Arbuscular-Mycorrhizal Networks Inhibit Eucalyptus tetrodonta Seedlings in Rain Forest Soil Microcosms

Eucalyptus tetrodonta, a co-dominant tree species of tropical, northern Australian savannas, does not invade adjacent monsoon rain forest unless the forest is burnt intensely. Such facilitation by fire of seedling establishment is known as the "ashbed effect." Because the ashbed effect might involve disruption of common mycorrhizal networks, we hypothesized that in the absence of fire, intact rain forest arbuscular mycorrhizal (AM) networks inhibit E. tetrodonta seedlings. Although arbuscular mycorrhizas predominate in the rain forest, common tree species of the northern Australian savannas (including adult E. tetrodonta) host ectomycorrhizas. To test our hypothesis, we grew E. tetrodonta and Ceiba pentandra (an AM-responsive species used to confirm treatments) separately in microcosms of ambient or methyl-bromide fumigated rain forest soil with or without severing potential mycorrhizal fungus connections to an AM nurse plant, Litsea glutinosa. As expected, C. pentandra formed mycorrhizas in all treatments but had the most root colonization and grew fastest in ambient soil. E. tetrodonta seedlings also formed AM in all treatments, but severing hyphae in fumigated soil produced the least colonization and the best growth. Three of ten E. tetrodonta seedlings in ambient soil with intact network hyphae died. Because foliar chlorosis was symptomatic of iron deficiency, after 130 days we began to fertilize half the E. tetrodonta seedlings in ambient soil with an iron solution. Iron fertilization completely remedied chlorosis and stimulated leaf growth. Our microcosm results suggest that in intact rain forest, common AM networks mediate belowground competition and AM fungi may exacerbate iron deficiency, thereby enhancing resistance to E. tetrodonta invasion. Common AM networks–previously unrecognized as contributors to the ashbed effect–probably help to maintain the rain forest–savanna boundary.     

Sandstone vegetation pattern in the Jim Jim Falls region,Northern Territory,Australia

Analysis of 58 floristic quadrats from two areas near Jim Jim Falls on the Arnhem Land Plateau revealed seven plant communities: wetland; monsoon forest; Allosyncarpia forest; broadleaf woodland; shrubby Eucalyptus woodland; Eucalyptus arnhemensis woodland; and sandstone scrub. The communities inter-grade with each other and form a complex mosaic. The patterning of vegetation is related to topography, rockiness, degree of fire protection and moisture supply. Surface soil nutrient concentrations were similar in all the communities with the exception of the monsoon forest which was substantially more fertile. We suggest that moisture supply is the primary determinant of vegetation and soil fertility patterns. Fire in the preceding dry season stimulated some herbaceous species, while a long unburnt shrubby Eucalyptus woodland was invaded by some monsoon forest seedlings. Conservation of the present mosaic of vegetation presents a major challenge to land managers because of this variable impact of fire.     

Munmarlary revisited: Response of a north Australian Eucalyptus tetrodonta savanna protected from fire for 20 years

Abstract A comparison was made between the total density of tree species recorded on three 1 ha plots that have been protected from fire for 20 years, and from three surrounding sites that have been subjected to the ambient fire regime. Both unburnt plots and ambient sites were in a lowland coastal Eucalyptus tetrodonta savanna in Kakadu National Park. Fire protection resulted in a substantial increase in the number of saplings (16 times more than ambient), poles (five times more than ambient) and trees (2. 5 times more than ambient), but slightly fewer (7%) sprouts than ambient. Of the 32 species recorded in the six 0. 2 ha samples, only nine species could legitimately be analysed using Chi-squared analysis to test for differences in the density of sprouts and saplings between unburnt and ambient samples; eight of these species had significantly different distributions. Typically the unburnt samples had a greater number of saplings compared to ambient conditions, but fewer sprouts. Eucalyptus miniata showed no significant difference in the density of sprouts and saplings between the unburnt and ambient samples. Chi-squared analyses of the frequency distribution among four size classes (sprouts, saplings, poles and trees) was possible for six species. The results for five of these species mirrored the findings of the comparison between sprouts and saplings. However, the fan palm Livistona humilus, which typically forms a component of the mid-layer in E. tetrodonta savannas, was found to have a large number of sprouts on the unburnt samples and a complete absence of stems in any of the other size classes. Dead L. humilus stems attested to the former occurrence of larger size classes of this species on the unburnt plots. No rainforest species were recorded in the unburnt samples. Minor differences in species composition between unburnt and ambient samples are thought to reflect sampling effects. The results of this study are consistent with the conclusions of an earlier study at the same site which also concluded that rainforest tree species do not readily colonize unburnt Eucalyptus savanna.     

Vegetation - environment relationships of old hay meadows at Sverveli, Telemark, S Norway

The vegetation of traditionally managed species-rich hay meadows at Sverveli, Telemark, S Norway was studied applying an indirect gradient approach. The vegetation in 93 randomly placed sample plots was analysed in order to detect the main vegetational gradients. Ecological measurements were recorded from each plot. The relationships between vegetation and environment were studied by DCA and LNMDS ordinations and non-parametric correlation analysis. Both ordinations revealed the same two ecologically interpretable vegetation gradients. Soil moisture was identified as the most important environmental factor in determining the species composition, followed by soil nutrient content. The contents of P, K. and Mg in the soil were more strongly correlated with the main vegetational gradients than was soil N. Differences in management history may explain some of the observed variation in species composition that was not accounted for by the recorded environmental variables.     

Rare plant species occurrence patterns are associated not with soil properties,but with frequent fire in a southeast Australian dry sclerophyll forest

Question

Soil properties can play a crucial role in influencing the abundance and distribution of plant species. Fire regimes can also have substantial impacts on plant community composition. However, few studies have examined the effects of both fire regimes and soil properties on the occurrence of rare plant species. Here, we asked if rare species have specific soil and fire regime associations relative to common species, and if soil properties may explain potential fire effects.

Location

Booderee National Park, southeastern Australia.

Methods

We collected soil cores and completed vegetation surveys on 42 sites in Sydney Coastal Dry Sclerophyll Forest vegetation. We tested for associations between the number of rare species and common species present in relation to three soil chemical properties (available phosphorus, ammonium and organic carbon), fire frequency, and time since fire.

Results

We found that rare and common species were not associated with any of the examined soil properties. However, rare species were associated with sites with a high fire frequency, while common species were negatively associated with time since fire.

Conclusions

Our results indicate that rare species’ occurrence patterns may be influenced by the direct effects of fire or mediated by multiple factors, rather than shaped solely by soil properties in our study area. Future work to understand the factors that underpin rare species’ occurrence patterns in response to fire is critical to develop fire management protocols that effectively conserve rare species in dry sclerophyll forests.     

Does low-intensity surface fire increase water- and nutrient-availability to overstorey Eucalyptus gomphocephala?

The objective of this study was to investigate how the management practices of prescribed fire and understorey vegetation removal affect water and nutrient relations of old, yet prematurely declining Eucalyptus gomphocephala. Long unburnt sites were established in Yalgorup National Park, Western Australia, adjacent to frequently burnt state forest sites. Trees were allocated to vegetation clearing, prescribed fire or no prescribed fire treatments. Prescribed fire was achieved in only one long unburnt national park site so that the results were pseudoreplicated but analysed accordingly. Soil chemistry, plant nutrient availability and tree foliar carbon and nitrogen isotope ratio and nutrient concentration were investigated. No effects of vegetation clearing were found. Prescribed fire sites were associated with sky exposure and bare ground whereas no prescribed fire sites were associated with shrub and litter cover and litter depth. Foliar carbon isotope ratios were significantly more negative in prescribed fire, relative to no prescribed fire, treatments on long unburnt sites. Soil exchangeable Zn and Mn and plant available (estimated by charged resin beads) Mg were higher on prescribed fire, relative to no prescribed fire, long unburnt sites. Seedling bioassays indicated elevated P and Cu availability on prescribed fire, relative to no prescribed fire, treatments. In overstorey E. gomphocephala, foliar N levels were elevated (but not to excessive levels), and there was a trend toward elevated foliar Mn, in prescribed fire relative to no prescribed fire treatments on long unburnt sites. In the context of our large-scale pseudoreplicated case study, prescribed fire provided a pulse of water and N, (with some indications towards provision of elevated Mn, Cu and Mg) availability to E. gomphocephala in decline on sites with a history of a long absence of fire that may in part underpin observations of elevated tree health on sites that have a history of relatively frequent fire.     

Impact of fire on leaf nutrients,arthropod fauna and herbivory of native and exotic eucalypts in Kings Park,Perth, Western Australia

The vegetation of Kings Park, near the centre of Perth, Western Australia, once had an overstorey of Eucalyptus marginata (jarrah) or Eucalyptus gomphocephala (tuart), and many trees still remain in the bushland parts of the Park. Avenues and roadsides have been planted with eastern Australian species, including Eucalyptus cladocalyx (sugar gum) and Eucalyptus botryoides (southern mahogany), both of which have become invasive. The present study examined the effect of a recent burn on the level of herbivory on these native and exotic eucalypts. Leaf damage, shoot extension and number of new leaves were measured on tagged shoots of saplings of each tree species in unburnt and burnt areas over an 8‐month period. Leaf macronutrient levels were quantified and the number of arthropods on saplings was measured at the end of the recording period by chemical knockdown. Leaf macronutrients were mostly higher in all four species in the burnt area, and this was associated with generally higher numbers of canopy arthropods and greater levels of leaf damage. It is suggested that the pulse of soil nutrients after the fire resulted in more nutrient‐rich foliage, which in turn was more palatable to arthropods. The resulting high levels of herbivory possibly led to reduced shoot extension of E. gomphocephala, E. botryoides and, to a lesser extent, E. cladocalyx. This acts as a negative feedback mechanism that lessens the tendency for lush, post‐fire regrowth to outcompete other species of plants. There was no consistent difference in the levels of the various types of leaf damage or of arthropods on the native and the exotic eucalypts, suggesting that freedom from herbivory is not contributing to the invasiveness of the two exotic species.     

Effects of fire regime on plant species richness and composition differ among forest,woodland and heath vegetation

Question

Do the effects of fire regimes on plant species richness and composition differ among floristically similar vegetation types?

Location

Booderee National Park, south‐eastern Australia.

Methods

We completed floristic surveys of 87 sites in Sydney Coastal dry sclerophyll vegetation, where fire history records have been maintained for over 55 years. We tested for associations between different aspects of the recent fire history and plant species richness and composition, and whether these relationships were consistent among structurally defined forest, woodland and heath vegetation types.

Results

The relationship between fire regime variables and plant species richness and composition differed among vegetation types, despite the three vegetation types having similar species pools. Fire frequency was positively related to species richness in woodland, negatively related to species richness in heath, and unrelated to species richness in forest. These different relationships were explained by differences in the associations between fire history and species traits among vegetation types. The negative relationship between fire frequency and species richness in heath vegetation was underpinned by reduced occurrence of resprouting species at high fire frequency sites (more than four fires in 55 years). However, in forest and woodland vegetation, resprouting species were not negatively associated with fire frequency.

Conclusions

We hypothesize that differing relationships among vegetation types were underpinned by differences in fire behaviour, and/or biotic and abiotic conditions, leading to differences in plant species mortality and post‐fire recovery among vegetation types. Our findings suggest that even when there is a high proportion of shared species between vegetation types, fires can have very different effects on vegetation communities, depending on the structural vegetation type. Both research and management of fire regimes may therefore benefit from considering vegetation types as separate management units.     

Soil–vegetation relationships in cerrados under different fire frequencies

Fire is an important ecological factor that structures savannas, such as the cerrado, by selecting plant species and altering soil nutrient content. In Emas National Park, central Brazil, we compared soils under three different fire regimes and their relationship to the cerrado species they support. We collected 25 soil and vegetation samples at each site. We found differences in soil characteristics (p?<?0.05), with fertility and fire frequency positively related: in the annually burned site we found higher values of organic matter, nitrogen, and clay, whereas in the protected site we detected lower values of pH and higher values of aluminum. We also observed differences in plant community structure, with distinct floristic compositions in each site. Floristic composition was more related to sand proportion (intra-set correlation?=?0.834). Different fire frequencies increase environmental heterogeneity and beta diversity in the Brazilian cerrado.     

Seasonal variation in seed bank composition and its interaction with nutrient enrichment in the Everglades wetlands

In the Florida Everglades, nutrient enrichment from agricultural outflow and the change in hydrology have collectively contributed to the expansion of cattails (Typha spp.). To assess the effectiveness of prescribed fire in controlling cattails and to predict vegetation dynamics after the fire, it is important to understand the seasonal variation of the soil seed bank and how the seed bank is affected by nutrient enrichment and fire. This paper investigates the effects of season, nutrient enrichment, and fire on soil seed bank species composition, richness, and density along a nutrient gradient in Water Conservation Area 2A (WCA 2A) of the Florida Everglades. Species richness was significantly affected by nutrient enrichment and season but not their interaction. Total seed density, however, was significantly affected by the interaction between nutrient enrichment and season. Yet, at species level, the relationship between seed density, nutrient enrichment and season varied. The highest seed density of cattail occurred in summer at highly enriched sites, but that of sawgrass occurred in fall regardless of enrichment; the seed density of water lily was very low regardless of season and nutrient enrichment, and the highest Amarathus seed density occurred at highly enriched sites year round. Moreover, germination timing differed greatly among species. While cattail seeds had a short incubation period and started to germinate 2–3 days after initiation of the germination assay, sawgrass seeds generally started to germinate 4 weeks later. Further, both the prescribed summer fire at the highly enriched site and the natural winter fire at the moderately enriched site reduced the seed density of cattail but not of sawgrass. Our results suggest that fire application for vegetation recovery in WCA 2A would benefit from explicitly considering seasonal dynamics of the seed bank.     

Impact of feral water buffalo and fire on growth and survival of mature savanna trees: An experimental field study in Kakadu National Park,northern Australia

Abstract The impact of feral Asian water buffalo (Bubalus bubalis) and season of fire on growth and survival of mature trees was monitored over 8 years in the eucalypt savannas of Kakadu National Park. Permanently marked plots were paired on either side of a 25‐km‐long buffalo‐proof fence at three locations on an elevational gradient, from ridge‐top to the edge of a floodplain; buffalo were removed from one side of the fence. All 750 trees ≥ 1.4 m height were permanently marked; survival and diameter of each tree was measured annually; 26 species were grouped into four eco‐taxonomic groups. The buffalo experiment was maintained for 7 years; trees were monitored an additional year. Fires were excluded from all sites the first 3 years, allowed to occur opportunistically for 4 years and excluded for the final year. Fires were of two main types: low‐intensity early dry season and high‐intensity late dry season. Growth rates of trees were size‐specific and positively related to diameters as exponential functions; trees grew slowest on the two ends of the gradient. Eucalypt mortality rates were 1.5 and 3 times lower than those of pantropics and of arborescent monocots, respectively, but the relative advantage was lost with fires or buffalo grazing. Without buffalo grazing, ground level biomass was 5–8 t ha^?1 compared with 2–3 t ha^?1, within 3 years. In buffalo‐absent plots, trees grew significantly slower on the dry ridge and slope, and had higher mortality across the entire gradient, compared with trees in buffalo‐present plots. At the floodplain margin, mortality of small palms was higher in buffalo‐present sites, most likely due to associated heavy infestations of weeds. Low‐intensity fires produced tree growth and mortality values similar to no‐fire, in general, but, like buffalo, provided a ‘fertilization’ effect for Eucalyptus miniata and Eucalyptus tetrodonta, increasing growth in all size classes. High‐intensity fires reduced growth and increased mortality of all functional groups, especially the smallest and largest (>35 cm d.b.h.) trees. When buffalo and fires were excluded in the final year, there were no differences in growth or mortality between paired sites across the environmental gradient. After 8 years, the total numbers of trees in buffalo‐absent plots were only 80% of the number in buffalo‐present plots, due to relatively greater recruitment of new trees in buffalo‐present plots; fire‐sensitive pantropics were particularly disadvantaged. Since the removal of buffalo is disadvantageous, at least over the first years, to savanna tree growth and survival due to a rebound effect of the ground‐level vegetation and subsequent changes in fire‐vegetation interactions, process‐orientated management aimed at reducing fuel loads and competitive pressure may be required in order to return the system to a previous state. The ‘footprint’ of 30 years of heavy grazing by buffalo has implications for the interpretation of previous studies on fire‐vegetation dynamics and for current research on vegetation change in these savannas.     

Effect of fire on soil nutrients and under storey vegetation in Chir pine forest in Garhwal Himalaya,India

The study was carried out in the Pinus roxburghii Sargent (Chir pine) forest in the sub-tropical region of Garhwal Himalaya to assess the effect of fire on soil nutrient status at different altitudes (700 m, 800 m and 1000 m), soil depths (0–20 cm, 20–40 cm and 40–60 cm) and on under storey vegetation. The soil nutrients and under storey vegetation were assessed before fire (pre-fire) and after fire (post-fire). The results of the study indicate that fire plays an important role in soil nutrient status and under storey vegetation. The nutrients (soil organic carbon, nitrogen, phosphorus and potassium), decreased in post-fire assessment and with increasing altitudes, and soil depths, compared to pre-fire assessment. The under storey vegetation diminished after fire in all forest sites. The study concludes that in Chir pine forest, fire plays a role in reducing soil nutrients along the altitudinal gradient, soil depths and under storey vegetation. Thus, these nutrients can be saved through some management practices e.g. by early controlled burning and by educating local villagers about the negative impacts of severe wild fires on soil and vegetation.     

Foliar and twig macronutrients (N,P, K,Ca and Mg) in selected species ofEucalyptus used in rehabilitation: sources of variation

Summary The macronutrient variation within four 6 year oldEucalyptus saligna and four 5 year oldE. wandoo growing on rehabilitated bauxite pits was determined. Significant differences in mean nutrient concentrations were generally recorded between good soil condition sites and poor soil sites, between tree individuals, branch height, and plant organ type; but mean nutrient values were not different among canopy aspects. Fully expanded leaves of the current year provided the most uniform nutrient levels among the plant organs and showed major differences between sites with good soil nutrient conditions and those with poor conditions. Differences in foliar and branch levels of N, P, K, Ca and Mg, the variation between sites, canopy heights and plant organ types, and the use of foliar nutrient levels to indicate deficiencies are discussed.     

Plant functional traits along environmental gradients in seasonally dry and fire‐prone ecosystem

Questions: How does the abundance and richness of plant assemblages with different functional (regeneration and nutrient acquisition) traits vary with fire regime, moisture availability and substrate fertility? What is the role of different functional traits in maintaining plant diversity under changing environmental conditions in seasonally dry and fire‐prone environments? Location: Southwest Western Australia. Methods: Plant species abundance and soil nutrients were determined at 16 forest sites with variable fire histories across an aridity gradient. All plant species were classified based on their functional traits as (1) perennial or annual, (2) ectomycorrhizal, arbuscular mycorrhizal, ericoid mycorrhizal, orchid mycorrhizal, proteoid or other non‐mycorrhizal, (3) resprouters or seeder, and (4) nitrogen fixer or non‐fixer. We used a multivariate (fourth‐corner) technique to simultaneously test the significance and direction of the relationship between each of these traits and fire frequency, fire interval length, aridity, and soil N, P and C fractions. Results: The functional response of the vegetation to fire regime was minor and restricted to annual species, which comprised only ～4% of taxa. Proteoid and ectomycorrhizal species dominated over species with arbuscular and orchid mycorrhizal roots, N‐fixers dominated over non‐fixers, and seeders dominated over resprouters when N fertility was low but organic labile P was high. Further, proteoid and ectomycorrhizal species richness increased with aridity, while arbuscular mycorrhizal species richness decreased. Conclusions: While the functional composition of southwest Australian vegetation is largely insensitive to changes in fire regime, nutrient acquisition and, to a lesser extent, regeneration traits provide mechanisms for the vegetation community to adjust to changes in resource availability. Thus, diversity responses to environmental change in seasonally dry and fire‐prone ecosystems are likely to be primarily mediated by the composition of nutrient acquisition traits in the vegetation community.     

The impact of experimental fire regimes on seed production in two tropical eucalypt species in northern Australia

Abstract This study investigated the effect of three experimental fire regimes on the fecundity, ovule development and seedfall of two common wet-dry tropical savanna eucalypts, Eucalyptus minima and Eucalyptus tetrodonta, in northern Australia. Both species flower early in the dry season and ovule development occurs during the dry season. This coincides with a period of frequent fires. The three fire regimes considered were applied for four years between 1990 and 1994. These regimes were (i) Unburnt, (ii) Early, fires lit early in the dry season, and (iii) Late, fires lit late in the dry season. The treatments were applied to nine catchments (15–20 km²) with each fire regime replicated three times. Fire intensity typically increases as the dry season proceeds. Therefore, early dry season fires generally differ from late dry season fires in both their intensity and their timing in relation to the reproductive phenology of the eucalypts. Late dry season burning significantly reduced the fecundity of both species, whereas Early burning had no significant effect. Ovule success was significantly reduced by the Early burning for both species. The Late burning significantly reduced ovule success in E. tetrodonta, but not in E. miniata. The results suggest that fire intensity and fire timing may both be important determinants of seed supply. Fire intensity may be a determinant of fecundity, whereas fire timing in relation to the reproduction phenology may have a significant impact on ovule survival. Both fire regimes resulted in a substantial reduction in seed supply compared with the Unburnt treatment. This may have a significant impact on seedling regeneration of these tropical savanna eucalypts.     

Factors Controlling Compositional Changes in a Northern Andean Páramo (La Rusia,Colombia)

In this study, we aimed to assess the processes controlling compositional change in a Northern Andean páramo highly affected by human‐induced disturbances over the last few decades (La Rusia, Colombia). Along the 3000–3800 m asl altitudinal range, we randomly sampled fifty 10 × 10 m plots. Therein, we measured altitude and variables related to soil conditions (i.e., moisture, nutrient contents, bulk density, and texture), occurrence of human‐induced disturbances (i.e., fire, vegetation clearing, potato cultivation, and cattle grazing), and land‐use history. We also recorded richness and abundance of plant species, identifying them as exotic or native. We differentiated four groups of plots according to their species composition. The groups had significant differences in altitude, soil conditions, land‐use history, and particularly, in richness of exotic species and exotic/native cover ratio. They could be ascribed to shrub‐ and grass‐páramo vegetation types based on their relative dominance of woody and herbaceous species; however, these groups were not arranged according to the hypothetical composition of altitudinal belts, but rather formed a mosaic of patches. This mosaic was determined not only by altitude but also by soil conditions and disturbance history of sites. Our results corroborate recent findings which highlight shrub‐ and grass‐páramo vegetation types as patches of contrasting species composition and structure that depend on local environmental variables, as well as human‐induced disturbances as a major determinant of compositional discontinuities in these ‘high mountain’ tropical ecosystems.     

Post‐fire recovery of revegetated woodland communities in south‐eastern Australia

The primary goal of restoration is to create self‐sustaining ecological communities that are resilient to periodic disturbance. Currently, little is known about how restored communities respond to disturbance events such as fire and how this response compares to remnant vegetation. Following the 2003 fires in south‐eastern Australia we examined the post‐fire response of revegetation plantings and compared this to remnant vegetation. Ten burnt and 10 unburnt (control) sites were assessed for each of three types of vegetation (direct seeding revegetation, revegetation using nursery seedlings (tubestock) and remnant woodland). Sixty sampling sites were surveyed 6 months after fire to quantify the initial survival of mid‐ and overstorey plant species in each type of vegetation. Three and 5 years after fire all sites were resurveyed to assess vegetation structure, species diversity and vigour, as well as indicators of soil function. Overall, revegetation showed high (>60%) post‐fire survival, but this varied among species depending on regeneration strategy (obligate seeder or resprouter). The native ground cover, mid‐ and overstorey in both types of plantings showed rapid recovery of vegetation structure and cover within 3 years of fire. This recovery was similar to the burnt remnant woodlands. Non‐native (exotic) ground cover initially increased after fire, but was no different in burnt and unburnt sites 5 years after fire. Fire had no effect on species richness, but burnt direct seeding sites had reduced species diversity (Simpson's Diversity Index) while diversity was higher in burnt remnant woodlands. Indices of soil function in all types of vegetation had recovered to levels found in unburnt sites 5 years after fire. These results indicate that even young revegetation (stands <10 years old) showed substantial recovery from disturbance by fire. This suggests that revegetation can provide an important basis for restoring woodland communities in the fire‐prone Australian environment.     

Modest enhancement of nitrogen conservation via retranslocation in response to gradients in N supply and leaf N status

Plant nutrient resorption, a ubiquitous mechanism of nutrient conservation, has often been proposed to be more pronounced in infertile than fertile habitats, and in species common to infertile compared to fertile habitats, because of the presumed advantage when nutrients are scarce. However, previous studies provide weak and inconsistent empirical support for these hypotheses, although few have examined intraspecific variation across well-quantified resource gradients. This study addresses intraspecific patterns of nutrient resorption for eight species across two N availability gradients on similar soils in an N-limited oak savanna ecosystem: a long-term fire frequency gradient with a negatively correlated N fertility gradient and a long-term N fertilization gradient. We hypothesized that both resorption proficiency (the minimum nutrient level retained in a senesced leaf) and efficiency (the proportional change in leaf nutrient concentration) would decrease with increasing soil N availability and plant N status. For the seven non-N fixers, either resorption proficiency or efficiency decreased modestly in treatments with higher N availability. In contrast, the legume Amorpha canescens Pursh had higher N levels in green and senesced leaves, and resorbed N much more weakly than the non-fixers, and did not respond in terms of proficiency or efficiency to soil N availability. Across all species and sites in each N fertility gradient, a scaling analysis showed greater resorption efficiency in plants with lower N concentrations. Our data suggest that species can have modest resorption responses reflective of soil nutrient availability and differences in resorption related to their N economy that represent mechanisms of nutrient conservation in nutrient-limited soils.     

Response of a monsoon forest-savanna boundary to fire protection,Weipa, northern Australia

A numerical floristic analysis of samples across a monsoon forest-savanna boundary, from an area that had been actively protected from fire for 15 years, at Weipa, northern Australia, revealed three communities: (i) a monsoon forest with a low closed canopy composed mainly of tree species with extra-Australian tropical affinities and a sparse ground layer; (ii) an ecotone with a distinct closed microphyll shrub layer beneath the open canopy of savanna trees; and (iii) a savanna dominated by Eucalyptus tetrodonta. The development of the ecotone has occurred since fire protection and is of limited extent within the fire protected block. The monsoon forest occurred on soils with significantly higher concentrations of bauxitic pisoliths than the other two communities. Soils under the monsoon forest had significantly higher concentrations of total K, S, C, N, exchangeable K and Ca, and higher pH and electrical conductivity than for soils of either of the other communities. A positive relationship between woody basal area and concentrations of surface soil total P, N, C, exchangeable Ca, CEC and gravel was detected across a 20 m transect from the ecotone community into the savanna. The invasion of monsoon forest seedlings was greatest in the ecotone, with few occurring in the savanna. It appears that the expansion of the monsoon forest requires the development of a layer of shrubs. The mechanism of this facilitation is unclear, although the possible role of nutrient enrichment by the shrubs requires further investigation.     

Drainage, soil fertility and fire frequency determine composition and structure of gumland heaths in northern New?Zealand

Vegetation and soils were sampled at remaining gumland heath ecosystems in northern New?Zealand to determine vegetation patterns, environmental controls and major threats to long-term persistence. Classification and ordination techniques identified six vegetation types reflecting differences in drainage, rainfall, altitude, nutrients, and time since fire. Two modal types reflected opposite ends of the main environmental spectra. Leptospermum scoparium (Myrtaceae) shrubland occurred on relatively better drained sites with lower rainfall, altitude, and soil nutrient levels, whereas Gleichenia dicarpa (Gleicheniaceae) fernland typically occurred on more poorly drained sites with higher rainfall, altitude, and nutrient levels. Another widespread vegetation type dominated by both Leptospermum scoparium and Gleichenia dicarpa occupied plots of intermediate drainage, rainfall, altitude and nutrients. The three remaining types were of limited distribution and reflected uncommon combinations of environmental conditions or recent fire. Low soil nutrients in gumlands (mean total N = 0.182%, total P = 0.004%, oven-dry weight) are reflected in low Leptospermum scoparium foliage nutrients (mean total N = 0.858%, total P = 0.034%, δ¹⁵N = δ6.06‰, oven-dry weight) and slow growth rates (mean annual height growth rate = 11.90 cm year^?1), as in heathlands in Australia and South Africa. Gumlands are threatened by non-native plant species invasion, especially Hakea sericea (Proteaceae); habitat destruction for agricultural, industrial, and suburban development; and nutrient enrichment from adjacent agricultural land. Currently, fire is much less common in gumlands (mean time since fire = 18.4 years) than during early European settlement and some communities are apparently reverting to forest. Research to investigate the use of fire as a management tool is recommended for long-term conservation of New?Zealand gumlands.