What limits herb biomass in grasslands: competition or herbivory?

Competition and herbivory are two of the main forces shaping plant communities. Although several studies have investigated their impact on plant populations separately, few investigations have examined how they might interact. With the purpose of clarifying the combined roles of competition and herbivory on herb biomass in a grassland, we assessed the effects of different herbivores with reduced grass competition. We conducted a field experiment in 2000-2001 in a British acid grassland (Oak Mead), where we experimentally reduced grass biomass and excluded rabbits, insects and molluscs in a factorial design. Removing the grasses from Oak Mead dramatically increased herb biomass and total above-ground biomass. Herbivore exclusions (i.e. rabbits, insects and molluscs) did not affect total above-ground biomass, but they altered relative abundance of several species. Grass removal and rabbit exclusion had positive interactions on biomass of several herb species, and there were some subtle interactions between different herbivore groups: monocots benefitted when both rabbits and molluscs were excluded, and some herb species had greater biomass when insects and rabbits were absent. We then compared the results with a 10-year experiment that manipulated similar variables in neighbouring grassland (Nashs Field). The comparison between Oak Mead and Nashs Field showed that cessation of herbicide application returns the system to its previous state of grass dominance after 3 years. Therefore, even when herbs were more abundant, they could not prevent reinvasion of the grasses once external factors were removed.     

Tree growth response along an elevational gradient: climate or genetics?

Environment and genetics combine to influence tree growth and should therefore be jointly considered when evaluating forest responses in a warming climate. Here, we combine dendroclimatology and population genetic approaches with the aim of attributing climatic influences on growth of European larch (Larix decidua) and Norway spruce (Picea abies). Increment cores and genomic DNA samples were collected from populations along a ~900-m elevational transect where the air temperature gradient encompasses a ~4 °C temperature difference. We found that low genetic differentiation among populations indicates gene flow is high, suggesting that migration rate is high enough to counteract the selective pressures of local environmental variation. We observed lower growth rates towards higher elevations and a transition from negative to positive correlations with growing season temperature upward along the elevational transect. With increasing elevation there was also a clear increase in the explained variance of growth due to summer temperatures. Comparisons between climate sensitivity patterns observed along this elevational transect with those from Larix and Picea sites distributed across the Alps reveal good agreement, and suggest that tree-ring width (TRW) variations are more climate-driven than genetics-driven at regional and larger scales. We conclude that elevational transects are an extremely valuable platform for understanding climatic-driven changes over time and can be especially powerful when working within an assessed genetic framework.     

Sympatric,parapatric or allopatric: the most important way to classify speciation?

The most common classification of modes of speciation begins with the spatial context in which divergence occurs: sympatric, parapatric or allopatric. This classification is unsatisfactory because it divides a continuum into discrete categories, concentrating attention on the extremes, and it subordinates other dimensions on which speciation processes vary, such as the forces driving differentiation and the genetic basis of reproductive isolation. It also ignores the fact that speciation is a prolonged process that commonly has phases in different spatial contexts. We use the example of local adaptation and partial reproductive isolation in the intertidal gastropod Littorina saxatilis to illustrate the inadequacy of the spatial classification of speciation modes. Parallel divergence in shell form in response to similar environmental gradients in England, Spain and Sweden makes this an excellent model system. However, attempts to demonstrate 'incipient' and 'sympatric' speciation involve speculation about the future and the past. We suggest that it is more productive to study the current balance between local adaptation and gene flow, the interaction between components of reproductive isolation and the genetic basis of differentiation.     

Effects of climate, tree age, dominance and growth on δ15N in young pinewoods

Needles, annual rings from basal stem discs and bark of three dominant and three suppressed Pinus pinaster from a 12-year-old pine stand (naturally regenerated after a wildfire) were analysed to study the effects of climate, tree age, dominance, and growth on tree δ¹⁵N. Foliar-N concentration in dominant pines (0.780–1.474% N) suggested that soil N availability was sufficient, a circumstance that allowed isotopic discrimination by plants and (greater) differences in δ¹⁵N among trees. The δ¹⁵N decreases in the order wood (−0.20 to +6.12‰), bark (−1.84 to +1.85‰) and needles (−2.13 to +0.77‰). In all trees, before dominance establishment (years 1–8), the N stored in each ring displayed a decreasing δ¹⁵N tendency as the tree grows, which is mainly due to a more “closed” N cycle or an increasing importance of N sources with lower δ¹⁵N. After dominance establishment (years 9–12), wood δ¹⁵N values were higher in suppressed than in dominant trees (2.62 and 1.46‰, respectively; P < 0.01) while the reverse was true for needles and bark; simultaneously, the absolute amount of N stored by suppressed pines in successive rings decreased, suggesting a lower soil N assimilation. These results could be explained by lignification acting as major N source for needles in suppressed pines because products released and reallocated during lignification are ¹⁵N-depleted compared with the source. According to principal component analysis, wood δ¹⁵N appears associated with wood N concentration and precipitation during the growing season, but clearly opposed to age, basal area increment and mean temperature in spring and summer.     

Is growth of soil microorganisms in boreal forests limited by carbon or nitrogen availability?

To study whether the biomass of soil microorganisms in a boreal Pinus sylvestris-Vaccinium vitis-idaea forest was limited by the availability of carbon or nitrogen, we applied sucrose from sugar cane, a C₄ plant, to the organic mor-layer of the C₃–C dominated soil. We can distinguish between microbial mineralization of the added sucrose and respiration of endogenous carbon (root and microbial) by using the C₄-sucrose as a tracer, exploiting the difference in natural abundance of ¹³C between the added C₄-sucrose (¹³C –10.8) and the endogenous C₃–carbon (¹³C –26.6 ). In addition to sucrose, NH₄Cl (340 kg N ha^–1) was added factorially to the mor-layer. We followed the microbial activity for nine days after the treatments, by in situ sampling of CO₂ evolved from the soil and mass spectrometric analyses of ¹³C in the CO₂. We found that microbial biomass was limited by the availability of carbon, rather than nitrogen availability, since there was a 50% increase in soil respiration in situ between 1 h and 5 days after adding the sucrose. However, no further increase was observed unless nitrogen was also added. Analyses of the ¹³C ratios of the evolved CO₂ showed that increases in respiration observed between 1 h and 9 days after the additions could be accounted for by an increase in mineralization of the added C₄–C.     

Is the biomass of water hyacinth lost through herbivory in native areas important?

The lamina area damaged and biomass per leaves removed by invertebrate herbivores were measured across seasons on water hyacinth, Eichhornia crassipes (Mart.) Solms (Pontederiaceae). The amount of the leaf biomass per meter square lost through herbivory was also assessed in different sampling dates in the plant population. Ten leaves of water hyacinth were sampled in each of 18 site-habitat-date combinations. Sampling dates were chosen to follow the plant phenology. The lamina area damaged (surface abrasions and holes) was measured with the visual estimation method; biomass removed by herbivores (surface abrasions and holes) was calculated indirectly from the damaged lamina area. Significant differences in total damaged area and removed biomass per lamina were found between sampling dates at each site, with highest values in March (end of growth period). Total damaged area per lamina (surface abrasions + holes) varied between 11% in March and 6% in July (decay period). Total removed biomass (surface abrasions + holes) varied between 27% in March and 13% in July. Significant differences in biomass removed by herbivory were found between sampling dates at each site. Biomass of lamina removed by herbivores per m⁻² varied between 26 and 13% in different seasons. The herbivore damage of discrete samples and the indirect method to calculate the biomass removed is useful in sites with aquatic free floating plants, where experimental exclusion of insects may be difficult to carry out.     

Size-dependent properties of matter: Is the size of a pill important?

This interdisciplinary scientific inquiry lesson specifically utilizes the 5E learning cycle to engage high school students in an investigation on size-dependent properties of matter. In particular, this inquiry lesson focuses on a biologically relevant phenomenon, namely accessibility to a pharmaceutical drug with respect to the size of the pill. In this context, students design and conduct a controlled experiment to test how the accessibility to an encapsulated drug is affected by the change in the size of the pill. Thus, through this investigation, students not only learn about the relationship between the size of a material in terms of surface area-to-volume ratio and the rate of diffusion of molecules, but also extend this knowledge to the importance of size in the context of nanoscale. Additionally, students practice the science process skills involved in undertaking a scientific inquiry.     

Island size, isolation, or interspecific competition? The breeding distribution of the Parus guild in the Danish archipelago

The Parus guild (Parus spp., Sitta, Certhia, and Regulus) is distributed as a complex mosaic within the Danish archipelago, with from one to eight species on different islands. We assessed the roles of island isolation, island size, and interspecific competition in determining the breeding species compositions of this guild on 53 Danish islands. Small, isolated islands supported fewer species than larger, nearshore islands. These effects, however, were largely restricted to a few sedentary species (P. cristatus, P. palustris, S. europaea) that are known to be poor dispersers/colonizers. In some cases, these three species were also absent from large, nearshore islands with suitable habitat, suggesting that habitat availability was not always responsible for the absence of a species. Monte Carlo simulations suggested that the pattern of species presence/absence was not a result of interspecific interactions. Thus, although a number of previous studies have documented interspecific competition among members of the Parus guild, our results suggest that such competition is not responsible for the unusual pattern of species distribution within the Danish archipelago. Received: 28 October 1996 / Accepted: 7 February 1997     

Is climate an important driver of post‐European vegetation change in the Eastern United States?

Many ecological phenomena combine to direct vegetation trends over time, with climate and disturbance playing prominent roles. To help decipher their relative importance during Euro‐American times, we employed a unique approach whereby tree species/genera were partitioned into temperature, shade tolerance, and pyrogenicity classes and applied to comparative tree‐census data. Our megadata analysis of 190 datasets determined the relative impacts of climate vs. altered disturbance regimes for various biomes across the eastern United States. As the Euro‐American period (ca. 1500 to today) spans two major climatic periods, from Little Ice Age to the Anthropocene, vegetation changes consistent with warming were expected. In most cases, however, European disturbance overrode regional climate, but in a manner that varied across the Tension Zone Line. To the north, intensive and expansive early European disturbance resulted in the ubiquitous loss of conifers and large increases of Acer, Populus, and Quercus in northern hardwoods, whereas to the south, these disturbances perpetuated the dominance of Quercus in central hardwoods. Acer increases and associated mesophication in Quercus‐Pinus systems were delayed until mid 20th century fire suppression. This led to significant warm to cool shifts in temperature class where cool‐adapted Acer saccharum increased and temperature neutral changes where warm‐adapted Acer rubrum increased. In both cases, these shifts were attributed to fire suppression rather than climate change. Because mesophication is ongoing, eastern US forests formed during the catastrophic disturbance era followed by fire suppression will remain in climate disequilibrium into the foreseeable future. Overall, the results of our study suggest that altered disturbance regimes rather than climate had the greatest influence on vegetation composition and dynamics in the eastern United States over multiple centuries. Land‐use change often trumped or negated the impacts of warming climate, and needs greater recognition in climate change discussions, scenarios, and model interpretations.     

Partitioning the components of relative growth rate: how important is plant size variation?

Plant growth plays a key role in the functioning of the terrestrial biosphere, and there have been substantial efforts to understand why growth varies among species. To this end, a large number of experimental analyses have been undertaken; however, the emergent patterns between growth rate and its components are often contradictory. We believe that these conflicting results are a consequence of the way growth is measured. Growth is typically characterized by relative growth rate (RGR); however, RGR often declines as organisms get larger, making it difficult to compare species of different sizes. To overcome this problem, we advocate using nonlinear mixed-effects models so that RGR can be calculated at a standard size, and we present easily implemented methods for doing this. We then present new methods for analyzing the traditional components of RGR that explicitly allow for the fact that (log)(RGR) is the sum of its components. These methods provide an exact decomposition of the variance in (log)(RGR). Finally, we use simple analytical and simulation approaches to explore the effect of size variation on growth and its components and show that the relative importance of the components of RGR is influenced by the extent to which analyses standardize for plant size.     

Is there genetic variation in the response to competition intensity in juvenile brown trout Salmo trutta?

Effects of intraspecific competition intensities on the relative performance (growth and movement) of juvenile brown trout Salmo trutta originating from nine different families were tested in tank experiments and in semi-natural streams. Both growth and movement differed consistently among families, indicating genetic variation in these traits. There were no significant interaction effects, however, between the intensity of competition and family origin on performance in either of the two experimental systems. Thus, genetic variation in response to competition intensity appeared to be limited in the population from which the juveniles used in this experiment originate.     

Is halogen content the most important factor in the removal of halogenated trace organics by MBR treatment?

This study investigated the relationship between physicochemical properties (namely halogen content and hydrophobicity) of halogenated trace organics and their removal efficiencies by a laboratory scale membrane bioreactor (MBR) under stable operating conditions. The reported results demonstrated a combined effect of halogen content and hydrophobicity on the removal. Compounds with high halogen content (>0.3) were well removed (>85%) when they possessed high hydrophobicity (Log D>3.2), while those with lower Log D values were also well removed if they had low halogen content (<0.1). General indices such as the BIOWIN index (which is based on only biodegradation) or a more specific index such as the halogen content (which captures a chemical aspect) appeared insufficient to predict the removal efficiency of halogenated compounds in MBR. Experimental data confirmed that the ratio of halogen content and Log D, which incorporates two important physico-chemical properties, is comparatively more suitable.     

What is the role of disturbance in catalyzing spatial shifts in forest composition and tree species biomass under climate change?

Mounting evidence suggests that climate change will cause shifts of tree species range and abundance (biomass). Abundance changes under climate change are likely to occur prior to a detectable range shift. Disturbances are expected to directly affect tree species abundance and composition, and could profoundly influence tree species spatial distribution within a geographical region. However, how multiple disturbance regimes will interact with changing climate to alter the spatial distribution of species abundance remains unclear. We simulated such forest demographic processes using a forest landscape succession and disturbance model (LANDIS-II) parameterized with forest inventory data in the northeastern United States. Our study incorporated climate change under a high-emission future and disturbance regimes varying with gradients of intensities and spatial extents. The results suggest that disturbances catalyze changes in tree species abundance and composition under a changing climate, but the effects of disturbances differ by intensity and extent. Moderate disturbances and large extent disturbances have limited effects, while high-intensity disturbances accelerate changes by removing cohorts of mid- and late-successional species, creating opportunities for early-successional species. High-intensity disturbances result in the northern movement of early-successional species and the southern movement of late-successional species abundances. Our study is among the first to systematically investigate how disturbance extent and intensity interact to determine the spatial distribution of changes in species abundance and forest composition.     

Herd size in large herbivores: encoded in the individual or emergent?

In large mammalian herbivores, the increase of group size with habitat openness was first assumed to be an adaptive response, encoded in the individual. However, it could, alternatively, be an emergent property: if groups were nonpermanent units, often fusing and splitting up, then any increase of the distance at which animals perceive one another could increase the rate of group fusion and thus mean group size. Dynamical models and empirical data support this second hypothesis. This is not to say that adaptive modifications of mean herd size cannot occur. However, this changes the way in which we can envisage the history of gregariousness in large herbivores during the Tertiary.     

On the influence of tree size on the climate–growth relationship of New Zealand kauri (Agathis australis): insights from annual, monthly and daily growth patterns

Many tree-ring-based climate reconstructions are based on the assumption that the climate reaction of trees is independent of their size. Here, we test this assumption for New Zealand kauri (Agathis australis), one of the longest tree ring-based proxies for the El Niño-Southern Oscillation (ENSO). The most recent kauri chronology contains a large amount of archaeological material, e.g. timber for which the original tree size is often unknown. We analyzed the climate–growth relationship of different-sized kauri in a pristine forest using different temporal scales, i.e. annually, monthly and daily data on tree growth and climate conditions. Trees of different life stages exhibited approximately the same seasonal growth peaks during austral spring (October and November). The dormancy period overlaps with the period where weekly air temperature maxima are below ca. 17–18 °C, and where the corresponding daily minima are below ca. 8 °C. However, both correlation functions between annual growth and seasonal climate as well as Kalman filter regressions between daily growth and climate conditions suggest an influence of tree size on the climate–growth relationship for kauri. Smaller trees (DBH < 40 cm) contain weaker climate signals than larger trees. Therefore, the precautionary stripping of near-pith material (first 20 cm) from kauri chronologies may result in more uniform responses to climate forcing and thus enhance the reliability of long-term climate reconstructions.     

Variation in growth and age at maturity in bluegill sunfish: genetic or environmental effects?

Populations of bluegill sunfish Lepomis macrochirus , experiencing heavy juvenile predation, showed increased growth rates and increased age and size at maturity relative to populations experiencing decreased predation on juveniles but increased predation on adults. This study examined bluegills experimentally from both types of populations and a cross between them in a common environment to determine if variation in growth and age at maturity is genetically or environmentally induced. Two factorial experiments, varying strain of bluegills and resource availability, were used to evaluate differences in growth rate. One experiment, varying strain of bluegills, was used to assess differences in age at maturity. Growth was strongly influenced by resource level, but growth rate did not vary among populations. Nearly all bluegills in each population matured at 1 year of age in a common environment. Thus, variation observed in source populations must be mostly attributable to differences in the environment between populations. At least three factors could potentially cause differences in growth and age at maturity: (1) variation in resource availability; (2) variation in demographic structure; and (3) variation in size-specific mortality rates caused by differences in predator abundance between populations. Observed patterns of variation between populations are best explained by effects of differences in predator populations.     

Is microbial community composition in boreal forest soils determined by pH,C-to-N ratio,the trees,or all three?

In Fennoscandian boreal forests, soil pH and N supply generally increase downhill as a result of water transport of base cations and N, respectively. Simultaneously, forest productivity increases, the understory changes from ericaceous dwarf shrubs to tall herbs; in the soil, fungi decrease whereas bacteria increase. The composition of the soil microbial community is mainly thought to be controlled by the pH and C-to-N ratio of the substrate. However, the latter also determines the N supply to plants, the plant community composition, and should also affect plant allocation of C below ground to roots and a major functional group of microbes, mycorrhizal fungi. We used phospholipid fatty acids (PLFAs) to analyze the potential importance of mycorrhizal fungi by comparing the microbial community composition in a tree-girdling experiment, where tree belowground C allocation was terminated, and in a long-term (34 years) N loading experiment, with the shifts across a natural pH and N supply gradient. Both tree girdling and N loading caused a decline of ca. 45% of the fungal biomarker PLFA 18:2ω6,9, suggesting a common mechanism, i.e., that N loading caused a decrease in the C supply to ectomycorrhizal fungi just as tree girdling did. The total abundance of bacterial PLFAs did not respond to tree girdling or to N loading, in which cases the pH (of the mor layer) did not change appreciably, but bacterial PLFAs increased considerably when pH increased across the natural gradient. Fungal biomass was high only in acid soil (pH < 4.1) with a high C-to-N ratio (>38). According to a principal component analysis, the soil C-to-N ratio was as good as predictor of microbial community structure as pH. Our study thus indicated the soil C-to-N ratio, and the response of trees to this ratio, as important factors that together with soil pH influence soil microbial community composition.     

Is the spatial pattern of a tree population in a seasonally dry tropical climate explained by density‐dependent mortality?

Spatial pattern of tropical plants is initially generated by limited seed dispersal, but the role of density‐dependent and independent mechanisms as modifiers of these patterns across ontogeny is poorly understood. We investigated whether density‐dependent mortality (DDM) and environmental heterogeneity can drive spatial pattern across the ontogeny of a tree in a seasonally dry tropical climate. We used Moran's I correlograms and spatial analysis by distance indices (SADIE) to assess the spatial patterns of the pre‐ and post‐germinative stages of Cordia oncocalyx (Boraginaceae), an abundant tree endemic in the deciduous thorny woodland in the northeastern Brazilian semiarid region. We also used RDA to analyse the effect of DDM and environmental heterogeneity (measured by microtopography and canopy openness) in the mortality and recruitment. Seeds, seedlings, juveniles and adults showed aggregated spatial patterns; infants and immatures were randomly distributed; adults, seeds and seedlings attracted each other while adult, juveniles and immatures repulsed each other. Infant and seedling mortality rates were related to DDM and the recruitment from infant to juvenile was more influenced by spatial heterogeneity. Attraction was determined by local dispersal; repulsion was related to DDM and environment heterogeneity, which allowed the return to aggregation in adult stage. Together, these results indicated that spatial pattern can change across ontogeny, in which the initial stages are responsive to DDM and the final stages are influenced by spatial heterogeneity.