Chilling and Forcing From Cut Twigs—How to Simplify Phenological Experiments for Citizen Science

Abstract

Low-cost phenological experiments with cut twigs are increasingly used to study bud development in response to spring warming and photoperiod. However, a broader variety of species needs to be tackled and in particular the influence of insufficient winter chilling deserves more attention. Therefore, we investigated if and how chilling requirements can be efficiently investigated by cut twigs and how this low-tech approach could be successfully implemented as a citizen science or school project. We conducted an experiment on bud burst and leaf development of Corylus avellana L. twigs, with natural chilling outdoors on a shrub (S) and another chilling treatment as cut twigs in containers (C), and subsequent forcing indoors. Subsampling of the number of cutting dates and number of twigs was used to infer minimum required sample sizes. Apart from insufficiently chilled twigs,~80% of the twigs (both S and C) reached leaf out. For multiple definitions of chilling and forcing, a negative exponential relationship was revealed between chilling and amount of forcing needed to reach certain developmental stages. At least 5 out of 15 cutting dates or alternatively half of the 10 twig repetitions, but especially those mirroring low chilling conditions, were needed to describe the chillingforcing relationship with some degree of robustness. In addition, for cutting dates with long chilling, i.e., from January onwards, freshly cut twigs (S) required significantly more forcing to reach bud burst than twigs from containers (C), although the effect was small. In general, chilling conditions of mature shrubs were well captured by cut twigs, therefore opening the possibility of chilling through refrigeration. We conclude that experimental protocols as outlined here are feasible for citizen scientists, school projects, and science education, and would have the potential to advance the research field if carried out on a large scale. We provide an easy-to-use Shiny simulation app to enable citizen scientists to build up a bud development model based on their own experimental data and then simulate future phenological development with winter and/or spring warming. This may encourage them to further study other aspects of climate change and the impacts of climate change.

Chilled to be forced: the best dose to wake up buds from winter dormancy

“…These two indicators provide both a sensitive (time to earliest budburst) and a robust (budburst percentage) indicator, which both reflected the same patterns of dormancy progression. In addition, the method used here has been proved to be reliable in reflecting the phenology of the donor trees (Vitasse & Basler, 2014; Menzel et al ., 2020) and has been applied to better understand chilling and photoperiod effects on temperate (Laube et al ., 2014; Flynn & Wolkovich, 2018), and even subtropical, trees (Du et al ., 2019). As the trees examined in this study were taken from only one population near Zurich, it is possible that the results of both chilling and forcing requirements found here may vary within the species distribution range (e.g.…”

Establishing the twig method for investigations on pollen characteristics of allergenic tree species

“…Branch experiments can simulate variation in chilling length using different harvest dates prior to actual flowering or higher temperatures in the climate chamber/window sill can be used to study the effect of accelerated warming (forcing) (see e.g. Menzel et al 2020a , b ).…”

“…The goal of these studies was among others to identify key influences on seasonal patterns and pollen emission, including the effects of climate change (Marselle et al 2019 ), and to study pollen characteristics, such as variation in allergic content among different grass species (Jung et al 2018 ). It is already known that climate change alters seasonal patterns of pollen, e.g., leading to an earlier start of pollen season (Frei and Gassner 2008 ; Menzel et al 2006 ; Menzel et al 2020a , b ; Rosenzweig et al 2008 ). Seasonal patterns and their trends are normally captured by phenological studies and/or longer time series of measured airborne pollen concentrations (e.g., Menzel et al 2021 ).…”

“…Thus, this method has the benefit that in the face of climate change different climatic scenarios and their impacts on pollen characteristics can be studied. The twig method has already been well described in previous studies (Basler and Körner 2012 ; Dantec et al 2014 ; Menzel et al 2020a , b ; Primack et al 2015 ). Vitasse and Basler ( 2014 ) observed for the tree species hornbeam, European beech, and sycamore that the timing of budburst is comparable between cut and uncut twigs from a donor tree.…”

Extreme temperatures help in identifying thresholds in phenological responses

“…An important feature of the climate system is the diversity of climatic conditions, and the picture of regional climate changes in recent decades has been shown to be complex and diverse. Thus, research on the long‐term dynamics of phenological events influenced by climatic factors remains among the most interesting topics in the natural sciences (Menzel, Yuan, Hamann et al., 2020).…”