Among Northern Hemisphere deciduous woody plants, conspicuous red autumn foliage — produced by de novo synthesis of anthocyanin pigments in senescing leaves — occurs in roughly 27% of North American, 25% of Asian, but only 9% of European species, a pattern robust under the deciduous-only, taxonomically controlled analysis (genus- and family-level random effects approximating phylogenetic non-independence) of Renner and Zohner (2020). Two adaptive explanations dominate the literature: an abiotic account, in which anthocyanins provide photoprotection during nutrient resorption under the higher autumn insolation of eastern North America and East Asia, and the less predictable autumn temperatures and shorter growing seasons of eastern North America, relative to Europe; and the signalling/co-evolution account, in which red colouration is an honest signal of defensive capacity directed at insect herbivores. The abiotic causal chain has never been causally tested at the continental scale, whether the signalling predictions are supported is disputed between the two camps, and the mechanism-level evidence accumulated since 2019 is conflicting. The problem asks for the causal explanation of the documented continental pattern, through phylogenetically informed, trait- and environment-linked comparative analyses and manipulative experiments that discriminate between the abiotic and signalling accounts.

Background

In temperate regions, deciduous trees and shrubs dismantle their photosynthetic machinery each autumn before shedding their leaves. As chlorophyll is degraded, the yellow-to-orange carotenoid pigments that were already present become visible, while some species additionally synthesize red-to-purple anthocyanins — water-soluble flavonoid pigments stored in the leaf vacuole — de novo in the senescing leaf. Because anthocyanin synthesis is costly and occurs during a period of active nutrient withdrawal, its function has been debated for over a century. During autumn senescence, nitrogen and other nutrients are resorbed from the leaf into storage organs; this resorption proceeds while the leaf is still exposed to strong light and increasingly cold, unpredictable temperatures, creating a risk of photooxidative stress in tissue whose chlorophyll is being dismantled.

Two adaptive explanations dominate. The photoprotection hypothesis holds that anthocyanins act as an internal sunscreen shielding senescing tissue from excess absorbed light, extending the period over which nutrient resorption can proceed, particularly at low temperature; it predicts more anthocyanin investment where autumn light is strong and cold snaps frequent. The signalling (or co-evolution) hypothesis, introduced by Hamilton and Brown (2001), holds that bright red autumn leaves are a costly handicap signal advertising the tree's defensive strength or nutritional unsuitability to herbivorous insects — classically aphids that migrate to trees in autumn to overwinter and reproduce — so that trees benefit by deterring settlers and insects benefit by choosing better hosts. Reviews of the competing hypotheses include Ougham, Thomas and Archetti (2008) and Archetti et al. (2009); Chittka and Döring (2007) questioned whether aphids can even perceive red light, striking at the mechanism the signal would require.

The biogeographic anchor of the debate is that red autumn foliage is strongly over-represented in some floras: Lev-Yadun and Holopainen (2009) highlighted the contrast between red-dominated autumn displays in America and yellow-dominated displays in northern Europe. Renner and Zohner (2019) consolidated the pattern and proposed an abiotic explanation: at comparable latitudes, eastern North America and East Asia receive higher autumn solar irradiation than Europe; eastern North America additionally experiences larger and less predictable autumn temperature swings, with cold snaps during senescence; and eastern North American species have on average about three-week shorter foliage periods. All three factors should favour investment in pigments that extend the time available for nutrient resorption before leaf abscission, explaining the higher frequency of coloured species outside Europe. Pena-Novas and Archetti (2020) disputed this comparative evidence — their reanalysis found no significant difference in the proportion of reddening species between Europe and eastern North America, and they argued that comparing whole floras is a weak proxy for phylogenetically controlled inter-specific comparisons — and maintained that the evidence for the photoprotection hypothesis is not solid while there is evidence for the co-evolution (signalling) hypothesis. In reply, Renner and Zohner (2020) merged both groups' species observations, restricted the dataset to deciduous species (those in which autumn senescence actually occurs, since evergreens lack the senescing-leaf context the photoprotection hypothesis concerns), and fitted a hierarchical Bayesian binomial regression with continent as a fixed effect and genus- and family-level random effects approximating phylogenetic nonindependence: conspicuous red autumn foliage occurs in about 9% of 201 European, 27% of 431 North American, and 25% of 900 Asian species. The continental pattern is therefore robust — with Europe the outlier — but its cause was explicitly left untested. Standard methodology in this area codes species-level pigment phenotype from floras and field observations, controls for shared ancestry with explicit phylogenies or taxonomic random effects, and links the trait to continental environmental covariates; manipulative work uses common gardens and controlled light, temperature, nutrient, and herbivore treatments during natural senescence.

Problem Statement

Determine the causal driver(s) of the documented continental-scale differences in the incidence of red autumn colouration among the deciduous woody floras of Europe, North America, and Asia (approximately 9%, 27%, and 25% of species, respectively), replacing the present correlational account with discriminating evidence. Concretely, test whether regional differences in incoming autumn irradiation, in the predictability and magnitude of autumn temperature swings, and in growing-season length causally increase investment in anthocyanic autumn foliage through the photoprotection-during-nutrient-resorption mechanism, and, in doing so, discriminate this abiotic explanation against the competing signalling/co-evolution account — that red colouration honestly signals defensive capacity or low nutritional quality to insect herbivores — whose adequacy for explaining the observed continental distribution is the live dispute in the source exchange: Renner and Zohner (2020) regard the signalling predictions as unsupported and insufficient to explain why reddening should be of greater selective benefit outside Europe, whereas Pena-Novas and Archetti (2020) regard those predictions as verified. Illustrative, non-binding approaches include phylogenetically informed, trait- and environment-linked comparative analyses across species and floras, common-garden or reciprocal-transplant comparisons of species from different continents, and manipulative experiments varying light, temperature variability, and nutrient availability during natural senescence. The overarching objective is a causal explanation of the continental pattern itself: re-documenting the pattern, or resolving the mechanism question within a single species without connecting it to the geographic distribution, would not answer the question.

References

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