Determinants of flammability in savanna grass species

1. Tropical grasses fuel the majority of fires on Earth. In fire-prone landscapes, enhanced flammabil-ity may be adaptive for grasses via the maintenance of an open canopy and an increase in spa-tiotemporal opportunities for recruitment and regeneration. In addit ion, by burning intensely butbriefly, high flammability may protect resprouting buds from lethal temperatures. Despite thesepotential benefits of high flammability to fire-prone grasses, variation in flammability among grassspecies, and how trait differences underpin this variation, remains unknown.2. By burning leaves and plant parts, we experimentally determined how five plant traits (biomassquantity, biomass density, biomass moisture content, leaf surface-area-to-volume ratio and leaf effec-tive heat of combustion) combined to determine the three components of flammability (ignitability,sustainability and combustibility) at the leaf and plant scales in 25 grass species of fire-pr one SouthAfrican grasslands at a time of peak fire occurrence. The influence of evolutionary history onflammability was assessed based on a phylogeny built here for the study species.3. Grass speci es differed significantly in all components of flammability. Accounting for evolution-ary history helped to explain patterns in leaf-scale combustibility and sustainability. The five mea-sured plant traits predicted components of flammability, particularly leaf ignitability and plantcombustibility in which 70% and 58% of variation, respectively, could be explained by a combina-tion of the traits. Total above-ground biomass was a key drive r o f combustibility and sustainabi litywith high biomass species burning more intensely and for longer, and producing the highest pre-dicted fire spread rates. Moisture content was the main influence on ignitability, where speci es withhigher moisture conten ts took longer to ignite and once alight burnt at a slower rate. Bioma ss den-sity, leaf surface-area-to-volume ratio and leaf effective heat of combustion were weaker predictorsof flammability components.4. Synthesis. We demonstrate that grass flammability is predicted from easily measurable plant func-tional traits and is influenced by evolutionary history with some components showing phylogeneticsignal. Grasses are not homogenous fuels to fire. Rather, species differ in functional traits that inturn demonstrably influence flammability. This diver sity is consistent with the idea that flammabilitymay be an adaptive trait for grasses of fire-prone ecosystems

Fermi's golden rule and exponential decay as a RG fixed point

We discuss the decay of unstable states into a quasicontinuum using models of the effective Hamiltonian type. The goal is to show that exponential decay and the golden rule are exact in a suitable scaling limit, and that there is an associated renormalization group (RG) with these properties as a fixed point. The method is inspired by a limit theorem for infinitely divisible distributions in probability theory, where there is a RG with a Cauchy distribution, i.e. a Lorentz line shape, as a fixed point. Our method of solving for the spectrum is well known; it does not involve a perturbation expansion in the interaction, and needs no assumption of a weak interaction. We use random matrices for the interaction, and show that the ensemble fluctuations vanish in the scaling limit. Thus the limit is the same for every model in the ensemble with probability one.Comment: 20 pages, 1 figur

Evolution of microstructure and crystallographic texture during dissimilar friction stir welding of duplex stainless steel to low carbon-manganese structural steel

Electron backscattered diffraction (EBSD) was used to analyze the evolution of microstructure and crystallographic texture during friction stir welding of dissimilar type 2205 duplex stainless steel (DSS) to type S275 low carbon-manganese structural steel. The results of microstructural analyses show that the temperature in the center of stirred zone reached temperatures between Ac 1 and Ac 3 during welding, resulting in a minor ferrite-to-austenite phase transformation in the S275 steel, and no changes in the fractions of ferrite and austenite in the DSS. Temperatures in the thermomechanically affected and shoulder-affected zones of both materials, in particular toward the root of the weld, did not exceed the Ac 1 of S275 steel. The shear generated by the friction between the material and the rotating probe occurred in austenitic/ferritic phase field of the S275 and DSS. In the former, the transformed austenite regions of the microstructure were transformed to acicular ferrite, on cooling, while the dual-phase austenitic/ferritic structure of the latter was retained. Studying the development of crystallographic textures with regard to shear flow lines generated by the probe tool showed the dominance of simple shear components across the whole weld in both materials. The ferrite texture in S275 steel was dominated by D 1, D 2, E, E¯ , and F, where the fraction of acicular ferrite formed on cooling showed a negligible deviation from the texture for the ideal shear texture components of bcc metals. The ferrite texture in DSS was dominated by D 1, D 2, I, I¯ , and F, and that of austenite was dominated by the A, A¯ , B, and B¯ of the ideal shear texture components for bcc and fcc metals, respectively. While D 1, D 2, and F components of the ideal shear texture are common between the ferrite in S275 steel and that of dual-phase DSS, the preferential partitioning of strain into the ferrite phase of DSS led to the development of I and I¯ components in DSS, as opposed to E and E¯ in the S275 steel. The formations of fine and ultrafine equiaxed grains were observed in different regions of both materials that are believed to be due to strain-induced continuous dynamic recrystallization (CDRX) in ferrite of both DSS and S275 steel, and discontinuous dynamic recrystallization (DDRX) in austenite phase of DSS

Postfire response of red alder, black cottonwood, and bigleaf maple to the Whatcom Creek fire, Bellingham, Washington

The June 1999 pipeline leak and subsequent explosion in Whatcom Creek resulted in a fire that scorched ~10 ha of coniferous and riparian forest in Bellingham, Washington, USA. This fire provided an opportunity to study the postfire responses of red alder (Alnus rubra), black cottonwood (Populus trichocarpa) [P. balsamifera subsp. trichocarpa], and bigleaf maple (Acer macrophyllum) in the 3.2-ha scorched riparian zone. Bigleaf maple endures all but the most intense fires, resprouting vigorously after fire. Red alder and black cottonwood are known resprouters that may survive low intensity fires, but data about their responses to fire are sparse. The postfire status of every tree in the burn zone was evaluated immediately after the fire in June 1999, and during the first postfire growing season in May 2000. Tree response, in relation to trunk diameter, percent crown scorch, and percent bark scorch, was categorized as (1) alive with a new leafy crown; (2) topkilled and resprouted; or (3) dead. Fire intensity was higher on the left bank (left side of the creek when facing downstream), where trees experienced greater mortality. More than half the 854 trees in the burn zone survived the fire, 57% by forming new leafy crowns and 43% by resprouting. Red alder, black cottonwood and bigleaf maple demonstrated abilities to survive a disturbance to which they are not ordinarily subjected. All three species resisted low intensity fire and reformed leafy crowns on the right bank, and they endured moderate to high intensity fire by resprouting after topkill on the left bank. On the left bank, probabilities of surviving 100% crown scorch were 0.24 for red alder, 0.66 for black cottonwood, and 0.68 for bigleaf maple; probabilities of surviving 100% bark scorch were 0.09 for red alder, 0.66 for black cottonwood, and 0.60 for bigleaf maple

Gradient analysis of vegetation on the north wall of the Columbia River Gorge, Washington

The Columbia River Gorge represents an ecocline of shifting environmental factors, species populations, and vegetation associations. Combinations of wind, temperature, and precipitation create different environmental regimes from the west to the east. The north wall of the Gorge supported four physiognomic associations: western conifer forest, central mixed forest, central broadleaf forest, and eastern steppe. Each association comprised species distributed individually along the ecocline. Douglas-fir (Pseudotsuga menziesii), western redcedar (Thuja plicata), bigleaf maple (Acer macrophyllum), dull Oregongrape (Berberis nervosa), sword fern (Polystichum munitum), bracken (Pteridium aquilinum), vine maple (Acer circinatum), and red bilberry (Vaccinium parviflorum) were centered in the western conifer forest. Garry oak (Quercus garryana), ponderosa pine (Pinus ponderosa), poison oak (Rhus diversiloba), and snowberry (Symphoricarpos albus) were centered in the central mixed forest. Cheatgrass (Bromus tectorum), northern buckwheat (Eriogonum compositum), Wyeth's lupine (Lupinus polyphyllus var. humicola), western serviceberry (Amelanchier alnifolia), mockorange (Philadelphus lewisii), and Sandberg's bluegrass (Poa sandbergii) were centered in the eastern steppe. Although the centre of distribution for each species was in one of the associations, few were confined to a specific association. The Columbia River Gorge is analogous to a mountain turned on its side, with the cool, moist summit to the west and the warm, dry base to the east. Plant species have migrated both eastward and westward along this ecocline during the lifetime of the Gorge, and each association has combinations of species derived from surrounding regions in Washington and Oregon

Community composition and floristic relationships in Montane wetlands in the North Cascades, Washington

Montane wetlands in the North Cascades, Washington, USA were dominated by small red peat moss (Sphagnum capillifolium), white marshmarigold (Caltha biflora), and many spiked cottongrass (Eriophorum polystachion), the species with highest cover and frequency. Overall, 139 species were recorded in the 24 wetlands studied. Of the common species, nine were distributed independently of margin vs. central expanse locations, eight were more prominent in the margin habitat, and six were more prominent in the central expanse. Species diversity was described by species richness, and by Shannon-Wiener and Simpson's indices. These data were interpreted according to three wetland sizes (1.5 ha) and margin vs. central expanse location. Species richness was significantly correlated with wetland size and was significantly greater in the larger wetlands. The Shannon-Wiener index was significantly correlated with wetland size and was significantly greater in wetland margins vs. central expanse. Simpson's index was unrelated to size or location in the wetland. Habitat heterogeneity in the wetlands was increased by buried logs, where conifer saplings were concentrated, even in the central expanse. North Cascades wetlands were distributed along gradients of elevation, annual precipitation, and degree of water movement. Lower elevation wetlands were characterized by higher annual precipitation and still water, whereas higher elevation wetlands were characterized by lower annual precipitation and moving water

Fire history of Douglas-fir forests in the Morse Creek drainage of Olympic National Park, Washington

In the Morse Creek drainage of the northeastern Olympic Mountains in Washington state, USA, montane forests dominated by Douglas fir (Pseudotsuga menziesii) owe their prominence to a complex fire regime that incorporates high severity stand-replacing fires and low/moderate severity ground fires. The fire history of these forests was quantified using dendrochronological methods to determine the role played by wildfire to favour dominance of Douglas fir rather than late-successional western hemlock (Tsuga heterophylla). Three matrix forest types reflect the influence of past wildfires. The youngest matrix type was 300 yr old. Germination dates and fire release markers were identified on increment cores from 318 Douglas firs, and used to date past fire events. A 600-yr fire history was developed for this 2500-ha area. Periods characterized by many small-scale, low and moderate severity fires were interrupted by 2 high severity, stand-replacing burning periods in 1687-1720 and 1897-1904. Mean fire return intervals (FRI) were calculated for various land units. The most informative size was 200 ha, the approximate mean size of lateral tributaries to Morse Creek. FRI was 21 yr at this spatial scale. For the entire 2500 ha drainage, mean FRI was estimated at 3 yr. Similar to Douglas-fir forests in central Oregon and northern California, small patchy fires were much more common in the eastern Olympics than previously thought. Instead of fire exclusion, a policy that uses management fires to burn many small patches of forest each year would approach the kind of fire regime typical of these forests

Fire history of Douglas-fir forests in the Morse Creek drainage of Olympic National Park, Washington

In the Morse Creek drainage of the northeastern Olympic Mountains in Washington state, USA, montane forests dominated by Douglas fir (Pseudotsuga menziesii) owe their prominence to a complex fire regime that incorporates high severity stand-replacing fires and low/moderate severity ground fires. The fire history of these forests was quantified using dendrochronological methods to determine the role played by wildfire to favour dominance of Douglas fir rather than late-successional western hemlock (Tsuga heterophylla). Three matrix forest types reflect the influence of past wildfires. The youngest matrix type was 300 yr old. Germination dates and fire release markers were identified on increment cores from 318 Douglas firs, and used to date past fire events. A 600-yr fire history was developed for this 2500-ha area. Periods characterized by many small-scale, low and moderate severity fires were interrupted by 2 high severity, stand-replacing burning periods in 1687-1720 and 1897-1904. Mean fire return intervals (FRI) were calculated for various land units. The most informative size was 200 ha, the approximate mean size of lateral tributaries to Morse Creek. FRI was 21 yr at this spatial scale. For the entire 2500 ha drainage, mean FRI was estimated at 3 yr. Similar to Douglas-fir forests in central Oregon and northern California, small patchy fires were much more common in the eastern Olympics than previously thought. Instead of fire exclusion, a policy that uses management fires to burn many small patches of forest each year would approach the kind of fire regime typical of these forests.Wetzel and Fonda "Fire history of Douglas-fir forests in the Morse Creek drainage of Olympic National Park, Washington." Northwest Science. 2000; 74(4): 263-27