Tannin,nitrogen, and cell wall composition of green vs. senescent Douglas-fir foliage

Summary Tannin, cell wall, and nitrogen composition of green foliage and needle litter of similar-aged Douglas-fir (Pseudotsuga menziesii Mirb. Franco) from two stands differing in density and crown closure were compared. Trees in the closed-canopy stand had a lower basal area growth rate than those in the open-canopy stand. Stands did not differ in wood basal area/ha or forest floor C/N ratios, but the closed-canopy stand had a significantly larger accumulation of forest floor biomass and significantly higher levels of field-extractable nitrogen and nitrogen mineralization rates. Green foliage from trees in the closed-canopy stand had significantly lower nitrogen, astringency, and lignin contents, but higher cellulose concentration than trees in the open-canopy stand. These trends, inconsistent with the inverse relationship often observed between nitrogen and polyphenol contents of foliage, may result from differences in relative resource availability in the two stands. In contrast to green foliage, needle litter from the two stands had comparable contents of nitrogen, cellulose, and lignin, but astringency was significantly higher in litter from the closed-canopy stand. It is suggested that, within the constraints imposed by site conditions, evergreens may alter the tannin composition of senescing foliage, potentially affecting herbivory and decomposition differently.     

Carbon and Nitrogen Dynamics of Boreal Jack Pine Stands With and Without a Green Alder Understory

We compared the species composition, structure and selected components of the carbon (C) and nitrogen (N) budgets of similar-aged, mature boreal jack pine (Pinus banksiana Lamb.) forests with and without green alder [Alnus crispa (Ait.) Pursh.] in two different boreal environments. The C and N content of the overstory biomass components (for example, stem, branch, and foliage), total vegetation, forest floor, and mineral soil were greater (P= 0.05 to P= 0.10) for jack pine with alder (JPA) stands than for jack pine without alder (JP) stands at both study areas. Jack pine foliage N isotopic discrimination (δ¹⁵N) and annual litterfall N content were significantly greater (P < 0.05) in the JPA than the JP stands at both study areas, suggesting that alder was fixing N and that N availability was greater in the JPA than the JP stands. The greater leaf area index (LAI) and overstory C accumulation in the JPA than the JP stands (P < 0.05) is likely because of the greater N availability in the JPA stands, but the effect of soil texture discontinuity on water availability in the JPA stands can not be dismissed. Percent ground cover by feathermoss varied among the jack pine communities and was positively correlated with overstory LAI (r ²= 0.83, P< 0.05). One index of N-use efficiency (NUE), defined as aboveground net primary productivity (ANPP) per litterfall N, was significantly greater (P < 0.05) for the JP than the JPA stands, but a second index of NUE, ANPP/N uptake, did not differ between the two jack pine communities. Jack pine trees growing without alder produced more organic matter per unit of N, but percent N retranslocation from senescing foliage and N mean residence time in the overstory did not differ between the JPA and the JP stands. A conceptual model is presented that illustrates the potential influence of alder on the species composition, structure, and function of boreal jack pine forests. Received 6 January 1998; accepted 15 April 1998.     

Nitrogen availability and weed invasion in a remnant native woodland in urban Melbourne

Abstract The woodlands of Studley Park, and the greater Yarra Bend Park, are within 4 km of the centre of the city of Melbourne and total more than 200 ha. Weeds are major contributors to the loss of diversity of native species. We examined the hypothesis that an increase in the availability of nitrogen (N) may be responsible for the increase in weed abundance. In a small field trial, nitrogen availability was greater in areas invaded by weeds and weeds had greater nitrate reductase activity than native species. Both invaded and uninvaded areas were well‐supplied with weed seed, although the composition of the seed bank varied greatly between sites. Diaspores of the weed Romulea rosea were abundant in uninvaded sites while those of Ehrharta erecta and Ehrharta longiflora were abundant in invaded sites. The abundance of weed diaspores suggested that increased N availability may be a precondition for invasion. Formation of a dense litter layer by senescent weeds is likely to promote the capture and retention of N added from anthropogenic sources including air pollution and overland flow from nearby roads and drainage channels.     

Optimal co-allocation of carbon and nitrogen in a forest stand at steady state

Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.     

Foliar and wood chemistry of sugar maple along a gradient of soil acidity and stand health

The decline of sugar maple (Acer saccharum Marsh.) in forest of north-eastern North America is an important environmental issue. In this study, relationships between, soil, wood and foliar chemistry were assessed for 17 stands distributed within a large area of the Quebec sugar maple forest and that were growing on soils with a strong gradient of acidity and base saturation. There were many significant relationships between variables describing the acid-base status of the top-B soil (Ca and Mg concentrations, exchangeable acidity and base saturation) and Ca and Mn concentrations and Ca/Mn and Mg/Mn in tree tissues. Manganese was the element that showed the strongest inverse non-linear relationships with top-B soil base saturation with variance explanation of 71 and 65%, for wood and foliage, respectively. The 17 sites were divided in two groups according to their level of decline. The declining stands had significantly higher wood Mn and Mg concentrations and lower Ca/Mn ratios and significantly higher foliar Mn and lower Ca and Al concentrations. It was impossible to determine if these differences were a cause or a symptom of sugar maple health. However, the increase in Mn concentrations in tree tissues with increasing soil acidity, as well as the higher Mn concentrations in declining as compared to healthy stands suggest that Mn, as well as low Ca availability, could be an important contributing factor in the sugar maple decline.     

Variation in foliar nutrients in Eucalyptus trees in eastern and Western Australia

Abstract Levels of nitrogen, phosphorus and potassium were measured for the foliage of two co-dominant eucalypts at each of two sites, one in eastern Australia and the other in Western Australia. In eastern Australia, foliage was sampled in the canopy and subcanopy for narrow-leaved ironbark Eucalyptus crebra and grey box E. mollucana and in Western Australia, for jarrah E. marginata and marri E. calophylla. The Western Australian trees were also sampled for ‘young’ and ‘old’ leaves. Both eucalypts in eastern Australia had greater nitrogen and phosphorus levels, but lower potassium, than E. marginata or E. calophylla. Eucalyptus calophylla foliage had greater levels of all three nutrients than E. marginata foliage as did E. crebra relative to E. mollucana. At both sites, foliar nutrient levels were greater in the canopy than subcanopy foliage, and, at least in Western Australia, the younger leaves had greater nutrient levels than the older leaves. The observed differences in foliar nutrient levels are consistent with observed trends in the abundance and diversity of foliage arthropods and the use of the trees as foraging substrates by birds.     

Decomposition Patterns of Foliar Litter and Deadwood in Managed and Unmanaged Stands: A 13-Year Experiment in Boreal Mixedwoods

Litter decomposition is a major driver of carbon (C) and nitrogen (N) cycles in forest ecosystems and has major implications for C sequestration and nutrient availability. However, empirical information regarding long-term decomposition rates of foliage and wood remains rare. In this study, we assessed long-term C and N dynamics (12–13 years) during decomposition of foliage and wood for three boreal tree species, under a range of harvesting intensities and slash treatments. We used model selection based on the second-order Akaike’s Information Criterion to determine which decomposition model had the most support. The double-exponential model provided a good fit to C mass loss for foliage of trembling aspen, white spruce, and balsam fir, as well as aspen wood. These litters underwent a rapid initial phase of leaching and mineralisation, followed by a slow decomposition. In contrast, for spruce and fir wood, the single-exponential model had the most support. The long-term average decay rate of wood was faster than that of foliage for aspen, but not of conifers. However, we found no evidence that fir and spruce wood decomposed at slower rates than the recalcitrant fraction of their foliage. The critical C:N ratios, at which net N mineralisation began, were higher for wood than for foliage. Long-term decay rates following clear-cutting were either similar or faster than those observed in control stands, depending on litter material, tree species, and slash treatment. The critical C:N ratios were reached later and decreased for all conifer litters following stem-only clear-cutting, indicating increased N retention in harvested sites with high slash loads. Partial harvesting had weak effects on C and N dynamics of decaying litters. A comprehensive understanding of the long-term patterns and controls of C and N dynamics following forest disturbance would improve our ability to forecast the implications of forest harvesting for C sequestration and nutrient availability.     

Spatial pattern and process in forest stands within the Virginia piedmont

Abstract. Question: Underlying ecological processes have often been inferred from the analysis of spatial patterns in ecosystems. Using an individual‐based model, we evaluate whether basic assumptions of species’life‐history, drought‐susceptibility, and shade tolerance generate dynamics that replicate patterns between and within forest stands. Location: Virginia piedmont, USA. Method: Model verification examines the transition in forest composition and stand structure between mesic, intermediate and xeric sites. At each site, tree location, diameter, and status were recorded in square plots ranging from 0.25 to 1.0 ha. Model validation examines the simulated spatial pattern of individual trees at scales of 1–25 m within each forest site using a univariate Ripley's K function. Results: 7512 live and dead trees were surveyed across all sites. All sites exhibit a consistent, significant shift in pattern for live trees by size, progressing from a clumped understorey (trees ± 0.1 m in diameter) to a uniform overstorey (trees > 0.25 m). Simulation results reflect not only the general shift in pattern of trees at appropriate scales within sites, but also the general transition in species composition and stand structure between sites. Conclusions: This shift has been observed in other forest ecosystems and interpreted as a result of competition; however, this hypothesis has seldom been evaluated using simulation models. These results support the hypothesis that forest pattern in the Virginia piedmont results from competition involving species’life‐history attributes driven by soil moisture availability between sites and light availability within sites.     

The Effects of Adjacent Land Use on Nitrogen Dynamics at Forest Edges in Northern Idaho

The effects of immediately adjacent agricultural fertilization on nitrogen (N) at upland forest edges have not been previously studied. Our objective was to determine whether N from fertilized agriculture enters northern Idaho forest edges and significantly impacts their N status. We stratified 27 forest edge sampling sites by the N fertilization history of the adjacent land: current, historical, and never. We measured N stable isotopes (δ¹⁵N), N concentration (%N), and carbon-to-nitrogen (C/N) ratios of conifer tree and deciduous shrub foliage, shrub roots, and bulk soil, as well as soil available N. Conifer foliage δ¹⁵N and %N, shrub root δ¹⁵N, and bulk soil N were greater and soil C/N ratios lower (P < 0.05) at forest edges than interiors, regardless of adjacent fertilization history. For shrub foliage and bulk soil δ¹⁵N, shrub root %N and C/N ratios, and soil nitrate, significant edge–interior differences were limited to forests bordering lands that had been fertilized currently or historically. Foliage and soil δ¹⁵N were most enriched at forest edges bordering currently fertilized agriculture, suggesting that these forests are receiving N fertilizer inputs. Shrub root %N was greater at forest edges bordering currently fertilized agriculture than at those bordering grasslands that had never been fertilized (P = 0.01). Elevated N at forest edges may increase vegetation growth, as well as susceptibility to disease and insects. The higher N we found at forest edges bordering agriculture may also be found elsewhere, given similar agricultural practices in other regions and the prevalence of forest fragmentation.     

Restoration of three forest herbs in the Liliaceae family by manipulating deer herbivory and overstorey and understorey vegetation

Background: Research on herbaceous vegetation restoration in forests characterised by overstorey tree harvests, excessive deer herbivory, and a dominant fern understorey is lacking. Most of the plant diversity found in Eastern hardwood forests in the United States is found in the herbaceous understorey layer. Loss of forest herbaceous species is an indicator of declining forest conditions.

Aims: The combined effects of deer herbivory, competitive understorey vegetation removal, and overstorey tree removal on the abundance and reproductive capacity of three understorey herbs in the Liliaceae family were evaluated.

Methods: A split-plot randomised block design was used with three replicates. Treatments included three harvest intensities, fenced/unfenced, herbicide/no herbicide-treated, prescribed burn/no prescribed burn, and all combinations. A generalised linear model was used to compare treatment effects over 8 years.

Results: Both fruit production and cover increased significantly in fenced areas for all three species. There was a significant 6-year recovery period for cover of the three species in response to herbicide. There was a significant 4-year recovery period of fire-treated plots for fruit production of the three species. The most intensively cut, fenced, and herbicide-treated plots had the greatest increases in sapling and Rubus spp. cover. Cover and fruit production of the three herbs were significantly greatest in the moderate-cut treatment.

Conclusions: Restoration of these three liliaceous species is most likely to occur in Eastern deciduous forests and similar forests using a combined fenced and moderate-cut treatment.     

Impacts of Hemlock Loss on Nitrogen Retention Vary with Soil Nitrogen Availability in the Southern Appalachian Mountains

The impacts of exotic insects and pathogens on forest ecosystems are increasingly recognized, yet the factors influencing the magnitude of effects remain poorly understood. Eastern hemlock (Tsuga canadensis) exerts strong control on nitrogen (N) dynamics, and its loss due to infestation by the hemlock woolly adelgid (Adelges tsugae) is expected to decrease N retention in impacted stands. We evaluated the potential for site variation in N availability to influence the magnitude of effects of hemlock decline on N dynamics in mixed hardwood stands. We measured N pools and fluxes at three elevations (low, mid, high) subjected to increasing atmospheric N deposition where hemlock was declining or absent (as reference), in western North Carolina. Nitrogen pools and fluxes varied substantially with elevation and increasing N availability. Total forest floor and mineral soil N increased (P?<?0.0001, P?=?0.0017, resp.) and forest floor and soil carbon (C) to N ratio decreased with elevation (P?<?0.0001, P?=?0.0123, resp.), suggesting that these high elevation pools are accumulating available N. Contrary to expectations, subsurface leaching of inorganic N was minimal overall (<1?kg?ha^?1 9 months^?1), and was not higher in stands with hemlock mortality. Mean subsurface flux was 0.16?±?0.04 (SE) (kg?N?ha^?1 100?days^?1) in reference and 0.17?±?0.05 (kg?N?ha^?1 100?days^?1) in declining hemlock stands. Moreover, although subsurface N flux increased with N availability in reference stands, there was no relationship between N availability and flux in stands experiencing hemlock decline. Higher foliar N and observed increases in the growth of hardwood species in high elevation stands suggest that hemlock decline has stimulated N uptake and growth by healthy vegetation within this mixed forest, and may contribute to decoupling the relationship between N deposition and ecosystem N flux.     

The influence of light and nutrient availability on herb layer species richness in oak-dominated forests in central Bohemia

In this study, we examined to what extent the internal site factors (light and soil conditions) are responsible for herb layer diversity in oak-dominated forest stands growing on different substrates in central Bohemia (Czech Republic). We collected data on herb layer diversity, light and nutrient availability at nine oak stands, representing the range of environmental variability for these types of forests in the region. We found that species richness increased with light availability, but only if the site occupied predominantly by fast-colonizing species was excluded from the analysis (P < 0.05). Species richness correlated positively with soil pH and negatively with nitrogen (N) concentration in humus (P < 0.05). The highest species richness was found at sites with not only low N soil concentration, but also simultaneously with high phosphorus (P) soil concentration. Despite this finding, however, herb layer diversity is evidently threatened much more in P-rich soils than in P-poor soils. It seems that the enhancement of N in an ecosystem due to litter accumulation and N deposition generally leads to only a minor increase in N availability at P-poor sites, but a considerable increase at P-rich sites. Therefore, species richness can be exceptionally high at P-rich sites, but only under conditions of strong N limitation.     

Recent carbon and nitrogen accumulation and acidification in soils of two Scots pine ecosystems in Southern Germany

Changes of the soil chemical status during the recent 22–30 years at two historically degraded forest sites in southern Germany (Pfaffenwinkel, Pustert) stocked with mature Scots pine (Pinus sylvestris L.) stands were studied by repeated soil inventories conducted in 1974, 1982–1984, 1994, and 2004 on replicated control plots of fertilization experiments, allowing a statistical analysis. Additionally, the nutritional status of the stands at all plots was monitored from 1964 until 2004 by annual or bi-annual analysis of current-year foliage, and stand growth was assessed by repeated stand inventories carried out in 3- to 9-year intervals. For both sites, a statistically significant systematic decrease of the forest floor C/N ratio between 1974 and 2004 from 35.4 to 29.2 (Pfaffenwinkel) and from 36.5 to 23.0 (Pustert) was observed. The soils at both sites also showed a considerable accumulation of organic carbon (210 and 400 kg C ha⁻¹ year⁻¹ for Pfaffenwinkel and Pustert, respectively) and nitrogen (13 and 18 kg N ha⁻¹ year⁻¹). In addition, the mineral topsoil at both sites has acidified considerably, indicated by significantly decreased pH values (Pustert only; mean decrease 0.1 units per decade), base saturation, and base cation stocks. The trend of N enrichment and base cation loss in the soils is mirrored by the trends of stand nutrition at both sites, which are characterized by improved N nutrition and reduced supply with K, Mg (Pustert only), and Ca. The results confirm findings of other studies indicating a recent N eutrophication and acidification of forest soils in Central Europe and southern Scandinavia. Since soils with historic degradation due to earlier non-sustainable forest utilization are widespread in Central Europe, the results obtained on our study sites probably apply for large forested areas, suggesting a significant potential of Central European forests to sequester atmospheric carbon and nitrogen not only in stand biomass, but also in the soil.     

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.     

Urbanization and fragmentation have opposing effects on soil nitrogen availability in temperate forest ecosystems

Nitrogen (N) availability relative to plant demand has been declining in recent years in terrestrial ecosystems throughout the world, a phenomenon known as N oligotrophication. The temperate forests of the northeastern U.S. have experienced a particularly steep decline in bioavailable N, which is expected to be exacerbated by climate change. This region has also experienced rapid urban expansion in recent decades that leads to forest fragmentation, and it is unknown whether and how these changes affect N availability and uptake by forest trees. Many studies have examined the impact of either urbanization or forest fragmentation on nitrogen (N) cycling, but none to our knowledge have focused on the combined effects of these co-occurring environmental changes. We examined the effects of urbanization and fragmentation on oak-dominated (Quercus spp.) forests along an urban to rural gradient from Boston to central Massachusetts (MA). At eight study sites along the urbanization gradient, plant and soil measurements were made along a 90 m transect from a developed edge to an intact forest interior. Rates of net ammonification, net mineralization, and foliar N concentrations were significantly higher in urban than rural sites, while net nitrification and foliar C:N were not different between urban and rural forests. At urban sites, foliar N and net ammonification and mineralization were higher at forest interiors compared to edges, while net nitrification and foliar C:N were higher at rural forest edges than interiors. These results indicate that urban forests in the northeastern U.S. have greater soil N availability and N uptake by trees compared to rural forests, counteracting the trend for widespread N oligotrophication in temperate forests around the globe. Such increases in available N are diminished at forest edges, however, demonstrating that forest fragmentation has the opposite effect of urbanization on coupled N availability and demand by trees.     

Vegetation structure and biodiversity along the eucalypt forest to rainforest continuum on the serpentinite soil catena in a subhumid area of Central Queensland,Australia

Abstract The deep lateritic earths that cap the serpentinite outcrop in the Rockhampton – Marlborough area on the Tropic of Capricorn in Central Queensland have been eroded to expose the underlying ultramafic rock. Water‐holding capacity of these nutrient‐poor soils increases in a gradient from the skeletal soils to the deep lateritic earths and results in a continuum of structural formations from open‐woodland to woodland to open‐forest. A couple of closed‐forest (rainforest) stands have developed where seepage into Marlborough Creek occurs throughout the year. Aerodynamic fluxes (frictional, thermal and evaporative) in the atmosphere as it flows over and through the vegetation influence the annual foliage growth in all strata in the continuum from skeletal soils to deep lateritic earths. The lateral growth of each plant is abraded so that the sum of the foliage projective covers of overstorey (FPCo) and understorey (FPCu) strata – that is Σ(FPCo + FPCu) – remains constant throughout the serpentinite soil catena. As more water becomes available in the soil catena, the mineral nutrient levels in overstorey leaves increase, making developing leaves more vulnerable to insect attack. Although the number of leaves produced annually on each vertical foliage shoot in the overstorey increases along the soil‐water gradient, Σ(FPCo + FPCu) remains constant in all stands. The carbon isotope ratios (a measure of stomatal resistance) and leaf specific weights (LSWs) (a measure of the proportion of structural to cytoplasmic content in a leaf) of overstorey and understorey strata, however, are constant throughout the continuum. The well‐watered rainforest pockets – where seepage occurs – form the end point of this serpentinite continuum. LSWs and carbon isotope ratios of the canopy trees are similar to those in the sheltered understorey in the eucalypt communities. A gradient of foliage attributes is observed from evergreen canopy trees (12 m) to subshrubs (2 m) in the sunlit life forms that compose the complex structure of the rainforest stands in the humid to subhumid climate of Central Queensland. As alpha diversity (number of species per hectare) is correlated with annual shoot growth per hectare, species richness along the serpentinite continuum is almost half that of nearby plant communities on medium‐nutrient soils. The one to two eucalypt species per hectare are about a tenth of the number recorded on adjacent medium‐nutrient soils.     

Assessment of the potential role of metals in sugar maple (Acer saccharum Marsh) decline in Ontario, Canada

Spatial and temporal differences in the crown condition of sugar maple (Acer saccharum Marsh) in Ontario remain largely unexplained. In this study, the potential role of metals in sugar maple dieback was explored by measuring metal concentrations in foliage and forest floor (LFH) at 35 forest stands in south-central Ontario that exhibit varying levels of decline and span a climatic, soil acidity and acid deposition gradient. Foliar and forest floor metal concentrations varied among sites by between two and ten-fold, with acidic sites exhibiting the highest concentrations of many metals in the forest floor and foliage. Sites with moderate decline symptoms (decline index (DI)?>?10, averaged between 1986 and 2004) had significantly greater Cd, Zn, Cu, Pb and Mn concentrations and lower Ca concentrations in the forest floor compared with healthy sites (DI?<?10). Foliar concentrations of Cd, Sr and Mn were also significantly greater and Ca was significantly lower in sites with moderate decline symptoms compared with healthy sites. However the highest metal concentrations in foliage and the forest floor found in this study are lower than critical values reported in the literature. The notable exception is Mn where values at acidic sites may be high enough to negatively impact sugar maple.     

Stand structure of the emergent conifer Agathis ovata in forest and maquis, Province Sud, New Caledonia

Abstract. The size structure of the endemic New Caledonian conifer Agathis ovata is reported for sample stands in forest and maquis from three areas on ultramafic substrates in the south of the main island (Grand Terre). In closed forest Agathis ovata is typically represented by a low density of emergent adult trees with only limited evidence of seedling recruitment. In maquis, Agathis ovata is represented by individuals of all sizes, with seedlings and saplings abundant in most sample stands. Preliminary evidence from tree-ring studies indicates that rings may be annual. Estimated diameter growth rate is about 2 mm y^?1 for trees ≥ 10 cm d.b.h., and ring counts suggest tree ages of up to 400 years in maquis and 500 years in forest. Agathis ovata, and three other members of the Araucariaceae found in New Caledonia (Araucaria laubenfelsii, A. montana and A. rulei), are the only tree species which regularly occur scattered in maquis in this way, creating an unusual structural assemblage. No angiosperm tree species show this behaviour. The circumstances under which the Agathis ovata– maquis stands arise and are maintained are the subject of further investigation. Preliminary evidence for tree ages indicates that these stands predate European arrival in New Caledonia and so are not the result of recent increases in the frequency and intensity of human disturbances. The presence of fire scars on many individuals, and location of most stands on slopes and spurs with outcropping laterite (cuirasse), suggests that this assemblage may owe its existence to the interplay of fire regime, topography and rockiness, and a resistance to fire in Agathis ovata which increases with plant size and age.     

Effect of forest canopy composition on soil nutrients and dynamics of the understorey: mixed canopies serve neither vascular nor bryophyte strata

Question: The effect of overstorey composition on above‐ground dynamics of understorey vegetation is poorly understood. This study examines the understorey biomass, production and turnover rates of vascular and non‐vascular plants along a conifer–broadleaf gradient of resource availability and heterogeneity. Location: Canadian boreal forests of northwest Quebec and Ontario. Methods: We sampled mature stands containing various proportions of black spruce (Picea mariana (Mill.) BSP), trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.). Above‐ground biomass of the understorey vegetation was assessed through harvesting; annual growth rates were calculated as the differences between biomass in 2007 and 2008, as estimated by allometric relationships, and turnover rates were estimated as net primary production divided by the biomass in 2007. Results: Higher aspen presence, linked to greater nutrient availability in the forest floor, was generally associated with higher vascular biomass and production in the understorey. This effect was less pronounced in sites of high intrinsic fertility. In contrast, bryophyte biomass was positively associated with conifer abundance, particularly in wet sites of the Quebec study area. Non‐linear responses resulted in total understorey biomass being lower under mixed canopies than under pure aspen or pure conifer canopies. Turnover rates did not differ with overstorey composition. Conclusions: While resource availability is a main driver of understorey productivity, resources as drivers appear to differ with differences in understorey strata components, i.e. vascular versus non‐vascular plants. Resource heterogeneity induced by a mixed canopy had overall negative effects on understorey above‐ground productivity, as this productivity seemed to rely on species adapted to the specific conditions induced by a pure canopy.     

Soil seed bank composition along a forest chronosequence in seasonally moist tropical forest,Panama

Abstract. We used a forest chronosequence at the Barro Colorado Nature Monument (BCNM) to examine changes in the abundance and species composition of seeds in the soil during forest succession. At each of eight sites varying from 20 yr to 100 yr since abandonment, and at two old-growth (> 500 yr) forest stands, we established two 160-m transects and sampled the surface 0–3 cm of soil in cores collected at each 5 m interval. Seed densities were estimated from the number of seedlings germinated from the soil over a six-week period. Contrary to expectation, neither the density of the soil seed bank, nor species richness or diversity were directly related to age since abandonment, but the density of the soil seed bank was correlated with the abundance of seed-bank-forming species in the standing vegetation. In marked contrast to published studies, herbaceous taxa were rare even in the youngest stands, and the common tree species, which accounted for most seeds in the soil, were present in all stands. The pioneer tree Miconia argentea (Melastomataceae) was the single most common species in the seed bank, accounting for 62% of seeds and present in 92% of soil samples. Rapid recovery of the vegetation of young regrowth stands on BCNM, when compared to sites elsewhere may be partly due to allochthonous seed rain from nearby mature forest stands and the lack of seed inputs of weeds and grasses from agricultural and pasture lands which may inhibit forest succession.