Facilitation and reproduction: Improving cone production thanks to conspecific tree neighbors in the alpine treeline

Background/Question/Methods

Cone production is known to decrease with altitude due to low growing season temperatures, which reduces the developmental period available to cone buds at the alpine treeline. However, the microclimate experienced by individual plants differs from ambient air temperature: wind has a direct negative effect on plant microclimate temperature, which can ultimately delay or even prevent full maturation of buds. Damage by snow blast during winter may also prevent cone production. In addition, facilitation (i.e. positive interactions) is thought to be more frequent in harsh environments such as alpine treeline ecotone. We therefore hypothesize that mature tree neighbors have a positive effect on cone production through sheltering from harsh winds in the alpine treeline ecotone. To test this hypothesis, we counted cone production in five altitudinal transects which cover the alpine treeline ecotone in two sites in the Canadian Appalachians. We characterized ten white spruces per transect with respect to cone production, elevation, tree height, wind mechanical damage, and distance to protection by tree neighbor. We developed a set of candidate models and we fitted them on our cone count using generalized linear models. We then compared model fit using the Akaike Information Criteria (AICc) to select the best models.

Results/Conclusions

Mean cone production decreased with elevation, but not as steeply for cones belonging to trees protected by neighbors. Based on AICc, the model with additive effect of elevation and distance to protection with a quadratic effect of elevation was the most appropriate for our data, accounting for 37% of the variance in cone production. Elevation and distance to protection had a negative linear effect on cone production (respectively -1.5 cone/m and -1.37 cone/m), meaning that facilitation increases reproductive output by 10%. Furthermore the distance to protection is more important at high elevations, increasing cone production by a factor of 2. Finally, the significance of quadratic effects suggests that the relationship between cone production, elevation and distance to protection is non-linear. These results indicate that facilitation by tree neighbours decreases the negative effects of elevation on cone production, likely through protection from wind. This study illustrates the importance of positive interactions between conspecific mature trees as well as for reproduction, neither of which is the focus of the majority of the facilitation literature.