Re-evaluating the Relevance of Vegetation Trimlines in the Canadian Arctic as an Indicator of Little Ice Age Paleoenvironments

The origin of trimlines associated with the so-called “lichen-free” areas in the Canadian Arctic has been attributed both to perennial snowfield expansion during the Little Ice Age (LIA) and to seasonally persistent snow cover in more recent times. Because of the disparate hypotheses (ecological versus paleoclimatic) regarding the formation of these trimlines, their use as a paleoclimatic indicator has been abandoned for more than two decades. We re-examine this debate and the validity of the opposing hypotheses in the light of new regional mapping of trimlines across the Queen Elizabeth Islands (QEI). The ecological hypothesis—insufficient duration of the growing season resulting from seasonally persistent snow cover—fails to account for the poikilohydric nature of lichens and their ability to endure short growing seasons. It cannot adequately explain the existence of sharp trimlines or account for the occurrence of those trimlines on sparsely vegetated carbonate terrain. Furthermore, trimlines outlining the former extent of thin plateau ice caps are accordant with trimlines associated with former perennial snowfields, indicating that these trimlines record snow and ice expansion during the LIA rather than the seasonal persistence of more recent snow cover. We suggest that these features represent an important LIA climate indicator and should therefore be used for paleoclimatic reconstruction.L’origine des épaulements propres aux zones dites sans lichen de l’Arctique canadien a été attribuée tant à l’expansion des champs de neige pérenne pendant le petit âge glaciaire qu’à la couverture de neige longévive d’époques plus récentes. Puisqu’il existe des hypothèses disparates (écologiques par opposition à paléoclimatiques) quant à la formation de ces épaulements, on a arrêté de s’en servir à titre d’indicateur paléoclimatique depuis plus d’une vingtaine d’années. Ici, ce débat fait l’objet d’un nouvel examen où l’on se penche sur la validité des hypothèses divergentes à la lumière du nouveau mappage régional des épaulements des îles de la Reine-Élisabeth. L’hypothèse d’ordre écologique —durée insuffisante de la saison de croissance découlant de la couverture de neige longévive en saison —omet de tenir compte de la nature poecilitique du lichen et de son aptitude à endurer de courtes saisons de croissance. Cette hypothèse ne permet pas d’expliquer adéquatement l’existence d’épaulements précis ou de tenir compte de la présence de ces épaulements en terrain carbonaté à végétation éparse. Par ailleurs, les épaulements qui délimitent l’ancienne étendue des minces calottes glaciaires des plateaux correspondent aux épaulements associés aux anciens champs de neige pérenne, ce qui indique que ces épaulements dénotent les expansions de neige et de glace du petit âge glaciaire et non pas de la couverture de neige longévive saisonnière plus récente. On suggère que ces caractéristiques représentent un important indicateur climatique du petit âge glaciaire et par conséquent, qu’on devrait s’en servir à des fins de reconstruction paléoclimatique

NONLINEARITIES, SCALE-DEPENDENCE, AND INDIVIDUALISM OF BOREAL FOREST TREES TO CLIMATE FORCING

Changes in climate are affecting tree growth, fire regimes and the geographic ranges of species (Beck et al. 2011; Kelly et al. 2013). Increasing our understanding of how boreal tree species respond to climate warming is critical for predicting the future states of the boreal forest and assessing the global impacts of these changes. Black spruce (Picea mariana [Mill.] B.S.P.) is the most abundant tree species in the Interior Alaskan boreal forest. Although it grows in a variety of community types (Hollingsworth et al. 2006), it is the only tree species found at the coldest, wettest sites on the landscape. Despite its abundance, very little is known about the climate-growth relationships of black spruce, as the majority of dendrochronological studies in Interior Alaska involve white spruce growing at treeline.Funding was provided by a National Science Foundation grant (ARC-0902169), the Scenarios Network for Alaska and Arctic Planning, and the Alaska Climate Science Center (Cooperative Agreement Number G10AC00588 from the U.S. Geological Survey)

Three Distinct Roles for Notch in Drosophila R7 Photoreceptor Specification

During specification of the R7 photoreceptor in the <I>Drosophila</I> eye, activation of Notch signaling leads to multiple responses within the cell, including antagonistic ones

The state of the Martian climate

60°N was +2.0°C, relative to the 1981–2010 average value (Fig. 5.1). This marks a new high for the record. The average annual surface air temperature (SAT) anomaly for 2016 for land stations north of starting in 1900, and is a significant increase over the previous highest value of +1.2°C, which was observed in 2007, 2011, and 2015. Average global annual temperatures also showed record values in 2015 and 2016. Currently, the Arctic is warming at more than twice the rate of lower latitudes

Setting benchmarks for modelling gas–surface interactions using coherent control of rotational orientation states

A fundamental and predictive understanding of molecule-surface interactions is challenging to obtain. Here the authors report an experimental technique allowing direct measurement of the scattering matrix, which reports on the coherent evolution of quantum states of a molecule scattering from a surface