Multidecadal change in Aspen experiencing long-unburned, mixed-severity wildfire, and reburn disturbance regimes

Quaking aspen (Populus tremuloides) is a valued, minor component on western landscapes. It provides a wide range of ecosystem services, and has been in decline for the last century. This decline may be explained partially by the lack of fire on the landscape as aspen benefit from fire that eliminates conifer competition and stimulates reproduction through root suckering. Managers are interested in aspen restoration but have a lack of knowledge about their spatial dynamics in response to fire. Improving our understanding of aspen stand spatial changes over time may be important for maintaining their presence on landscapes where their role is unique from neighboring ecotypes. Our study area in northeastern California on the Lassen, Modoc and Plumas National Forests has experienced recent large mixed-severity wildfires where aspen was present, providing an opportunity to study the re-introduction of fire. We observed two time periods; a 54-year absence of fire from 1941 to 1993 preceding a 24-year period of wildfire activity from 1993 to 2017. We utilized historical (1941, 1993) and current (2017) aerial photos to delineate aspen stand size, location and succession to conifers. We chose aspen stands in areas where wildfires overlapped (twice burned), where only a single wildfire burned, or areas that did not burn within the recent 24-year period. We looked at these same stands within the first period of fire exclusion for comparison (i.e., 1941-1993). In the absence of fire, all aspen stand areas declined and all stands experienced increases in conifer composition. After wildfire, stands that burned experienced a release from conifer competition and increased in stand area. Stands that burned twice or at high severity experienced a larger removal of conifer competition than stands that burned once at low severity, promoting aspen recovery and expansion. Stands with less edge:area ratio also expanded more with fire present. Across both time periods, stand movement, where aspen stand footprints were mostly in new areas compared to footprints of previous years, was highest in smaller stands. In the fire exclusion period, smaller stands exhibited greater changes in area and location (movement), highlighting their vulnerability to loss in the absence of disturbances that provide adequate growing space for aspen over time

A Nexus of Evil: An Analysis of the Terror-Crime Link at the U.S.-Mexican Border

As the nexus between terrorism and crime continues to expand, the threat of terrorism is now on our doorstep. The collaboration between terrorist groups and cartel operations at the U.S.-Mexican border is stimulated by the mutual benefits gained by both parties, and the instability in Mexico and the simultaneous market for drugs in the United States have created fertile soil for the growth of this nexus. The terrorist group, Hezbollah, has already demonstrated its alliance with Mexican cartels, and more alliances are likely to continue to be revealed. The ramifications of these alliances go beyond money laundering and fundraising, as terrorist groups can use the drug trade to disintegrate American society, gain access to new groups for recruitment and radicalization, and infiltrate the United States to execute terrorist attacks

An experimental investigation of a resonance phenomenon in tubes with turbulent flow

This investigation was aimed at observing the attenuation and dispersion of sinusoidal disturbances in a 5/16 I.D., 54\u27 length of straight copper tubing conducting a mean turbulent flow of air. This theory consisted of three models applying to three different frequency bands

76-Year Decline and Recovery of Aspen Mediated by Contrasting Fire Regimes: Long-Unburned, Infrequent and Frequent Mixed-Severity Wildfire

Quaking aspen (Populus tremuloides) is a valued, minor component on northeastern California landscapes. It provides a wide range of ecosystem services and has been in decline throughout the region for the last century. This decline may be explained partially by the lack of fire on the landscape due to heavier fire suppression, as aspen benefit from fire that eliminates conifer competition and stimulates reproduction through root suckering. However, there is little known about how aspen stand area changes in response to overlapping fire. Our study area in northeastern California on the Lassen, Modoc and Plumas National Forests has experienced recent large mixed-severity wildfires where aspen was present, providing an opportunity to study the re-introduction of fire. We observed two time periods; a 52-year absence of fire from 1941 to 1993 preceding a 24-year period of wildfire activity from 1993 to 2017. We utilized aerial photos and satellite imagery to delineate aspen stands and assess conifer cover percent. We chose aspen stands in areas where wildfires overlapped (twice-burned), where only a single wildfire burned, and areas that did not burn within the recent 24-year period. We observed these same stands within the first period of fire exclusion for comparison (i.e., 1941–1993). In the absence of fire, all aspen stand areas declined and all stands experienced increases in conifer composition. After wildfire, stands that burned experienced a release from conifer competition and increased in stand area. Stands that burned twice or at high severity experienced a larger removal of conifer competition than stands that burned once at low severity, promoting expansion of aspen stand area. Stands with less edge:area ratio also expanded in area more with fire present. Across both time periods, stand movement, where aspen stand footprints were mostly in new areas compared to footprints of previous years, was highest in smaller stands. In the fire exclusion period, smaller stands exhibited greater loss of area and changes in location (movement) than in the return of fire period, highlighting their vulnerability to loss via succession to conifers in the absence of disturbances that provide adequate growing space for aspen over time

A recurrence formula in reciprocal frame structures: the Da Vinci bridge

This article studies a reciprocal system referred to as the “Da Vinci bridge” and first proposed by Leonardo Da Vinci in the Codex Atlanticus written in 1495. The concept of reciprocal systems consists in transferring forces using planar or elongated elements in such a way to form a closed circuit. Reciprocal systems are usually built from the repetition of a building block. In the case of the Da Vinci bridge, the building block is an assembly of four beams. This article uncovers a recurrence formula linking the number of building blocks to the reaction forces on the most loaded beams of the bridge. It is interesting to note that the recurrence formula differs if the system consists in an even or odd number of building blocks. The various hypotheses of the strength of material computations that led to the recurrence formula are validated by comparison with an experimental realization. This studies shows that, when designing reciprocal systems, an optimization has to be carried out in order to chose between using long beams or using a large number of building elements