• Energy Research

AbstractUnderstanding how a warmer and more variable climate will affect the productivity and stability of perennial forages in the establishment year is essential for determining forage management strategies resilient to the changing climate. We conducted an experiment from 2020–2021 and repeated it in 2021–2022 to determine how warmer and more variable temperatures affect yield of an alfalfa (Medicago sativa L.)–orchardgrass (Dactylis glomerata L.) mixture, as well as on the abundance of a winter annual weed, chickweed (Stellaria media), during the establishment year. We utilized open‐top chambers (OTCs) to manipulate air temperature in situ to impose three treatments: (1) constant warming (OTC present year‐round), (2) fluctuating warming (OTC present year‐round but temporarily removed prior to abrupt cold snaps), and (3) control (no OTC, ambient air temperature). OTCs increased air temperature within the alfalfa–orchardgrass canopy by an average of 0.45 and 0.72°C in Year 1 and Year 2, respectively; however, the effects of OTCs on air temperature varied temporally within and between years. Constant warming decreased alfalfa frost injury compared with the control, while the control and fluctuating warming resulted in similar levels of frost injury. Warmer temperature within the OTCs neither affected chickweed biomass nor the biomass of all other weed species. Both constant and fluctuating warming increased forage dry matter yield at the first harvest timepoint in the spring of both years, the period when preceding ambient air temperature tended to be cooler than other harvest timepoints. We found that the constant warming increased total forage yield (summed over the entire year) in Year 1, when precipitation was well above the 30‐year normal for our region, but decreased total forage yield in Year 2, when summer precipitation was substantially below the 30‐year normal. As global temperature continues to rise, yield loss of cool‐season perennial forages, such as alfalfa and orchardgrass, may be exacerbated in years without adequate precipitation. Therefore, exploring heat‐ and drought‐tolerant alternative forages, as well as management strategies that improve resilience to weather variability, will be increasingly important.

AbstractUnderstanding how a warmer and more variable climate will affect the productivity and stability of perennial forages in the establishment year is essential for determining forage management strategies resilient to the changing climate. We conducted an experiment from 2020–2021 and repeated it in 2021–2022 to determine how warmer and more variable temperatures affect yield of an alfalfa (Medicago sativa L.)–orchardgrass (Dactylis glomerata L.) mixture, as well as on the abundance of a winter annual weed, chickweed (Stellaria media), during the establishment year. We utilized open‐top chambers (OTCs) to manipulate air temperature in situ to impose three treatments: (1) constant warming (OTC present year‐round), (2) fluctuating warming (OTC present year‐round but temporarily removed prior to abrupt cold snaps), and (3) control (no OTC, ambient air temperature). OTCs increased air temperature within the alfalfa–orchardgrass canopy by an average of 0.45 and 0.72°C in Year 1 and Year 2, respectively; however, the effects of OTCs on air temperature varied temporally within and between years. Constant warming decreased alfalfa frost injury compared with the control, while the control and fluctuating warming resulted in similar levels of frost injury. Warmer temperature within the OTCs neither affected chickweed biomass nor the biomass of all other weed species. Both constant and fluctuating warming increased forage dry matter yield at the first harvest timepoint in the spring of both years, the period when preceding ambient air temperature tended to be cooler than other harvest timepoints. We found that the constant warming increased total forage yield (summed over the entire year) in Year 1, when precipitation was well above the 30‐year normal for our region, but decreased total forage yield in Year 2, when summer precipitation was substantially below the 30‐year normal. As global temperature continues to rise, yield loss of cool‐season perennial forages, such as alfalfa and orchardgrass, may be exacerbated in years without adequate precipitation. Therefore, exploring heat‐ and drought‐tolerant alternative forages, as well as management strategies that improve resilience to weather variability, will be increasingly important.

Climate zones play a significant role in shaping the forest ecosystems located within them by influencing multiple ecological processes, including growth, disturbances, and species interactions. Therefore, delineation of current and future climate zones is essential to establish a framework for understanding and predicting shifts in forest ecosystems. In this study, we developed and applied an efficient approach to delineate regional climate zones in the northeastern United States and maritime Canada, aiming to characterize potential shifts in climate zones and discuss associated changes in forest ecosystems. The approach comprised five steps: climate data dimensionality reduction, sampling scenario design, cluster generation, climate zone delineation, and zone shift prediction. The climate zones in the study area were delineated into four different orders, with increasing subzone resolutions of 3, 9, 15, and 21. Furthermore, projected climate normals under Shared Socioeconomic Pathways 4.5 and 8.5 scenarios were used to predict the shifts in climate zones until 2100. Our findings indicate that climate zones characterized by higher temperatures and lower precipitation are expected to become more prevalent, potentially becoming the dominant climate condition across the entire region. Thes changes are likely to alter regional forest composition, structure, and productivity. In short, such shifts in climate underscore the significant impact of environmental change on forest ecosystem dynamics and carbon sequestration potential.

Climate zones play a significant role in shaping the forest ecosystems located within them by influencing multiple ecological processes, including growth, disturbances, and species interactions. Therefore, delineation of current and future climate zones is essential to establish a framework for understanding and predicting shifts in forest ecosystems. In this study, we developed and applied an efficient approach to delineate regional climate zones in the northeastern United States and maritime Canada, aiming to characterize potential shifts in climate zones and discuss associated changes in forest ecosystems. The approach comprised five steps: climate data dimensionality reduction, sampling scenario design, cluster generation, climate zone delineation, and zone shift prediction. The climate zones in the study area were delineated into four different orders, with increasing subzone resolutions of 3, 9, 15, and 21. Furthermore, projected climate normals under Shared Socioeconomic Pathways 4.5 and 8.5 scenarios were used to predict the shifts in climate zones until 2100. Our findings indicate that climate zones characterized by higher temperatures and lower precipitation are expected to become more prevalent, potentially becoming the dominant climate condition across the entire region. Thes changes are likely to alter regional forest composition, structure, and productivity. In short, such shifts in climate underscore the significant impact of environmental change on forest ecosystem dynamics and carbon sequestration potential.

AbstractNorthern temperate ecosystems are experiencing warmer and more variable winters, trends that are expected to continue into the foreseeable future. Despite this, most studies have focused on climate change impacts during the growing season, particularly when comparing responses across different vegetation cover types. Here we examined how a perennial grassland and adjacent mixed forest ecosystem in New Hampshire, United States, responded to a period of highly variable winters from 2014 through 2017 that included the warmest winter on record to date. In the grassland, record‐breaking temperatures in the winter of 2015/2016 led to a February onset of plant growth and the ecosystem became a sustained carbon sink well before winter ended, taking up roughly 90 g/m2more carbon during the winter to spring transition than in other recorded years. The forest was an unusually large carbon source during the same period. While forest photosynthesis was restricted by leaf‐out phenology, warm winter temperatures caused large pulses of ecosystem respiration that released nearly 230 g C/m2from February through April, more than double the carbon losses during that period in cooler years. These findings suggest that, as winters continue to warm, increases in ecosystem respiration outside the growing season could outpace increases in carbon uptake during a longer growing season, particularly in forests that depend on leaf‐out timing to initiate carbon uptake. In ecosystems with a perennial leaf habit, warming winter temperatures are more likely to increase ecosystem carbon uptake through extension of the active growing season. Our results highlight the importance of understanding relationships among antecedent winter conditions and carbon exchange across land‐cover types to understand how landscape carbon exchange will change under projected climate warming.

AbstractNorthern temperate ecosystems are experiencing warmer and more variable winters, trends that are expected to continue into the foreseeable future. Despite this, most studies have focused on climate change impacts during the growing season, particularly when comparing responses across different vegetation cover types. Here we examined how a perennial grassland and adjacent mixed forest ecosystem in New Hampshire, United States, responded to a period of highly variable winters from 2014 through 2017 that included the warmest winter on record to date. In the grassland, record‐breaking temperatures in the winter of 2015/2016 led to a February onset of plant growth and the ecosystem became a sustained carbon sink well before winter ended, taking up roughly 90 g/m2more carbon during the winter to spring transition than in other recorded years. The forest was an unusually large carbon source during the same period. While forest photosynthesis was restricted by leaf‐out phenology, warm winter temperatures caused large pulses of ecosystem respiration that released nearly 230 g C/m2from February through April, more than double the carbon losses during that period in cooler years. These findings suggest that, as winters continue to warm, increases in ecosystem respiration outside the growing season could outpace increases in carbon uptake during a longer growing season, particularly in forests that depend on leaf‐out timing to initiate carbon uptake. In ecosystems with a perennial leaf habit, warming winter temperatures are more likely to increase ecosystem carbon uptake through extension of the active growing season. Our results highlight the importance of understanding relationships among antecedent winter conditions and carbon exchange across land‐cover types to understand how landscape carbon exchange will change under projected climate warming.

AbstractClimate change is altering the timing and duration of the vernal window, a period that marks the end of winter and the start of the growing season when rapid transitions in ecosystem energy, water, nutrient, and carbon dynamics take place. Research on this period typically captures only a portion of the ecosystem in transition and focuses largely on the dates by which the system wakes up. Previous work has not addressed lags between transitions that represent delays in energy, water, nutrient, and carbon flows. The objectives of this study were to establish the sequence of physical and biogeochemical transitions and lags during the vernal window period and to understand how climate change may alter them. We synthesized observations from a statewide sensor network in New Hampshire,USA, that concurrently monitored climate, snow, soils, and streams over a three‐year period and supplemented these observations with climate reanalysis data, snow data assimilation model output, and satellite spectral data. We found that some of the transitions that occurred within the vernal window were sequential, with air temperatures warming prior to snow melt, which preceded forest canopy closure. Other transitions were simultaneous with one another and had zero‐length lags, such as snowpack disappearance, rapid soil warming, and peak stream discharge. We modeled lags as a function of both winter coldness and snow depth, both of which are expected to decline with climate change. Warmer winters with less snow resulted in longer lags and a more protracted vernal window. This lengthening of individual lags and of the entire vernal window carries important consequences for the thermodynamics and biogeochemistry of ecosystems, both during the winter‐to‐spring transition and throughout the rest of the year.

AbstractClimate change is altering the timing and duration of the vernal window, a period that marks the end of winter and the start of the growing season when rapid transitions in ecosystem energy, water, nutrient, and carbon dynamics take place. Research on this period typically captures only a portion of the ecosystem in transition and focuses largely on the dates by which the system wakes up. Previous work has not addressed lags between transitions that represent delays in energy, water, nutrient, and carbon flows. The objectives of this study were to establish the sequence of physical and biogeochemical transitions and lags during the vernal window period and to understand how climate change may alter them. We synthesized observations from a statewide sensor network in New Hampshire,USA, that concurrently monitored climate, snow, soils, and streams over a three‐year period and supplemented these observations with climate reanalysis data, snow data assimilation model output, and satellite spectral data. We found that some of the transitions that occurred within the vernal window were sequential, with air temperatures warming prior to snow melt, which preceded forest canopy closure. Other transitions were simultaneous with one another and had zero‐length lags, such as snowpack disappearance, rapid soil warming, and peak stream discharge. We modeled lags as a function of both winter coldness and snow depth, both of which are expected to decline with climate change. Warmer winters with less snow resulted in longer lags and a more protracted vernal window. This lengthening of individual lags and of the entire vernal window carries important consequences for the thermodynamics and biogeochemistry of ecosystems, both during the winter‐to‐spring transition and throughout the rest of the year.