Local-scale synchrony and variability in mast seed production patterns of Picea glauca

1. Mast seeding is the synchronous and highly variable production of seed by a population of plants. Mast seeding results from the behaviour of individuals; however, little is known about the synchrony of individuals at local scales. 2. We address two primary questions at a within-population (17-36 ha study plots) and individual level: (i) How variable is seed production between and within years? (ii) How synchronized is seed production between individuals? 3. We monitored annual cone production of 356 Picea glauca (white spruce) from 1990 to 2005 within four plots spanning a total distance of 5.3 km in the Yukon Territory, Canada. 4. Spearman correlations (r(s)) were conducted to test for synchrony. Overall, the trees were moderately synchronous (mean r(s) (+/- SE) of 0.52 +/- 0.14), and synchrony was statistically detectable (r(s) > 0) over all distances. Individuals < 75 m apart were highly synchronous (0.64 +/- 0.18), and correlations dropped to 0.33 +/- 0.07 for trees > 3 km apart. There was considerable variation in cone production patterns among pairs of individuals. 5. The number of mast years per plot varied from one to three. During a mast year, many individuals within plots produced large cone crops, with more variability between individuals in low mean cone years. Individual trees had dominant endogenous cycles varying from none to 1-5 years. Forty-four per cent of trees had no significant lag, 23% a negative 1-year lag, and 20% a positive 3-year lag. Basal area did not influence lags, but trees with higher mean cone production throughout the study were more likely to have a 3-year lag compared with no lag. 6. The scale of highest synchrony coincided with the scale at which the dominant seed predator in the area, the territorial red squirrel (Tamiasciurus hudsonicus), operates. This may be the scale at which selection for synchrony occurs. 7. Based on high synchrony locally, high synchrony within a mast year, and multiple lags in cone production by individuals, both available resources and strong weather cues appear to play roles in the observed patterns.