Climatic determinants of white spruce cone crops in the boreal forest of southwestern Yukon

Abstract

White spruce ( Picea glauca (Moench) Voss) cone crops were measured from 1986 to 2011 in the Kluane region of southwestern Yukon to test the hypothesis that the size of cone crops could be predicted from spring and summer temperature and rainfall of years t, t – 1, and t – 2. We counted cones in the top 3 m of an average of 700 white spruce trees each year spread over 3–14 sites along 210 km of the Alaska Highway and the Haines Highway. We tested the conventional explanation for white spruce cone crops that implicates summer temperatures and rainfall in years t and t – 1 and rejected it, since it explained very little of the variation in our 26 years of data. We used exploratory data analysis with robust multiple regressions coupled with Akaike’s information criterion corrected (AICc) analysis to determine the best statistical model to predict the size of cone crops. We could statistically explain 54% of the variation in cone crops from July and August temperatures of years t – 1 and t – 2 and May precipitation of year t – 2. There was no indication of a periodicity in cone crops, and years of large cone crops were synchronous over the Kluane region with few exceptions. This is the first quantitative model developed for the prediction of white spruce cone crops in the Canadian boreal forest and has the surprising result that weather conditions 2 years prior to the cone crop are the most significant predictors.

Reproduction as a bottleneck to treeline advance across the circumarctic forest tundra ecotone

“…Our analyses support the concept of a sequential model of seed production, in which successful production of viable seed is influenced by a series of weather conditions occurring over multiple seasons (Juday et al 2003, Krebs et al 2012, Roland et al 2014. Favourable climate conditions driven by synoptic weather patterns have been identified as triggers of regionally synchronous pulses of high seed production (e.g., 'mast' years; Koenig and Knops 1998).…”

“…Favourable climate conditions driven by synoptic weather patterns have been identified as triggers of regionally synchronous pulses of high seed production (e.g., 'mast' years; Koenig and Knops 1998). All seven species in this study exhibit masting dynamics that are hypothesized to be the result of some combination of warm, dry conditions in the growing seasons up to two years prior to cone development (see examples for our study species in Woodward et al 1994, Houle 1999, Sirois 2000, Krebs et al 2012, Bisi et al 2016, Gallego Zamorano et al 2018. However, documentation of seed production in mast years requires long time series and our study design was insufficient to assess controls over mast years.…”

“…The position of the treeline ecotone has been explicitly linked to temperature limitations (Timoney et al 1992, Sveinbjörnsson et al 2002, Körner 2012, and we expect that climate constraints should be apparent across ecotonal boundaries in most years, regardless of annual fluctuations in productivity. A greater accumulation of growing degree days (GDD) enables an individual tree to extract more resources and accumulate more biomass than it would otherwise, increasing the likelihood that it will allocate resources to reproduction (Krebs et al 2012). Cooler years may limit pollination and seed viability at the treeline ecotone (Hofgaard 1993, Roland et al 2014.…”

Climatic dipoles drive two principal modes of North American boreal bird irruption

“…In lieu of such data, which are unfortunately too sparse to match the spatial and temporal scales of this study, we made inferences about conifer seed crop size that are consistent with tree reproductive theory and output from models developed to make weather-based conifer seed crop predictions (17)(18)(19)(20)(21)(22)(34)(35)(36)(37). Specifically, an anomalously warm or dry spring or summer favors budding and pollination, yielding a large seed crop (masting) during the next year's autumn, whereas an anomalously cool or wet spring or summer yields a small seed crop during autumn of the following year.…”

“…Reproductive cycles typically span 2 to 3 y in boreal conifers, and climatic conditions in the years preceding a masting event have been shown to be critical in pines (Pinus spp.) (18,19), balsam fir (Abies balsamea) (20), white spruce (Picea glauca) (21,22), and black spruce (Picea mariana) (23). Many of these studies suggest that masting is favored by a warm, dry spring and summer in the year of cone initiation and a warm summer in the subsequent year.…”