High Fire Frequency Research Articles

Recuperación post-fuego de la comunidad de líquenes epífitos en la provincia de Córdoba, Argentina

Los líquenes son reconocidos bioindicadores de la calidad del ambiente. Estudios de comunidades de líquenes en relación a los incendios forestales, han demostrado como responden al fuego por encima de las variables microambientales que modelan la comunidad en ausencia de este disturbio. El objetivo de la presente contribución es reconocer el efecto del fuego en la comunidad de líquenes del Bosque Serrano en Córdoba. Se seleccionaron dos zonas, una testigo y otra sometida a fuego en 1996. A través de un muestreo sistemático y estandarizado se relevó la presencia y cobertura de especies de líquenes en árboles y arbustos en las dos áreas estudiadas. Entre el área testigo y el incendiado la diversidad y cobertura de líquenes es similar pero varia su composición. Las especies en la zona incendiadas corresponden a líquenes heliófilos y adaptados a situaciones de estrés. El fuego como disturbio frecuente favorece la presencia de pocas especies con importantes coberturas que resistan las condiciones que el disturbio impone. Los incendios de alta intensidad y frecuencia dejan a la comunidad de líquenes sin posibilidades de desarrollo, disminuyendo la diversidad general y la calidad de los sistemas forestales en los que viven.

Synchronization enhances synaptic efficacy through spike timing-dependent plasticity in the olfactory system

Synaptic modifications are measured in biological experiments with respect to spike timings. Spike timing-dependent plasticity is the latest development in refinements of Hebbian learning. We have applied additive and multiplicative STDP synaptic learning rules to a biologically inspired olfactory network. The olfactory system recognizes odorant patterns by synchronization of mitral cells. Synchronization enhances synaptic connections between mitral cells and cortical cells. Both STDP rules exhibit unimodal weight distributions which is biologically realistic. As a result, cortical cells respond with a wider range of variability and higher firing frequency. This property has potential for the improvement of artificial odor recognition through ongoing selection of mitral cells.

A comparison of the resilience of four habitats to fire, and the implications of changes in community composition for conservation: a case study from the Serra de Monchique, Portugal

Background: The occurrence of fire is natural in many ecosystems but recently the number and frequency of fires has increased. Aim: To provide an assessment of the resilience of four habitats in the Serra de Monchique to fire and to assess the implications of changes in community composition for conservation. Methods: Plots were established in heathland, rhododendron scrub, rocky outcrops and woodland and the vegetation recorded pre-fire and two and four years post-fire. A one-off survey of a range of heathland plots with different fire history was also conducted. Results: The rarer habitats (rhododendron scrub, woodland and rocky outcrops) were less resilient to fire than the more abundant heathland habitat. Fire caused an increase in the cover and number of therophyte and hemicryptophyte species, and the more typical dominant species failed to reach their natural cover. In the heathlands Erica australis (an indicator species for the ‘dry heathland’ habitat of European community interest) increased and Cistus salvifolius and Ulex minor decreased in cover with time since fire. Conclusions: The rarer habitats were least resilient to fire, which is of considerable conservation importance since fire frequency is predicted to increase with climate change. Higher fire frequencies caused a change towards a less valuable heathland type.

Ant community responses to experimental fire and logging in a eucalypt forest of south-eastern Australia

Sustainable forest management requires a sound understanding of the impacts of forestry management practices, especially prescribed fire and timber harvesting, on biodiversity. Many studies have examined the impacts of fire and logging separately, but few have considered them together. Here we describe the combined effects of selective logging and repeated prescribed fire on ants in eucalypt forest near Eden in south-eastern Australia. The study was conducted within 18 experimental coupes comprising three replicates of each of six treatments, representing combinations of two logging (selectively logged and unlogged) and three prescribed burning (unburnt, routine and high fire frequency over 20 years) practices. Ants were sampled using terrestrial and arboreal pitfall traps in two plots within each of the 18 coupes. A total of 92 species from 35 genera were recorded, with ant species richness increasing with both increasing fire frequency and selective harvesting. We also found significant differences in ant species composition between management treatments; however, these differences were rather modest, and apparent only between extremes of fire and harvesting combinations. Logging on its own was not a significant factor contributing to variation in ant species composition. However, the responses of two common species and a key functional group to fire depended on logging treatment; in each case their abundance decreased with increasing fire frequency at unlogged sites, but increased at logged sites. The general resilience of ant communities suggests that forests remain in good functional health even when subject to a combination of selective logging and frequent burning. However, our study under-sampled species with specialist microhabitat requirements, and these might have been more heavily impacted. We have also revealed some significant interactions between the effects of fire and logging, which highlights the importance of considering combined impacts of forest management practices.

Compound vesicle fusion increases quantal size and potentiates synaptic transmission

Exocytosis at synapses generally refers to fusion between vesicles and the plasma membrane1. Although compound fusion between vesicles2,3 was proposed at ribbon-type synapses4,5, whether it exists, how it is mediated, and what role it plays at conventional synapses remain unclear. Here we addressed this issue at a nerve terminal containing conventional active zones. High potassium application and high frequency firing induced giant capacitance up-steps reflecting exocytosis of vesicles larger than regular ones, followed by giant down-steps reflecting bulk endocytosis. They also induced giant vesicle-like structures, as observed with electron microscopy, and giant miniature EPSCs (mEPSCs) reflecting more transmitter release. Calcium and its sensor for vesicle fusion, synaptotagmin, were required for these giant events. After high frequency firing, calcium/synaptotagmin-dependent mEPSC size increase was paralleled by calcium/synaptotagmin-dependent post-tetanic potentiation (PTP). These results suggest that calcium/synaptotagmin mediates compound fusion between vesicles, that exocytosis of compound vesicles increases quantal size which enhances synaptic strength and thus contributes to the generation of PTP, and that exocytosed compound vesicles may be retrieved via bulk endocytosis. We suggest to include a new vesicle cycling route, compound exocytosis followed by bulk endocytosis, into models of synapses, where currently only vesicle fusion with the plasma membrane is considered (Fig. S1)1.

Fire frequency influences composition and structure of the shrub layer in an Australian subcoastal temperate grassy woodland

AbstractLittle is known about the relationship between fire regimes and plant diversity in Australia's temperate grassy woodlands. The effect of fire frequency on shrubs in grassy woodland remnants across Western Sydney's Cumberland Plain was examined. Shrub species richness and composition were compared in sites that had experienced a high, moderate or low frequency of fire over the previous 20 years. Nine sites were surveyed, three in each fire frequency category; most sites, including all low‐fire‐frequency sites, had burnt 9–36 months prior to sampling. Fire frequency had a profound effect on the composition and structure of the shrub layer. Per cent frequency and density of the prickly shrub Bursaria spinosa (Pittosporaceae) was considerably higher in low‐fire‐frequency sites than where fires had occurred at least once a decade. In sites where fire had been absent for decades prior to a recent fire, this species dominated the landscape, while elsewhere it occurred as clumps in a grassy matrix. Per cent frequency of other native shrubs, particularly obligate seeders, was greatest at moderate fire frequencies. Exotic shrubs were recorded most often where fire had been rare. While ordination clearly separated out the low‐fire‐frequency sites, complete separation between high‐ and moderate‐fire‐frequency blocks was not achieved. The increase in Bursaria in the absence of fire mirrors the encroachment of woody plants into a range of grassy ecosystems around the world. The sensitivity of obligate seeder species, many of them short‐lived legumes with fire‐cued seeds, to both very frequent and very infrequent fire shows the vulnerability of these species to extreme fire regimes, despite the safeguards conferred by hard‐seededness. Competition from Bursaria, as well as loss of viable seed in the soil, may have contributed to the low frequency of these species after a long inter‐fire interval.

Block of Tetrodotoxin-sensitive, Nav1.7, and Tetrodotoxin-resistant, Nav1.8, Na+ Channels by Ranolazine

Evidence supports a role for the tetrodotoxin-sensitive Na(V)1.7 and the tetrodotoxin-resistant Na(V)1.8 in the pathogenesis of pain. Ranolazine, an anti-ischemic drug, has been shown to block cardiac (Na(V)1.5) late sodium current (I(Na)). In this study, whole-cell patch-clamp techniques were used to determine the effects of ranolazine on human Na(V)1.7 (hNa(V)1.7 + beta(1) subunits) and rat Na(V)1.8 (rNa(V)1.8) channels expressed in HEK293 and ND7-23 cells, respectively. Ranolazine reduced hNa(V)1.7 and rNa(V)1.8 I(Na) with IC50 values of 10.3 and 21.5 microM (holding potential = -120 or -100 mV, respectively). The potency of I(Na) block by ranolazine increased to 3.2 and 4.3 microM when 5-sec depolarizing prepulses to -70 (hNa(V)1.7) and -40 (rNa(V)1.8) mV were applied. Ranolazine caused a preferential hyperpolarizing shift of the steady-state fast, intermediate and slow inactivation of hNa(V)1.7 and intermediate and slow inactivation of rNa(V)1.8, suggesting preferential interaction of the drug with the inactivated states of both channels. Ranolazine (30 microM) caused a use-dependent block (10-msec pulses at 1, 2 and 5 Hz) of hNa(V)1.7 and rNa(V)1.8 I(Na) and significantly accelerated the onset of, and slowed the recovery from inactivation, of both channels. An increase of depolarizing pulse duration from 3 to 200 msec did not affect the use-dependent block of I(Na) by 100 microM ranolazine. Taken together, the data suggest that ranolazine blocks the open state and may interact with the inactivated states of Na(V)1.7 and Na(V)1.8 channels. The state-and use-dependent modulation of hNa(V)1.7 and rNa(V)1.8 Na+ channels by ranolazine could lead to an increased effect of the drug at high firing frequencies, as in injured neurons.

Distinct Functional Effects of Kv3.3 Mutations Associated with Spinocerebellar Ataxia Type 13

Spinocerebellar Ataxia Type 13 is an autosomal dominant genetic disease characterized by ataxia, oculomotor abnormalities, and the death of cerebellar neurons. SCA13 is caused by mutations in the Kv3.3 voltage-gated K+ channel gene, including R366H in S2; R420H and R423H in S4; and F448L in S5. SCA13 exists in two forms with infant or adult onset. There is a strong genotype-phenotype correlation between the disease-causing mutation and the age of onset of symptoms. The functional effects of the mutations fall into two categories. R366H, R420H, and R423H are non-functional when expressed alone and exert strong dominant negative effects when co-expressed with wild type Kv3.3 or other members of the Kv3 subfamily. The stoichiometry of suppression differs among the three R→H mutations, with R366H subunits less disruptive to function than R420H or R423H. In contrast, F448L is a dominant gain of function mutation that affects channel gating. F448L shifts the voltage dependence of activation in the hyperpolarized potential and dramatically slows deactivation. These changes in Kv3.3 gating are not significantly different whether F448L is expressed alone or co-expressed with wild type subunits. Kv3 channels, including Kv3.3, facilitate high frequency firing in neurons. The SCA13 mutations are expected to alter the excitability of cerebellar neurons, which express high levels of Kv3.3. Since the functional effects of the mutations are distinct, it is likely that they will have distinct effects on the excitability of cerebellar neurons. This hypothesis is supported by mathematical modeling of firing behavior in cerebellar neurons. Differential effects on neuronal excitability are likely to underlie the symptoms of SCA13 and may help to explain the differences in the age of disease onset.This work was supported by NIH grant R01-NS058500 to DMP.

Influence of fire frequency on <i>Colophospermum mopane</i> and <i>Combretum apiculatum</i> woodland structure and composition in northern Gonarezhou National Park, Zimbabwe

We investigated the long-term effects of fire frequency on Colophospermum mopane and Combretum apiculatum woodland structure and composition in northern Gonarezhou National Park (GNP), Zimbabwe. Fire frequency was categorised as high (every 1–2 years), medium (every 3–4 years) and low (every 5–6 years). The following variables were measured or recorded: plant height, species name, canopy depth and diameter, basal circumference, number of stems per plant, plant status (dead or alive) and number of woody plants in a plot. There was a positive correlation (r = 0.55, P = 0.0007) between annual area burnt (total from January to December) and annual rainfall (average over two rain stations per rain year, July to June) between 1972 and 2005. A total of 64 woody species were recorded from C. mopane and C. apiculatum woodlands. Mean plant height increased from 4.5 to 8.2 meters in C. mopane woodland and from 4.5 to 5.1 meters in C. apiculatum woodland in areas subjected to high and low fire frequencies. In C. mopane woodland, low fire frequency was characterised by a significantly low density of woody plants (P < 0.001), however, with a significantly high mean basal area (P < 0.001). Fire frequency had no significant effect on species diversity (P > 0.05). Our results suggest that C. mopane and C. apiculatum woodlands are in a state of structural transformation. Fire frequency effects, however, appear to be woodland specific. Fire management strategies in GNP should take into consideration annual rainfall and the different vegetation types.Conservation implication: This study provides valuable information on fire frequency effects on woody vegetation in northern GNP, which can be used in fire management programmes for the park. The positive relationship between annual rainfall and annual area burnt emphasises the need for wildlife managers to consider annual rainfall in fire management.

TRPV1 Expression Defines Functionally Distinct Pelvic Colon Afferents

Changes in primary sensory neurons are likely to contribute to the emergence of chronic visceral pain. An important step in understanding visceral pain is the development of comprehensive phenotypes that combines functional and anatomical properties for these neurons. We developed a novel ex vivo physiology preparation in mice that allows intracellular recording from colon sensory neurons during colon distension, in the presence and absence of pharmacologic agents. This preparation also allows recovery of functionally characterized afferents for histochemical analysis. Recordings obtained from L6 dorsal root ganglion cells in C57BL/6 mice identified two distinct populations of distension-responsive colon afferents: high-firing frequency (HF) and low-firing frequency (LF) cells. Fluid distension of the colon elicited rapid firing (>20 Hz) in HF cells, whereas LF cells seldom fired >5 Hz. Distension response thresholds were significantly lower in HF cells (LF, 17.5 +/- 1.1 cmH(2)O; HF, 2.6 +/- 1.0 cmH(2)O). Responses of most LF afferents to colon distension were sensitized by luminal application of capsaicin (1 microm; 8 of 9 LF cells), mustard oil (100 microm; 10 of 12 LF cells), and low pH (pH 4.0; 5 of 6 LF cells). In contrast, few HF afferents were sensitized by capsaicin (3 of 9), mustard oil (2 of 7), or low pH (1 of 6) application. Few HF afferents (4 of 23) expressed the capsaicin receptor, TRPV1. In contrast, 87% (25 of 29) of LF afferents expressed TRPV1. TRPV1 has been shown to be required for development of inflammatory hyperalgesia. These results suggest a unique functional role of TRPV1-positive colon afferents that could be exploited to design specific therapies for visceral hypersensitivity.

Italian Subalpine Forest

We found that the expansion of Pinus cembra, a long-lived species, occurred in a context of moderate fire frequency during the early Holocene. Nevertheless, a high fire frequency period could have depleted the local population in the short term. The predicted increase in drought in the future could thus affect the cembra pine ecosystem in the Alps only if intervals are reduced to less than 80 years. Both photos show subalpine forest dominated by cembra pine (Pinus cembra L.) in the Stelvio National Park, Valtellina Valley, North Italy. These photographs illustrate the article “The effect of fire frequency on local cembra pine populations,” by A. Genries, L. Mercier, M. Lavoie, S. D. Muller, O. Radakovitch, and C. Carcaillet, tentatively scheduled to appear in Ecology 90(2), February 2009.

Synaptic vesicle pool size, release probability and synaptic depression are sensitive to Ca2+ buffering capacity in the developing rat calyx of Held

The calyx of Held, a specialized synaptic terminal in the medial nucleus of the trapezoid body, undergoes a series of changes during postnatal development that prepares this synapse for reliable high frequency firing. These changes reduce short-term synaptic depression during tetanic stimulation and thereby prevent action potential failures during a stimulus train. We measured presynaptic membrane capacitance changes in calyces from young postnatal day 5-7 (p5-7) or older (p10-12) rat pups to examine the effect of calcium buffer capacity on vesicle pool size and the efficiency of exocytosis. Vesicle pool size was sensitive to the choice and concentration of exogenous Ca2+ buffer, and this sensitivity was much stronger in younger animals. Pool size and exocytosis efficiency in p5-7 calyces were depressed by 0.2 mM EGTA to a greater extent than with 0.05 mM BAPTA, even though BAPTA is a 100-fold faster Ca2+ buffer. However, this was not the case for p10-12 calyces. With 5 mM EGTA, exocytosis efficiency was reduced to a much larger extent in young calyces compared to older calyces. Depression of exocytosis using pairs of 10-ms depolarizations was reduced by 0.2 mM EGTA compared to 0.05 mM BAPTA to a similar extent in both age groups. These results indicate a developmentally regulated heterogeneity in the sensitivity of different vesicle pools to Ca2+ buffer capacity. We propose that, during development, a population of vesicles that are tightly coupled to Ca2+ channels expands at the expense of vesicles more distant from Ca2+ channels.

Effects of fire frequency on species composition in a Mediterranean shrubland

The effect of high fire frequency on the species composition of Mediterranean-type plant communities is reported on the basis of shrubland stands located in Catalonia (Spain) that have experienced from 1 to 5 fires over the last 31 years. We focused on changes in the abundance of species, grouped according to post-fire regenerative traits (resprouting and seed germination) and life form (shrubs, herbaceous graminoids, and herbaceous non-graminoids). High fire frequency was related to a low abundance of obligate resprouter and obligate seeder shrubs. In the latter group, Cistus sp. disappeared in stands with high fire frequency and showed a maximum abundance in stands with low fire frequency. The abundance of facultative resprouter shrubs did not change with fire frequency. This group thus became dominant in the shrub layer at high fire frequency due to the low abundance of obligate resprouter and obligate seeder shrubs. These changes in the abundance of shrub species involve changes in the patterns of relative dominance of regenerative syndromes, in line with fire frequency. An examination of life forms revealed that the abundance of herbaceous non-graminoids and herbaceous graminoids was higher in stands with high fire frequency and the graminoid Brachypodium retusum was dominant at all fire frequencies. These results suggest a loss in the resilience of shrubs after frequent fires, leading to a simplification of vegetation structure, with a shift from shrubland to grassland-type communities, thereby probably enhancing a potential positive herb-fire feedback.

Block of Tetrodotoxin-sensitive, Nav1.7, and Tetrodotoxin-resistant, Nav1.8, Na+ channels by Ranolazine

Evidence supports a role for the tetrodotoxin-sensitive Nav1.7 and the tetrodotoxin-resistant Nav1.8 in the pathogenesis of pain. Ranolazine, an anti-ischemic drug, has been shown to block cardiac (Nav1.5) late sodium current (INa). In this study, whole-cell patch-clamp techniques were used to determine the effects of ranolazine on human Nav1.7 (hNav1.7+β1 subunits) and rat Nav1.8 (rNav1.8) channels expressed in HEK293 and ND7-23 cells, respectively. Ranolazine reduced hNav1.7 and rNav1.8 INa with IC50 values of 10.3 and 21.5 μM (holding potential=-120 or -100 mV, respectively). The potency of INa block by ranolazine increased to 3.2 and 4.3 μM when 5-sec depolarizing prepulses to -70 (hNav1.7) and -40 (rNav1.8) mV were applied. Ranolazine caused a preferential hyperpolarizing shift of the steady-state fast, intermediate and slow inactivation of hNav1.7 and and intermediate and slow inactivation of rNav1.8, suggesting preferential interaction of the drug with the inactivated states of both channels. Ranolazine (30 μM) caused a use-dependent block (10-msec pulses at 1, 2 and 5 Hz) of hNav1.7 and rNav1.8 INa and significantly accelerated the onset of, and slowed the recovery from inactivation, of both channels. An increase of depolarizing pulse duration from 3 to 200 msec did not affect the use-dependent block of INa by 100 μM ranolazine. Taken together, the data suggest that ranolazine blocks the open state and may interact with the inactivated states of Nav1.7 and Nav1.8 channels. The state-and use-dependent modulation of hNav1.7 and rNav1.8 Na+ channels by ranolazine could lead to an increased effect of the drug at high firing frequencies, as in injured neurons.

A new method for analyzing the refill process and fabrication of a piezoelectric inkjet printing head for LCD color filter manufacturing

This paper presents a method for analyzing the refill process of a piezoelectric inkjet printing head with a high firing frequency for color filter manufacturing. Theoretical and experimental studies on the equivalent length (Leq) versus jetting characteristics were performed. The new model has shown quantitatively the same result compared with a commercialized simulation code. Also it is identified that the refill time increases with the equivalent liquid length (Leq) because the viscous force increases. The inkjet printing head has been designed with a lumped model analysis and fabricated with a silicon wafer (1 0 0) by a MEMS process. To investigate how the equivalent length (Leq) influences the firing frequency, an experiment was conducted using a stroboscope. In the case of colorant ink, it is possible to eject an ink droplet up to 5 kHz with a 40 pl drop volume. On the other hand, the firing frequency calculated with the new model is about 3 kHz under the condition of the equivalent liquid length (Leq), 250 µm. The difference between the new model and experiment may be a result of a mismatch of initial meniscus position due to the meniscus oscillation. Experimentally the meniscus oscillation is observed through an optical measurement with a visualization apparatus and a transparent nozzle. Hence the efficiency of the new model may be enhanced in a high viscosity range. The methods for increasing the firing frequency are to reduce the equivalent length (Leq) and to modify the ink property. Because the former tends to decrease a viscous loss and the latter tends to increase a viscous damping, two parameters should be combined adequately within an allowable drop volume.

The relevance of fire frequency for the floodplain vegetation of the Okavango Delta, Botswana

AbstractThe study investigates the relevance of fire frequency for the floodplain vegetation of the Okavango Delta in Botswana, considering species composition, species richness, vegetation structure and tree density. Based on flood and fire frequency data derived from a series of satellite images, active and drying floodplains were separated and study plots were selected with fire frequencies up to ten burns between 1989 and 2003. Both for drying and active floodplains, no typical species assemblage could be associated with fire frequency. On drying floodplains, fire frequency showed no effect on species composition, but rather on vegetation structure. While small woody species showed higher cover values on high fire frequency, large trees showed significantly lower cover values on high fire frequency. For active floodplains, a significant response to fire frequency, both positively and negatively could be determined for specific species. But as almost none of these species appear at low or high frequency only, no differences in species composition could be associated with fire frequency. Rather flood frequency and specific annual flood cycles could be described as the determining factors for the vegetation on active floodplains.

FIRE REDUCES MORPHOSPACE OCCUPATION IN PLANT COMMUNITIES

The two main assembly processes claimed to structure plant communities are habitat filtering and competitive interactions. The set of species growing in fire-prone communities has been filtered in such a way that species without fire-persistence traits have not successfully entered the community. Because plant traits are evolutionarily conserved and fire traits are correlated with other plant traits, communities under high fire frequency should not include all possible trait combinations, and thus the morphospace occupation by species in these communities should be lower than expected by chance (underoccupied). In contrast, communities under low fire frequency would lack the filtering factor, and thus their underoccupation of the morphospace is not expected. We test this prediction by comparing the morphospace occupation by species in communities located in the western Mediterranean Basin, five of them subject to high fire frequency (HiFi) and four to low fire frequency (LowFi). We first compile a set of morphological and functional traits for the species growing on the nine sites, then we compute the morphospace occupation of each site as a convex hull volume, and finally, to assert that our results are not a product of a random branching pattern of evolution, we simulate our traits under a null model of neutral evolution and compare the morphospace occupation of the simulated traits with the results from the empirical data. The results suggest that, as predicted, there is a clear differential morphospace occupation between communities under different fire regimes in such a way that the morphospace is underoccupied in HiFi communities only. The simulation of a neutral evolutionary model does not replicate the observed pattern of differential morphospace occupation, and thus it should be attributed to assembly processes. In conclusion, our results suggest that fire is a strong community assembling process, filtering the species that have fire-persistent traits and thus assembling phenotypically and phylogenetically clustered communities with vacant zones in the morphospace.

Initial segment Kv2.2 channels mediate a slow delayed rectifier and maintain high frequency action potential firing in medial nucleus of the trapezoid body neurons

The medial nucleus of the trapezoid body (MNTB) is specialized for high frequency firing by expression of Kv3 channels, which minimize action potential (AP) duration, and Kv1 channels, which suppress multiple AP firing, during each calyceal giant EPSC. However, the outward K(+) current in MNTB neurons is dominated by another unidentified delayed rectifier. It has slow kinetics and a peak conductance of approximately 37 nS; it is half-activated at -9.2 +/- 2.1 mV and half-inactivated at -35.9 +/- 1.5 mV. It is blocked by several non-specific potassium channel antagonists including quinine (100 microm) and high concentrations of extracellular tetraethylammonium (TEA; IC(50) = 11.8 mM), but no specific antagonists were found. These characteristics are similar to recombinant Kv2-mediated currents. Quantitative RT-PCR showed that Kv2.2 mRNA was much more prevalent than Kv2.1 in the MNTB. A Kv2.2 antibody showed specific staining and Western blots confirmed that it recognized a protein approximately 110 kDa which was absent in brainstem tissue from a Kv2.2 knockout mouse. Confocal imaging showed that Kv2.2 was highly expressed in axon initial segments of MNTB neurons. In the absence of a specific antagonist, Hodgkin-Huxley modelling of voltage-gated conductances showed that Kv2.2 has a minor role during single APs (due to its slow activation) but assists recovery of voltage-gated sodium channels (Nav) from inactivation by hyperpolarizing interspike potentials during repetitive AP firing. Current-clamp recordings during high frequency firing and characterization of Nav inactivation confirmed this hypothesis. We conclude that Kv2.2-containing channels have a distinctive initial segment location and crucial function in maintaining AP amplitude by regulating the interspike potential during high frequency firing.

Spatio‐temporal trends in tree cover of a tropical mesic savanna are driven by landscape disturbance

1. How tree cover in tropical savannas changes through time and space is a major unresolved issue in ecology due to the complexity of these systems where fire, grazing, climatic variability and landscape variation interact, determining the spatial patterns of biomass. We aimed to assess the relative importance of fire and grazing in determining multi-decadal and landscape-scale patterns of savanna tree cover and biomass in a mesic savanna. 2. Assessment of digitized aerial photography, from 1964, 1984, 1991 and 2004, was undertaken for 40 sites at the Kapalga fire experimental area (∼1470 mm mean annual rainfall) in Kakadu National Park in monsoonal northern Australia. 3. Spatio-temporal changes in tree cover were analysed using linear mixed effects modelling, with multi-model inference in an information theoretic framework. 4. Savanna tree cover fluctuated greatly over the 40-year period, and varied spatially amongst the 40 sites, by 20-30% in a given time period. Tree cover was most likely to decline when initially high, and with a high frequency of fire, especially late dry season fire. The presence of a feral grazer had little effect other than through reducing fire frequency. Our results are consistent with an overall increase in tree cover in these mesic savannas during drier-than-average periods. 5. Ground-based measurements of change in stand basal area under experimental burning regimes agreed with the estimated effects of fire on tree cover at the same site based on our analyses of aerial photography. 6. Synthesis and applications. The dynamism of these mesic savannas is driven by variation in fire frequency and severity, which is influenced by feral grazers, creating a mosaic of tree cover that shifts over time in response to localized variation in disturbance. These findings are relevant to the management of this biome. We show that changing disturbance regimes alter the dynamics of these systems. Fire management incorporating a variety of fire regimes will promote the persistence of a savanna patch mosaic. © 2008 The Authors.

Nitrogen Enhances the Competitive Ability of Cheatgrass (Bromus tectorum) Relative to Native Grasses

Invasion by cheatgrass and the associated high fire frequency can displace native plant communities from a perennial to an annual grass driven system. Our overall objective of this study was to determine the potential to favor desired native perennial bunchgrasses over annual grasses by altering plant available mineral nitrogen (N). In the first study, we grew cheatgrass and three native bunch grasses (native grasses were combined in equal proportions) in an addition series experimental design and applied one of three N treatments (0, 137, and 280 mg N/kg soil). Regression models were used to derive the effects of intra- and interspecific competition on individual plant yield of cheatgrass and the native bunch grasses (combined). In our second study, we compared the absolute growth rate of the four plant species grown in isolation in a randomized complete block design for 109 days under the same soil N treatments as the competition study. Predicted mean average weight of isolated individuals increased with increasing soil N concentrations for both cheatgrass and the three native perennials (P < 0.05). Biomass of cheatgrass and its competitive ability increased with increasing soil N concentrations (P < 0.0001) compared to the combined native bunchgrasses. However, the greatest resource partitioning occurred at the 137 mg N/kg soil N treatment compared to the 0 (control) and 280 mg N/kg soil treatments, suggesting there may be a level of N that minimizes competition. In the second study, the absolute growth of cheatgrass grown in isolation also increased with increasing N levels (P = 0.0297). Results and ecological implications of this study suggest that increasing soil N leads to greater competitive ability of cheatgrass, and that it may be possible to favor desired plant communities by modifying soil nutrient levels.Nomenclature: Bluebunch wheatgrass, Pseudoroegneria spicata (Pursh) A. Love PSSP6; Idaho fescue, Festuca idahoensis Elmer FEID; needle and thread, Hesperostipa comata (Trin. and Rupr.) Barkworth HECO26; cheatgrass, Bromus tectorum L BRTE.