Shelly A. Rayback

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

Reconstruction of Summer Temperature for a Canadian High Arctic Site from Retrospective Analysis of the Dwarf Shrub, Cassiope tetragona

Abstract We used retrospective analysis of the widespread evergreen dwarf-shrub, Cassiope tetragona, to reconstruct average summer air temperature for Alexandra Fiord, Ellesmere Island, Canada. Retrospective analysis is a technique based on dendrochronological methods. In this study, chronologies are based on the morphological characteristics of the plant stems. Two growth and two reproduction chronologies, ranging from 80 to 118 years long, were developed from each of two populations at the High Arctic site. We used multiple regression models to develop a 100-year-long (1895–1994) reconstruction of July–September average air temperature that explained 45% of the climatic variance in the instrumental record. The reconstruction revealed an increase in summer temperature from ∼1905 to the early 1960s, a cooling trend from the mid-1960 to the 1970s, and an increase in temperature after 1980. These historical temperature patterns correspond well with those from other climate proxies from sites on Ellesmere and Devon Islands. As well, the similarity between our model and an arctic-wide proxy temperature time series suggests that the Cassiope-based reconstruction contains a large-scale temperature signal. There is great potential for the development of proxy climate data using Cassiope tetragona from sites throughout the Arctic.

Dendrochronological Potential of the Arctic Dwarf-Shrub Cassiope tetragona

In this report, we describe the use of dendrochronological techniques on the circumpolar, evergreen dwarf-shrub, Cassiope tetragona. Using techniques such as crossdating and standardization, and the software programs COFECHA and ARSTAN, we developed C. tetragona growth and reproduction chronologies for sites in the Canadian High Arctic. High-resolution chronologies may be used to reconstruct past climate and phase changes in large-scale modes of atmospheric circulation (e.g. Arctic Oscillation, North Atlantic Oscillation), to investigate the growth and reproductive responses of the plant to ambient and manipulated environmental variables, and to reconstruct the plants past ecohydrology (δ18O, δD, δ13C), gas exchange (δ13C) and mineral nutrition (δ15N). As C. tetragona is a circumpolar species, chronologies may be developed throughout the Arctic at sites where no trees exist, and thus provide new information on the past climate and environmental history of sites and regions previously unstudied.

THE DENDROCLIMATOLOGICAL POTENTIAL OF AN ALPINE SHRUB, CASSIOPE MERTENSIANA, FROM MOUNT RAINIER, WA, USA

Abstract The application of dendrochronological techniques to shrubs found in arctic and alpine plant communities is opening previously untapped regions to the exploration of plant‐climate ecological relationships and climate reconstruction. In this pilot study, we present growth (1963–2004), reproduction (1963–2004), and stable carbon isotope ratio (1975–2004) chronologies for Cassiope mertensiana from a subalpine site in Mount Rainier National Park, Washington, USA. Based on simple linear correlation analysis, positive correlations characterize plant growth and previous year mean maximum temperature in April and June, suggesting the influence of temperature on snowpack and, in turn, on growing season length, plant and soil insulation, and nutrient and moisture availability. Plant growth and reproduction are significantly correlated with current year July mean maximum temperature and total precipitation, indicating the importance of a warm and extended growing season for optimal plant development. Using step‐wise multiple linear regression analysis, we developed a preliminary calibration model for July mean maximum temperature (R=0.63), extending over the 1974–2004 time period. This archive has the potential to elucidate multi‐scale, spatially‐explicit, ecological and climatic information for alpine ecosystems situated along a north‐south transect from the southern Yukon to the Pacific Northwest of the United States.

Multiple Climate Signals Characterize Cassiope Mertensiana Chronologies for a Site on Mount Rainier, Washington, USA

This study investigates the nature and strength of the climate signals that characterize an alpine dwarf-shrub, Cassiope mertensiana (mountain bell heather). For the first time, we present six new C. mertensiana chronologies (two growth, two reproduction, 1963-2004; δ13C, δ18O, 1975-2005) based on 14 plants from a subalpine meadow site on Mount Rainier, Washington, USA. Correlation, factor, and multivariate regression analyses revealed that multiple and different climate factors control the chronologies. Annual growth and leaf production are controlled by previous- and current- year growing season and previous-year February temperature. Warm growing season temperatures positively influence soil nutrient uptake rates, while winter temperatures may temporally alter moisture distribution resulting in reduced plant growth. Annual flower bud production is controlled by previous and current-year mean summer, growing season, and annual dewpoint temperature. Annual flower production is controlled by current-year mean spring, summer, growing-season, and annual dewpoint temperature. Increased atmospheric moisture facilitates open stomata, higher photosynthetic activity, and increased reproduction. Previous-year growing season and annual dew-point temperature, fall relative humidity, and temperature control δ18O values, indicating the influence of stomatal conductance on evaporative enrichment. Previous-year spring dewpoint and annual maximum temperature control δ13C values, suggesting the values are a product of photosynthetic rate controlled by temperature or photon flux.

Inconclusive evidence of Juniperus virginiana recovery following sulfur pollution reductions

Thomas et al. (1) address a question of great scientific interest: have pollution reductions mandated by the Clean Air Act improved forest health and productivity? Although answers to this question are of great importance, various aspects of this work limit its ability to address this question.

The surprising recovery of red spruce growth shows links to decreased acid deposition and elevated temperature

Following growth declines and increased mortality linked to acid deposition-induced calcium depletion, red spruce (Picea rubens Sarg.) in the northeastern United States are experiencing a recovery. We found that more than 75% of red spruce trees and 90% of the plots examined in this study exhibited increasing growth since 2001. To understand this change, we assessed the relationship between red spruce radial growth and factors that may influence growth: tree age and diameter, stand dynamics, plot characteristics (elevation, slope, aspect, geographical position), and a suite of environmental variables (temperature, precipitation, climate and precipitation indices (degree days, SPEI [standardized precipitation evapotranspiration index], and acid deposition [SO42-, NO3-, pH of rainfall, cation:anion ratio of rainfall]) for 52 plots (658 trees) from five states (spanning 2.5°N × 5°W). Examining the growth relationships from 1925 to 2012, we found that while there was variability in response to climate and acid deposition (limited to 1980-2012) by elevation and location, plot and tree factors did not adequately explain growth. Higher temperatures outside the traditional growing season (e.g., fall, winter, and spring) were related to increased growth. Nitrogen deposition (1980-2012) was associated with lower growth, but the strength of this relationship has lessened over time. Overall, we predict sustained favorable conditions for red spruce in the near term as acid deposition continues to decline and non-traditional growing season (fall through spring) temperatures moderate, provided that overall temperatures and precipitation remain adequate for growth.

Drought timing and local climate determine the sensitivity of eastern temperate forests to drought

Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi-arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad-scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors-the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)-are stronger drivers of drought sensitivity than soil and stand characteristics. Drought-induced reductions in tree growth were greatest when the droughts occurred during early-season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and ψ50 ) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in early-season PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors.

Students' Understanding of Sustainability and Climate Change Across Linked Service-Learning Courses

ABSTRACT College and university faculty are increasingly being called upon to teach about sustainability. Many of these faculty members are incorporating content related to climate change because climate change is arguably the biggest threat to global sustainability. However, the concept of sustainability is complex, interdisciplinary, and potentially difficult to teach. Further, students may feel paralyzed in the face of climate change. Thus, delivering a course that effectively covers the concept of sustainability while also empowering students to take action against climate change is difficult. The goal of this article is to describe a joint effort between four college courses in different disciplines that used service-learning projects focused on climate change as a tool to teach sustainability concepts. Although the four courses were rooted in different disciplines, they intentionally shared common components and assignments, including community service projects, shared readings and reflections, and a symposium that brought all students together. Eighty preproject and 77 postproject reflections were qualitatively analyzed for themes related to learning outcomes. The results demonstrate that students in these classes gained a more sophisticated understanding of climate change and how it affects their respective disciplines, as well as a newfound sense of personal responsibility and agency for addressing climate change. Service-learning is an increasingly popular pedagogy on college campuses. This article highlights how the pedagogy can be used as a tool for integrating climate change into courses from multiple disciplines to teach about sustainability and empower students.