Natalie T. Boelman

Urban environment of New York City promotes growth in northern red oak seedlings

Urbanization is accelerating across the globe, elevating the importance of studying urban ecology. Urban environments exhibit several factors affecting plant growth and function, including high temperatures (particularly at night), CO(2) concentrations and atmospheric nitrogen deposition. We investigated the effects of urban environments on growth in Quercus rubra L. seedlings. We grew seedlings from acorns for one season at four sites along an urban-rural transect from Central Park in New York City to the Catskill Mountains in upstate New York (difference in average maximum temperatures of 2.4 °C; difference in minimum temperatures of 4.6 °C). In addition, we grew Q. rubra seedlings in growth cabinets (GCs) mimicking the seasonal differential between the city and rural sites (based on a 5-year average). In the field experiment, we found an eightfold increase in biomass in urban-grown seedlings relative to those grown at rural sites. This difference was primarily related to changes in growth allocation. Urban-grown seedlings and seedlings grown at urban temperatures in the GCs exhibited a lower root: shoot ratio (urban ~0.8, rural/remote ~1.5), reducing below-ground carbon costs associated with construction and maintenance. These urban seedlings instead allocated more growth to leaves than did rural-grown seedlings, resulting in 10-fold greater photosynthetic area but no difference in photosynthetic capacity of foliage per unit area. Seedlings grown at urban temperatures in both the field and GC experiments had higher leaf nitrogen concentrations per unit area than those grown at cooler temperatures (increases of 23% in field, 32% in GC). Lastly, we measured threefold greater (13)C enrichment of respired CO(2) (relative to substrate) in urban-grown leaves than at other sites, which may suggest greater allocation of respiratory function to growth over maintenance. It also shows that lack of differences in total R flux in response to environmental conditions may mask dramatic shifts in respiratory functioning. Overall, our findings indicating greater seedling growth and establishment at a critical regeneration phase of forest development may have important implications for the ecology of urban forests as well as the predicted growth of the terrestrial biosphere in temperate regions in response to climate change.

Greater deciduous shrub abundance extends tundra peak season and increases modeled net CO2 uptake

Satellite studies of the terrestrial Arctic report increased summer greening and longer overall growing and peak seasons since the 1980s, which increases productivity and the period of carbon uptake. These trends are attributed to increasing air temperatures and reduced snow cover duration in spring and fall. Concurrently, deciduous shrubs are becoming increasingly abundant in tundra landscapes, which may also impact canopy phenology and productivity. Our aim was to determine the influence of greater deciduous shrub abundance on tundra canopy phenology and subsequent impacts on net ecosystem carbon exchange (NEE) during the growing and peak seasons in the arctic foothills region of Alaska. We compared deciduous shrub-dominated and evergreen/graminoid-dominated community-level canopy phenology throughout the growing season using the normalized difference vegetation index (NDVI). We used a tundra plant-community-specific leaf area index (LAI) model to estimate LAI throughout the green season and a tundra-specific NEE model to estimate the impact of greater deciduous shrub abundance and associated shifts in both leaf area and canopy phenology on tundra carbon flux. We found that deciduous shrub canopies reached the onset of peak greenness 13 days earlier and the onset of senescence 3 days earlier compared to evergreen/graminoid canopies, resulting in a 10-day extension of the peak season. The combined effect of the longer peak season and greater leaf area of deciduous shrub canopies almost tripled the modeled net carbon uptake of deciduous shrub communities compared to evergreen/graminoid communities, while the longer peak season alone resulted in 84% greater carbon uptake in deciduous shrub communities. These results suggest that greater deciduous shrub abundance increases carbon uptake not only due to greater leaf area, but also due to an extension of the period of peak greenness, which extends the period of maximum carbon uptake.

The effect of extreme spring weather on body condition and stress physiology in Lapland longspurs and white-crowned sparrows breeding in the Arctic.

Highlights • The spring of 2013 was extreme with record low temperatures and snow cover.• Arrival of migrant birds in Arctic was significantly delayed in 2013 compared to 3 other years.• Body condition was negatively affected in white-crowned sparrows and Lapland longspurs.• Stress physiology was increased in Lapland longspurs but not white-crowned sparrows.• Extreme events have the capacity to affect phenology, body condition and stress physiology.

Understanding burn severity sensing in Arctic tundra: exploring vegetation indices, suboptimal assessment timing and the impact of increasing pixel size

Little is known about how satellite imagery can be used to describe burn severity in tundra landscapes. The Anaktuvuk River Fire (ARF) in 2007 burned over 1000 km2 of tundra on the North Slope of Alaska, creating a mosaic of small (1 m2) to large (>100 m2) patches that differed in burn severity. The ARF scar provided us with an ideal landscape to determine if a single-date spectral vegetation index can be used once vegetation recovery began and to independently determine how pixel size influences burn severity assessment. We determine and explore the sensitivity of several commonly used vegetation indices to variation in burn severity across the ARF scar and the influence of pixel size on the assessment and classification of tundra burn severity. We conducted field surveys of spectral reflectance at the peak of the first growing season post-fire (extended assessment period) at 18 field sites that ranged from high to low burn severity. In comparing single-date indices, we found that the two-band enhanced vegetation index (EVI2) was highly correlated with normalized burn ratio (NBR) and better distinguished among three burn severity classes than both the NBR and the normalized difference vegetation index (NDVI). We also show clear evidence that shortwave infrared (SWIR) reflectivity does not vary as a function of burn severity. By comparing a Quickbird scene (2.4 m pixels) to simulated 30 and 250 m pixel scenes, we are able to confirm that while the moderate spatial resolution of the Landsat Thematic Mapper (TM) sensor (30 m) is sufficient for mapping tundra burn severity, the coarser resolution of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor (250 m) is not well matched to the fine scale of spatial heterogeneity in the ARF burn scar.

Applying terrestrial lidar for evaluation and calibration of airborne lidar-derived shrub biomass estimates in Arctic tundra

ABSTRACT Monitoring of climate-driven expansion of low-stature shrubs in Arctic tundra can be improved through application of high-resolution remote sensing. However, the destructive nature of harvest sampling that is usually performed for validation of these data is resource intensive and can limit future comparisons by destroying benchmark measurements. We compared aboveground shrub biomass estimates derived from terrestrial laser scanning (TLS) and airborne laser scanning (ALS) with the goal of determining whether TLS data can be used to accurately calibrate ALS estimates of shrub biomass in Arctic tundra. We used a leave-one-out cross-validation calibration of canopy volume against harvested shrub biomass to establish predictive relationships between TLS canopy volume and harvested shrub biomass, and between ALS canopy volume and TLS-derived shrub biomass estimates. TLS produced more accurate predictions of shrub biomass (R2 = 0.78; root mean square deviation [RMSD] = 102 g) than did ALS, but the accuracy of ALS-derived shrub biomass predictions was the same whether they were calibrated directly against harvest biomass or against TLS-derived estimates of biomass (R2 = 0.62; RMSD = 140 g). Our results suggest that once the initial TLS-harvest relationship is known, TLS can provide valid ground reference data for calibration of ALS-derived estimates of shrub biomass without the need for additional destructive harvest.

Hill Slope Variations in Chlorophyll Fluorescence Indices and Leaf Traits in a Small Arctic Watershed

Abstract Physiological processes responsible for ecosystem carbon and nitrogen cycling may vary across hill slopes and be controlled by watershed hydrology and the associated nutrient transport. Mass transport of nutrients down slope and into water tracks may increase nutrient delivery to plant roots, nutrient uptake, and perhaps photosynthetic activity. Small arctic watersheds are commonly characterized by increased biomass, particularly of woody deciduous shrubs, both down slope and in water tracks. We ask if photosynthetic physiology varies with hill slope position and if it is correlated to observed changes in above ground biomass. Chlorophyll fluorescence surveys from six common species reveal that maximum photosynthetic electron transport decreased significantly (by as much as 85%) down slope in 4 species. Leaf nitrogen concentrations varied from 1 to 2.5% across all leaves sampled, but show little trend with hill slope position, and as a result are not well correlated with photosynthetic electron transport. We hypothesize that trace metal concentrations may have increased in the leaves of plants growing in down slope positions and that this may be responsible for the reduction in electron transport. The relationship between the measured maximum energy conversion by photosystem II and maximum electron transport rate is species specific and indicative of light adaptation in these arctic species. Increased plant growth down slope and in water tracks does not appear to be correlated to the physiological parameters measured and instead are more likely a product of increased canopy nitrogen concentrations and leaf area accumulation.

Temperature response of respiration and respiratory quotients of 16 co-occurring temperate tree species

The forests of the northeastern US are globally, one of the fastest growing terrestrial carbon sinks due to historical declines in large-scale agriculture, timber harvesting and fire disturbance. However, shifting range distributions of tree species with warming air temperatures are altering forest community composition and carbon dynamics. Here, we focus on respiration, a physiological process that is strongly temperature and species dependent. We specifically examined the response of respiration (R; CO2 release) to temperature in 10 broadleaved and six conifer species, as well as the respiratory quotient (RQ; ratio of CO2 released to O2 consumed) of nine broadleaved species that co-occur in the Hudson Highlands Region of New York, USA. The relationships between these physiological measurements and associated leaf traits were also explored. The rates of respiration at 20 °C were 71% higher in northern-ranged broadleaved species when compared with both central- and southern-ranged species. In contrast, the rates of respiration at 20 °C in northern-ranged conifers were 12% lower than in central-ranged conifers. The RQ of broadleaved species increased by 14% as temperatures increased from 15 °C to 35 °C. When RQ values were pooled across temperature, northern-ranged broadleaved species had 12% and 9% lower RQ values than central, and southern-ranged species, respectively, suggesting a reliance on alternative (non-carbohydrate) substrates to fulfill respiratory demands. A Pearson correlation analysis of leaf traits and respiration revealed strong correlations between leaf nitrogen, leaf mass area and R for both broadleaved and conifer species. Our results elucidate leaf trait relationships with tree physiology and reveal the various form and function strategies for species from differing range distributions. Compounded with predicted range distribution shifts and species replacement, this may reduce the carbon storage potential of northeast forests.

Weathering the storm: Do arctic blizzards cause repeatable changes in stress physiology and body condition in breeding songbirds?

Highlights • Stress physiology was analyzed in response to multi-day snowstorms in 5 years.• Baseline corticosterone was unaffected by snowstorms in all but two instances.• Stress-induced corticosterone were typically elevated during snowstorms.• Stress physiology was significantly different across a multiday storm in only one year.• Body condition tended to increase on the first day of the storm.

Shrub shading moderates the effects of weather on arthropod activity in arctic tundra: Shrub cover affects arctic arthropod activity

1. Rapid warming has facilitated an increase in deciduous shrub cover in arctic tundra. Because shrubs create a cooler microclimate during the growing season, shrub cover could modulate the effects of global warming on the phenology and activity of ectotherms, including arthropods. This possibility was explored here using two dominant arthropod groups (flies and wolf spiders) in Alaskan tundra.