Modelling the Timing of Betula Pubescens Budburst. I. Temperature and Photoperiod: a Conceptual Model
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Vol. 46: 147–157, 2011 CLIMATE RESEARCH Published online March 8 doi: 10.3354/cr00980 Clim Res Modelling the timing of Betula pubescens budburst. I. Temperature and photoperiod: a conceptual model Amelia Caffarra1,*, Alison Donnelly2, Isabelle Chuine3, Mike B. Jones2 1Research and Innovation Centre, Agriculture Area, Fondazione Edmund Mach, San Michele all’Adige, 38100 Trento, Italy 2Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland 3CEFE-CNRS, 1919 route de Mende, 34293 Montpellier, France ABSTRACT: The main factors triggering and releasing bud dormancy are photoperiod and tempera- ture. Their individual and combined effects are complex and change along a transition from a dor- mant to a non-dormant state. Despite the number of studies reporting the effects of temperature and photoperiod on dormancy release and budburst, information on the parameters defining these rela- tionships is scarce. The aim of the present study was to investigate the effects and interaction of tem- perature and photoperiod on the rates of dormancy induction and release in Betula pubescens (Ehrh.) in order to develop a conceptual model of budburst for this species. We performed a series of con- trolled environment experiments in which temperature and photoperiod were varied during different phases of dormancy in B. pubescens clones. Endodormancy was induced by short days and low tem- peratures, and released by exposure to a minimal period of chilling temperatures. Photoperiod dur- ing exposure to chilling temperatures did not affect budburst. Longer exposure to chilling increased growth capability (growth rate at a given forcing temperature) and decreased the time to budburst. During the forcing phase, budburst was promoted by photoperiods above a critical threshold, which was not constant, but decreased upon longer chilling exposures. These relationships between photoperiod and temperature have, as yet, not been integrated into the commonly used process- based phenological models. We suggest models should account for these relationships to increase the accuracy of their predictions under future climate conditions. KEY WORDS: Betula pubescens · Controlled environment experiments · Phenology · Photoperiod · Temperature · Dormancy · Budburst Resale or republication not permitted without written consent of the publisher 1. INTRODUCTION In order to predict and attempt to mitigate the im- pacts of climate change, in particular rising tempera- A growing number of studies worldwide have re- ture, it is crucial to understand the effect climate plays ported changes in plant and animal life cycles in re- on plant growth, of which budburst is one aspect. sponse to current climate change (for reviews see Peñu- However, the lack of understanding of the physiologi- elas & Filella 2001, Parmesan & Yohe 2003, Root et al. cal mechanisms regulating dormancy makes it difficult 2003, Menzel et al. 2006). Most of these phenological to develop widely applicable predictive models of bud- changes are highly correlated with changes in temper- burst (Hänninen 1995, Arora et al. 2003). ature, and a trend for advanced spring events (such as Experimental evidence shows that in many tree spe- leaf unfolding or flowering) is apparent for many spe- cies, the main factors triggering and releasing dor- cies (Sparks & Menzel 2002, Root et al. 2003, Menzel et mancy are photoperiod and temperature (Downs & al. 2006). Borthwick 1956, Murray et al. 1989, Myking & Heide *Email: [email protected] © Inter-Research 2011 · www.int-res.com 148 Clim Res 46: 147–157, 2011 1995, Thomas & Vince Prue 1997). The action of these interactions between the factors conditioning budburst environmental drivers is complex and changes along timing, and their control of the transitions from one the transition from a dormant to a non-dormant state. sub-phase of dormancy to the next. The integration of In addition, it has been shown that photoperiod and these interactions in a coherent model structure can temperature interact at various stages during dor- only be obtained through inter-comparable experi- mancy induction, release and quiescence (Håbjørg ments linking together observed processes and mea- 1972, Junttila 1980, Heide 1993, 2003, Myking & Heide suring the rates of plant response to changes in envi- 1995, Partanen et al. 2001). ronmental drivers (Hänninen 2006). Photoperiod affects the time at which buds enter a The aim of the present study was to investigate the phase of winter rest. Short days (ShDs) signal the onset effects and interaction of temperature and photoperiod of winter, which, in turn, triggers decreasing ‘growth on the rates of dormancy induction and release in competence’ (growth capability). Once the plant Betula pubescens, through a set of experiments, in has been exposed to a certain number of dormancy- order to develop a conceptual model of dormancy for inductive days (dormancy induction requirement), the this species. phase of endodormancy is reached, a period during More specifically, we aimed: (1) to quantify the which buds do not grow even under favourable envi- effects of photoperiod and temperature on dormancy ronmental conditions (Downs & Borthwick 1956, Håb- induction requirements and determine the effect of jørg 1972, Howe et al. 1996, Thomas & Vince Prue their interactions on subsequent bud growth, (2) to 1997, Welling et al. 1997). quantify chilling requirements, (3) to test the interac- Chilling temperatures are the main trigger for tion between photoperiod and chilling exposure on endodormancy release (Perry 1971, Sarvas 1974, Can- budburst rate and percentage, (4) to parameterise the nell & Smith 1983, Battey 2000). The concept of ‘chill- sigmoidal function describing the rate of budburst at ing temperature’ is not clearly defined, and a consis- different forcing temperatures and (5) to integrate tent response to chilling has not been detected in these experimental results with previous findings in dormant plants. To date, it is not known whether pho- order to develop a conceptual model of dormancy, toperiod plays a role in chilling fulfilment, i.e. if it which has been mathematically formalized (Caffarra affects chilling efficiency for a given temperature dur- et al. 2011, this issue) ing endodormancy release. The present study was conducted on Betula pubes- After the release of endodormancy, warm tempera- cens (birch) because it is widely distributed at boreal tures stimulate growth in buds, which then enter the and temperate latitudes (Atkinson 1992), where cli- phase of ecodormancy, i.e. have the ability to resume mate is warming more rapidly than in other regions growth when exposed to growth-promoting signals (IPCC 2007). Alteration in the phenology of B. pubes- (forcing conditions). During this phase the rate of cens is thus likely and may have important conse- growth is proportional to temperature. Ecodormant quences for ecosystem dynamics in these regions. buds burst upon accumulation of a certain heat re- quirement (Perry 1971, Hänninen 1990, Battey 2000). However, it has been shown in some species that addi- 2. MATERIALS AND METHODS tional chilling during this period decreases the heat requirement for budburst to occur (Nienstaedt 1966, 2.1. Selection and vegetative propagation of the Landsberg 1974, Couvillon & Erez 1980, Heide 1993, experimental material Myking & Heide 1995). Photoperiod has been found to interact with temperature during the ecodormant The experiments were conducted on the Betula pu- period. In Betula pubescens and B. pendula, long days bescens (Ehrh.) (birch) clone grown in the Interna- (LDs) appear to substitute partially for the effect of tional Phenological Gardens (IPGs) network, which low temperatures and decrease the heat requirement comprises 89 garden sites throughout Europe (www. for budburst to occur (Myking & Heide 1995). agrar.hu-berlin.de/struktur/institute/nptw/agrarmet/ The response of the rate of bud growth of young phaenologie/ipg/ipg_allg-e). The clone under study, seedlings to increasing temperature has a sigmoidal originally from Germany, was propagated vegeta- shape (Sarvas 1972). tively, in order to obtain experimental material with a Despite the number of studies reporting the effects homogenous genotype, and therefore any observed of temperature and photoperiod on dormancy release differences could be attributed to environmental con- and budburst, information on the parameters defining ditions rather than genotypical differences among these relationships is scarce (Kramer 1994, Hänninen plants. The clones in the current experiments were 2006). The development of a process-based model of propagated using softwood cuttings, taken from the budburst requires a quantitative description of the soft, new growth of the tree, as described by Hartmann Caffarra et al.: Modelling the timing of B. pubescens budburst. I. 149 et al. (1997). In 2003 we took 300 softwood cuttings 2.3. Manipulations from the B. pubescens IPG clone growing at the John Fitzgerald Kennedy Arboretum in Wexford (Ireland, 2.3.1. Expt 1 — Effects of forcing temperature IPG No.14). After collection, the softwood cuttings were treated with rooting powder (0.4% 1-naphtha- On 28 February of Year 1, 60 dormant twigs of Betula leneacetic acid, NAA) and placed in compost-filled pubescens were removed from the parent plant, which trays before being transferred to a mist unit until root- was growing outdoors in IPG 14. Thus, up to that date, ing took place (approximately 6 wk later). The rooted the twigs had