Increase In Firing Frequency Research Articles

Responses to apical and basolateral application of glutamate in mouse fungiform taste cells with action potentials.

In taste bud cells, glutamate may elicit two types of responses, as an umami tastant and as a neurotransmitter. Glutamate applied to apical membrane of taste cells would elicit taste responses whereas glutamate applied to basolateral membrane may act as a neurotransmitter. Using restricted stimulation to apical or basolateral membrane of taste cells, we examined responses of taste cells to glutamate stimulation, separately. Apical application of monosodium glutamate (MSG, 0.3 M) increased firing frequency in some of mouse fungiform taste cells that evoked action potentials. These cells were tested with other basic taste compounds, NaCl (salty), saccharin (sweet), HCl (sour), and quinine (bitter). MSG-sensitive taste cells could be classified into sweet-best (S-type), MSG-best (M-type), and NaCl or other electrolytes-best (N- or E/H-type) cells. Furthermore, S- and M-type could be classified into two sub-types according to the synergistic effect between MSG and inosine-5'-monophosphate (S1, M1 with synergism; S2, M2 without synergism). Basolateral application of glutamate (100 μM) had almost no effect on the mean spontaneous firing rates in taste cells. However, about 10% of taste cells tested showed transient increases in spontaneous firing rates (>mean + 2 standard deviation) after basolateral application of glutamate. These results suggest the existence of multiple types of umami-sensitive taste cells and the existence of glutamate receptor(s) on the basolateral membrane of a subset of taste cells.

Are Cav1.3 pacemaker channels in chromaffin cells? Possible bias from resting cell conditions and DHP blockers usage

Mouse and rat chromaffin cells (MCCs, RCCs) fire spontaneously at rest and their activity is mainly supported by the two L-type Ca2+ channels expressed in these cells (Cav1.2 and Cav1.3). Using Cav1.3-/- KO MCCs we have shown that Cav1.3 possess all the prerequisites for carrying subthreshold currents that sustain low frequency cell firing near resting (0.5 to 2 Hz at -50 mV)1: low-threshold and steep voltage dependence of activation, slow and incomplete inactivation during pulses of several hundreds of milliseconds. Cav1.2 contributes also to pacemaking MCCs and possibly even Na+ channels may participate in the firing of a small percentage of cells. We now show that at potentials near resting (–50 mV), Cav1.3 carries equal amounts of Ca2+ current to Cav1.2 but activates at 9 mV more negative potentials. MCCs express only TTX-sensitive Nav1 channels that activate at 24 mV more positive potentials than Cav1.3 and are fully inactivating. Their blockade prevents the firing only in a small percentage of cells (13%). This suggests that the order of importance with regard to pacemaking MCCs is: Cav1.3, Cav1.2 and Nav1. The above conclusions, however, rely on the proper use of DHPs, whose blocking potency is strongly holding potential dependent. We also show that small increases of KCl concentration steadily depolarize the MCCs causing abnormally increased firing frequencies, lowered and broadened AP waveforms and an increased facility of switching “non-firing” into “firing” cells that may lead to erroneous conclusions about the role of Cav1.3 and Cav1.2 as pacemaker channels in MCCs.2

Mating-induced differential coding of plant odour and sex pheromone in a male moth

Innate behaviours in animals can be influenced by several factors, such as the environment, experience, or physiological status. This behavioural plasticity originates from changes in the underlying neuronal substrate. A well-described form of plasticity is induced by mating. In both vertebrates and invertebrates, males experience a post-ejaculatory refractory period, during which they avoid new females. In the male moth Agrotis ipsilon, mating induces a transient inhibition of responses to the female-produced sex pheromone. To understand the neural bases of this inhibition and its possible odour specificity, we carried out a detailed analysis of the response characteristics of the different neuron types from the periphery to the central level. We examined the response patterns of pheromone-sensitive and plant volatile-sensitive neurons in virgin and mated male moths. By using intracellular recordings, we showed that mating changes the response characteristics of pheromone-sensitive antennal lobe (AL) neurons, and thus decreases their sensitivity to sex pheromone. Individual olfactory receptor neuron (ORN) recordings and calcium imaging experiments indicated that pheromone sensory input remains constant. On the other hand, calcium responses to non-pheromonal odours (plant volatiles) increased after mating, as reflected by increased firing frequencies of plant-sensitive AL neurons, although ORN responses to heptanal remained unchanged. We suggest that differential processing of pheromone and plant odours allows mated males to transiently block their central pheromone detection system, and increase non-pheromonal odour detection in order to efficiently locate food sources.

Characterization of adult mouse muscle spindle afferents in an in vitro isolated muscle nerve preparation

Changes in muscle spindle afferent activity has been demonstrated in models of muscle pain and diabetes and may also be altered in Restless Legs Syndrome. Here we describe a novel in vitro adult mouse muscle nerve preparation ideally suited to measure plasticity in muscle afferent activity. Briefly, we removed the Extensor Digitorum Longus (EDL) and its nerve from adult (2-month old) male C57Bl/6 mice. The isolated muscle was attached to a force and length controller (Aurora). The length at which the highest force following twitch contraction occurred was set as the resting length (Lo). Muscle afferent activity was recorded using a suction electrode and individual units discriminated using WaveClus (Quiroga, 2004). A battery of stretches and vibrations were performed at Lo, Lo+ 5%, and Lo-5%. We collected the responses of at least 2 stretch sensitive units in 12 muscles at room temperature. Group Ia afferents were found to have clear dynamic sensitivity and showed entrainment to vibration with amplitudes as low 5 μM. Muscles were stretched 2.5%, 5%, and 7.5% Lo and increased firing frequency by a constant number of impulses per mm stretch, similar to that reported in vivo for rats and cats. Health of the preparation was confirmed following stretch in a subset of animals by determining the maximal isometric tension (23.4 ± 2.5 N/cm2, n = 6). This research was supported by K12 GM000680 NIH/NIGMS More Division (KAW) and Pfizer (SH).

TRENDS IN RECOVERY OF MEDITERRANEAN SOIL CHEMICAL PROPERTIES AND MICROBIAL ACTIVITIES AFTER INFREQUENT AND FREQUENT WILDFIRES

ABSTRACTSince the 1970s, increase in fire frequency has been observed in all European Mediterranean regions. The objectives of this study were (1) to determine the effects of wildfire frequency on the recovery at short‐ and long‐term of soil chemical and microbial properties and (2) to identify the mechanisms underlying the recovery of these sites properties. Soils from 17 plots (Maures mountains range, Var, France) were classified into 5 wildfire regimes (i.e. not burned since at least 57 years ago, infrequently and frequently burned‐with time since fire between 4 and 17 years). Soil samples from these plots were analysed for their nutrient content, chemical functions of soil organic matter (SOM) using FT‐MIR spectroscopy and microbial mineralising activities. Our results showed that the frequent wildfire regime slowed down the recovery in the short term of SOM spectroscopic properties and nutrient availability. Both low quantity and low quality (i.e. high percentage of aromatic and phenolic organic forms) of soil organic matter were found to be related to soil microbial recovery at 4 years after frequent wildfires. The frequent wildfires improved the recovery in net nitrification and nitrate content, leading to an increase in catabolic evenness and a recovery in microbial C‐substrate utilisation profiles between 4 and 17 years. However, frequent wildfires slowed down the recovery of hydrolytic enzyme pool (i.e. FDA hydrolases) and phenol oxidase activity, both involved in soil C cycling. Overall, our observations suggest that 4 fires in 50 years is a threshold beyond which soil quality may be endangered. Copyright © 2011 John Wiley & Sons, Ltd.

Effects of dendritic load on the firing frequency of oscillating neurons

We study the effects of passive dendritic properties on the dynamics of neuronal oscillators. We find that the addition of a passive dendrite can sometimes have counterintuitive effects on firing frequency. Specifically, the addition of a hyperpolarized passive dendritic load can either increase, decrease, or have negligible effects on firing frequency. We use the theory of weak coupling to derive phase equations for "ball-and-stick" model neurons and two-compartment model neurons. We then develop a framework for understanding how the addition of passive dendrites modulates the frequency of neuronal oscillators. We show that the average value of the neuronal oscillator's phase response curves measures the sensitivity of the neuron's firing rate to the dendritic load, including whether the addition of the dendrite causes an increase or decrease in firing frequency. We interpret this finding in terms of to the slope of the neuronal oscillator's frequency-applied current curve. We also show that equivalent results exist for constant and noisy point-source input to the dendrite. We note that the results are not specific to neurons but are applicable to any oscillator subject to a passive load.

How does increased fire frequency affect carbon loss from fire? A case study in the northern boreal forest

Fire frequency is expected to increase due to climate warming in many areas, particularly the boreal forests. An increase in fire frequency may have important effects on the global carbon cycle by decreasing the size of boreal carbon stores. Our objective was to quantify and compare the amount of carbon consumed during and the amount of carbon remaining following fire in black spruce (Picea mariana (Mill.) BSP) forests burned after long v. short intervals. We hypothesised that stands with a shortened fire return interval would have a higher carbon consumption than those experiencing a historically typical fire return interval. Using field measurements of forest canopy, soil organic horizons and adventitious roots, we reconstructed pre-fire stand conditions to estimate the biomass lost in each fire and the effects on post-fire residual carbon stores. We found evidence of a higher loss of carbon following two fire events that recurred after a short interval, resulting in a much greater total reduction in carbon relative to pre-fire or mature stand conditions. Consequently, carbon storage across disturbance intervals was dramatically reduced following short-interval burns. Recovery of these stores would require a subsequent lengthening of the fire cycle, which appears unlikely under future climate scenarios.

Landscape-level differences in fire regime between block and patch-mosaicburning strategies in Mkuzi Game Reserve, South Africa

Patch-mosaic burning (PMB) is commonly advocated to create a mosaic of fire regimes that is believed to be more beneficial for maintaining biodiversity than the relatively homogeneous environment produced by block burning. This premise was examined for the 23 651 ha Mkuzi Game Reserve, in which all fires have been mapped for a period of block burning (1966–1984) and for a period of PMB (1985–1999), using a geographic information system. Patch-mosaic burning met with the expectation of creating a more heterogeneous fire regime as evidenced by a four-fold increase in average fire frequency, a greater area of reserve burnt per annum, an increase in the perimeter-to-area ratio of individual burns, and a greater proportion of fires occurring throughout the dry season as opposed to being concentrated toward the end of the dry season. In contradiction of expectation, size of individual burns did not differ between strategies, both periods were characterised by intense fires, and natural barriers to fire did not increase in importance under PMB. There was a reduced importance of arson fires and a markedly increased proportion of prescribed burns under PMB. Patch-mosaic burning has met in part its objective of creating a more heterogeneous fire regime.

Response to Comment on “The Incidence of Fire in Amazonian Forests with Implications for REDD”

Balch et al . suggest that the increased fire frequency reported in our study is principally due to post-deforestation activities. We present a new analysis demonstrating that for the majority of grid cells with positive fire trends, there is a low likelihood that these trends have resulted exclusively from post-deforestation activities. We therefore confirm that fires pose a growing threat to reducing emissions from deforestation and degradation (REDD) policies.

Multinodal regulation of the arcuate/paraventricular nucleus circuit by leptin.

Melanocortin-4 receptor (MC4R) is critical for energy homeostasis, and the paraventricular nucleus of the hypothalamus (PVN) is a key site of MC4R action. Most studies suggest that leptin regulates PVN neurons indirectly, by binding to receptors in the arcuate nucleus or ventromedial hypothalamus and regulating release of products like α-melanocyte-stimulating hormone (α-MSH), neuropeptide Y (NPY), glutamate, and GABA from first-order neurons onto the MC4R PVN cells. Here, we investigate mechanisms underlying regulation of activity of these neurons under various metabolic states by using hypothalamic slices from a transgenic MC4R-GFP mouse to record directly from MC4R neurons. First, we show that in vivo leptin levels regulate the tonic firing rate of second-order MC4R PVN neurons, with fasting increasing firing frequency in a leptin-dependent manner. We also show that, although leptin inhibits these neurons directly at the postsynaptic membrane, α-MSH and NPY potently stimulate and inhibit the cells, respectively. Thus, in contrast with the conventional model of leptin action, the primary control of MC4R PVN neurons is unlikely to be mediated by leptin action on arcuate NPY/agouti-related protein and proopiomelanocortin neurons. We also show that the activity of MC4R PVN neurons is controlled by the constitutive activity of the MC4R and that expression of the receptor mRNA and α-MSH sensitivity are both stimulated by leptin. Thus, leptin acts multinodally on arcuate nucleus/PVN circuits to regulate energy homeostasis, with prominent mechanisms involving direct control of both membrane conductances and gene expression in the MC4R PVN neuron.

KChIP1 modulation of Kv4.3-mediated A-type K+ currents and repetitive firing in hippocampal interneurons

Neuronal A-type K+ channels regulate action potential waveform, back-propagation and firing frequency. In hippocampal CA1 interneurons located at the stratum lacunosum-moleculare/radiatum junction (LM/RAD), Kv4.3 mediates A-type K+ currents and a Kv4 β-subunit of the Kv channel interacting protein (KChIP) family, KChIP1, appears specifically expressed in these cells. However, the functional role of this accessory subunit in A-type K+ currents and interneuron excitability remains largely unknown. Thus, first we studied KChIP1 and Kv4.3 channel interactions in human embryonic kidney 293 (HEK293) cells and determined that KChIP1 coexpression modulated the biophysical properties of Kv4.3 A-type currents (faster recovery from inactivation, leftward shift of activation curve, faster rise time and slower decay) and this modulation was selectively prevented by KChIP1 short interfering RNA (siRNA) knockdown. Next, we evaluated the effects of KChIP1 down-regulation by siRNA on A-type K+ currents in LM/RAD interneurons in slice cultures. Recovery from inactivation of A-type K+ currents was slower after KChIP1 down-regulation but other properties were unchanged. In addition, down-regulation of KChIP1 levels did not affect action potential waveform and firing, but increased firing frequency during suprathreshold depolarizations, indicating that KChIP1 regulates interneuron excitability. The effects of KChIP1 down-regulation were cell-specific since CA1 pyramidal cells that do not express KChIP1 were unaffected. Overall, our findings suggest that KChIP1 interacts with Kv4.3 in LM/RAD interneurons, enabling faster recovery from inactivation of A-type currents and thus promoting stronger inhibitory control of firing during sustained activity.

Differential organization of excitatory and inhibitory synapses within the rat dorsal vagal complex

The dorsal motor nucleus of the vagus (DMV) is pivotal in the regulation of upper gastrointestinal functions, including motility and both gastric and pancreatic secretion. DMV neurons receive robust GABA- and glutamatergic inputs. Microinjection of the GABA(A) antagonist bicuculline (BIC) into the DMV increases pancreatic secretion and gastric motility, whereas the glutamatergic antagonist kynurenic acid (KYN) is ineffective unless preceded by microinjection of BIC. We used whole cell patch-clamp recordings with the aim of unveiling the brain stem neurocircuitry that uses tonic GABA- and glutamatergic synapses to control the activity of DMV neurons in a brain stem slice preparation. Perfusion with BIC altered the firing frequency of 71% of DMV neurons, increasing firing frequency in 80% of the responsive neurons and decreasing firing frequency in 20%. Addition of KYN to the perfusate either decreased (52%) or increased (25%) the firing frequency of BIC-sensitive neurons. When KYN was applied first, the firing rate was decreased in 43% and increased in 21% of the neurons; further perfusion with BIC had no additional effect in the majority of neurons. Our results indicate that there are several permutations in the arrangements of GABA- and glutamatergic inputs controlling the activity of DMV neurons. Our data support the concept of brain stem neuronal circuitry that may be wired in a finely tuned organ- or function-specific manner that permits precise and discrete modulation of the vagal motor output to the gastrointestinal tract.

Spatiotemporal Variations of Fire Frequency in Central Boreal Forest

Determination of the direct causal factors controlling wildfires is key to understanding wildfire–vegetation–climate dynamics in a changing climate and for developing sustainable management strategies for biodiversity conservation and maintenance of long-term forest productivity. In this study, we sought to understand how the fire frequency of a large mixedwood forest in the central boreal shield varies as a result of temporal and spatial factors. We reconstructed the fire history of an 11,600-km2 area located in the northwestern boreal forest of Ontario, using archival data of large fires occurring since 1921 and dendrochronological dating for fires prior to 1921. The fire cycle decreased from 295 years for the period of 1820–1920 to approximately 100 years for the period of 1921–2008. Spatially, fire frequency increased with latitude, attributable to higher human activities that have increased fragmentation and fire suppression in the southern portion of the study area. Fire frequency also increased with distance to waterbodies, and was higher on Podzols that were strongly correlated with moderate drainage and coniferous vegetation. The temporal increase of fire frequency in the central region, unlike western and eastern boreal forests where fire frequency has decreased, may be a result of increased warm and dry conditions associated with climate change in central North America, suggesting that the response of wildfire to global climate change may be regionally individualistic. The significant spatial factors we found in this study are in agreement with other wildfire studies, indicating the commonality of the influences by physiographic features and human activities on regional fire regimes across the boreal forest. Overall, wildfire in the central boreal shield is more frequent than that in the wetter eastern boreal region and less frequent than that in the drier western boreal region, confirming a climatic top-down control on the fire activities of the entire North American boreal forest.

Network effects of frequency dependent phase response curves

Phase response curves (PRCs) are experimentally obtainable measures of the effects of small perturbations to a neuron on the timing of its firing. The PRC assumes that the neuron is repetitively firing at a constant frequency and that the perturbation is small relative to the period of the oscillation. Theoretical and simulation results have shown that neurons whose PRCs show a region of phase delay for early stimulus phases and a region of phase advance for late stimulus phases, called Type II PRCs, tend to synchronize more readily when synaptically coupled than neurons whose PRCs show only phase advances at all stimulus phases, called Type I PRCs. It has not been determined if PRC-induced properties of synchronization continue to hold in networks where the neurons are not intrinsically oscillatory, the synaptic coupling is not uniform, and neurons receive multiple inputs during each spike cycle. To investigate the influence of the neuronal PRC on network dynamics, we consider the effects upon synchronization of changes in the amplitude of the PRC that occur with changes in intrinsic firing frequency of neurons in large, noisy, non-uniform networks. We show that as the average network firing frequency increases, networks consisting of neurons with Type I PRCs show an increase in synchronization over large ranges of synaptic connectivity parameters. This is expected, as an increase in average synaptic current leads to greater effective coupling among neurons. In contrast, networks of neurons with Type II PRCs show a decrease in synchronization as frequency increases, implicating the influence of frequency-dependent changes in the PRC upon network synchronization.

Activation of Bombesin Receptor Subtype-3 Influences Activity of Orexin Neurons by Both Direct and Indirect Pathways

The neuropeptides orexin A and orexin B (also known as hypocretin 1 and hypocretin 2), produced in lateral hypothalamic neurons, are critical regulators of feeding behavior, the reward system, and sleep/wake states. Orexin-producing neurons (orexin neurons) are regulated by various factors involved in regulation of energy homeostasis and sleep/wakefulness states. Bombesin receptor subtype 3 (BRS3) is an orphan receptor that might be implicated in energy homeostasis and is highly expressed in the hypothalamus. However, the neural pathway by which BRS3 regulates energy homeostasis is largely unknown. We examined whether BRS3 is involved in the regulation of orexin neurons. Using a calcium imaging method, we found that a selective BRS3 agonist [Ac-Phe-Trp-Ala-His-(tauBzl)-Nip-Gly-Arg-NH2] increased the intracellular calcium concentration of orexin neurons. However, intracellular recordings from slice preparations revealed that the BRS3 agonist hyperpolarized orexin neurons. The BRS3 agonist depolarized orexin neuron in the presence of tetrodotoxin. Moreover, in the presence of GABA receptor blockers, picrotoxin and CGP55845, the BRS3 agonist induced depolarization and increased firing frequency. Additionally, double-label in situ hybridization study revealed that Brs3 mRNA was expressed in almost all orexin neurons and many cells around these neurons. These findings suggest that the BRS3 agonist indirectly inhibited orexin neurons through GABAergic input and directly activated orexin neurons. Inhibition of activity of orexin neurons through BRS3 might be an important pathway for regulation of feeding and sleep/wake states. This pathway might serve as a novel target for the treatment of obesity.

Rebound Discharge in Deep Cerebellar Nuclear Neurons In Vitro

Neurons of the deep cerebellar nuclei (DCN) play a critical role in defining the output of cerebellum in the course of encoding Purkinje cell inhibitory inputs. The earliest work performed with in vitro preparations established that DCN cells have the capacity to translate membrane hyperpolarizations into a rebound increase in firing frequency. The primary means of distinguishing between DCN neurons has been according to cell size and transmitter phenotype, but in some cases, differences in the firing properties of DCN cells maintained in vitro have been reported. In particular, it was shown that large diameter cells in the rat DCN exhibit two phenotypes of rebound discharge in vitro that may eventually help define their functional roles in cerebellar output. A transient burst and weak burst phenotype can be distinguished based on the frequency and pattern of rebound discharge immediately following a hyperpolarizing stimulus. Work to date indicates that the difference in excitability arises from at least the degree of activation of T-type Ca2+ current during the immediate phase of rebound firing and Ca2+-dependent K+ channels that underlie afterhyperpolarizations. Both phenotypes can be detected following stimulation of Purkinje cell inhibitory inputs under conditions that preserve resting membrane potential and natural ionic gradients. In this paper, we review the evidence supporting the existence of different rebound phenotypes in DCN cells and the ion channel expression patterns that underlie their generation.

An 11 000-year-long record of fire and vegetation history at Beaver Lake, Oregon, central Willamette Valley

High-resolution macroscopic charcoal and pollen analysis were used to reconstruct an 11 000-year-long record of fire and vegetation history from Beaver Lake, Oregon, the first complete Holocene paleoecological record from the floor of the Willamette Valley. In the early Holocene (ca 11 000–7500 calendar years before present [cal yr BP]), warmer, drier summers than at present led to the establishment of xeric woodland of Quercus, Corylus, and Pseudotsuga near the site. Disturbances (i.e., floods, fires) were common at this time and as a result Alnus rubra grew nearby. High fire frequency occurred in the early Holocene from ca 11 200–9300 cal yr BP. Riparian forest and wet prairie developed in the middle Holocene (ca 7500 cal yr BP), likely the result of a decrease in the frequency of flooding and a shift to effectively cooler, wetter conditions than before. The vegetation at Beaver Lake remained generally unchanged into the late Holocene (from 4000 cal yr BP to present), with the exception of land clearance associated with Euro-American settlement of the valley (ca 160 cal yr BP). Middle-to-late Holocene increases in fire frequency, coupled with abrupt shifts in fire-episode magnitude and charcoal composition, likely indicate the influence anthropogenic burning near the site. The paleoecological record from Beaver Lake, and in particular the general increase in fire frequency over the last 8500 years, differs significantly from other low-elevation sites in the Pacific Northwest, which suggests that local controls (e.g., shifts in vegetation structure, intensification of human land-use), rather than regional climatic controls, more strongly influenced its environmental history.

Late 20th century mangrove encroachment in the coastal Australian monsoon tropics parallels the regional increase in woody biomass

In Kakadu National Park, a World Heritage property in the Australian monsoon tropics 250 km to the east of Darwin, a number of recent studies have shown that woody encroachment (expansion of woody communities) and densification (increased biomass in woody communities) has occurred in the last 40 years. The cause of this increase in woody biomass is poorly understood, but possibly associated with the control of invasive Asian water buffalo, trend to higher rainfall, and increased frequency of fires. Mangroves provide an important context to understand these landscape changes, given that they are unaffected by fire or feral water buffalo. We examine change in mangrove distribution in a series of coastal tropical swamps fringing Darwin, Northern Territory, Australia over a 30-year period using a series of 7 aerial photographs spanning 23 years from 1974 and a 2004 high-resolution satellite image. In late 1974, Darwin was impacted by an intense tropical cyclone. Vegetation at 3,000 randomly placed points was manually classified, and a multinomial logistic model was used to asses the impact of landscape position (coastal, intertidal, and upper-tidal) and swamp on mangrove change between 1974 and 2004. Over the study period, there was instability and slight mangrove loss at the coast, stability in the intertidal zone, and mangrove gain in the upper-tidal zone, with an overall increase in mangrove presence of 16.2% above the pre-cyclone distribution. A swamp that was impacted by drainage works for mosquito control and the construction of a sewage treatment plant showed a greater mangrove increase than the two unmodified swamps. The mangrove expansion is consistent with woody encroachment observed in nearby but ecologically distinct systems. Plausible causes for this change include changed local hydrology, changes in sea level, and elevated atmospheric CO2 concentrations.

Conductance mesophyllienne pour le CO2 et caractéristiques morphologiques des feuilles sous stress hydrique pendant la repousse de Quercus ilex L.

Quercus ilex L., the dominant species in Mediterranean forests and one with a great capacity for resprouting after disturbances, is threatened by the expected increase in fire frequency and drought associated with climate change. • The aim of this study was to determine the contribution of photosynthesis limitants, especially mes- ophyll conductance (gmes) during this species' resprouting and under summer drought. • Resprouts showed 5.3-fold increased gmes and 3.8-fold increased stomatal conductance (gs )a t mid- day with respect to leaves of undisturbed individuals. With increased drought, structural changes (decreased density and increased thickness) in resprouts contributed to the observed higher photosyn- thesis and increased gmes .H owever,gmes only partially depended on leaf structure, and was also under physiological control. Resprouts also showed lower non-stomatal limitations (around 50% higher car- boxylation velocity (Vc,max) and capacity for ribulose-1,5-bisphosphate regeneration (Jmax)). A sig- nificant contribution of gmes to leaf carbon isotope discrimination values was observed. • gmes exhibits a dominant role in photosynthesis limitation in Q. ilex and is regulated by factors other than morphology. During resprouting after disturbances, greater capacity to withstand drought, as evidenced by higher gmes, gs and lower non-stomatal limitants, enables increased photosynthesis and rapid growth.

Long-term decrease of organic and inorganic nitrogen concentrations due to pine forest wildfire

• Growing concerns about fires and the increase of fire frequency and severity due to climate change have stimulated a large number of scientific papers about fire ecology. Most researchers have focused on the short-term effects of fire, and the knowledge about the long-term consequences of fires on ecosystem nutrient dynamics is still scarce. • Our aim was to improve the existing knowledge about the long-term effects of wildfires on forestlabile N concentrations. We hypothesized that fires may cause an initial decline in organic and inorganic N availability, and in the amount of microbial biomass-N; this should be followed by the recovery of pre-fire N concentrations on a long-term basis. We selected a fire chronosequence in Pinus canariensis forests on La Palma Island (Canary Islands, Spain). These forests are under low anthropogenic atmospheric deposition, and forest management is completely lacking; wildfires are therefore the only significant disturbance. Soil samples were collected during the winter and spring at 22 burned and unburned plots. • Fire produced a significant decrease in microbial biomass N, mineral N and dissolved organic N. Almost 20 y after fire, pre-fire levels of N concentrations had not recovered. • These results demonstrate that P. canariensis forest soils have a lower resilience against fire than expected. The magnitude of these observed changes suggests that pine forest wildfires may induce long-term (2 decades) changes in soil and in plant primary production. Mots-cles : azote organique dissous / azote labile / biomasse microbienne / azote mineral / Pinus canariensis / incendie