Biologically‐Relevant Trends in Springtime Temperatures Across the United States

Abstract

Long‐term trends in temperature—a primary driver of phenology—are typically evaluated using monthly or seasonal averages. However, accumulated warmth, rather than average temperature, cues phenological events; further, the amount of heat necessary to trigger activity is species‐specific. We evaluated trends in the timing of three heat accumulation thresholds encompassing spring‐season biological activity in the conterminous United States over a 70‐year period to document changes from a biologically relevant perspective. The Southwest, Northeast, and Northwest regions exhibit the strongest advancements. Rates of change vary among thresholds within many regions, resulting in temporal compression and lengthening within the season. Further, in the Eastern United States, the days between when a single threshold is met in the south and north are decreasing; in the West, the opposite pattern is occurring. These trends generally match long‐term observations of species' phenology, underscoring the value of this approach for documenting biologically relevant changes in temperature.

Multi‐season climate projections forecast declines in migratory monarch butterflies

“…Our analysis of uncertainties suggests that we can best improve & Crimmins, 2019;IPCC, 2014) would also be beneficial, as forecasts are less likely to be reliable when projected climate differs markedly from that used to estimate population responses to climatic change (Fitzpatrick & Hargrove, 2009;Saunders et al, 2016).…”

“…In particular, using newly available data from volunteer‐based networks to better understand lesser‐studied portions of the migratory cycle (e.g., relationships between local climate conditions and spatiotemporal variation in abundance of monarchs in eastern Texas) would be especially valuable and is likely to improve the accuracy of population forecasts. Instigating studies in regions where climate is highly variable and/or changing rapidly (e.g., central Michigan; Figures 2 and 3; Crimmins & Crimmins, 2019; IPCC, 2014) would also be beneficial, as forecasts are less likely to be reliable when projected climate differs markedly from that used to estimate population responses to climatic change (Fitzpatrick & Hargrove, 2009; Saunders et al, 2016). Finally, recent efforts to systematically monitor monarchs and milkweed throughout the summer breeding range, like the Integrated Monarch Monitoring Program (Cariveau et al, 2019; Weiser et al, 2019), could improve precision of population forecasts by providing high‐quality data on monarch recruitment in previously unsampled areas.…”

Higher temperature sensitivity of flowering than leaf‐out alters the time between phenophases across temperate tree species

“…Higher variation of spring temperature before flowering than leaf‐out could partly explain the larger advance in flowering than leaf‐out for LO‐FL species, but could not account for the decreased temporal differences between leaf‐out and flowering for FL‐LO species. On the second, woody plants have been well documented to require a specific amount of heat during spring to break dormancy, and a lower heat requirement may lead to earlier development under climatic warming (Crimmins & Crimmins, 2019; Hänninen, 2016). Our results confirmed the second explanation, since we indeed found that the earlier phenological events required fewer GDD than later events for both the LO‐FL and FL‐LO species, as well as higher sensitivity to heat for flowering than leaf‐out, which suggested that in addition to S T , the sensitivity of the heat requirement could also partially account for the decreasing temporal difference between leaf‐out and flowering.…”

Geographic variation in offspring size: Long‐ and short‐term climate affect mean seed mass ofStreptanthuspopulations

“…Given that climate change is likely to affect many biologically important environmental variables including both growing season length (through increases in temperature) and precipitation, we need to gain a more complete understanding of how these climatic parameters—individually and in combination—affect mean seed mass (Cayan et al, 2008 ; Crimmins & Crimmins, 2019 ; Polade et al, 2017 ). Few studies have assessed the importance of interactions between climatic parameters on seed mass (Larios et al, 2014 ; Soper Gorden et al, 2016 ), and despite the well‐documented independent effects of both precipitation and growing season length on mean seed mass (Crouch & Vander Kloet, 1980 ; de Frenne et al, 2010 ; Konarzewski et al, 2012 ; Murray et al, 2004 ), to our knowledge, no studies have investigated how these variables interact to affect population mean seed mass.…”