A CULTURAL METHOD OF REDUCING POCKET GOPHER IMPACT ON ALFALFA YIELD

Low Input Sustainable Agriculture (LISA) strives to minimize input of agrichemicals for farmers while maintaining profits. Alfalfa fits into this scheme in 2 ways. First, the plains pocket gophers (Geomys bursarius) can reduce yield of alfalfa, thus an effective, economical means of control with minimal environmental impact would be desirable. Second, the increased use of alfalfa in rotation with row crops to increase soil nitrogen may increase pocket gopher problems by increasing their habitat. Our objective was to evaluate a cultural method to control pocket gopher damage, namely, by comparing 2 different varieties of alfalfa. One variety is tap-rooted (Wrangler) while the other has a more fibrous-rooted system (Spredor 2). We hypothesized that damage would be less in the fibrous-rooted alfalfa as it is capable of vegetative reproduction and could recolonize areas. We released live-trapped pocket gophers on 4 treatment areas in each alfalfa variety. Pocket gophers were present on plots of each variety from the fall of 1988 through the fall of 1989. Damage caused by pocket gophers was measured by clipping 80 samples/harvest during the 1989 growing season. Yields were 15 to 19% less in treatment areas than in control areas for both varieties. Sampling continued through the 1990 growing season to determine the recovery rate of each variety after gophers had been removed. The tap-rooted variety showed no improvement in 1990 over 1989. On the other hand, the fibrous-rooted alfalfa exhibited a 4% increase in treatment over control areas after gopher removal

A CULTURAL METHOD OF REDUCING POCKET GOPHER IMPACT ON ALFALFA YIELD

Low Input Sustainable Agriculture (LISA) strives to minimize input of agrichemicals for farmers while maintaining profits. Alfalfa fits into this scheme in 2 ways. First, the plains pocket gophers (Geomys bursarius) can reduce yield of alfalfa, thus an effective, economical means of control with minimal environmental impact would be desirable. Second, the increased use of alfalfa in rotation with row crops to increase soil nitrogen may increase pocket gopher problems by increasing their habitat. Our objective was to evaluate a cultural method to control pocket gopher damage, namely, by comparing 2 different varieties of alfalfa. One variety is tap-rooted (Wrangler) while the other has a more fibrous-rooted system (Spredor 2). We hypothesized that damage would be less in the fibrous-rooted alfalfa as it is capable of vegetative reproduction and could recolonize areas. We released live-trapped pocket gophers on 4 treatment areas in each alfalfa variety. Pocket gophers were present on plots of each variety from the fall of 1988 through the fall of 1989. Damage caused by pocket gophers was measured by clipping 80 samples/harvest during the 1989 growing season. Yields were 15 to 19% less in treatment areas than in control areas for both varieties. Sampling continued through the 1990 growing season to determine the recovery rate of each variety after gophers had been removed. The tap-rooted variety showed no improvement in 1990 over 1989. On the other hand, the fibrous-rooted alfalfa exhibited a 4% increase in treatment over control areas after gopher removal

The Wilson-Polchinski exact renormalization group equation

The critical exponent $\eta$ is not well accounted for in the Polchinski exact formulation of the renormalization group (RG). With a particular emphasis laid on the introduction of the critical exponent $\eta$, I re-establish (after Golner, hep-th/9801124) the explicit relation between the early Wilson exact RG equation, constructed with the incomplete integration as cutoff procedure, and the formulation with an arbitrary cutoff function proposed later on by Polchinski. I (re)-do the analysis of the Wilson-Polchinski equation expanded up to the next to leading order of the derivative expansion. I finally specify a criterion for choosing the ``best'' value of $\eta$ to this order. This paper will help in using more systematically the exact RG equation in various studies.Comment: Some minor changes, a reference added, typos correcte

Self-avoiding walks and connective constants in small-world networks

Long-distance characteristics of small-world networks have been studied by means of self-avoiding walks (SAW's). We consider networks generated by rewiring links in one- and two-dimensional regular lattices. The number of SAW's $u_n$ was obtained from numerical simulations as a function of the number of steps $n$ on the considered networks. The so-called connective constant, $\mu = \lim_{n \to \infty} u_n/u_{n-1}$, which characterizes the long-distance behavior of the walks, increases continuously with disorder strength (or rewiring probability, $p$). For small $p$, one has a linear relation $\mu = \mu_0 + a p$, $\mu_0$ and $a$ being constants dependent on the underlying lattice. Close to $p = 1$ one finds the behavior expected for random graphs. An analytical approach is given to account for the results derived from numerical simulations. Both methods yield results agreeing with each other for small $p$, and differ for $p$ close to 1, because of the different connectivity distributions resulting in both cases.Comment: 7 pages, 5 figure

Critical exponents and equation of state of the three-dimensional Heisenberg universality class

We improve the theoretical estimates of the critical exponents for the three-dimensional Heisenberg universality class. We find gamma=1.3960(9), nu=0.7112(5), eta=0.0375(5), alpha=-0.1336(15), beta=0.3689(3), and delta=4.783(3). We consider an improved lattice phi^4 Hamiltonian with suppressed leading scaling corrections. Our results are obtained by combining Monte Carlo simulations based on finite-size scaling methods and high-temperature expansions. The critical exponents are computed from high-temperature expansions specialized to the phi^4 improved model. By the same technique we determine the coefficients of the small-magnetization expansion of the equation of state. This expansion is extended analytically by means of approximate parametric representations, obtaining the equation of state in the whole critical region. We also determine a number of universal amplitude ratios.Comment: 40 pages, final version. In publication in Phys. Rev.

Crossover phenomena in spin models with medium-range interactions and self-avoiding walks with medium-range jumps

We study crossover phenomena in a model of self-avoiding walks with medium-range jumps, that corresponds to the limit $N\to 0$ of an $N$-vector spin system with medium-range interactions. In particular, we consider the critical crossover limit that interpolates between the Gaussian and the Wilson-Fisher fixed point. The corresponding crossover functions are computed using field-theoretical methods and an appropriate mean-field expansion. The critical crossover limit is accurately studied by numerical Monte Carlo simulations, which are much more efficient for walk models than for spin systems. Monte Carlo data are compared with the field-theoretical predictions concerning the critical crossover functions, finding a good agreement. We also verify the predictions for the scaling behavior of the leading nonuniversal corrections. We determine phenomenological parametrizations that are exact in the critical crossover limit, have the correct scaling behavior for the leading correction, and describe the nonuniversal lscrossover behavior of our data for any finite range.Comment: 43 pages, revte

Critical behavior of the three-dimensional XY universality class

We improve the theoretical estimates of the critical exponents for the three-dimensional XY universality class. We find alpha=-0.0146(8), gamma=1.3177(5), nu=0.67155(27), eta=0.0380(4), beta=0.3485(2), and delta=4.780(2). We observe a discrepancy with the most recent experimental estimate of alpha; this discrepancy calls for further theoretical and experimental investigations. Our results are obtained by combining Monte Carlo simulations based on finite-size scaling methods, and high-temperature expansions. Two improved models (with suppressed leading scaling corrections) are selected by Monte Carlo computation. The critical exponents are computed from high-temperature expansions specialized to these improved models. By the same technique we determine the coefficients of the small-magnetization expansion of the equation of state. This expansion is extended analytically by means of approximate parametric representations, obtaining the equation of state in the whole critical region. We also determine the specific-heat amplitude ratio.Comment: 61 pages, 3 figures, RevTe

Estimate of the Critical Exponents from the Field-Theoretical Renormalization Group: Mathematical Sense of the "Standard Values"

New estimates of the critical exponents have been obtained from the field-theoretical renormalization group using a new method for summing divergent series. The results almost coincide with the central values obtained by Le Guillou and Zinn-Justin (the so-called "standard values"), but have lower uncertainty. It has been shown that usual field-theoretical estimates implicitly imply the smoothness of the coefficient functions. The last assumption is open for discussion in view of the existence of the oscillating contribution to the coefficient functions. The appropriate interpretation of the last contribution is necessary both for the estimation of the systematic errors in the "standard values" and for a further increase in accuracy.Comment: PDF, 12 page