous tree taxa, despite the wide range of woodland types lepidoptera and passerines breed and live in. Could some tree species and woodland compositions support the caterpillar population at a higher abundance or across a longer period of time, thus buffering the negative consequences of mismatch?

This study uses data on caterpillar abundance and biomass collected throughout spring across 44 sites in Scotland over 10 years. Sites varied in woodland composition with 10 main deciduous trees: alder, ash, beech, birch, elm, hazel, oak, rowan, sycamore, willow. The results found that birch, oak and willow support the greatest numbers of caterpillars, with the peak on oak and willow also lasting roughly a week longer than average. Oak also stood out as an exceptional host through a knock-on effect on abundance throughout the local woodland, with more caterpillars found on branches of any tree species when oak trees were present or at higher density.

In considering whether woodland tree composition is likely to buffer the negative consequences of mismatch, the study concludes this is unlikely to occur to great effect, as the caterpillar peak is not distinctly timed on different trees, with would maximise the overall spread of the peak within a woodland. However, when mistimed and breeding in a woodland lacking oak, birch or willow, birds able to travel to and forage in woodland containing these trees, with higher and longer lasting abundances of caterpillars, may be buffered more than those who are unable to.

This study highlights the value of broadening the range of habitats in which we study key impacts of climate change, with implications for conservation, resilient forestry planting and management decisions.

Abstract
Phenological shifts are well documented biological responses to warming. While many studies have focused on the mean timing of an event, there is growing appreciation that the height and width of the phenological distribution will also impact on species interactions. A temperate deciduous forest food chain of oak trees – arboreal caterpillars – insectivorous passerines has become paradigmatic in research on phenological mismatch. This focus on oak-dominated woodlands means that we have limited insight into whether 1) caterpillar phenological distributions vary among tree taxa and habitats and 2) oak is an exceptional host, which has implications for the potential for buffering of interactions on a local and landscape scale. Here, we survey caterpillar abundance and mass throughout spring on 10 tree taxa for 10 years across 44 Scottish woodland sites. We found substantial variation in caterpillar abundance among host taxa, with oak, birch and willow yielding similarly high numbers of caterpillars, and evidence that caterpillar abundance increases with the density of oak foliage within a woodland stand, but not with the density of other taxa. Considering variation in the phenological distribution of caterpillars on different host taxa, we found the main axis of variation to be the maximum abundance/total biomass reached, which was highest on oak. We found significant variation in the mean timing of abundance and duration of abundance and total biomass among hosts, though effect sizes were quite small, and little evidence for among host variation in the phenological distribution of individual caterpillar mass. In woodlands where oak is abundant, our findings are consistent with the presence of other tree taxa providing little local buffering of phenological mismatch. Whereas, in the absence of oak, birch and willow have the potential to support similarly substantial caterpillar abundances. These findings have implications for conservation, resilient forestry planting and management decisions.

Notes

This work was funded by a Natural Environment Research Council (NERC) advanced fellowship (NE/I020598/1O) and NERC grant no. (NE/P011802/1) to ABP and a NERC E3 DTP PhD to KHM.
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