Comparative development of a thinned and a natural Douglas-fir stand from 45 to 60 years of age

An understanding of the dynamics of individual tree and stand growth, development, and structural patterns during the immature formative years of a commercial species is essential to determine correct thinning practices necessary to attain desired objectives. This study compared a natural, medium-site Douglas-fir stand and an adjacent similar stand released by a severe crown thinning in 1955 at 45 years of age. Individual tree measurement and classification data collected in 1955 and in 1970 from all stems 1.5 inches and larger on permanent one-acre research plots within each stand were used to compare growth and developmental changes occurring during the interval. In addition, the Douglas-fir on both plots were measured in 1970 with a Barr and Stroud optical dendrometer to determine volumetric and morphological differences. The interplot and intraplot characteristics manifested during and at the end of the 15-year period on each study area were examined. Structural changes of crown classes and components were delineated and investigated. Regression analyses were performed to compare basal area growth rates and volume similarities or differences by crown classes within and between the plots. After 15 years, the thinned stand was superior in site index, stand restructuring for dominance, basal area growth, and d.b.h.- volume relationships based on total cubic foot volume inside bark. Neither the ratio of live crown length to total height nor defect due to crown breakage and resulting Fomes cajanderi had an influence on subsequent crown class movement or basal area growth rate. Mortality was caused by suppression, and occurred in the intermediate and overtopped crown classes. The dynamics and magnitude of crown class movement, especially stability in the dominant class and the proportion of upward movement by the intermediate and codominant classes were greater than previously observed in other studies. This phenomenon has a major impact on silvicultural decisions, and warrants further intensive investigation. Intermediate trees showed the greatest vertical crown class movement; overtopped trees were candidates for mortality in the near future. The greatest actual basal area growth on an individual tree basis occurred in the dominant class on the thinned plot, but the larger codominants and the smaller dominants from the same plot exhibited superior potential for response to release with proportionally larger basal area increases. All crown classes except overtopped on the thinned study area were capable o relatively greater basal area growth than their unthinned counterparts. Fifteen years after release, the volumes of the dominant and codominant trees on the thinned area were significantly greater than the dominants and codominants of corresponding d.b.h.s on the unthinned plot. The d.b.h., height, crown length, and crown surface area were also substantially greater in these crown classes on the thinned plot. At 60 years of age, both plots were forming into three crown classes--a dominant, a codominant, and an intermediate-overtopped group based on crown class structure, basal area growth, and volume characteristics. The thinned stand, however, had relatively superior characteristics in these categories. The need for intensive periodic remeasurement of single-tree plots to determine the effects of genetics, tree physiology, microsite, and the spatial-distribution relationship on growth and development is discussed

Photonic chip-based low noise microwave oscillator

Numerous modern technologies are reliant on the low-phase noise and exquisite timing stability of microwave signals. Substantial progress has been made in the field of microwave photonics, whereby low noise microwave signals are generated by the down-conversion of ultra-stable optical references using a frequency comb. Such systems, however, are constructed with bulk or fiber optics and are difficult to further reduce in size and power consumption. Our work addresses this challenge by leveraging advances in integrated photonics to demonstrate low-noise microwave generation via two-point optical frequency division. Narrow linewidth self-injection locked integrated lasers are stabilized to a miniature Fabry-P\'{e}rot cavity, and the frequency gap between the lasers is divided with an efficient dark-soliton frequency comb. The stabilized output of the microcomb is photodetected to produce a microwave signal at 20 GHz with phase noise of -96 dBc/Hz at 100 Hz offset frequency that decreases to -135 dBc/Hz at 10 kHz offset--values which are unprecedented for an integrated photonic system. All photonic components can be heterogeneously integrated on a single chip, providing a significant advance for the application of photonics to high-precision navigation, communication and timing systems

Forest fertilization : (a state-of-the-art review and description of environmental effects) /

"Program Element 1B2037.""August 1972."Includes bibliographical references (pages 51-57).Mode of access: Internet

Low-noise microwave generation with an air-gap optical reference cavity

We demonstrate a high finesse, microfabricated mirror-based, air-gap cavity with volume less than 1 ml, constructed in an array, that can support low-noise microwave generation through optical frequency division. We use the air-gap cavity in conjunction with a 10 nm bandwidth mode-locked laser to generate low phase noise 10 GHz microwaves, exhibiting a phase noise of −95 and −142 dBc/Hz at 100 Hz and 10 kHz offset frequencies, respectively. This is accomplished using the 2-point lock optical frequency division method, where we exploit 40 dB common-mode rejection of two lasers separated by 1.29 THz and locked to the same air-gap cavity. If used with an octave spanning comb, the air-gap cavity is capable of supporting 10 GHz phase noise below −160 dBc/Hz at 10 kHz offset, a level significantly lower than electronic synthesizers. These results show how extremely small optical reference cavities, operated without the benefit of vacuum enclosures or thermal insulation, can, nonetheless, support state-of-the-art microwave phase noise in compact and portable systems

Photonic chip-based low-noise microwave oscillator.

Numerous modern technologies are reliant on the low-phase noise and exquisite timing stability of microwave signals. Substantial progress has been made in the field of microwave photonics, whereby low-noise microwave signals are generated by the down-conversion of ultrastable optical references using a frequency comb1-3. Such systems, however, are constructed with bulk or fibre optics and are difficult to further reduce in size and power consumption. In this work we address this challenge by leveraging advances in integrated photonics to demonstrate low-noise microwave generation via two-point optical frequency division4,5. Narrow-linewidth self-injection-locked integrated lasers6,7 are stabilized to a miniature Fabry-Pérot cavity8, and the frequency gap between the lasers is divided with an efficient dark soliton frequency comb9. The stabilized output of the microcomb is photodetected to produce a microwave signal at 20 GHz with phase noise of -96 dBc Hz-1 at 100 Hz offset frequency that decreases to -135 dBc Hz-1 at 10 kHz offset-values that are unprecedented for an integrated photonic system. All photonic components can be heterogeneously integrated on a single chip, providing a significant advance for the application of photonics to high-precision navigation, communication and timing systems

Photonic chip-based low noise microwave oscillator

Numerous modern technologies are reliant on the low-phase noise and exquisite timing stability of microwave signals. Substantial progress has been made in the field of microwave photonics, whereby low noise microwave signals are generated by the down-conversion of ultra-stable optical references using a frequency comb. Such systems, however, are constructed with bulk or fiber optics and are difficult to further reduce in size and power consumption. Our work addresses this challenge by leveraging advances in integrated photonics to demonstrate low-noise microwave generation via two-point optical frequency division. Narrow linewidth self-injection locked integrated lasers are stabilized to a miniature Fabry-P\'{e}rot cavity, and the frequency gap between the lasers is divided with an efficient dark-soliton frequency comb. The stabilized output of the microcomb is photodetected to produce a microwave signal at 20 GHz with phase noise of -96 dBc/Hz at 100 Hz offset frequency that decreases to -135 dBc/Hz at 10 kHz offset--values which are unprecedented for an integrated photonic system. All photonic components can be heterogeneously integrated on a single chip, providing a significant advance for the application of photonics to high-precision navigation, communication and timing systems