Plant succession on gopher mounds in Western Cascade meadows: consequences for species diversity and heterogeneity

Pocket gophers have the potential to alter the dynamics of grasslands by creating mounds that bury existing vegetation and locally reset succession. Gopher mounds may provide safe sites for less competitive species, potentially increasing both species diversity and vegetation heterogeneity (spatial variation in species composition). We compared species composition, diversity and heterogeneity among gopher mounds of different ages in three montane meadows in the Cascade Range of Oregon. Cover of graminoids and forbs increased with mound age, as did species richness. Contrary to many studies, we found no evidence that mounds provided safe sites for early successional species, despite their abundance in the soil seed bank, or that diversity peaked on intermediate-aged mounds. However, cover of forbs relative to that of graminoids was greater on mounds than in the adjacent meadow. Variation in species composition was also greater within and among mounds than in adjacent patches of undisturbed vegetation, suggesting that these small-scale disturbances increase heterogeneity within meadows

Computations in Large N Matrix Mechanics

The algebraic formulation of Large N matrix mechanics recently developed by Halpern and Schwartz leads to a practical method of numerical computation for both action and Hamiltonian problems. The new technique posits a boundary condition on the planar connected parts X_w, namely that they should decrease rapidly with increasing order. This leads to algebraic/variational schemes of computation which show remarkably rapid convergence in numerical tests on some many- matrix models. The method allows the calculation of all moments of the ground state, in a sequence of approximations, and excited states can be determined as well. There are two unexpected findings: a large d expansion and a new selection rule for certain types of interaction.Comment: 27 page

Combining local- and large-scale models to predict the distributions of invasive plant species

Habitat-distribution models are increasingly used to predict the potential distributions of invasive species and to inform monitoring. However, these models assume that species are in equilibrium with the environment, which is clearly not true for most invasive species. Although this assumption is frequently acknowledged, solutions have not been adequately addressed. There are several potential methods for improving habitat-distribution models. Models that require only presence data may be more effective for invasive species, but this assumption has rarely been tested. In addition, combining modeling types to form ‘ensemble’ models may improve the accuracy of predictions. However, even with these improvements, models developed for recently invaded areas are greatly influenced by the current distributions of species and thus reflect near- rather than long-term potential for invasion. Larger scale models from species’ native and invaded ranges may better reflect long-term invasion potential, but they lack finer scale resolution. We compared logistic regression (which uses presence/absence data) and two presence-only methods for modeling the potential distributions of three invasive plant species on the Olympic Peninsula in Washington State, USA. We then combined the three methods to create ensemble models. We also developed climate-envelope models for the same species based on larger scale distributions and combined models from multiple scales to create an index of near- and long-term invasion risk to inform monitoring in Olympic National Park (ONP). Neither presence-only nor ensemble models were more accurate than logistic regression for any of the species. Larger scale models predicted much greater areas at risk of invasion. Our index of near- and long-term invasion risk indicates that \u3c4% of ONP is at high near-term risk of invasion while 67-99% of the Park is at moderate or high long-term risk of invasion. We demonstrate how modeling results can be used to guide the design of monitoring protocols and monitoring results can in turn be used to refine models. We propose that by using models from multiple scales to predict invasion risk and by explicitly linking model development to monitoring, it may be possible to overcome some of the limitations of habitat-distribution models

Level and pattern of overstory retention shape the abundance and long-term dynamics of natural and created snags

Standing dead trees, or snags, serve myriad functions in natural forests, but are often scarce in forests managed for timber production. Variable retention (VR), the retention of live and dead trees through harvest, has been adopted globally as a less intensive form of regeneration harvest. In this study, we explore how two key elements of VR systems — level (amount) and spatial pattern of live-tree retention — affect the carryover and post-harvest dynamics of natural and artificially created snags. We present nearly two decades of data from the DEMO Study, a regional-scale experiment in VR harvests of Douglas-fir-dominated forests in the Pacific Northwest. Snag losses to harvest were greater at 15 than at 40% retention (67 vs. 47% declines in density) and greater in dispersed than in aggregated treatments (64 vs. 50% declines). Densities of hard and tall (≥5 m) snags were particularly sensitive to low-level dispersed retention, declining by 76 and 81%, respectively. Despite these losses, post-harvest densities correlated with pre-harvest densities for most snag size and decay classes. In contrast to initial harvest effects, snag densities changed minimally over the post-harvest period (years 1 to 18 or 19), with low rates of recruitment offsetting low rates of loss. Post-harvest survival of snags was greater at 15 than at 40% retention (79 vs. 69%), as were rates of decay (68 vs. 52% of hard snags transitioned to soft). However, pattern had no effect on either process. Snag recruitment did not vary with retention level or pattern at the scale of the 13-ha harvest unit, but was several-fold greater in the 1-ha aggregates (14.3–27.8 snags ha -1) than in the corresponding dispersed treatments (4.2–5.3 snags ha -1). Snag size (diameter) distributions showed greater change in dispersed than in aggregated treatments, reflecting greater loss of smaller snags and recruitment biased toward larger snags. Created snags showed uniformly high survival (97%), irrespective of treatment, but rates of decay were greater at lower retention. If a goal of VR is to sustain snag abundance and diversity through harvest, emphasis should be placed on minimizing initial losses, either by reducing the intensity of felling in areas of dispersed retention or locating forest aggregates in areas of greater initial snag density, diversity, or incipient decay

Genetic Interactions in Zebrafish Midline Development

AbstractMutational analyses have shown that the genesno tail(ntl, Brachyuryhomolog),floating head(flh,aNothomeobox gene), andcyclops(cyc) play direct and essential roles in the development of midline structures in the zebrafish. In bothntlandflhmutants a notochord does not develop, and incycmutants the floor plate is nearly entirely missing. We made double mutants to learn how these genes might interact. Midline development is disrupted to a greater extent incyc;flhdouble mutants than in eithercycorflhsingle mutants; their effects appear additive. Both the notochord and floor plate are completely lacking, and other phenotypic disturbances suggest that midline signaling functions are severely reduced. On the other hand, trunk midline defects inflh;ntldouble mutants are not additive, but are most often similar to those inntlsingle mutants. This finding reveals that loss ofntlfunction can suppress phenotypic defects due to mutation atflh,and we interpret it to mean that the wild-type allele ofntl(ntl+) functions upstream toflhin a regulatory hierarchy. Loss of function ofntlalso strongly suppresses the floor plate deficiency incycmutants, for we found trunk floor plate to be present incyc;ntldouble mutants. From these findings we propose thatntl+plays an early role in cell fate choice at the dorsal midline, mediated by the Ntl protein acting to antagonize floor plate development as well as to promote notochord development

Influence of forest structure and experimental green-tree retention on northern flying squirrel (Glaucomys sabrinus) abundance

In many regions of the world, forest management has shifted from practices emphasizing timber production to more sustainable harvesting that integrates ecological values, including maintenance of biodiversity, wildlife habitat, and ecological goods and services. To this end, management strategies emphasize retention of stand structures that meet the needs of forest-obligate wildlife species and enhance connectivity across landscapes. However, little is known about the effects on arboreal rodents of varying the amount or spatial distribution of retained structures. We quantified the responses of northern flying squirrels (Glaucomys sabrinus) to retention harvests of varying levels (15%, 40%, 75%, and 100% of original basal area) and spatial patterns (trees uniformly dispersed vs. aggregated in 1-ha patches), using six experimental treatments replicated at three locations in southwestern Oregon and Washington. Relative abundance of northern flying squirrels decreased following harvest; minimum number of squirrels known alive (MNKA) in the control (100%) and 75% retention treatment was significantly higher than in the 15% or 40% treatments. In mixed-effects regression models, MNKA increased with treatment-unit basal area and amount of surrounding mature (>80-year-old) forest, suggesting that squirrel abundance was influenced by local structure and landscape-scale variables. However, only basal area contributed to best-fit models of reproductive female abundance. Our results suggest a threshold response of northern flying squirrels to green-tree retention somewhere between 40% and 75% that is likely to be influenced by the spatial pattern of retention and landscape context. This study underscores previous conclusions that northern flying squirrels are sensitive to logging at both local and larger landscape scales, and demonstrates the current minimum retention standard of 15% will not provide suitable habitat for this species.Keywords: Douglas-fir (Pseudotsuga menziesii) forests, Glaucomys sabrinus, Landscape context, Biodiversity, Harvest intensity, Green-tree retentio

Fermi LAT observations of the Geminga pulsar

We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the second brightest non-variable GeV source in the $\gamma$-ray sky and the first example of a radio-quiet $\gamma$-ray pulsar. The observations cover one year, from the launch of the $Fermi$ satellite through 2009 June 15. A data sample of over 60,000 photons enabled us to build a timing solution based solely on $\gamma$ rays. Timing analysis shows two prominent peaks, separated by $\Delta \phi$ = 0.497 $\pm$ 0.004 in phase, which narrow with increasing energy. Pulsed $\gamma$ rays are observed beyond 18 GeV, precluding emission below 2.7 stellar radii because of magnetic absorption. The phase-averaged spectrum was fitted with a power law with exponential cut-off of spectral index $\Gamma$ = (1.30 $\pm$ 0.01 $\pm$ 0.04), cut-off energy $E_{0}$ = (2.46 $\pm$ 0.04 $\pm$ 0.17) GeV and an integral photon flux above 0.1 GeV of (4.14 $\pm$ 0.02 $\pm$ 0.32) $\times$ 10$^{-6}$ cm$^{-2}$ s$^{-1}$. The first uncertainties are statistical and the second are systematic. The phase-resolved spectroscopy shows a clear evolution of the spectral parameters, with the spectral index reaching a minimum value just before the leading peak and the cut-off energy having maxima around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is present at all rotational phases. The spectral shape, broad pulse profile, and maximum photon energy favor the outer magnetospheric emission scenarios.Comment: 32 pages, 12 figures, 3 tables. Accepted for publication in The Astrophysical Journal. Corresponding authors: Denis Dumora ([email protected]), Fabio Gargano ([email protected]), Massimiliano Razzano ([email protected]

Fermi-LAT Search for Pulsar Wind Nebulae around gamma-ray Pulsars

The high sensitivity of the Fermi-LAT (Large Area Telescope) offers the first opportunity to study faint and extended GeV sources such as pulsar wind nebulae (PWNe). After one year of observation the LAT detected and identified three pulsar wind nebulae: the Crab Nebula, Vela-X and the PWN inside MSH 15-52. In the meantime, the list of LAT detected pulsars increased steadily. These pulsars are characterized by high energy loss rates from ~3 \times 10^{33} erg s$^{-1}$ to 5 \times 10$^{38}$ erg s$^{-1}$ and are therefore likely to power a PWN. This paper summarizes the search for PWNe in the off-pulse windows of 54 LAT-detected pulsars using 16 months of survey observations. Ten sources show significant emission, seven of these likely being of magnetospheric origin. The detection of significant emission in the off-pulse interval offers new constraints on the gamma-ray emitting regions in pulsar magnetospheres. The three other sources with significant emission are the Crab Nebula, Vela-X and a new pulsar wind nebula candidate associated with the LAT pulsar PSR J1023-5746, coincident with the TeV source HESS J1023-575. We further explore the association between the H.E.S.S. and the Fermi source by modeling its spectral energy distribution. Flux upper limits derived for the 44 remaining sources are used to provide new constraints on famous PWNe that have been detected at keV and/or TeV energies.Comment: Accepted for publication in Astrophysical Journal, 42 pages, 17 figure