Power-law behavior reveals phase transitions in landscape controls of fire regimes

In low-severity fire regimes of the American West and elsewhere, landscape memory of fire events is registered in fire-scarred trees, with temporal record lengths often exceeding 200 years^1-5^. Understanding the environmental controls on historical wildfires, and how they changed across spatial scales, is difficult because there are no surviving explicit records of either weather or vegetation (fuels). We show how power laws associated with fire-event time series arise in limited domains of parameters that represent critical transitions in the controls on landscape fire. We used stochastic simulations iteratively with Monte Carlo inference to replicate the spatio-temporal structure of historical fire-scar records in forested watersheds of varying topographic complexity. We find that the balance between endogenous and exogenous controls on fire spread shifts with topographic complexity, where in the most complex landscapes the endogenous controls dominate and the pattern exhibits criticality. Comparison to an self-organized criticality (SOC) model^6,7^ shows that the latter mimics historical fire only in a limited domain of criticality, and is not an adequate mechanism to explain landscape fire dynamics, which are shaped by both endogenous and exogenous controls. Our results identify a continuous phase transition in landscape controls, marked by power laws, and provide an ecological analogue to critical behavior in physical and chemical systems^8-11^. This explicitly cross-scale analysis provides a paradigm for identifying critical thresholds in landscape dynamics that may be crossed in a rapidly changing climate

IDENTIFICATION OF MARKET POTENTIAL FOR PASTURE-RAISED PORK IN THE MISSISSIPPI DELTA OF ARKANSAS-1998

Agribusiness,

DOMESTIC SUPPORT: ISSUES AND OPTIONS IN THE AGRICULTURAL NEGOTIATIONS

International Relations/Trade,

QUANTITATIVE ASPECTS OF TRANSMITTER RELEASE

The opener-stretcher motor neuron in crayfish makes 50 endings upon each of 1200 muscle fibers. We have calculated the quantal content of junctional potentials produced by individual terminals and by the whole cell at various physiological frequencies. The results show that when the motor neuron is active at 20 impulses/second, it releases 50 quanta/impulse per muscle fiber, or a total of 4.5 x 109 quanta/hr. These figures are similar to those for vertebrate muscles per fiber, but larger for the entire neuron because the opener motor unit is so large. On the basis that the quanta correspond to synaptic vesicles each containing 103–104 molecules of transmitter, the release rate must be around 10-11 mole/hr. This value is within an order of magnitude of the release figures obtained for mammalian neurons by collecting transmitter in perfusates, but it is far lower than the value reported for a crustacean inhibitory neuron. If the membrane materials surrounding each vesicle were lost in the release process, the replacement synthesis would involve 24 mm2 of membrane/hr. We conclude that the metabolic load in terms of transmitter synthesis is probably sustainable, but that the release mechanism must operate in such a way that vesicle membrane materials are neither lost nor incorporated into the terminal membrane