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The phenological timing of leaf out in temperate forests is a critical transition point each year that alters the global climate system, which in turn, feeds back to plants, driving leaf out to occur nearly 3 d earlier per decade as temperatures rise. To improve predictions of leaf out timing, urban heat islands (UHIs) or densely developed areas that are hotter than surrounding undeveloped regions are often used to approximate warming via space-for-time substitutions (i.e., rural-to-urban temperature gradients). However, more than just environment changes along these gradients—urban regions are highly managed systems with limited-to-no within species diversity. We demonstrate here that recent observations that UHI gradients underpredict leaf out response to temperature when compared to temperature gradients through time is likely because both genetics and environment are changing across rural-to-urban gradients, whereas only environment is changing through time. We tested this hypothesis using genomic, phenological, and temperature data of northern red oak (Quercus rubra) over several years between an urban and rural site. Across our gradient, models that included just temperature predicted moderate advancement of leaf out. However, if we account for the genetic diversity of our trees in our model, leaf out phenology is predicted to advance significantly more in response to temperature. We demonstrate that this stronger relationship between phenological timing and climate is because urban trees have reduced genetic diversity as they are planted from limited stock by humans and, moreover, are most closely related to individuals at the rural site that leaf out later on average. 

Biological soil crusts (biocrusts) are critical components of dryland and other ecosystems worldwide, and are increasingly recognized as novel model ecosystems from which more general principles of ecology can be elucidated. Biocrusts are often diverse communities, comprised of both eukaryotic and prokaryotic organisms with a range of metabolic lifestyles that enable the fixation of atmospheric carbon and nitrogen. However, how the function of these biocrust communities varies with succession is incompletely characterized, especially in comparison to more familiar terrestrial ecosystem types such as forests. We conducted a greenhouse experiment to investigate how community composition and soil-atmosphere trace gas fluxes of CO₂, CH₄, and N₂O varied from early-successional light cyanobacterial biocrusts to mid-successional dark cyanobacteria biocrusts and late-successional moss-lichen biocrusts and as biocrusts of each successional stage matured. Cover type richness increased as biocrusts developed, and richness was generally highest in the late-successional moss-lichen biocrusts. Microbial community composition varied in relation to successional stage, but microbial diversity did not differ significantly among stages. Net photosynthetic uptake of CO₂by each biocrust type also increased as biocrusts developed but tended to be moderately greater (by up to ≈25%) for the mid-successional dark cyanobacteria biocrusts than the light cyanobacterial biocrusts or the moss-lichen biocrusts. Rates of soil C accumulation were highest for the dark cyanobacteria biocrusts and light cyanobacteria biocrusts, and lowest for the moss-lichen biocrusts and bare soil controls. Biocrust CH₄and N₂O fluxes were not consistently distinguishable from the same fluxes measured from bare soil controls; the measured rates were also substantially lower than have been reported in previous biocrust studies. Our experiment, which uniquely used greenhouse-grown biocrusts to manipulate community composition and accelerate biocrust development, shows how biocrust function varies along a dynamic gradient of biocrust successional stages. 

The ability to automatically delineate individual tree crowns using remote sensing data opens the possibility to collect detailed tree information over large geographic regions. While individual tree crown delineation (ITCD) methods have proven successful in conifer-dominated forests using Light Detection and Ranging (LiDAR) data, it remains unclear how well these methods can be applied in deciduous broadleaf-dominated forests. We applied five automated LiDAR-based ITCD methods across fifteen plots ranging from conifer- to broadleaf-dominated forest stands at Harvard Forest in Petersham, MA, USA, and assessed accuracy against manual delineation of crowns from unmanned aerial vehicle (UAV) imagery. We then identified tree- and plot-level factors influencing the success of automated delineation techniques. There was relatively little difference in accuracy between automated crown delineation methods (51–59% aggregated plot accuracy) and, despite parameter tuning, none of the methods produced high accuracy across all plots (27—90% range in plot-level accuracy). The accuracy of all methods was significantly higher with increased plot conifer fraction, and individual conifer trees were identified with higher accuracy (mean 64%) than broadleaf trees (42%) across methods. Further, while tree-level factors (e.g., diameter at breast height, height and crown area) strongly influenced the success of crown delineations, the influence of plot-level factors varied. The most important plot-level factor was species evenness, a metric of relative species abundance that is related to both conifer fraction and the degree to which trees can fill canopy space. As species evenness decreased (e.g., high conifer fraction and less efficient filling of canopy space), the probability of successful delineation increased. Overall, our work suggests that the tested LiDAR-based ITCD methods perform equally well in a mixed temperate forest, but that delineation success is driven by forest characteristics like functional group, tree size, diversity, and crown architecture. While LiDAR-based ITCD methods are well suited for stands with distinct canopy structure, we suggest that future work explore the integration of phenology and spectral characteristics with existing LiDAR as an approach to improve crown delineation in broadleaf-dominated stands. 

Summary Leaf optical properties impact leaf energy balance and thus leaf temperature. The effect of leaf development on mid‐infrared (MIR) reflectance, and hence thermal emissivity, has not been investigated in detail.We measured a suite of morphological characteristics, as well as directional‐hemispherical reflectance from ultraviolet to thermal infrared wavelengths (250 nm to 20 µm) of leaves from five temperate deciduous tree species over the 8 wk following spring leaf emergence.By contrast to reflectance at shorter wavelengths, the shape and magnitude of MIR reflectance spectra changed markedly with development. MIR spectral differences among species became more pronounced and unique as leaves matured. Comparison of reflectance spectra of intact vs dried and ground leaves points to cuticular development – and not internal structural or biochemical changes – as the main driving factor. Accompanying the observed spectral changes was a drop in thermal emissivity from about 0.99 to 0.95 over the 8 wk following leaf emergence.Emissivity changes were not large enough to substantially influence leaf temperature, but they could potentially lead to a bias in radiometrically measured temperatures of up to 3 K. Our results also pointed to the potential for using MIR spectroscopy to better understand species‐level differences in cuticular development and composition. 

Abstract How variations in carbon supply affect wood formation remains poorly understood in particular in mature forest trees. To elucidate how carbon supply affects carbon allocation and wood formation, we attempted to manipulate carbon supply to the cambial region by phloem girdling and compression during the mid‐ and late‐growing season and measured effects on structural development, CO₂efflux and nonstructural carbon reserves in stems of mature white pines. Wood formation and stem CO₂efflux varied with a location relative to treatment (i.e., above or below the restriction). We observed up to twice as many tracheids formed above versus below the treatment after the phloem transport manipulation, whereas the cell‐wall area decreased only slightly below the treatments, and cell size did not change relative to the control. Nonstructural carbon reserves in the xylem, needles and roots were largely unaffected by the treatments. Our results suggest that low and high carbon supply affects wood formation, primarily through a strong effect on cell proliferation, and respiration, but local nonstructural carbon concentrations appear to be maintained homeostatically. This contrasts with reports of decoupling of source activity and wood formation at the whole‐tree or ecosystem level, highlighting the need to better understand organ‐specific responses, within‐tree feedbacks, as well as phenological and ontogenetic effects on sink‐source dynamics.