Structure and function in two tropical gallery forest communities: implications for forest conservation in fragmented systems

1. Composition, growth and turnover of trees in two species-rich tropical gallery forests were examined to evaluate what community reorganization may be needed to transform recently created tropical forest fragments into stable refugia for regional forest biotas.
2. Rates of tree growth and turnover over a 5-year interval were comparable to those recorded in continuous forests and in both communities there had been some tree species turnover in the measured stem size classes during the 5-year interval.
3. The more abundant tree species in both communities formed three functional groups along gradients between streams and forest edges: edge-concentrators, core-concentrators and generalists.
4. Soil fertility showed no consistent increase close to streams and neither tree growth nor recruitment rates were increased in this zone. In contrast, forest edge zones exhibited increased rates of tree growth and recruitment indicating that growth processes in these forests are light-limited rather than soil-limited, and that forest edge zones are generally favourable habitats for tree populations.
5. Both communities showed signs of past fire incursions, and the tendency of a subset of tree species to concentrate in the more growth-limited core habitats is attributed to their fire sensitivity.
6. Rapid development of an edge zone of fire-insensitive tree species is considered to be essential to the survival of forest community fragments in the fire-prone landscapes of the tropics, and the edges of gallery forests are recommended as potential sources of species with which to fashion these protective ecotones.
7. Preservation of a diverse forest biota in the fire-protected interiors of fragments will require natural or artificially enhanced immigration rates that are sufficient to offset local extinctions.     

Spatial hints of forest ecotone indicating forest succession, a case of larch forests in Baihuashan Reserve, north China

In cold or alpine areas of northern China, birch forests and larch forests are the two primary forest types. These forests are also characteristic of a south branch of boreal forests in Asia. Some ecologists argue that larch forests can replace birch forests, but this still remains a question due to fragmentary or short observations. The ecotone between a larch forest patch and a birch forest patch is the arena in which the two species interplay and compete with each other, and studies of these areas are meaningful to understanding forest succession. In the alpine area of the Baihuashan Reserve, northern China, we sampled a larch-birch forest ecotone with eight plots in four transects and then analyzed population structures of larches and birches. The results show that the edges of the larch forest patch are composed of many larch saplings or young trees, but the edges of the birch forest patch are mainly composed of old birches. Across the ecotone, the larches, on average, are taller than the birches. These facts suggest that larch saplings can permeate into birch forest patches, probably by seed dispersal, germination, success-ful competition and growth, but birch saplings cannot permeate into larch forest patches. Therefore, on the ecotone, larch forest patches can steadily expand by unceasing permeation into birch forest patches, whereas birch forest patches progressively recede due to ultimate death of the old and poor recruitment of the young. Larch forest patches replace birch forest patches in a stepwise manner, causing succession from birch forests to larch forests. This study not only confirms that larch forests can naturally replace birch forests, but also introduces a simple and reliable method, employing spatial hints, to study forest succession. Additionally, the findings are of benefit to cultivation or development of larch forests in cold or alpine areas of the North Temperate Zone, which can be a huge carbon sink.     

Aboriginal fire regimes in Queensland, Australia: analysis of the explorers' record

ABSTRACT. The record of eighteenth and nineteenth century explorers' references to Aboriginal fire in Queensland was stratified according to fourteen vegetation typcs and season of fire. It was demonstrated that references to 'current' fire (i.e. flames or smoke) may not represent traditional Aboriginal activity and that many fires were lit to frighten or harm, to protect themselves from, or to signal to kinfolk the presence of the European intruders. Because of this interpretational difficulty the records to 'current' fire were treated separately from 'past' fire (i.e. burnt ground). The data were analysed as the number of observations per 100 km spent in each vegetation type for any one season to compensate for bias created by differing amounts of travel. The record suggests highest frequency of burning in grassland around the Gulf of Carpentaria, relatively high fire frequency of most coastal and subcoastal vegetation types and relatively infrequent burning of inland Queensland. The analysis indicates a propensity for winter and autumn fue relative to spring and summer fire in all vegetation types combined and in most individual vegetation types.     

Simulating forest succession along ecological gradients in southern Central Europe

Forest stand development was simulated using a forest succession model of the JABOWA/FORET type. The environmental conditions are representative for a wide spectrum of Swiss forest sites ranging from 220 m to 1 700 m a.s.l. Each model run covers a period of 1 200 yr and is based on the averaged successional characteristics of 50 forest plots with an individual size of 1/12 ha. These small forest plots serve as basic units to simulate establishment, growth, and death of individual trees of 29 species. Existing light in the forest stand, climatic conditions, soil properties, and other environmental factors control the growth of each individual tree. Compared with previous simulation studies, some major modifications were made, including the incorporation of the indicator values of Ellenberg (1979) to describe the ecophysiological behaviour of the species considered. As a test, the simulated species composition through time was compared with the actual vegetation and the potentially natural species composition on the corresponding site types. The extensive comparison revealed that approximately 80% of the simulations match the expected species configurations. Thus, it was concluded that the model is valid for the purpose of evaluating impacts of natural and human disturbances on forest communities.     

Fire weather in the wet-dry tropics of the World Heritage Kakadu National Park,Australia

Abstract Seasonal changes of weather and fuels in the wet-dry tropics are dramatic; fires follow suit. In this paper, we examine quantitatively rainfall, evaporation, wind, temperature and humidity information, and indices derived from them, for Kapalga Research Station and nearby Jabiru in World Heritage Kakadu National Park, Northern Australia. At Kapalga, the average annual rainfall of about 1200mm mostly falls during a 6 month wet season. Grasses, green in the wet, begin to desiccate during the early dry season. Perennial grasses cure more slowly than the annuals, and grasses in drainages cure later than those on ridges. Fire weather is usually most severe in September-October (late dry season) and least severe in January-February (late wet season). As the dry season progresses to its peak, daily wind patterns change, daily maximum temperatures increase to an average of 36°C, dew points drop to a minimum, and soil moisture is severely depleted. In the early dry season (cf. later), fires have a greater tendency to go out at night compared with later perhaps because winds then are calmer, fuels are more discontinuous, and relights from burning logs are less likely to occur. Fire weather in the north of Australia appears less severe than that in the southeast of the continent where socially disastrous fires occur periodically.     

The effect of fire on nutrients in a pine forest soil

The effect of a hot summer fire on soil nutrient contents in the upper 2 cm of Aleppo pine forest with a dense woody understory was studied from September 1985 to May 1986. In comparison with the adjacent unburned forest, total nitrogen decreased by 25% but available forms of nitrogen were much higher. In burned and unburned soils there was a similar trend to increase and decrease in NH ₄ ⁺ −N, However, while (NO ₂ ⁻ +NO ₃ ⁻ −N decreased in the unburned soil it rose rapidly in the burned ash soil. Total phosphorus increased by 300% after the fire but decreased again 2 months later. Also water-soluble P increased up to November and then decreased to the levels of the unburned soils. The same was true for electrical conductivity and pH, increasing immediately after the fire and then leveling off again. This increase in nutrient levels in the “ash soil” was reflected in the striking increase in shoot and root biomass and in the content of N, P, Mg, K, Ca, Zn and Fe in wheat and clover plants grown in pots in these soils. These nutrient levels were much higher in the wheat plants, which also produced 12 times more seeds in the “ash soil.” It seems that fire in these pine forests causes a short-term flush of the mineral elements in the upper “ash soil” layer which is reverted gradually via the herbaceous post-fire to the ecosystem.     

Early fire and forest change in the Baliem Valley, Irian Jaya, Indonesia

Abstract. A 3.5 m section of organic sediment was obtained from a karstic pond on a hill in the centre of the Baliem Valley, one of the major settled intermontane highland areas of New Guinea. The material spans two time periods each of approximately 2 millennia, one from about 2000 BP to the present and the other from 33,500 to 31,500 BP. The pollen analysis of the earlier section showed that it formed when the valley was forested by Nothofagus forest, but a carbonized particle input was consistently present after about 32,500 years ago. The recent section covers a period when the hill was totally cleared except for grassland and some open shrubby regrowth. The early burning and associated clearances are tentatively ascribed to a human origin. Fire is associated with slope erosion on the hill at 28,000 BP which supports the hypothesis of long term human settlement in the area.     

Successional change of forest pattern along topographical gradients in warm-temperate mixed forests in Mt Kiyosumi,central Japan

Forest patterns along topographical gradients were compared between second- and old-growth forested watersheds in a warm-temperate zone of Mt Kiyosumi, central Japan. Three community types were distinguished depending on the topographical habitat type in each watershed, for example, conifer forest was dominated byAbies firma andTsuga sieboldii on ridge sites, evergreen broad-leaved forest was dominated byQuercus acuta, Q. salicina andCastanopsis cuspidata var.sieboldii on slope sites, and deciduous forest was dominated byEuptelea polyandra andCornus controversa in valley sites. Beta diversity and distinctiveness of each topographical community type increased with progression of secondary succession. Conifers and evergreen broad-leaved trees, which were intermingled with each other on ridges and slopes of the second-growth watershed, were in turn restricted to the ridge and slope habitat type, respectively, in the old-growth watershed. The process of this differentiation can be explained by the continuous regeneration of conifers on ridge sites, and its absence on slope sites due to different light conditions caused by progressive canopy closure of evergreen trees on the slope sites toward the old-growth watershed. In the valley type habitat, frequent soil disturbance, such as landslides and soil creep, hinder the continuous growth of late successional evergreen trees, and thus seral or pioneer deciduous trees can persist in the habitat.     

The role of seedling age and size in the recovery of Allosyncarpia ternata following fire

Abstract This paper examines the effects of seedling size and age on fire tolerance of Allosyncarpia ternata (Myrtaceae), a dominant tree in patches of monsoon rainforest of the wet-dry tropics in the Northern Territory, Australia. We address the following questions: how large does a seedling have to be to tolerate fire; how old does it have to be to reach this fire-tolerant size; and how can land-management authorities best manage fire regimes to maintain Allosyncarpia forest? In a field experiment, shadehouse-grown seedlings aged from 8 months to 5 years were subjected to low- and high-intensity fires in September 1994. Among 5-year-old seedlings, mortality was independent of fire intensity. However, mortality of young (8-month-old) seedlings was significantly higher in the high-intensity fire. Three-year-old seedlings behaved in an intermediate manner; their survivorship and growth were marginally favoured by low-intensity fire, rather than high-intensity fire or no fire at all, and were dependent on pre-treatment seedling height. Thus, the critical age that distinguishes fire-tolerant from fire-sensitive seedlings is somewhat more than 3 years for relatively short seedlings and somewhat less than 3 years for taller seedlings. In August 1993, a wildfire penetrated several hundred metres into Allosyncarpia forest growing on a steep, rocky escarpment, where it caused severe damage to A. ternata seedlings. More than three-quarters of the ≥ 3.5-year-old seedlings (including some that had suffered the total loss of above-ground parts) recovered during the following wet season and showed higher growth rates than their unburned neighbours. New growth was also promoted in those tall seedlings and saplings that had sustained only partial leaf scorch. In contrast, all 18-month-old seedlings were killed by the fire. Measurements of leaf-scorch height in burned Allosyncarpia forest on the escarpment indicated a general uphill decrease in fire intensity, matching trends in increasing site rockiness and decreasing fuel density. An important implication for land management is that a fire-free interval of at least 3 years following a seed-fall event is required for a new generation of A. ternata germinants to progress into the cohort of established seedlings.     

Reproductive Phenology of Woody Species in a North Australian Tropical Savanna1

Interspecific and interannual variation in reproductive phenology was quantified for 50 common species of trees and shrubs from a mesic savanna near Darwin, northern Australia. The presence of buds, flowers, and fruit was noted over a 30-month period, from September 1992 to February 1995. Surveys were undertaken at monthly intervals for the less common species, and at bimonthly intervals for ten of the common trees and tall shrubs. The majority of species flowered each year at about the same time. There was no evidence of sub-annual or continuous regimes of reproductive phenology. There was no supra-annual carryover of seed-bearing fruit in the canopy of any species. The peak flowering periods were the mid to late dry season (July–August) and the transition between the dry season and the wet season (October–November). The two dominant trees–Eucalyptus miniata and E, tetrodonta– flowered during the dry season, thereby providing resources for some elements of the vertebrate fauna. Flowering and fruiting were uncommon at the end of the wet season (February/March), although two species that flower and fruit at this time (E. porrecta and Terminalia ferdinandianas may provide resources to consumers at a time when floral or fruit resources are otherwise scarce. Because the peak of reproductive activity takes place during the late dry season, fruit maturity and seed dispersal have occurred prior to the onset of the rainy season for most species, and germination and seedling establishment potentially may take effect in response to the first rains. Late dry season fires, which tend to be extensive and intense, are a potential threat to the floral and fruit reserves within these savannas.     

Migration of herbaceous plant species across ancient–recent forest ecotones in central Belgium

1 We studied the migration of forest plant species using their percentage cover and frequency in 197 plots distributed over 26 transects across ecotones between ancient and recent deciduous forests in the Meerdaal forest complex in central Belgium. The recent forest stands varied in age between 36 and 132 years, and all occurred on silty, well-drained soils.
2 The total cover, number and diversity of field layer species did not differ significantly between ancient and recent forest stands.
3 The number and cover of the ancient forest plant species and of ant-dispersed species correlated positively with the age of the recent forest and negatively with both the duration of its former agricultural land use and the distance to the ancient forest. This implies a slow colonization of the recent forest stands by these species; all species were, however, able to migrate across the ecotones.
4 The cover of four species ( Anemone nemorosa , Lamium galeobdolon , Convallaria majalis and Polygonatum multiflorum ) declined along the transect, suggesting that they are limited by seed dispersal. Their colonization rates, calculated from the occurrence of the farthest individual, ranged from < 0.05 to 1.15 m year⁻¹ and for other measures from < 0.05 to 0.65 m year⁻¹. Anemone and Lamium appeared to colonize the recent forest by establishment of isolated individuals, while Polygonatum and Convallaria expanded populations from existing patches on the border between ancient and recent forest.
5 Several forest species were able to colonize the recent forest rapidly, where some of them even reached a higher abundance, due to the increased availability of colonization sites with a higher nutrient content and a thinner organic layer.     

Environmental correlates of vegetation patterns and species richness in the northern Grampians,Victoria

Abstract Plant species cover-abundance and density data were collected for 94 sample plots across a gradient from rocky uplands to sandy outwash plains in the northern part of Grampians (Gariwerd) National Park in western Victoria. Detrended correspondence analysis (DCA) was used to identify dominant gradients in species composition. A range of static (e.g. substrate type, soil depth, microclimate indicators) and dynamic (e.g. elapsed time since last fire) environmental variables were measured. Correlations were sought between these variables and vegetation patterns including those for richness (R) and Shannon-Weiner diversity (H′). The dominant gradient of vegetation change identified by DCA separated rocky sites and sites near ephemeral streams, from well-drained, sandy sites. Secondary gradients identified time since last fire as important for sandy sites, and altitude and aspect-related microclimate for rocky sites. Diversity was highest in the first 2 years after fire but showed no further decline in older sites. Overall, R and H' were negatively correlated with soil nutrient concentrations. On sandy sites R was high, but was low on rocky sites and near streams. Within the rocky sites, R was highest on cool, moist south and east slopes, and lowest on hot, dry north and west slopes. Explanations of diversity patterns based on inhibition of competitive exclusion due to stress and recurrent disturbance best fit the results presented here.     

On variation in forest floor thickness across four red pine plantations in Pennsylvania,USA

We studied variation in forest floor thickness in four plantations of red pine (Pinus resinosa Ait.) which were similar in age, soil type and associated vegetation. The plantations were located (west to east) in the Clear Creek, Moshannon, Sproul and Tiadaghton State Forests of Pennsylvania, USA. A gradient in forest floor thickness exists across these plantations; the forest floor is thickest in the west and it becomes progressively thinner toward the east. Decomposition of imported litter increased from west to east, suggesting that the variation in forest floor thickness is related to variation in the rate of decomposition. Decomposition rates were related to saprotroph abundance. Variation in forest floor N and phenolic concentrations, in overall mycorrhiza density and in the relative proportions of three common mycorrhiza morphotypes could not explain the variation in decomposition rate. The P concentrations and pH of the forest floor were significantly lower at Clear Creek and Moshannon, where decomposition rates were lowest, compared to Sproul and Tiadaghton, where decomposition rates were most rapid. This suggests that P concentration and pH may have exerted some control on decomposition.     

Effects of forest clearing and succession on the carbon and nitrogen content of soils in Puerto Rico and US Virgin Islands

Soil samples from mature and secondary forests and agricultural sites in three subtropical life zones of Puerto Rico and the US Virgin Islands were collected to determine the effects of forest conversion to agriculture and succession on soil organic carbon (C) and nitrogen (N) contents. Site characteristics that may affect soil C and N (slope, elevation, aspect, and texture) were as uniform as possible. Carbon contents (to 50 cm depth or bedrock) of cultivated sites, as a percent of corresponding mature forests, were lower in the wet (44%) and moist (31%) than in the dry (86%) life zones whereas N contents were relatively high regardless of life zone (60–130% of the mature forests). Conversion of forests to pasture resulted in less soil C and N loss than conversion to crops. The time for recovery of soil C and N during succession was approximately the same in all three life zones, about 40–50 yr for C about 15–20 yr for N. However, the rate of recovery of soil C was faster in the wet and moist life zone, whereas N appeared to recover faster in the dry life zone. Evidence for loss of soil C during cultivation and gain during succession to soil depths of 50–100 cm is presented.     

Comparison of regeneration following burning,clearing or mineral sand mining at Tomago,NSW: II. Succession of ant assemblages in a coastal forest

Abstract The dry sclerophyll forest community of the Tomago Sandbeds, near Newcastle in New South Wales, has been subject to regular disturbances due to fire, clearing and strip mining for over 18 years. In this study we use chronosequence analysis to examine whether the structure of the ant community varies with the type of disturbance and the time since disturbance. We treat the recovery trajectory after fire as a control trajectory because fire is an endogenous disturbance. The main analyses were based on an ant fauna comprising 72 species sampled from 44 sites surveyed in December 1992. Comparison with samples taken in April and December 1991, and for cumulative records for all sites over this 20 month period, all show quantitatively similar responses. Results suggest that while fire has a minor effect on the composition of the ant community over time, the impact of clearing and mining is much more severe. Ant species richness at cleared and mined sites recovers rapidly, overshoots controls in mid-succession and returns to control levels by 18 years after disturbance. The cumulative number of species recorded over all sites (from the total recorded fauna of 82 species) for each different disturbance type were: burned, 61; cleared, 55; and mined 56. Species composition at cleared or mined sites, after 18 years, approaches but does not match controls. The recovery trend for mined sites lags slightly behind that for cleared sites, which have reached 49% similarity with the oldest burned sites, while mined sites have not exceeded 39% similarity of species composition. The main patterns in the ant community appear to be related to habitat variables. These results provide further evidence that the ant community may be used as a reliable bio-indicator for evaluating the extent of habitat damage and recovery after disturbance in these Australian forests.     

Biomass and carbon accumulation in a fire chronosequence of a seasonally dry tropical forest

Seasonally dry tropical forests (SDTF) are a widely distributed vegetation type in the tropics, characterized by seasonal rainfall with several months of drought when they are subject to fire. This study is one of the first attempts to quantify above- and belowground biomass (AGB and BGB) and above- and belowground carbon (AGC and BGC) pools to calculate their recovery after fire, using a chronosequence approach (six forests that ranged form 1 to 29 years after fire and mature forest). We quantified AGB and AGC pools of trees, lianas, palms, and seedlings, and BGB and BGC pools (Oi, Oe, Oa soil horizons, and fine roots). Total AGC ranged from 0.05 to nearly 72 Mg C ha⁻¹, BGC from 21.6 to nearly 85 Mg C ha⁻¹, and total ecosystem carbon from 21.7 to 153.5 Mg C ha⁻¹; all these pools increased with forest age. Nearly 50% of the total ecosystem carbon was stored in the Oa horizon of mature forests, and up to 90% was stored in the Oa-horizon of early successional SDTF stands. The soils were shallow with a depth of <20 cm at the study site. To recover values similar to mature forests, BGC and BGB required <19 years with accumulation rates greater than 20 Mg C ha⁻¹ yr⁻¹, while AGB required 80 years with accumulation rates nearly 2.5 Mg C ha⁻¹ yr⁻¹. Total ecosystem biomass and carbon required 70 and 50 years, respectively, to recover values similar to mature forests. When belowground pools are not included in the calculation of total ecosystem biomass or carbon recovery, we estimated an overestimation of 10 and 30 years, respectively.     

Fine roots, arbuscular mycorrhizal hyphae and soil nutrients in four neotropical rain forests: patterns across large geographic distances

* It is commonly hypothesized that stand-level fine root biomass increases as soil fertility decreases both within and among tropical forests, but few data exist to test this prediction across broad geographic scales. This study investigated the relationships among fine roots, arbuscular mycorrhizal (AM) fungi and soil nutrients in four lowland, neotropical rainforests. * Within each forest, samples were collected from plots that differed in fertility and above-ground biomass, and fine roots, AM hyphae and total soil nutrients were measured. * Among sites, total fine root mass varied by a factor of three, from 237+/-19 g m-2 in Costa Rica to 800+/-116 g m-2 in Brazil (0-40 cm depth). Both root mass and length were negatively correlated to soil nitrogen and phosphorus, but AM hyphae were not related to nutrients, root properties or above-ground biomass. * These results suggest that understanding how soil fertility affects fine roots is an additional factor that may improve the representation of root functions in global biogeochemical models or biome-wide averages of root properties in tropical forests.     

Mitochondrial DNA and allozymes reveal high dispersal abilities and historical movement across drainage boundaries in two species of freshwater fishes from inland rivers in Queensland, Australia

This study used allozymes and mtDNA variation to test that: 1) populations of two fish species, Nematolosa erebi and Retropinna semoni , in lowland rivers in central Australia were highly connected within drainages, 2) populations in different drainages were highly differentiated and 3) there was evidence of historical connections between two major lowland drainages in inland Australia. Levels of genetic differentiation among populations within drainages were low, but still statistically significant, indicating that populations were not as highly connected as had been predicted. Populations from the Murray–Darling and the Lake Eyre drainages were highly differentiated, indicating no contemporary dispersal across drainage boundaries. Both species showed evidence of historical connections between the two drainage basins, although estimates of the time that these last occurred differed between the two species. Nematolosa erebi populations from the two drainages were estimated to have been separated c . 150 000 years ago, whereas populations of R. semoni , were estimated to have been separated c . 1.5 million years ago.