Frequent fires reduce tree growth in northern Australian savannas: implications for tree demography and carbon sequestration

Tropical savannas are typically highly productive yet fire‐prone ecosystems, and it has been suggested that reducing fire frequency in savannas could substantially increase the size of the global carbon sink. However, the long‐term demographic consequences of modifying fire regimes in savannas are difficult to predict, with the effects of fire on many parameters, such as tree growth rates, poorly understood. Over 10 years, we examined the effects of fire frequency on the growth rates (annual increment of diameter at breast height) of 3075 tagged trees, at 137 locations throughout the mesic savannas of Kakadu, Nitmiluk and Litchfield National Parks, in northern Australia. Frequent fires substantially reduced tree growth rates, with the magnitude of the effect markedly increasing with fire severity. The highest observed frequencies of mild, moderate and severe fires (1.0, 0.8 and 0.4 fires yr^?1, respectively) reduced tree growth by 24%, 40% and 66% respectively, relative to unburnt areas. These reductions in tree growth imply reductions in the net primary productivity of trees by between 0.19 t C ha^?1 yr^?1, in the case of mild fires, and 0.51 t C ha^?1 yr^?1, in the case of severe fires. Such reductions are relatively large, given that net biome productivity (carbon sequestration potential) of these savannas is estimated to be just 1–2 t C ha^?1 yr^?1. Our results suggest that current models of savanna tree demography, that do not account for a relationship between severe fire frequency and tree growth rate, are likely to underestimate the long‐term negative effects of frequent severe fires on tree populations. Additionally, the negative impact of frequent severe fires on carbon sequestration rates may have been underestimated; reducing fire frequencies in savannas may increase carbon sequestration to a greater extent than previously thought.     

Isolation of polymorphic tetranucleotide microsatellite markers for the large‐billed scrubwren (Sericornis magnirostris)

We isolated 12 polymorphic microsatellite markers for the large‐billed scrubwren Sericornis magnirostris from genomic libraries enriched for (AAGG)_n and (AACC)_n repetitive elements and characterized them in 11 individuals. The number of alleles ranged from four to 15 per locus with the observed heterozygosity ranging from 0.14 to 0.91. These markers will be useful to address questions concerning population genetic structure and models of speciation.     

Fire weather risk differs across rain forest—savanna boundaries in the humid tropics of north‐eastern Australia

Alternative stable state theory has been applied to understanding the control by landscape fire activity of pyrophobic tropical rain forest and pyrophytic eucalypt savanna boundaries, which are often separated by tall eucalypt forests. We evaluate the microclimate of three vegetation types across an elevational gradient and their relative fire risk as measured by McArthur's Forest Fire Danger Index (FFDI). Microclimatic data were collected from rain forest, tall eucalypt forest and savanna sites on eight vegetation boundaries throughout the humid tropics in north Queensland over a 3‐year period and were compared with data from a nearby meteorological station. There was a clear annual pattern in daily FFDI with highest values in the austral winter dry season and lowest values in the austral summer wet season. There was a strong association of the meteorological station FFDI values with those from the three vegetation types, albeit they were substantially lower. The rank order of FFDI values among the vegetation types decreased from savanna, tall eucalypt forest, then rain forest, a pattern that was consistent across each transect. Only very rarely would rain forest be flammable, despite being adjacent to highly flammable savannas. These results demonstrate the very strong effect of vegetation type on microclimate and fire risk, compared with the weak effect of elevation, consistent with a fire–vegetation feedback. This study is the first demonstration of how vegetation type influences microclimate and fire risk across a topographically complex tropical forest–savanna gradient.     

Dehiscence of Fruit in Oilseed Rape (Brassica nap us L.): II. THE ROLE OF CELL WALL DEGRADING ENZYMES AND ETHYLENE

Pod shatter in oilseed rape is accompanied by the degradationof the cell wall at the site of fruit dehiscence. Cell separationis preceded by an increase in the activity of the hydrolyticenzyme cellulase (ß 1,4-glucanase, E.C. 3.1.2.4 [EC] ),and this rise in enzyme activity is restricted to the dehiscencezone cells. In contrast, the activity of the cell wall degradingenzyme polygalacturonase (E.C. 3.2.1.15 [EC] ) exhibits no correlationeither temporally or spatially with pod dehiscence. An analysisof the ethylene production profile by intact pods during maturationhas revealed the existence of an ethylene climacteric and thisis temporally correlated with the tissue-specific increase incellulase activity. The major site of ethylene production bythe fruit has been identified to be the developing seed. Sincemaintenance of intact pods in ethylene accelerates both thesenescence and dehiscence of the tissue, it is possible thatthis gaseous regulator plays an important role in the processof pod shatter in vivo. Key words: Oilseed rape, Brassica napus, pod shatter, cellulase, polygalacturonase, ethylene     

Responsiveness of Pseudaletia unipuncta males to the female sex pheromone

ABSTRACT. In wind tunnel experiments male response to different concentrations of synthetic pheromone varied with age. At 25°C responsiveness increased up to day 5 and then declined slightly on day 7. For any given age the level of response generally increased with an increase in pheromone concentration. Males held at 15°C (since emergence) showed a continual increase in responsiveness with age; however, overall response levels were less than at 25°C. At 25°C male response to low concentrations of pheromone (10 and 30 μg) varied markedly over a 24 h period but no differences were observed at higher concentrations (100 and 300 μg).     

Anatomical and Biochemical Changes Associated with the Induction of Oilseed Rape (Brassica napus) Pod Dehiscence by Dasineura brassicae (Winn.)

Dehiscence of pods at an early stage of development is a characteristicof oilseed rape pods damaged by Dasineura brassicae (pod midge).Anatomical examination of pods exhibiting symptoms of infestationrevealed a loss of cohesion between intact cells of the dehiscencezone, a narrow tissue of thin-walled cells present between thevalve margins. Determination of hydrolytic enzyme activity inpericarp tissues of damaged siliquae showed localized enhancementof both polygalacturonase (EC 3.2.1.15 [EC] ) and cellulase (ß1,4 glucanase, EC 3.1.2.4 [EC] ) activity, positionally consistent withthese factors being responsible for the observed cell wall degradation.Mechanistic similarities of midge-induced and maturation-associatedpod dehiscence are discussed. Oilseed rape, Brassica napus L. cv., Bienvenu, pod shatter, pod midge, Dasineura brassicae (Winn.), cellulase, polygalacturonase     

Puddling in butterflies: sodium affects reproductive success in Thymelicus lineola*

ABSTRACT. Adults of many species of Lepidoptera, principally the males, frequent mud puddles, edges of streams, carrion and animal excreta where they imbibe moisture, an activity referred to as 'puddling' Sodium ions are the only known stimulus present which cause males of at least two lepidopteran species to drink for extended periods. In the European skipper Thymelicus lineola (Ochsenheimer) (Lepidoptera: Hesperiidae), only males puddle, even though they have concentrations of abdominal sodium 2–3 times that of females at emergence. During their first mating, males transfer 32% of their abdominal sodium to females. This could be of considerable importance given that an average egg complement contains >50% of the total body sodium of females at emergence. Virgin females, as well as having reduced fecundity, have reduced longevity. This is attributed to virgins not obtaining important nutrients which males transfer to females during mating. Access to sodium ions increases the total number of matings by c. 50% for males living >15 days. Access to sodium ions by once-mated males increases the percentage of males which re-mate on the day following first mating; the percentage of females, mated to the twice-mated males, which lay >50% fertile eggs; and the drought resistance of eggs laid by those females.     

Frequency-dependent selection on aposematic prey: some experiments

Experiments to measure the strength of frequency-dependent selection on dimorphic populations of distasteful, brightly coloured prey were carried out, using pastry cylinders as prey and wild birds as predators. When the two forms were haphazardly intermingled, the birds took a relative excess of whichever form happened to be the rarer if all prey in the population were distasteful but selected the forms independently of frequency if they were all neutral in flavour. They did not take an excess of the rarer form from populations of distasteful prey when that form was distributed in a single clump rather than evenly dispersed among the commoner form. This confirms the expectation that rarer forms of aposematic animals are at a frequency-dependent disadvantage when they are intermingled with commoner forms but not when they occur in clusters.