Sugar maple responses to climate change: We’ll boil it down for you

Abstract In our recent article, we use in situ ecophysiological data from individual sugar maple trees across the species’ range to identify climate conditions that maximize the volume and sugar concentration of sap. Houle and Duchesne present a critique of our research that hinges on their own analysis of industry aggregate data on syrup production, from within the latitudinal range where the industry is currently concentrated. Their approach falls short of both a proper validation of our ecological model and a rigorous comparison of two potentially complementary contributions. Notably, the aggregate dataset that Houle and Duchesne analyze includes an arbitrary mix of traditional gravity tapping and vacuum tubing extraction of sap, where the latter can maintain sap flow in the absence of freeze/thaw dynamics. In contrast, we hold the collection method constant, avoiding the confounding effects of vacuum tubing collection. Thus, their model conflates historical climate data with ongoing changes in sap extraction methods, and thereby masks relationships between climate and the volume and sugar concentration of sap. Moreover, by using regionally constrained aggregate data, Houle and Duchesne fail to capture the breadth of the species’ responses to climate, which are represented in our dataset by populations at the edges of the species range. By ignoring these and other deep methodological discrepancies between their analysis and ours, Houle and Duchesne inappropriately put forth their own study as ‘validation’ of our ecological model. Unfortunately, this approach distracts from collaborative interdisciplinary discourse that could help refine predictions about sugar maple responses to climate. In our reply below, we refute Houle and Duchesne’s unfounded ad hominem claims that our paper predicts a collapse of the maple syrup industry or is ‘alarmist’ in any way, and we clarify our contribution where they otherwise mischaracterize or misunderstand our work. We maintain that our model suggests a climate optimum for syrup production, based on range-wide data on the underlying ecophysiological responses of individual trees. We further point out where future research could address gaps in our knowledge of climate effects on tree physiology (the focus of our research) and on the syrup industry itself (which appears to be the aim of Houle and Duchesne’s analysis here). We close by inviting researchers and syrup producers to join ACER-net, our collaborative science and outreach platform for understanding sugar maple and its ecosystem services in a changing world.