Impact of insect defoliation on forest carbon balance as assessed with a canopy assimilation model

Disturbances such as fire, hurricanes, and herbivory often result in the net release of CO₂ from forests to the atmosphere, but the magnitude of carbon (C) loss is poorly quantified and difficult to predict. Here, we investigate the carbon balance of an oak/pine forest in the New Jersey Pine Barrens using the Canopy Conductance Constrained Carbon Assimilation (4C‐A) model. The 4C‐A model utilizes whole‐tree sap‐flux and leaf‐level photosynthetic gas exchange measurements at distinct canopy levels to estimate canopy assimilation. After model parameterization, sensitivity analyses, and evaluation against eddy flux measurements made in 2006, the model was used to predict C assimilation for an undisturbed year in 2005, and in 2007 when the stand was completely defoliated for 2–3 weeks during an infestation of gypsy moths (Lymantria dispar L.). Following defoliation, only 50% of the foliage reemerged in a second flush. In 2007, canopy net assimilation (A_nC), as modeled with the 4C‐A, was reduced to approximately 75% of A_nC in 2006 (940 vs. 1240 g C m^?2 a^?1). Overall, net primary production (NPP) in 2007 was approximately 240 g C m^?2 a^?1 (vs. 250 g C m^?2 a^?1 in 2006), with 60% of NPP allocated to foliage production, a short‐term carbon pool. Woody biomass accumulation, a long‐term carbon pool, was reduced by 20% compared with the previous year (72 vs. 57 g C m^?2 a^?1 in 2006 and 2007, respectively). The overall impact of the defoliation spanned 21% of upland forests (320 km²) in the New Jersey Pine Barrens, representing a significant amount of overall C not being taken up from the atmosphere by the forest, thus not accumulated in the biosphere.     

Invasive insects impact forest carbon dynamics

Invasive insects can impact ecosystem functioning by altering carbon, nutrient, and hydrologic cycles. In this study, we used eddy covariance to measure net CO₂ exchange with the atmosphere (NEE), and biometric measurements to characterize net ecosystem productivity (NEP) in oak‐ and pine‐dominated forests that were defoliated by Gypsy moth (Lymantria dispar L.) in the New Jersey Pine Barrens. Three years of data were used to compare C dynamics; 2005 with minimal defoliation, 2006 with partial defoliation of the canopy and understory in a mixed stand, and 2007 with complete defoliation of an oak‐dominated stand, and partial defoliation of the mixed and pine‐dominated stands. Previous to defoliation in 2005, annual net CO₂ exchange (NEE_yr) was estimated at ?187, ?137 and ?204 g C m^?2 yr^?1 at the oak‐, mixed‐, and pine‐dominated stands, respectively. Annual NEP estimated from biometric measurements was 108%, 100%, and 98% of NEE_yr in 2005 for the oak‐, mixed‐, and pine‐dominated stands, respectively. Gypsy moth defoliation strongly reduced fluxes in 2006 and 2007 compared with 2005; NEE_yr was ?122, +103, and ?161 g C m^?2 yr^?1 in 2006, and +293, +129, and ?17 g C m^?2 yr^?1 in 2007 at the oak‐, mixed‐, and pine‐dominated stands, respectively. At the landscape scale, Gypsy moths defoliated 20.2% of upland forests in 2007. We calculated that defoliation in these upland forests reduced NEE_yr by 41%, with a 55% reduction in the heavily impacted oak‐dominated stands. ‘Transient’ disturbances such as insect defoliation, nonstand replacing wildfires, and prescribed burns are major factors controlling NEE across this landscape, and when integrated over time, may explain much of the patterning of aboveground biomass and forest floor mass in these upland forests.     

Distributional patterns of sunfishes on the New Jersey coastal plain

Synopsis Sunfishes of the genera Lepomis and Enneacanthus are characteristic inhabitants of quiet waters on the New Jersey coastal plain. In southern New Jersey, E. chaetodon and E. obesus are now almost totally restricted to the Pine Barrens region of the Outer Coastal Plain. In contrast, E. gloriosus is widely distributed. Lepomis gibbosus and L. macrochirus are also widespread, but have established few populations in the more acidic waters of the Pine Barrens. Factor analysis was used to determine underlying patterns of distribution among these 5 species, using 6 habitat variables, measured over 54 collection sites in New Jersey. Two general factors account for 41% of the distributional variation. Habitat variables most strongly associated with factor 1 suggest an underlying trophic gradient (dystrophy to eutrophy) with its associated species. Factor 2 suggests an underlying current-bottom gradient. The basis for the scarcity of Lepomis spp. from acidic waters was explored by examining the tolerance of recently hatched embryos to reduced pH. No increase in mortality was observed at pH 4.25 for L. gibbosus eleutheroembryos and pH 4.5 for L. macrochirus eleutheroembryos. The two species appear to be poorly suited to dystrophic habitats; young Lepomis are primarily planktivores, a dietary niche that is conspicuously reduced in dystrophic waters. Fish of the genus Enneacanthus glean invertebrates on substrate or vegetation, a behavior well suited for dystrophic habitats where most primary and secondary production is associated with aquatic macrophytes and substrate.     

Effects of fire season on flowering of forbs and shrubs in longleaf pine forests

Summary Effects of variation in fire season on flowering of forbs and shrubs were studied experimentally in two longleaf pine forest habitats in northern Florida, USA. Large, replicated plots were burned at different times of the year, and flowering on each plot was measured over the twelve months following fire. While fire season had little effect on the number of species flowering during the year following fire, fires during the growing season decreased average flowering duration per species and increased synchronization of peak flowering times within species relative to fires between growing seasons. Fires during the growing season also increased the dominance of fall flowering forbs and delayed peak fall flowering. Differences in flowering resulting from variation in fire season were related to seasonal changes in the morphology of clonal forbs, especially fall-flowering composites. Community level differences in flowering phenologies indicated that timing of fire relative to environmental cues that induced flowering was important in determining flowering synchrony among species within the ground cover of longleaf pine forests. Differences in fire season produced qualitatively similar effects on flowering phenologies in both habitats, indicating plant responses to variation in the timing of fires were not habitat specific.     

Total phosphorus inference models and indices for coastal plain streams based on benthic diatom assemblages from artificial substrates

We investigated the potential for using diatoms to monitor and assess nutrient enrichment in coastal plain streams using weighted-averaging inference models and diatom trophic indices. Samples were collected from low-gradient, clay- to sand-bottom streams in New Jersey (NJ), USA, using artificial substrates (diatometers). Multivariate analysis showed that conductivity was overall the most important explanatory variable. Total phosphorus (TP) explained a significant proportion of the variation in diatom species composition. There was statistical justification for development of inference models for TP but not for total nitrogen (TN). We developed and tested models for inferring TP using weighted-averaging (WA) and weighted-averaging partial least squares (WA-PLS) regression and calibration techniques. We also created a diatom TP index by rescaling the inferred TP values. WA-PLS provided the best model (n = 38), which showed moderate predictive ability (r _boot² = 0.43; RMSEP_boot = 0.30 log₁₀ μg l⁻¹ TP); it performed best at lower TP concentrations and tended to underestimate values above 100 μg l⁻¹. The TP index performed well; it assigned the majority of the index scores to the correct nutrient category. TP models and indices developed for the Coastal Plain had lower predictive ability than those developed for northern NJ and streams in other comparable geographic regions of the US. This lower performance can be attributed primarily to a data gap in the TP gradient in the calibration dataset (lack of sites with TP concentrations between 240 and 560 μg l⁻¹), and a smaller number of samples. We conclude that diatom-based TP inference models and artificial substrate sampling are useful for assessing and monitoring nutrient enrichment in coastal plain streams. Given the worldwide distribution of streams similar to those in this study, these tools should be widely applicable. Handling editor: D. Ryder     

Vertical gradients in leaf trait diversity in a New Zealand forest

Leaves come in a remarkable diversity of sizes and shapes. However, spatial patterns in leaf trait diversity are rarely investigated and poorly resolved. We used a hierarchical approach to evaluate vertical variability in leaf morphology (i.e., leaf trait diversity) in 16 common tree and shrub species inhabiting a New Zealand forest. Height-related heterogeneity in leaf area, specific leaf area, circularity and length to width ratio was analyzed at three scales: (1) among leaves within plants, (2) among plants within species and (3) among species within functional groups (i.e., trees vs. shrubs). Results were scale dependent. Among-leaf morphological diversity was unrelated to plant height. Among-individual morphological diversity increased with the average height of each species, indicating that taller plant species express a greater range of leaf traits than shorter species. Among-species morphological diversity was higher in shrubs than in trees. We hypothesize that scale-dependent patterns in leaf trait diversity result from scale-dependent adaptations to forest environmental conditions. As trees grow from the forest floor into the canopy, they are exposed to a range of environmental conditions, which may select for a range of leaf traits through ontogeny. Conversely, shrubs never reach the forest canopy and may instead be differentially adapted to suites of environmental conditions associated with different stages of forest recovery from tree-fall disturbances. Overall results indicate that vertical patterns in leaf trait diversity exist. However, their strength and directionality are strongly scale-dependent, suggesting that different processes govern leaf shape diversity at different levels of ecological organization.     

Parasites boost productivity: effects of mistletoe on litterfall dynamics in a temperate Australian forest

The importance of litter in regulating ecosystem processes has long been recognised, with a growing appreciation of the differential contribution of various functional plant groups. Despite the ubiquity of mistletoes in terrestrial ecosystems and their prominence in ecological studies, they are one group that have been overlooked in litter research. This study evaluated the litter contribution from a hemiparasitic mistletoe, Amyema miquelii (Lehm. ex Miq.) Tiegh., in an open eucalypt forest (Eucalyptus blakelyi, E. dwyeri and E. dealbata), at three scales; the forest stand, single trees and individual mistletoes. Litter from mistletoes significantly increased overall litterfall by up to 189%, the amount of mistletoe litter being proportional to the mistletoe biomass in the canopy. The high litter input was due to a much higher rate of mistletoe leaf turnover than that of host trees; the host litterfall and rate of leaf turnover was not significantly affected by mistletoe presence. The additional litter from mistletoes also affected the spatial and temporal distribution of litterfall due to the patchy distribution of mistletoes and their prolonged period of high litterfall. Associated with these changes in litterfall was an increase in ground litter mass and plant productivity, which reflects similar findings with root-parasitic plants. These findings represent novel mechanisms underlying the role of mistletoes as keystone resources and provide further evidence of the importance of parasites in affecting trophic dynamics.     

Comparative physiology and demography of three Neotropical forest shrubs: alternative shade-adaptive character syndromes

A suite of functionally-related characters and demography of three species of Neotropical shadeadapted understory shrubs (Psychotria, Rubiaceae) were studied in the field over five years. Plants were growing in large-scale irrigated and control treatments in gaps and shade in old-growth moist forest at Barro Colorado Island, Panama. Irrigation demonstrated that dry-season drought limited stomatal conductance, light saturated photosynthesis, and leaf longevity in all three species. Drought increased mortality of P. furcata. In contrast, irrigation did not affect measures of photosynthetic capacity determined with an oxygen electrode or from photosynthesis-CO₂ response curves in the field. Drought stress limited field photosynthesis and leaf and plant survivorship without affecting photosynthetic capacity during late dry season. Leaves grown in high light in naturally occurring treefall gaps had higher photosynthetic capacity, dark respiration and mass per unit area than leaves grown in the shaded understory. P. furcata had the lowest acclimation to high light for all of these characters, and plant mortality was greater in gaps than in shaded understory for this species. The higher photosynthetic capacity of gap-grown leaves was also apparent when photosynthetic capacity was calculated on a leaf mass basis. Acclimation to high light involved repackaging (higher mass per unit leaf area) as well as higher photosynthetic capacity per unit leaf mass in these species. The three species showed two distinct syndromes of functionally-related adaptations to low light. P. limonensis and P. marginata had high leaf longevity (3 years), high plant survivorship, low leaf nitrogen content, and high leaf mass per unit area. In contrast, P. furcata had low leaf survivorship (1 year), high plant mortality (77–96% in 39 months), low leaf mass per unit area, high leaf nitrogen content, and the highest leaf area to total plant mass; the lowest levels of shelf shading, dark respiration and light compensation; and the highest stem diameter growth rates. This suite of characters may permit higher whole-plant carbon gain and high leaf and population turnover in P. furcata. Growth in deep shade can be accomplished through alternative character syndromes, and leaf longevity may not be correlated with photosynthetic capacity in shade adapted plants.     

Below-ground spatial pattern of rhizomes in a grassland community and its relevance to above-ground spatial pattern

Studies of spatial patterns in grassland plant communities have focused on above-ground patterns, ignoring the fact that in clonal plant communities, such as those found in grasslands, above-ground spatial structure must reflect below-ground horizontal growth. The present study examines, at both a fine and a coarse spatial scale, relationships between rhizome and ramet distribution. At the coarse scale, the dominance of species differed between above- and below-ground; some species dominated only above- or below-ground, and others dominated in both layers. At the fine scale, a single species' ramet aggregation above-ground significantly differed from its rhizome aggregation below-ground, for many species. Even for a given species, quantitative relationships between above- and below-ground dominance varied among localities. The variation in spatial pattern among species can be explained by attributes of clonal growth form, including rhizome size, rhizome origin and pattern of above-ground ramet aggregation. Using these parameters of clonal growth, four major space occupation patterns were described for mountain grassland species. For species with a high abundance of evenly distributed rhizomes, ramets either i) reflect rhizome distribution, or ii) do not fully reflect rhizome distribution, but rather are spatially aggregated, and rhizomes are typically developed below-ground. For species with a low abundance of rhizomes, ramets either iii) reflect rhizome distribution and rhizomes are typically of above-ground origin, or iv) do not reflect rhizome distribution and are aggregated only at the growing tips of rhizomes. Spatial correlation above-ground among species was the same as below-ground for some pairs of species but was significantly different for other pairs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of carbon fluxes to drought in a coastal plain loblolly pine forest

Full accounting of ecosystem carbon (C) pools and fluxes in coastal plain ecosystems remains less studied compared with upland systems, even though the C stocks in these systems may be up to an order of magnitude higher, making them a potentially important component in regional C cycle. Here, we report C pools and CO₂ exchange rates during three hydrologically contrasting years (i.e. 2005–2007) in a coastal plain loblolly pine plantation in North Carolina, USA. The daily temperatures were similar among the study years and to the long‐term (1971–2000) average, whereas the amount and timing of precipitation differed significantly. Precipitation was the largest in 2005 (147 mm above normal), intermediate in 2006 (48 mm below) and lowest in 2007 (486 mm below normal). The forest was a strong C sink during all years, sequestering 361 ± 67 (2005), 835 ± 55 (2006) and 724 ± 55 (2007) g C m^?2 yr^?1 according to eddy covariance measurements of net ecosystem CO₂ exchange (NEE). The interannual differences in NEE were traced to drought‐induced declines in canopy and whole tree hydraulic conductances, which declined with growing precipitation deficit and decreasing soil volumetric water content (VWC). In contrast, the interannual differences were small in gross ecosystem productivity (GEP) and ecosystem respiration (ER), both seemingly insensitive to drought. However, the drought sensitivity of GEP was masked by higher leaf area index and higher photosynthetically active radiation during the dry year. Normalizing GEP by these factors enhanced interannual differences, but there were no signs of suppressed GEP at low VWC during any given year. Although ER was very consistent across the 3 years, and not suppressed by low VWC, the total respiratory cost as a fraction of net primary production increased with annual precipitation and the contribution of heterotrophic respiration (R_h) was significantly higher during the wettest year, exceeding new litter inputs by 58%. Although the difference was smaller during the other 2 years (R_h : litterfall ratio was 1.05 in 2006 and 1.10 in 2007), the soils lost about 109 g C m^?2 yr^?1, outlining their potential vulnerability to decomposition, and pointing to potential management considerations to protect existing soil C stocks.     

Contrasting patterns of photosynthetic acclimation and photoinhibition in two evergreen herbs from a winter deciduous forest

The relationship between the microclimate within an Oak-Hickory forest and photosynthetic characters of two resident evergreen herbs with contrasting leaf phenologies was investigated on a monthly basis for 1 full year. Heuchera americana has leaf flushes in the spring and fall, with average leaf life spans of 6–7 months. Hexastylis arifolia produces a single cohort of leaves each spring with a leaf life span of 12–13 months. We predicted that among evergreen plants inhabiting a seasonal habitat, a species for which the frequency of leaf turnover is greater than the frequency of seasonal extremes would have a greater annual range in photosynthetic capacity than a species that only produced a single flush of leaves during the year. Photosynthetic parameters, including apparent quantum yield, maximum photosynthetic capacity (P_max), temperature of maximum photosynthesis, photochemical efficiency of PSII and leaf nitrogen (N) and chlorophyll concentrations, were periodically measured under laboratory conditions in leaves sampled from natural populations of both species. Mature leaves of both species acclimated to changing understory conditions with the mean seasonal differences being significantly greater for Heuchera than for Hexastylis. Area based maximum photosynthetic rates at 25°C were approximately 250% and 100% greater in winter leaves than summer leaves for Heuchera and Hexastylis respectively. Nitrogen concentrations were highest in winter leaves. Chlorophyll concentrations were highest in summer leaves. Low P_max/N values for these species suggest preferential allocation of leaf nitrogen into non-photosynthetic pools and/or light-harvesting function at the expense of photosynthetic enzymes and electron transport components. Despite the increase in photosynthetic capacity, there was evidence of chronic winter photoinhibition in Hexastylis, but not in Heuchera. Among these ecologically similar species, there appears to be a trade-off between the frequency of leaf production and the balance of photosynthetic acclimation and photoinhibition.     

Growth pattern and modular reiteration of a hardy coloniser Polytrichum commune Hedw.

Heathlands in Brittany are subject to recurrent summer burning. Recolonisation after large fires begins with bryophytes, but species composition during succession may be altered by colonies of Polytrichum commune Hedw. which appears to be able to prevent reestablishment of the typical heathland formation. Three complementary levels of investigation were adopted in attempting to understand how P. commune, manages to persist and succeed in space open to rapid saturation.Analysis at the level of the local plant community, showed radical changes in the distribution and composition of the herbaceous and muscinal layers over two years. Between May 1992 and May 1994, P. commune cover increased from 13% to 77% accompanied by an appreciable decrease in other pioneer species.We compared individual colony growth of P. commune with that of Polytrichum piliferum, another pioneer moss. In both species, horizontal and vertical growth occurred in spring and autumn. During 1995, P. commune colony diameter had increased by a factor of 4.0 compared with 3.6 for P. piliferum. Over the same interval, colony height increased by a factor of 1.3 for P. commune compared with 1.8 for P. piliferum.Analysis at the level of the module showed that 79% of experimentally cultured stems produced branches, 25% producing a single branch, 32% two branches, and 25% 4 to 7 branches. The lower the location of new branches on culture stems, the longer their leafed section and the greater their overall length. These new branches were morphologically and functionally new ramets.These observations on P. commune conform to the inhibition model of community succession (Connell & Slatyer 1977). Successional patterns recorded in the present study could be interpreted in terms of differences in species colony growth ability. Dense colony growth would allow a species to saturate one of the more important ecosystem compartment. Polytric capacity to grow simultaneously and efficiently in the few centimetres in and above the soil surface could prevent seedling establishment and decrease fitness of coexisting Poaceae.     

Factors involved in potential sulfur accumulation in litter and soil from a coastal pine forest

Samples of 01/02, A1, E2 and Bh horizons collected from a coastal pine forest were assayed for the potential capacity to adsorb and to form organic sulfur from added sulfate. The subsequent mobilization (mineralization) of organic S was also assayed to determine potential capacities of the samples for organic S accumulation. Organic and inorganic forms of S were quantified, as were total carbon and nitrogen levels. Relationships between these parameters and the above S processing capacities for each sample were examined, and fluctuations were statistically analyzed. Of the organic S present, sulfonate-S (non-Raney Ni-reducible S) was found to be the major component irrespective of horizon. Adsorbed and soluble S were found to be low, suggesting that loss by leaching may be an important fate of incoming sulfate at this site. Results from this study were compared with those obtained for two additional sites, which have been examined in detail and documented previously to be S accumulating. Soils from the pine forest site adsorbed substantially less sulfate than those from the other two sites. The organic S accumulation capacity was relatively low for the pine site, averaging less than one third of the potential established for the other two sites.requests for offprints     

Water relations and growth of shrubs before and after fire in a semi-arid woodland

Summary Plant water relations and shoot growth rate of shrubs resprouting after fire or unburnt were measured in a semi-arid poplar box (Eucalyptus populnea) shrub woodland of eastern Australia. In vegetation unburnt for about 60 years, the dawn xylem water potential (_x) of the dominant shrub species was about-1.0 MPa when the soil was wet and-8.0 MPa when the soil was very dry. At any one time, the dominant shrub species,Eremophila mitchellii, E. sturtii, Geijera parviflora andCassia nemophila, were similar in _x butAcacia aneura andDodonaea viscosa were consistently higher in _x than this group when the soil was moist and lower when the soil was dry. The dominant tree species,Eucalyptus populnea andE. intertexta, appeared to have access to additional water beneath the hardpan which is located 60–80 cm below the surface. When shrubs were under extreme water stress (_x of-8 MPa), the trees had a _x of-3 to-3.6 MPa. Following a fire, both _x and leaf stomatal conductance (g _s) of resprouting shrubs were higher for about 5 years than comparable-aged unburnt vegetation, with relative differences in _x increasing with drought stress. Elongation rate of resprouts was positively linked to prefire shrub height in 3 of 4 species. However, shrubs resprouting after high intensity fires had substantially higher rates of shoot elongation than after low intensity fires which were in turn higher than for foliar expansion of unburnt shrubs. It is concluded that the growth rate of resprouting shrubs is primarily determined by physiological/ morphological factors associated with plant size but is also assisted by greater availability of water and possibly nutrients for a period after fire.     

Effects of growth temperature and winter duration on leaf phenology of Erythronium japonicum,a forest spring geophyte

The effects of growth temperature and winter duration on the leaf phenology of Erythronium japonicum were examined in two experiments. Bulbs wintered in the field were cultivated at 10 and 20° C and the bulbs were cultivated at 15° C after chilling treatment at 3° C for 60 and 120 days and without chilling in winter. The plants cultivated at 20° C showed significantly earlier leaf emergence, a more rapid rate of leaf extension and shorter leaf longevity than those cultivated at 10° C. The decrease in the leaf longevity at 20° C resulted from the decreases in the durations of all of the developmental, mature, and senescent phases. The bulbs without chilling treatment did not sprout leaves and those with chilling treatment sprouted leaves. The increase in the length of chilling treatment from 60 to 120 days affected leaf phenology in same manner as the increase in the growth temperature from 10 to 20° C.     

Spatial structure of stoloniferous herbs: an interplay between structural blue-print, ontogeny and phenotypic plasticity

Plant form and spatial structure reflect the basic architectural blue-print of a plant. In most plant species, the expression of the structural blue-print is systematically altered during ontogeny resulting in predictable changes in the allometry of plant structures and in the types of structures that are produced. The expression of the structural blue-print or the timing of ontogenetic changes is also frequently altered by environmental conditions. This latter source of variability, referred to as phenotypic plasticity, is manifested through changes in the timing and rates of meristem initiation and development, the likelihood that meristems will remain dormant or commit to different demographic fates (i.e., vegetative vs. reproductive structures), or the size and structure of the organs formed. Complex interactions among these components can result in considerable differences in form and spatial structure among individuals of the same species. This paper focuses on the importance of these different components in determining the architecture of clonal plants with long internode connections between ramets.A case study is presented that attempts to separate ontogenetic variation and phenotypic plasticity in two stoloniferous species with different structural blue-prints, in their responses to shading. In both species the rate of ontogenetic development responded to intermediate shading levels, but only at very low levels of light availability did plastic changes in branch formation occur. Under shaded conditions the two species achieved similar changes in their architecture in conspicuously different ways. We discuss how different mechanisms leading to a given architecture can be distinguished and what the ecological implications of this are.     

Depth of water acquisition by invading shrubs and resident herbs in a Sierra Nevada meadow

Woody plant encroachment into semiarid ecosystems has become a global trend in recent decades. Due to stream channel incision, the semiarid riparian montane meadows of the southern Sierra Nevada Mountains, USA are experiencing long-term declines in soil moisture. A woody shrub, Artemisia rothrockii A. Gray (Rothrock sagebrush, Asteraceae) is invading these herbaceous meadows. We used an analysis of the stable oxygen isotope ratios of plant and soil water to measure the depth of plant water acquisition during the early stages of this woody plant encroachment. Sagebrush used deeper water on average than most herbs, but it also acquired 10–30% of its water from shallow (<30 cm) soil. Most of the young sagebrush seedlings (1–3 years old, <15 cm) that we sampled used deep water like the older shrubs. Many, but not all of the herb species we sampled were also able to acquire deep water. These findings are consistent with a scenario of shrub encroachment in which channel incision causes shallow-water-dependent herbs to die back, allowing shrub seedlings to establish in disturbed areas during wet years. At least during the early stages of the invasion, some herbs appear to coexist with sagebrush by using deep root systems to cope with the declining shallow soil moisture.     

Allelopathy as a competitive strategy in persistent thickets of Lantana camara L. in three Australian forest communities

Field experiments were established to assess possible allelopathic suppression by Lantana camara L. of two indigenous tree species. The design allowed comparison of allelopathic effects with density-dependent resource competition effects. Fire and its role in competitive interactions was included as an experimental treatment. Allelopathic responses were measured in L. camara thickets by germinating and growing Alectryon subcinereus (A. Gray) Radlk. in dry rainforest ecotones (Macleay River) or Cryptocarya rigida (Meissner) in warm temperate rainforest and wet sclerophyll forest (Lake Macquarie) at 10, 20 and 30 seedlings m^-2, where L. camara was either physically removed (LR), burnt (LB), or cut and left in place (LT). Germination for both species increased significantly by completely removing L. camara (LR) whereas burning (LB) was significant only for C. rigida. Seedling growth for both species was negatively related to increasing density when all L. camara was removed (LR) but was positively related in the other two treatments (LB and LT). C. rigida seedling biomass increased 47.4% (1.75%2.58 g) and 68.6% (1.98%2.95 g) with increasing seedling density for LT and LB respectively and decreased 23.2% (2.93–2.25 g) for LR. A. subcinereus seedling biomass increased 29.7% (1.95–2.53 g) and 34.7% (2.25–3.03 g) with increasing seedling density for LT and LB respectively and decreased 27.9% (3.30–2.38 g) for LR. Phytotoxin dilution effects were inferred in LT and LB rather than density-dependent intraspecific competition, whereas the reverse was true for LR. Seedling biomass for C. rigida resulting from potential phytotoxin dilution at high seedling density was not significantly different from the response of LR at low seedling density but, for A. subcinereus, the phytotoxin dilution response was significantly less than LR at low seedling density. Moderately intense fire (LB) was not significantly different from the LT treatment at both locations, emphasising that moderate to low intensity fires should not be used to control existing invasions of L. camara. Competitive strategies for invasive populations are identified that may modify succession following disturbance, thereby allowing thicket formation and long-term persistence to affect community dynamics. Such strategies need to be recognised in managing natural communities, particularly for biodiversity conservation.Nomenclature: Harden (1990).