Tradeoffs between chilling and forcing in satisfying dormancy requirements for Pacific Northwest tree species

Abstract

Many temperate and boreal tree species have a chilling requirement, that is, they need to experience cold temperatures during fall and winter to burst bud normally in the spring. Results from trials with 11 Pacific Northwest tree species are consistent with the concept that plants can accumulate both chilling and forcing units simultaneously during the dormant season and they exhibit a tradeoff between amount of forcing and chilling. That is, the parallel model of chilling and forcing was effective in predicting budburst and well chilled plants require less forcing for bud burst than plants which have received less chilling. Genotypes differed in the shape of the possibility line which describes the quantitative tradeoff between chilling and forcing units. Plants which have an obligate chilling requirement (Douglas-fir, western hemlock, western larch, pines, and true firs) and received no or very low levels of chilling did not burst bud normally even with long photoperiods. Pacific madrone and western redcedar benefited from chilling in terms of requiring less forcing to promote bud burst but many plants burst bud normally without chilling. Equations predicting budburst were developed for each species in our trials for a portion of western North America under current climatic conditions and for 2080. Mean winter temperature was predicted to increase 3.2–5.5°C and this change resulted in earlier predicted budburst for Douglas-fir throughout much of our study area (up to 74 days earlier) but later budburst in some southern portions of its current range (up to 48 days later) as insufficient chilling is predicted to occur. Other species all had earlier predicted dates of budburst by 2080 than currently. Recent warming trends have resulted in earlier budburst for some woody plant species; however, the substantial winter warming predicted by some climate models will reduce future chilling in some locations such that budburst will not consistently occur earlier.

Photoperiod cues and patterns of genetic variation limit phenological responses to climate change in warm parts of species’ range: Modeling diameter‐growth cessation in coast Douglas‐fir

“…Thus, photoperiod cues and patterns of genetic variation will likely allow for coast Douglas-fir to phenologically track climate change in the cool parts of the range by extending the growing season later into autumn, but will also limit its ability to do so in the warm parts, which might lead to its growth period becoming decoupled from the timing of suitable growing conditions. If co-occurring species in these warm parts of the range (either current residents or newly arrived migrants) better track climate change phenologically, they could gain a competitive advantage over coast Douglas-fir Harrington & Gould, 2015), which could be one factor contributing to possible range contractions along the warm edges of the species' distribution (also see Amano et al, 2014;Chuine, 2010;Morin et al, 2008). Initiation events are expected to occur earlier in the year with warming at high latitudes and elevations, allowing trees to capture newly favorable conditions in the early spring and track climate change, but to occur at similar or later times of the year toward low-latitude and low-elevation range limits, so that shifts in growth-initiation phenology lag behind climate change (Ford et al, 2016).…”

“…In temperate and boreal trees, numerous efforts have documented and modeled the effects of climate on spring budburst (e.g., Cannell & Smith, 1983;H€ anninen & Kramer, 2007;Harrington, Gould, & St. Clair, 2010;Laube et al, 2014;Polgar & Primack, 2011;Romberger, 1963;Sarvas, 1974;Vegis, 1964) and have illuminated how this particular phenological event might mediate climate change impacts on trees in important and sometimes surprising ways (e.g., Cannell & Smith, 1986;Chuine, 2010;H€ anninen, 2006;Harrington & Gould, 2015). In temperate and boreal trees, numerous efforts have documented and modeled the effects of climate on spring budburst (e.g., Cannell & Smith, 1983;H€ anninen & Kramer, 2007;Harrington, Gould, & St. Clair, 2010;Laube et al, 2014;Polgar & Primack, 2011;Romberger, 1963;Sarvas, 1974;Vegis, 1964) and have illuminated how this particular phenological event might mediate climate change impacts on trees in important and sometimes surprising ways (e.g., Cannell & Smith, 1986;Chuine, 2010;H€ anninen, 2006;Harrington & Gould, 2015).…”

“…These types of spatial patterns have been identified for budburst and diameter-growth initiation, with trees in cool locations (high latitude/elevation) exhibiting strong shifts in initiation to earlier in the year (likely a favorable response to climate change), while trees in warmer locations (low latitude/elevation) exhibit much weaker shifts or even initiate growth later in the year due to low chilling (likely an unfavorable response) (Ford, Harrington, Bansal, Gould, & St. Clair, 2016;Harrington & Gould, 2015;Murray, Cannell, & Smith, 1989). These types of spatial patterns have been identified for budburst and diameter-growth initiation, with trees in cool locations (high latitude/elevation) exhibiting strong shifts in initiation to earlier in the year (likely a favorable response to climate change), while trees in warmer locations (low latitude/elevation) exhibit much weaker shifts or even initiate growth later in the year due to low chilling (likely an unfavorable response) (Ford, Harrington, Bansal, Gould, & St. Clair, 2016;Harrington & Gould, 2015;Murray, Cannell, & Smith, 1989).…”

Temperature Range Shifts for Three European Tree Species over the Last 10,000 Years

“…Here, we use January temperature (Tjan), quantitatively reconstructed over the past 10,000 years in Europe, in recognition of its ecological importance to fulfillment of chilling requirements for budburst and growth (Nienstaedt, 1967; Heide, 1993; Kramer, 1994, 1995; Harrington and Gould, 2015) and the influence of winter temperature on soil temperature, an important factor in determining treeline globally (Körner and Paulsen, 2004). Winter frost may damage needles and wood and, therefore, restrict the range of A. alba and F. sylvatica in areas where temperature descends below −10°C.…”

Provenance Variation in Phenology and Frost Tolerance in Subalpine Fir (Abies lasiocarpa) Planted in Denmark and Iceland

“…Requirements for chilling may delay flushing in many boreal tree species when winter temperatures become warmer or when a tree species or provenance is moved into a new environment with warmer winter temperatures [51,52,55]. Harrington and Gould [56] studied chilling requirements of Pacific Northwest tree species and found Pacific silver fir (Abies amabilis Douglas ex J.Forbes), grand fir (Abies grandis (Douglas ex D.Don) Lindl.) and noble fir all had obligate chilling requirements.…”