Observations on the gynoecial pathway for pollen tube growth in sweet lowbush blueberry (Vaccinium angustifolium Ait.)

Gynoecial structure in sweet lowbush blueberry, Vaccinium angustifolium Ait., was investigated in order to characterize the pollen tube pathway in order to provide a framework for further studies on pollination and fungal infection. Closed flower buds and pollinated open flowers were collected from managed lowbush blueberry fields in Colchester County, Nova Scotia, Canada. Following chemical fixation, the tissue samples were examined histologically using light and scanning electron microscopy. The continuous pathway was characterized by a fluted, exudate-filled stylar canal that connects the wet stigmatic surface with the exudate covered surface of the ovarian placentae. Following pollen deposition and germination, tubes growing along the pathway eventually arrive at the micropyles of the anatropous ovules; ovule penetration by pollen tubes and fertilization of the female gametophytes ensue. The pollen tube pathway of this taxon conforms to the general pattern reported from other ericalean taxa

Submarine slides from the walls of Smeerenburgfjorden, NW Svalbard

The steep slopes of glacially eroded fjord walls are potential sites for slope failure and mass wasting (Syvitski et al. 1987). Slides of sedimentary material are relatively common, but require that sediment is present on fjord sides, often deposited from glaciers before and during their retreat through fjord systems. This debris then fails to produce slides that run out onto fjord floors and leave scars on fjord sidewalls.This is the author accepted manuscript. The final version is available from the Geological Society of London via https://doi.org//10.1144/M46.2

Quantifying the effects of harvesting on carbon fluxes and stocks in northern temperate forests

Harvest disturbance has substantial impacts on forest carbon (C) fluxes and stocks. The quantification of these effects is essential for the better understanding of forest C dynamics and informing forest management in the context of global change. We used a process-based forest ecosystem model, PnET-CN, to evaluate how, and by what mechanisms, clear-cuts alter ecosystem C fluxes, aboveground C stocks (AGC), and leaf area index (LAI) in northern temperate forests. We compared C fluxes and stocks predicted by the model and observed at two chronosequences of eddy covariance flux sites for deciduous broadleaf forests (DBF) and evergreen needleleaf forests (ENF) in the Upper Midwest region of northern Wisconsin and Michigan, USA. The average normalized root mean square error (NRMSE) and the Willmott index of agreement (d) for carbon fluxes, LAI, and AGC in the two chronosequences were 20% and 0.90, respectively. Simulated gross primary productivity (GPP) increased with stand age, reaching a maximum (1200–1500 g C m−2 yr−1) at 11–30 years of age, and leveled off thereafter (900–1000 g C m−2 yr−1). Simulated ecosystem respiration (ER) for both plant functional types (PFTs) was initially as high as 700–1000 g C m−2 yr−1 in the first or second year after harvesting, decreased with age (400–800 g C m−2 yr−1) before canopy closure at 10–25 years of age, and increased to 800–900 g C m−2 yr−1 with stand development after canopy recovery. Simulated net ecosystem productivity (NEP) for both PFTs was initially negative, with net C losses of 400–700 g C m−2 yr−1 for 6–17 years after clear-cuts, reaching peak values of 400–600 g C m−2 yr−1 at 14–29 years of age, and eventually stabilizing in mature forests (\u3e 60 years old), with a weak C sink (100–200 g C m−2 yr−1). The decline of NEP with age was caused by the relative flattening of GPP and gradual increase of ER. ENF recovered more slowly from a net C source to a net sink, and lost more C than DBF. This suggests that in general ENF may be slower to recover to full C assimilation capacity after stand-replacing harvests, arising from the slower development of photosynthesis with stand age. Our model results indicated that increased harvesting intensity would delay the recovery of NEP after clear-cuts, but this had little effect on C dynamics during late succession. Future modeling studies of disturbance effects will benefit from the incorporation of forest population dynamics (e.g., regeneration and mortality) and relationships between age-related model parameters and state variables (e.g., LAI) into the model

Holocene glacial history of Svalbard: Status, perspectives and challenges

© 2020 The Author(s) We synthesize the current understanding of glacier activity on Svalbard from the end of the Late Pleistocene (12,000 yrs. before present) to the end of the Little Ice Age (c. 1920 AD). Our glacier history is derived from the SVALHOLA database, the first compilation of Holocene geochronology for Svalbard and the surrounding waters, including over 1,800 radiocarbon, terrestrial cosmogenic nuclide and optically stimulated luminescence ages. Data have been categorized by geological setting, uniformly (re-)calibrated, quality assessed and ultimately used to constrain glacier fluctuations (deglaciation, ice free conditions, glacier re-advances and ice marginal positions). We advance existing knowledge by mapping the extent and distribution of ice-cover during the Holocene glacial maximum and the glacial minimum, as well as present retreat rates (and percentages) within Early Holocene fjord-systems. Throughout the Holocene, Svalbard glaciers have responded to a varying combination of climatic, environmental and dynamic driving factors which influence both the extent and behavior of ice margins. We discuss the complexities of glacier systems and their dynamics in response to changes in climate. This review provides a holistic state of the art of Holocene glaciers on Svalbard, suitable for orienting future works which address gaps in our current knowledge

Subglacial Water Flow Over an Antarctic Palaeo‐Ice Stream Bed

The subglacial hydrological system exerts a critical control on the dynamic behavior of the overlying ice because its configuration affects the degree of basal lubrication between the ice and the bed. Yet, this component of the glaciological system is notoriously hard to access and observe, particularly over timescales longer than the satellite era. In Antarctica, abundant evidence for past subglacial water flow over former ice-sheet beds exists around the peripheries of the ice sheet including networks of huge channels carved into bedrock (now submarine) on the Pacific margin of West Antarctica. Here, we combine detailed bathymetric investigations of a channel system in Marguerite Trough, a major palaeo-ice stream bed, with numerical hydrological modeling to explore subglacial water accumulation, routing and potential for erosion over decadal-centennial timescales. Detailed channel morphologies from remotely operated vehicle surveys indicate multiple stages of localized incision, and the occurrence of potholes, some gigantic in scale, suggests incision by turbulent water carrying a significant bedload. Further, the modeling indicates that subglacial water is available during deglaciation and was likely released in episodic drainage events, from subglacial lakes, varying in magnitude over time. Our observations support previous assertions that these huge bedrock channel systems were incised over multiple glacial cycles through episodic subglacial lake drainage events; however, here we present a viable pattern for subglacial drainage at times when the ice sheet existed over the continental shelf and was capable of continuing to erode the bedrock substrate

Marginal Fluctuations of a Svalbard Surge-Type Tidewater Glacier, Blomstrandbreen, since the Little Ice Age: A Record of Three Surges

© 2016 Regents of the University of Colorado. Previous advances and retreats of Blomstrandbreen within the cold period known as the Little Ice Age, between approximately 1400 and 1920, are relatively well documented. The seafloor characteristics associated with these glacier fluctuations, and their importance for the identification of similar surge-type tidewater glaciers, are discussed. We use detailed multibeam-bathymetric data acquired within Nordvågen, the marine area offshore of Blomstrandbreen, to provide a new understanding of the style and pattern of deglaciation around Blomstrandhalvøya since Blomstrandbreen's neoglacial maximum. Glacial landforms on the seafloor of Nordvågen comprise overridden moraines, glacial lineations, terminal moraines, and annual recessional moraines. Crevasse-fill ridges, which are often regarded as a characteristic landform of surging tidewater glaciers, are present on only restricted areas of Nordvågen. Significantly, this study shows that large terminal surge moraines and numerous crevasse-fill ridges may not always be well developed in association with glacier surges, with implications for the identification of surges in the geological record. Using historical observations, aerial photographs, and satellite imagery of Blomstrandbreen, we have correlated former ice-marginal positions with mapped submarine landforms. Three surge events occurred during a pattern of overall retreat, with a spacing of about 50 years between active advance phases; this represents a relatively short quiescent phase for Svalbard glaciers. Average retreat rates of 10-50 m yr-1 are typical of the quiescent phase of the surge cycle, whereas surge advances vary from 200 m to over 725 m