• Energy Research

SummaryClimate change causes both temporal (e.g. advancing spring phenology) and geographic (e.g. range expansion poleward) species shifts, which affect the photoperiod experienced at critical developmental stages (‘experienced photoperiod’). As photoperiod is a common trigger of seasonal biological responses – affecting woody plant spring phenology in 87% of reviewed studies that manipulated photoperiod – shifts in experienced photoperiod may have important implications for future plant distributions and fitness. However, photoperiod has not been a focus of climate change forecasting to date, especially for early‐season (‘spring’) events, often assumed to be driven by temperature. Synthesizing published studies, we find that impacts on experienced photoperiod from temporal shifts could be orders of magnitude larger than from spatial shifts (1.6 h of change for expected temporal vs 1 min for latitudinal shifts). Incorporating these effects into forecasts is possible by leveraging existing experimental data; we show that results from growth chamber experiments on woody plants often have data relevant for climate change impacts, and suggest that shifts in experienced photoperiod may increasingly constrain responses to additional warming. Further, combining modeling approaches and empirical work on when, where and how much photoperiod affects phenology could rapidly advance our understanding and predictions of future spatio‐temporal shifts from climate change.

SummaryClimate change causes both temporal (e.g. advancing spring phenology) and geographic (e.g. range expansion poleward) species shifts, which affect the photoperiod experienced at critical developmental stages (‘experienced photoperiod’). As photoperiod is a common trigger of seasonal biological responses – affecting woody plant spring phenology in 87% of reviewed studies that manipulated photoperiod – shifts in experienced photoperiod may have important implications for future plant distributions and fitness. However, photoperiod has not been a focus of climate change forecasting to date, especially for early‐season (‘spring’) events, often assumed to be driven by temperature. Synthesizing published studies, we find that impacts on experienced photoperiod from temporal shifts could be orders of magnitude larger than from spatial shifts (1.6 h of change for expected temporal vs 1 min for latitudinal shifts). Incorporating these effects into forecasts is possible by leveraging existing experimental data; we show that results from growth chamber experiments on woody plants often have data relevant for climate change impacts, and suggest that shifts in experienced photoperiod may increasingly constrain responses to additional warming. Further, combining modeling approaches and empirical work on when, where and how much photoperiod affects phenology could rapidly advance our understanding and predictions of future spatio‐temporal shifts from climate change.

AbstractTemperature sensitivity—the magnitude of a biological response per °C—is a fundamental concept across scientific disciplines, especially biology, where temperature determines the rate of many plant, animal and ecosystem processes. Recently, a growing body of literature in global change biology has found temperature sensitivities decline as temperatures rise (Fuet al., 2015; Güsewell et al., 2017; Piao et al., 2017; Chen et al., 2019; Dai et al., 2019). Such observations have been used to suggest climate change is reshaping biological processes, with major implications for forecasts of future change. Here we present a simple alternative explanation for observed declining sensitivities: the use of linear models to estimate non-linear temperature responses. We show how linear estimates of sensitivities will appear to decline with warming for events that occur after a cumulative thermal threshold is met—a common model for many biological events. Corrections for the non-linearity of temperature response in simulated data and long-term phenological data from Europe remove the apparent decline. Our results show that rising temperatures combined with linear estimates based on calendar time produce observations of declining sensitivity—without any shift in the underlying biology. Current methods may thus undermine efforts to identify when and how warming will reshape biological processes.Significance statementRecently a growing body of literature has observed declining temperature sensitivities of plant leafout and other events with higher temperatures. Such results suggest that climate change is already reshaping fundamental biological processes. These temperature sensitivities are often estimated as the magnitude of a biological response per °C from linear regression. The underlying model for many events—that a critical threshold of warmth must be reached to trigger the event—however, is non-linear. We show that this mismatch between the statistical and biological models can produce the illusion of declining sensitivities with warming using current methods. We suggest simple alternative approaches that can better identify when and how warming will reshape biological processes.

AbstractTemperature sensitivity—the magnitude of a biological response per °C—is a fundamental concept across scientific disciplines, especially biology, where temperature determines the rate of many plant, animal and ecosystem processes. Recently, a growing body of literature in global change biology has found temperature sensitivities decline as temperatures rise (Fuet al., 2015; Güsewell et al., 2017; Piao et al., 2017; Chen et al., 2019; Dai et al., 2019). Such observations have been used to suggest climate change is reshaping biological processes, with major implications for forecasts of future change. Here we present a simple alternative explanation for observed declining sensitivities: the use of linear models to estimate non-linear temperature responses. We show how linear estimates of sensitivities will appear to decline with warming for events that occur after a cumulative thermal threshold is met—a common model for many biological events. Corrections for the non-linearity of temperature response in simulated data and long-term phenological data from Europe remove the apparent decline. Our results show that rising temperatures combined with linear estimates based on calendar time produce observations of declining sensitivity—without any shift in the underlying biology. Current methods may thus undermine efforts to identify when and how warming will reshape biological processes.Significance statementRecently a growing body of literature has observed declining temperature sensitivities of plant leafout and other events with higher temperatures. Such results suggest that climate change is already reshaping fundamental biological processes. These temperature sensitivities are often estimated as the magnitude of a biological response per °C from linear regression. The underlying model for many events—that a critical threshold of warmth must be reached to trigger the event—however, is non-linear. We show that this mismatch between the statistical and biological models can produce the illusion of declining sensitivities with warming using current methods. We suggest simple alternative approaches that can better identify when and how warming will reshape biological processes.

AbstractForecasts of widespread range shifts with climate change stem from assumptions that climate drives species' distributions. However, local adaptation and biotic interactions also influence range limits and thus may impact range shifts. Despite the potential importance of these factors, few studies have directly tested their effects on performance at range limits. We address how population‐level variation and biotic interactions may affect range shifts by transplanting seeds and seedlings of western North American conifers of different origin populations into different competitive neighborhoods within and beyond their elevational ranges and monitoring their performance. We find evidence that competition with neighboring trees limits performance within current ranges, but that interactions between adults and juveniles switch from competitive to facilitative at upper range limits. Local adaptation had weaker effects on performance that did not predictably vary with range position or seed origin. Our findings suggest that competitive interactions may slow species turnover within forests at lower range limits, whereas facilitative interactions may accelerate the pace of tree expansions upward near timberline.

AbstractForecasts of widespread range shifts with climate change stem from assumptions that climate drives species' distributions. However, local adaptation and biotic interactions also influence range limits and thus may impact range shifts. Despite the potential importance of these factors, few studies have directly tested their effects on performance at range limits. We address how population‐level variation and biotic interactions may affect range shifts by transplanting seeds and seedlings of western North American conifers of different origin populations into different competitive neighborhoods within and beyond their elevational ranges and monitoring their performance. We find evidence that competition with neighboring trees limits performance within current ranges, but that interactions between adults and juveniles switch from competitive to facilitative at upper range limits. Local adaptation had weaker effects on performance that did not predictably vary with range position or seed origin. Our findings suggest that competitive interactions may slow species turnover within forests at lower range limits, whereas facilitative interactions may accelerate the pace of tree expansions upward near timberline.

• Premise of the study: The extent to which climate controls species’ range limits is a classic biological question that is particularly relevant given anthropogenic climate change. While climate is known to play a role in species distributions, biotic interactions such as competition also affect range limits. Furthermore, climatic and biotic controls of ranges may vary in strength across life stages, implying complex range shift dynamics with climate change.• Methods: We quantified climatic and competitive influences on growth of juvenile and adult trees of three conifer species on Mt. Rainier, Washington, United States. We collected annual growth data of these trees, which we compared to the competitive environment and annual climate (100 years of data) experienced by each individual.• Key results: We found that the relationships between growth and climate and between growth and competition differed by life stage and location. Growth was sensitive to heavy snowpack and cold temperatures at high elevation upper limits (treeline), but growth was poorly explained by climate in low elevation closed‐canopy forests. Competitive effects on growth were more important for saplings than adults, but did not become more important at either upper or lower range limits.• Conclusions: In all, our results suggest that range shifts under climate change will differ at leading vs. trailing edges. At treeline, warmer temperatures will lead to increased growth and likely to range expansion. However, climate change will have less dramatic effects in low elevation closed‐canopy forest communities, where growth is less strongly limited by climate, especially at young life stages.

• Premise of the study: The extent to which climate controls species’ range limits is a classic biological question that is particularly relevant given anthropogenic climate change. While climate is known to play a role in species distributions, biotic interactions such as competition also affect range limits. Furthermore, climatic and biotic controls of ranges may vary in strength across life stages, implying complex range shift dynamics with climate change.• Methods: We quantified climatic and competitive influences on growth of juvenile and adult trees of three conifer species on Mt. Rainier, Washington, United States. We collected annual growth data of these trees, which we compared to the competitive environment and annual climate (100 years of data) experienced by each individual.• Key results: We found that the relationships between growth and climate and between growth and competition differed by life stage and location. Growth was sensitive to heavy snowpack and cold temperatures at high elevation upper limits (treeline), but growth was poorly explained by climate in low elevation closed‐canopy forests. Competitive effects on growth were more important for saplings than adults, but did not become more important at either upper or lower range limits.• Conclusions: In all, our results suggest that range shifts under climate change will differ at leading vs. trailing edges. At treeline, warmer temperatures will lead to increased growth and likely to range expansion. However, climate change will have less dramatic effects in low elevation closed‐canopy forest communities, where growth is less strongly limited by climate, especially at young life stages.