The bear’s habitat may also have included honeybees, but this is speculative. The genus appears to have originated in Europe dispersing into Asia, Africa as well as North America. In North America the fossil record of this lineage is represented by a single species from the Miocene of Nevada. The most likely route that the Apis lineage took to arrive in North America would have been via the Bering Isthmus, which was present throughout the Neogene until ~5–7Ma. This land connection would have allowed for the existence of expansive high-latitude terrestrial continuity, spanning the northern reaches of the Eurasian and North American continents. Thus Apis in North America may have originally inhabited this Arctic biome before dispersing southward into the mid-latitudes of North America. In which case, the polar P. abstrusus may have had opportunity to supplement its diet with honey. Aside from the Beaver Pond site fossil bear, all other basal ursines are known from the northern mid-latitudes of Eurasia and North America . The lack of fossil bears in the intervening latitudes reflects the scarcity of northern Neogene vertebrate fossil sites in these regions. Thus, the discovery of the Beaver Pond site P. abstrusus at 78°N fills a substantial geographical gap. The finding also shows that early ursines were adapted to northern forests with snowy winters. Moreover, the Beaver Pond site bear is a small-bodied bear with dental caries and associated with a polar forest, rich in seasonal fruits ,large pots with drainage suggesting that the northern populations of P. abstrusus likely consumed large amounts of sugar-rich foods in the fall, a pattern consistent with preparation for hibernation seen in modern bears.
If so, the Beaver Pond site bear represents the earliest known, and most primitive bear, to have hibernated. Modern ursid hibernators include high latitude/ altitude Asian black bears , northern American black bears , all brown bears , and female polar bears. Also, the fossil species of cave bears are inferred to have hibernated. All living bears also employ a reproductive strategy of embryonic diapause with implicit adaptive value of reducing the cost of reproduction by truncating embryonic development and of optimizing birth season at the most appropriate time. Furthermore, these reproductive cycles may regulate metabolism by facilitating earlier entry of pregnant females into winter-dormancy state. In the context of the phylogeny of modern bears the northern americanus-arctos-spelaeus-maritimus clade appears to have acquired hibernation from a single ancestor. The case for Asian black bear is ambiguous because its nearest relatives are not known to hibernate; namely the sloth bear of India, the sun bear of Southeast Asia. The early diverging spectacled bear of South America is also a non-hibernator . If the northern adapted Beaver Pond bear was a hibernator, then hibernation can be traced to the ancestor of all modern bears. This would imply that the Asian black bear retains the primitive condition, and the Eurasian ancestor of the spectacled bear, which would have passed through cold Beringian habitat when it first immigrated to North America, also employed hibernation as part of its repertoire for winter survival. In this evolutionary scenario modern non-hibernating bears, are interpreted to have secondarily lost this trait, in association with adaptation to warmer habitats.Fire-induced turbulence has considerable impact on scalar dispersion and momentum transport both during and after the passage of the fire-front across the fuel in wild land fire environments.
The dispersion of scalars such as smoke in turn affects visibility and human health, especially for residents in the wild land-urban interface, while the transport of firebrands poses a severe risk of spotting up to several kilometers away from the fire-front . Moreover, fire behavior is closely linked with the ambient turbulence . Depending on the environment, turbulence in the canopy or the atmospheric boundary layer demonstrably affects fire spread-rates , occasionally in unexpected ways. With a deep necessity to understand fire-induced turbulence both in grassland and forested environments, fire simulations as well as in-situ measurements from burns on small as well as management scales have gained importance. However, the excessive computational overhead involved in simulating fires in forested and grassland environments make them less amenable for extended use. While a fast-running simulation platform like QUIC-fire is able to circumvent this issue, its wind solver outputs the time-averaged flow field, which makes it difficult to analyze the turbulence-driving fire behavior. Given these constraints on computational models, careful examination of observed data from burn experiments can be useful in investigating the coherent motions that characterize fire-induced turbulence. Such examination can potentially also allow for the simplification of the governing equations describing fire behavior based on the relative importance of the different terms involved. In the past, empirically derived models based on the widely used model by Rothermel have been successful in simplifying the complex set of governing equations for successful operational use via the parameterization of several physical quantities. However, these models are based on laboratory experiments so that their application on management scales is limited.
Another limitation is their inability to provide insights into the complex turbulent environment arising from fire-atmosphere interaction. Despite their usefulness in understanding the physics underlying fire-induced turbulence, studies relying on experimental data have only recently gathered momentum because of the high risk of damage posed to equipment during data collection in the presence of a flame . Here, we review the key findings of some of the few leading empirical studies that attempted to untangle the complex dynamics underlying fire-atmosphere interaction. Recent studies by Heilman et al. explored turbulent kinetic energy levels at different heights of a 20 m measurement tower, the energy spectrum of the velocity components, and patterns in the turbulent heat and momentum fluxes for two different backing surface fires in the New Jersey Pinelands National Reserve , one of high intensity in 2011 and the other of lower intensity in 2012. For the higher intensity fire, fire-induced TKE near the canopy top was found to be much more substantial compared to the lower heights, making turbulent diffusion of smoke near the canopy top more important than mixing at lower heights within the canopy. Analysis of 1 minute averaged turbulent momentum fluxes showed that vertical turbulent fluxes contributed 40%–80% to the net momentum flux before, during, and after fire-front-passage . The relative contribution of the vertical turbulent heat and momentum fluxes to the net heat and momentum fluxes was found to be highest at the mid-canopy height when compared to near the surface or the canopy top. Estimates of the turbulence anisotropy obtained by Heilman et al. demonstrated the significance of horizontal velocity perturbations over vertical velocity perturbations pre- and post-FFP as well as during FFP periods at all measurement heights. Analysis of the relative contribution of the terms in the TKE budget equations showed that shear production was higher than the buoyancy production at all measurement heights within the over story vegetation layer pre- and post-FFP. Furthermore,square pot the turbulent transport term resulted in a loss of TKE at all measurement heights during FFP in 2011, while its effect was found to be greatly diminished in the lower-intensity 2012 burn. In a separate study, Heilman et al. studied the contribution to turbulent fluxes of sweep-ejection events before, during, and after the passage of a heading surface fire-front through a network of 20 m measurement towers during an operational burn in the New Jersey Pine Barren. It was found that pre-FFP and post-FFP periods were characterized by sweeps and ejections, while FFP periods were characterized by sweeps and outward interactions . These studies encapsulate some of the most recent turbulence analyses on experimental surface fires in forested regions. The FireFlux experiment was a pioneering study on fire–atmosphere interactions in a grassland fire and set a benchmark for similar studies in the future . It comprised a heading experimental grassland fire conducted at the Houston Coastal Center in Texas on 23 February, 2006. Temperature and velocity measurements taken during the experiment by a 43 m instrumented tower were utilized by Clements et al. for a first-order analysis. Derived quantities such as the velocity spectra, turbulent momentum fluxes, and TKE were analyzed to comment on the turbulence levels induced by the fire-front passage. The turbulence intensity generated by the fire was found to be four to five times greater than the ambient atmospheric turbulence. Furthermore, turbulent fluxes demonstrated a downward transfer of high momentum downstream of the fire-front. Heilman et al. took the analysis further with their study of sweep-ejection-like events, which also included results from the three prescribed fires in the New Jersey Pinelands described above.
Their focus was on examining how the frequencies of occurrence of sweep-ejection-like events and their contributions to the mean vertical turbulent fluxes of heat and momentum were modulated by the vicinity or presence of the fire. Contrary to conditions in the absence of surface fires, it was found that ejections were completely dominant in their contribution to heat-flux events and outward interaction events substantially overshadowed the contribution of sweeps and ejections to momentum-flux events in both environments. The most frequent events during FFP were heat-flux inward interaction events and momentum-flux outward interaction events in grassland and forested environments, respectively. As seen above, previous empirical and semi-empirical studies have taken a keen interest in exploring turbulent fluxes and terms of TKE budget equation pre-, during, and post-FFP for insights into the dominant physical processes characterizing fire-induced turbulence. However, the interpretation of coherent structures inferred from the turbulent fluxes and the TKE budget terms is sensitive to the averaging scheme employed to decompose turbulent signals into the mean and turbulent fluctuations from the mean. Additionally, the effect of the fire on the mean kinetic energy budget terms remains to be explored using the existing data. Furthermore, several differences in the fire-turbulence behavior between surface fires in forested and grassland environments remain to be articulated. For instance, we expect the nature of the turbulent transport term to be different, considering that it assumes different levels of importance within the canopy and in the open atmospheric boundary layer in no-fire conditions . Heilman et al. also reflected on the need to understand differences in the behavior of turbulent fluxes for backing and heading fires across all the measurement heights in the canopy. Evidently, there is a need to unify and synthesize observations from burn experiments that have already been conducted via an inter-comparison to develop an over-arching description of the process involved. The analyses presented by Clements et al. and Heilman et al. for four experimental management scale burns conducted in Texas in 2006 , in the New Jersey Pinelands National Reserve in 2011 and 2012, and in the New Jersey Pinelands National Reserve in 2019 are synthesized and re-investigated in this study. Here, we aim to fill the gaps in the previous analyses on these data regarding the coherent motions arising out of fire-atmosphere interaction when juxtaposed with the background canopy turbulence or open atmospheric turbulence, depending on the surface-fire environment. High-frequency 10 Hz measurements in the case of the sub-canopy fires and 20 Hz measurements in the case of the grassland fire for velocity and temperature are utilized. We first introduce the data by exploring the slowly-varying parts of the velocity components and temperature for changes occurring in the flow over relatively longer time scales. We then investigate pre-, during, and post- FFP wind-rose statistics for a visual representation of the combined changes induced by the presence of the fire and ambient wind shifts. Finally, we compute the turbulent fluxes and terms of the TKE budget equation; we comment on the effect of the former on the latter and on the physical significance of both. From these quantities, computed at different heights above ground level , we seek insights into the more persistent coherent structures, the key fire-spread mechanisms, and the dominant turbulence generation and TKE redistribution processes, along with the relevant vertical length scales. Another important and relatively unexplored question of interest is the effect of FFP on the terms of the MKE budget equation, which is investigated here for the sub-canopy surface fires. We aim to draw a comparison of insights among all the three scenarios considered here: heading surface fire beneath the canopy, backing surface fire beneath the canopy, and heading surface fire in a grassland.