High-density Stands Research Articles

Dendroclimatic analysis of red pine affected by Diplodia shoot blight in different latitudinal regions in Michigan

Drought and intraspecific competition in red pine (Pinus resinosa Ait.) stands in Michigan have been implicated as predisposing factors for increased shoot blight risk caused by Diplodia pinea Desm. The overall objective of this study is to examine the interactive effects of drought, other climatic factors, and biotic factors (intraspecific competition) on productivity and growth of red pine forests affected by Diplodia shoot blight. This study incorporates a dendroclimatic approach that allows the retrospective examination of past annual diameter growth in relation to historical climate and the assessment of the potential impacts of future climate change. A total of 20 red pine stands were sampled in two latitudinal regions (the Upper Peninsula (UP) and the Lower Peninsula (LP) regions) of Michigan across two levels of initial stand density (low vs. high) and two levels of forest health condition (healthy vs. D. pinea affected). The full dendroclimatic relationships revealed in this study indicated that other climatic factors, in addition to summer drought stress, impacted red pine radial growth. Overall, red pine radial growth was generally more affected by precipitation and moisture index than solely by temperature variables. The radial growth response to climate depended on latitudinal region: summer moisture stress was more influential in the LP; cold spring and early summer temperatures negatively impacted growth more in the UP; and the degree of winter harshness was more of a factor in the UP. Crown damage caused by winter damage may have predisposed red pine stands to D. pinea affection by providing an easier entry point for fungal infection. Negative relations with precipitation in D. pinea affected stands may be due to increased dispersal of spores in D. pinea affected stands with significant rain and any increased storm and wind activity. Projections of radial growth under future scenarios of climate change indicate that climate warming has the potential to increase growth mainly in red pine stands in the UP region where growth has historically been limited by cool temperatures early in the growing season. Under the moisture index model, growth of red pine under the drier (A2) climate change scenario, only the D. pinea impacted high-density stands in the LP region are projected to show a significant decrease in growth by the middle (2041–2070) and final (2071–2100) projection periods. Model-based projections of forest growth in Michigan generally do not account for forest health issues such as invasive pathogens. The current study therefore provides a new understanding of the role of forest pathogens under future climate change.

Limited Pollen Dispersal Contributes to Population Genetic Structure but Not Local Adaptation in Quercus oleoides Forests of Costa Rica.

Background Quercus oleoides Cham. and Schlect., tropical live oak, is a species of conservation importance in its southern range limit of northwestern Costa Rica. It occurs in high-density stands across a fragmented landscape spanning a contrasting elevation and precipitation gradient. We examined genetic diversity and spatial genetic structure in this geographically isolated and genetically distinct population. We characterized population genetic diversity at 11 nuclear microsatellite loci in 260 individuals from 13 sites. We monitored flowering time at 10 sites, and characterized the local environment in order to compare observed spatial genetic structure to hypotheses of isolation-by-distance and isolation-by-environment. Finally, we quantified pollen dispersal distances and tested for local adaptation through a reciprocal transplant experiment in order to experimentally address these hypotheses.ResultsHigh genetic diversity is maintained in the population and the genetic variation is significantly structured among sampled sites. We identified 5 distinct genetic clusters and average pollen dispersal predominately occurred over short distances. Differences among sites in flowering phenology and environmental factors, however, were not strictly associated with genetic differentiation. Growth and survival of upland and lowland progeny in their native and foreign environments was expected to exhibit evidence of local adaptation due to the more extreme dry season in the lowlands. Seedlings planted in the lowland garden experienced much higher mortality than seedlings in the upland garden, but we did not identify evidence for local adaptation.ConclusionOverall, this study indicates that the Costa Rican Q. oleoides population has a rich population genetic history. Despite environmental heterogeneity and habitat fragmentation, isolation-by-distance and isolation-by-environment alone do not explain spatial genetic structure. These results add to studies of genetic structure by examining a common, tropical tree over multiple habitats and provide information for managers of a successional forest in a protected area.

Soil microbial community resilience with tree thinning in a 40-year-old experimental ponderosa pine forest

Establishment of native grasses is a primary objective of restoration in Pinus ponderosa var. scopulorum (P. & C. Lawson) forests in the southwestern United States. Interactions among native grasses and soil microorganisms generate feedbacks that influence the achievement of this objective. We examined soil chemical properties and communities of plants and soil microorganisms in clear-cuts and P. ponderosa stands thinned and maintained at low and medium tree densities for over 40 years along with high density (unthinned) stands. Phospholipid fatty acids (PLFA) in soils were analyzed to examine arbuscular mycorrhizal (AM) fungi and microbial communities in the three thinning treatments and the unthinned stands with and without a recent broadcast burn. Additionally, two native bunchgrasses, Festuca arizonica and Muhlenbergia wrightii were grown in containers filled with intact soil cores collected from each field plot to more thoroughly compare the abundance of AM fungi and microbial communities across different stand densities and burn treatments. Tree thinning decreased litter cover and increased the abundance and diversity and altered community composition of both herbaceous vegetation and AM fungi. In the mineral soil layer, the pH, total carbon, nitrogen, phosphorus and PLFA profiles did not differ significantly among the four stand density or burn treatments. Mycorrhizal colonization of the container grown grasses did not significantly differ with tree density or burn treatments; however, F. arizonica roots had a strong trend for decreased colonization when grown in soil from high density (unthinned) tree cover. Soil from the containers with F. arizonica had a greater abundance of AM fungal spores. Furthermore, bacterial community composition varied with grass species. Concentration of biomarkers for bacteria were higher in soil that supported F. arizonica compared to soil in which M. wrightii was grown. Our results indicate that the creation of clear-cut openings in forests may increase the abundance and richness of AM fungal propagules and soil bacterial communities were surprisingly resilient to tree thinning and low-intensity fire treatments. These results suggest managing forests to create clear-cut openings generate conditions that favor understory native grasses and AM fungi that are linked to soil bacterial communities.

Biological flora of New Zealand 14: Metrosideros excelsa, pōhutukawa, New Zealand Christmas tree

We review the biology and ecology of Metrosideros excelsa (Myrtaceae), an endemic angiosperm evergreen tree. Metrosideros excelsa belongs to a conspicuous and widely distributed Pacific Basin genus, with centres of diversity in both New Zealand and New Caledonia. Metrosideros excelsa is an iconic tree species that forms a significant component of northern New Zealand's exposed coastal headland and cliff vegetation. Where conditions are more favourable, M. excelsa forms tall coastal forest, ranging from simple young high-density stands to diverse mature forest. Inland, M. excelsa stands are confined to the margins of lakes and rivers on the Central Volcanic Plateau, where some may originate from early Māori plantings. Metrosideros excelsa is reliant on stochastic disturbance events (e.g. landslides, volcanic eruptions) to create open sites necessary for regeneration. Mass flowering (December–January), followed by abundant production of wind-dispersed seed maximises chance colonisation of such sites. Since human settlement in New Zealand, the distribution of M. excelsa forest has declined by c. 90% and the southern limit of the species has retreated north. Natural regeneration on the mainland is limited by the infrequency of large-scale disturbances and increased anthropogenic and herbivore pressures. Consequently, M. excelsa forest has become rare and localised on the mainland; monitoring and active management are fundamental to the species' long-term conservation.

Carbon and nitrogen dynamics in a Pinus densiflora forest with low and high stand densities

Aims understanding carbon ( c) and nitrogen (N) dynamics and their dependence on the stand density of an even-aged, mature forest provides knowledge that is important for forest management. this study investigated the differences in ecosystem total c and N stor age and flux between a low-density stand (D) and a high-density stand (HD) and examined the effects of stand density on above ground net primary productivity (

Green Revolution Trees: Semidwarfism Transgenes Modify Gibberellins, Promote Root Growth, Enhance Morphological Diversity, and Reduce Competitiveness in Hybrid Poplar

Semidwarfism has been used extensively in row crops and horticulture to promote yield, reduce lodging, and improve harvest index, and it might have similar benefits for trees for short-rotation forestry or energy plantations, reclamation, phytoremediation, or other applications. We studied the effects of the dominant semidwarfism transgenes GA Insensitive (GAI) and Repressor of GAI-Like, which affect gibberellin (GA) action, and the GA catabolic gene, GA 2-oxidase, in nursery beds and in 2-year-old high-density stands of hybrid poplar (Populus tremula × Populus alba). Twenty-nine traits were analyzed, including measures of growth, morphology, and physiology. Endogenous GA levels were modified in most transgenic events; GA(20) and GA(8), in particular, had strong inverse associations with tree height. Nearly all measured traits varied significantly among genotypes, and several traits interacted with planting density, including aboveground biomass, root-shoot ratio, root fraction, branch angle, and crown depth. Semidwarfism promoted biomass allocation to roots over shoots and substantially increased rooting efficiency with most genes tested. The increased root proportion and increased leaf chlorophyll levels were associated with changes in leaf carbon isotope discrimination, indicating altered water use efficiency. Semidwarf trees had dramatically reduced growth when in direct competition with wild-type trees, supporting the hypothesis that semidwarfism genes could be effective tools to mitigate the spread of exotic, hybrid, and transgenic plants in wild and feral populations.

Short- and long-term responses to seasonal drought in ponderosa pines growing at different plantation densities in Patagonia, South America

Trees drought responses could be developed in the short- or in the long-term, aiming at sustaining carbon fixation and water use efficiency (WUE). The objective of this study was to examine short- and long-term adjustments occurring in different size Pinus ponderosa Dougl. ex P. & C. Laws trees in response to seasonal drought when they are growing under different competition level. The following variables were studied: branch and stem hydraulic conductivity, canopy and stomatal conductance (gc, gs), transpiration (E), photosynthesis (Amax), wood δ13C (as a proxy of intrinsic WUE), leaf to sapwood area ratio (AL:As) and growth in the biggest (B) and the smallest (S) trees of high (H) and low (L) density stands. AL:As was positively correlated with tree size and negatively correlated with competition level, increasing leaf hydraulic conductance in H trees. Accordingly, higher gc and E per unit AL were found in H than in L trees when soil water availability was high, but decreased abruptly during dry periods. BL trees maintained stable gc and E values even during the summer drought. The functional adjustments observed in H trees allow them to maintain their hydraulic integrity (no apparent ks losses), but their stem and leaf growth were severely affected by drought events. iWUE was similar between all tree groups in a wet season, whereas it significantly decreased in SH trees in a dry season suggesting that when radiation and water are co-limiting gas exchange, functional adjustments not only affect absolute growth, but also WUE.

Natural regeneration of Spanish black pine [Pinus nigra Arn. ssp. salzmannii (Dunal) Franco] at contrasting altitudes in a Mediterranean mountain area

AbstractInformation on plant seed dispersal, natural loss dynamics of seeds and germination are critical for understanding natural regeneration mechanisms. The aim of this study was to assess the effect of different forest stand densities on seedfall, seed predation, and seedling germination of two populations of the endangered Spanish black Pine forests located at lower (Central population) and higher elevation near the limit of the species’ range (peripheral population) in the Cuenca Mountains of Central Spain. The seed predation and germination experiment also included a nested site preparation treatment. Seed fall varied significantly between 2006 and 2005 or 2007 in both populations. During the only mast year of 2006, higher seedfall was observed at lower elevation and in higher density stands. Predation rates were influenced by the seed crop since predators consumed more than 75 % of seeds in years with lower production and less than 15 % in a mast year. Seed germination is influenced by forest habitat, stand density and soil scalping. For common habitat types, and in a high seed production year, better seed germination rates were observed in medium and dense stands (25–30 and 35–40 m2 ha−1, respectively, in terms of basal area). No statistical difference in seed germination rate was found for Spanish black pine forest at its ecological distribution limit between lower and higher densities (15–20 and 35–40 m2 ha−1, in terms of basal area). In both sites, closed stands with soil scalping exhibited higher germination rates.

Carbon Accumulation Patterns During Post-Fire Succession in Cajander Larch (Larix cajanderi) Forests of Siberia

Increased fire activity within boreal forests could affect global terrestrial carbon (C) stocks by decreasing stand age or altering tree recruitment, leading to patterns of forest regrowth that differ from those of pre-fire stands. To improve our understanding of post-fire C accumulation patterns within boreal forests, we evaluated above- and belowground C pools within 17 Cajander larch (Larix cajanderi) stands of northeastern Siberia that varied in both years since fire and stand density. Early-successional stands ( 70-year old), aboveground larch biomass, ANPPtree, and soil organic layer C pools increased with stand age. These stands were low density and multi-aged, containing both mature trees and new recruits. The rapid accumulation of aboveground larch biomass in high-density, mid-successional stands allowed them to obtain C stocks similar to those in much older low-density stands (~8,000 g C m−2). If fire frequency increases without altering stand density, landscape-level C storage could decline, but if larch density also increases, large aboveground C pools within high-density stands could compensate for a shorter successional cycle.

Invasive acacias experience higher ant seed removal rates at the invasion edges

Abstract. Seed dispersal is a key process for the invasion of new areas by exotic species. Introduced plants often take advantage of native generalist dispersers. Australian acacias are primarily dispersed by ants in their native range and produce seeds bearing a protein and lipid rich reward for ant mutualists (elaiosome). Nevertheless, the role of myrmecochory in the expansion of Australian acacias in European invaded areas is still not clear. We selected one European population of Acacia dealbata and another of A. longifolia and offered elaiosome-bearing and elaiosome-removed seeds to local ant communities. For each species, seeds were offered both in high-density acacia stands and in low-density invasion edges. For both acacia species, seed removal was significantly higher at the low-density edges. For A. longifolia, manual elimination of elaiosomes reduced the chance of seed removal by 80% in the low-density edges, whereas it made no difference on the high-density stands. For A. dealbata, the absence of elaiosome reduced seed removal rate by 52%, independently of the acacia density. Our data suggests that invasive acacias have found effective ant seed dispersers in Europe and that the importance of such dispersers is higher at the invasion edges.

Shrub fuel characteristics estimated from overstory variables in NW Spain pine stands

Understanding the relationships between forest overstory and the understory shrub layer is important for predicting biomass changes in response to forest management. The shrub fuel stratum is a critical fire hazard in the Mediterranean region, hence managing the canopy cover to control the understory could be an option for managing fire hazard. In this study we determined the relationship between overstory and understory shrub variables for Pinus pinaster Ait., Pinus radiata D. Don and Pinus sylvestris L. in a large geographical area (NW Spain) using data from the Spanish National Forest Inventory. CHAID and quantile regression procedures were used for obtaining mean and maximum response models, respectively, of shrub characteristics (cover, height and fine fuel load). Overstory variables, especially stand basal area, explained the variation in the mean response of shrub variables. According to maximum response models, maximum shrub development was also limited by overstory variables in P. pinaster and P. radiata stands. This does not apply to P. sylvestris, suggesting that altitude or other non-measured environmental factors are more active limiting constraints for shrub development in stands of this species. Simulation of surface fire behaviour and crown fire initiation was exemplified from P. pinaster data, using quantile regression models as inputs. Fire behaviour outputs revealed that the control of understory shrubs through high overstory basal area can decrease surface fire intensity. Nonetheless, the effect of overstory basal area on crown fire initiation is negligible, and is unlikely to increase fire suppression effectiveness, at least in young stands. Moreover, the potential for crown fire spread and intensity is expected to increase in high-density stands because of the higher canopy bulk density. Forest management can benefit from fire behaviour models, namely in the design of silvicultural treatments targeting crown fire hazard mitigation.

Monitoring the state of a large boreal forest region in eastern Canada through the use of multitemporal classified satellite imagery

Multitemporal classification of Landsat imagery was used to measure and monitor the state of the forest over a large area (11.6 million ha) of boreal forest in eastern Canada using four criteria for a 20 year period (1985–2005). The Enhancement-Classification Method was used in this study. Forty-eight thematic classes based on Canada's National Forest Inventory were identified, then grouped into 13 indicators, and reorganized within four main criteria: (i) forest versus nonforest land cover, (ii) forest development stage, (iii) forest cover type, and (iv) forest cover density. Validation based on 2973 high-resolution geo-referenced digital aerial colour photos of the 2005 classified images showed an overall accuracy of the four criteria of 83%, 68%, 58%, and 62%, respectively. The change in each indicator between 1985 and 2005 could be summarized as: (i) a decrease in productive forest area of 0.4% (approx. 43 000 ha); (ii) a 4.6% decrease in mature stand area, with a concomitant increase in areas classified as vegetated (1.3%) and regenerated (3.4%); (iii) concentration of harvesting pressure on coniferous and mixed stands with respective reductions of 8.2% and 0.8%, due to their conversion to deciduous stands; and (iv) an increase in low-density stands (3.1%) and a decrease in high-density stands (8.3%). These results demonstrate that medium-resolution (30 m) remote sensing tools can be used both to monitor the state of the boreal forest and to produce key indicators, which were extracted from the multidate Landsat satellite imagery.

Genetic diversity and mating system of bracatinga (Mimosa scabrella) in a re-emergent agroforestry system in southern Brazil

Bracatinga (Mimosa scabrella) is a legume tree species common in the early stages of succession in Araucaria angustifolia forests in southern Brazil. Bracatinga can form high-density monospecific stands called bracatingais. Its traditional management for charcoal production involves maintenance of the seed bank. Our objective was to analyze the genetic diversity and structure of bracatingais to understand the mechanisms by which intraspecific diversity of M. scabrella is created and maintained in landscapes managed by family farmers in their agroforestry mosaics. We analyzed 14 bracatingais using 8 allozyme loci. We compared parental and progeny generation indices (7 loci) and described the mating system (9 loci) of two progenies. Overall diversity was high: A = 2.69, H o = 0.257, H e = 0.382 and similar between populations. Overall fixation (F = 0.364) was similar to the fixation index (f = 0.329). The genetic divergence among populations was low (Θp = 0.052) but significant. The progenies’ genetic diversity values were similar to those of the previous generation (H e pop11 = 0.342 vs. 0.420/H e pop10 = 0.432 vs. 0.400). Progenies were compatible with half-sib and full-sib crossing expectations (θ xy = 0.204 and 0.194). Our data showed that there is a tendency for genetic structuring caused not only by the reproductive system but also by genetic drift. It is very likely that the high genetic diversity is amplified by internal migration within each bracatingal. This study showed that current landscape management can contribute to maintaining high levels of bracatinga genetic diversity, which contributes to its regional conservation.

Modelling silviculture alternatives for managing Pinus pinea L.

A yield model was developed to simulate silviculture alternatives for Pinus pinea L. in north-east Spain (Catalonia). The model uses several functions to estimate the main silvicultural parameters at stand level and a disaggregation system to predict diameter distributions. From a network of 75 temporary plots a system of equations to predict stand variables was simultaneously fitted for two stand density types, namely low and high density stands, using the threestage least-squares method (3SLS). The diameter distributions were estimated by the Weibull distribution function using the parameter recovery method (PRM) and the method of moments. Based on this yield model, two silviculture alternatives were simulated for each stand density type and site class, resulting in 16 silviculture scenarios. The yield model and silviculture alternatives offer a management tool and a guide for the sustainable forest management of evenaged Pinus pinea forests in this region.

New cropping designs for globe artichoke industry

A two-year experiment was carried-out in order to evaluate the effects of two plant arrangements (single vs. twin rows) and four plant densities (1.0 -1.2 -1.4 and 1.8 plant m–2) on the agronomical behaviour and head characteristics of three globe artichoke genotypes ( Violetto di Sicilia , Harmony F1 and Madrigal F1 ). The change of the cultivation format toward a high density stand significantly increase yield and yield synchronicity. The twin rows plant arrangement, although reduced total yield, increased the yield synchronicity. Moreover, the cultivation of seed-propagated genotypes ( Harmony F1 and Madrigal F1 ) allowed extending significantly the availability of the heads across the year. On the basis of our results, we can assert that the implementation of a specific scheduling cultivation, based on higher density stands, twin rows plant arrangement and the integration of the traditional early genotypes with the new seed-propagated cultivars, is a promising way to match the requirements of a globe artichoke industrial crop, and to predispose a better mechanization of the cultural practices.

Relationships between fire severity and post-fire landscape pattern following a large mixed-severity fire in the Valle Vidal, New Mexico, USA

The predominant fire regime associated with ponderosa pine ( Pinus ponderosa) forests in the southwestern US has shifted from the historic norm of frequent, low-severity fires to less frequent mixed-severity and crown fires. This change in the severity of fire has altered ponderosa pine forests from the open stands typical of pre-settlement times to even-aged, high-density stands at increased risk of crown fire. As a result, restoration plans and post-fire management practices must consider the spatial and temporal variability of fire severity in both mixed-severity and crown fire events because fire-severity patterns strongly influence post-fire ecological conditions. This study examines the landscape pattern of fire severity in the Ponil Complex Fire and applies a moving-window approach to post-fire landscape pattern measurement. The moving-window approach allows examination of the quantitative and spatial variability of landscape pattern, producing a more nuanced description of forest pattern when compared to whole-landscape or patch-based metrics. The fire resulted in a complex mosaic of fire patches and forest-structure changes. In high-severity fire patches, mean and median values of many post-fire landscape metrics were markedly different from those in low and moderate-severity patches. Landscape pattern in high-severity patches also had the greatest variability of metric values, suggesting that high-severity fire patches require a spatially mediated management response to fire. Categorical fire-severity maps and traditional landscape-pattern assessment would not be able to identify these spatially variable post-fire conditions.

Long-term responses of mammalian herbivores to stand thinning and fertilization in young lodgepole pine (Pinus contortavar.latifolia) forest

Snowshoe hares (Lepus americanus Exrleben, 1777), mule deer (Odocoileus hemionus (Rafinesque, 1817)), and moose (Alces alces (L., 1758)) commonly occur in young coniferous forests. This study was designed to test the hypothesis that large-scale pre-commercial thinning (PCT) and repeated fertilization 15–20 years after the onset of treatments in young lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Wats.) stands would enhance relative habitat use by hares, deer, and moose compared with unmanaged stands. Study areas were located in south-central British Columbia, Canada. Habitat use was measured by fecal pellet and pellet-group counts. Understory vegetation and coniferous stand structure were measured in all stands. Habitat use by deer and moose was highest in heavily thinned stands, probably due to the higher levels of forage and cover provided by understory shrubs and conifers in thinned stands. Habitat use by snowshoe hares was highest in high-density stands, but also in lower-density (≤1000 stems·ha–1) stands where an increase in understory conifers provided essential cover for hares. Managers should consider the long-term nature of understory development in young stands managed for timber production. Heavy thinning (≤1000 stems·ha–1) will generate suitable understory habitat for these herbivores sooner than conventional PCT at higher stand densities.

Agricultural legacy, climate, and soil influence the restoration and carbon potential of woody regrowth in Australia

Opportunities for dual restoration and carbon benefits from naturally regenerating woody ecosystems in agricultural landscapes have been highlighted recently. The restoration capacity of woody ecosystems depends on the magnitude and duration of ecosystem modification, i.e., the "agricultural legacy." However, this legacy may not influence carbon sequestration in the same way as restoration because carbon potential depends primarily on biomass accumulation, with little consideration of other attributes and functions of the ecosystem. Our present study simultaneously assesses the restoration and carbon potential of Acacia harpophylla regrowth, an extensive regrowth ecosystem in northeastern Australia. We used a landscape-scale survey of A. harpophylla regrowth to test the following hypotheses: (1) management history, in combination with climatic and edaphic factors, has long-term effects on stem densities, and (2) higher-density stands have lower restoration and carbon potential, which is also influenced by climatic and edaphic factors. We focused on the restoration of forest structure, which was characterized using stem density, aboveground biomass, stem heights, and stem diameters. Data were analyzed using multilevel models within the hierarchical Bayesian model (HBM) framework. We found strong support for both hypotheses. Repeated attempts at clearing Brigalow (A. harpophylla ecosystem) regrowth increases stem densities, and these densities remain high over the long term, particularly in high-rainfall areas and on gilgaied, high-clay soils (hypothesis 1). In models testing hypothesis 2, interactions between stem density and stand age indicate that higher-density stands have slower biomass accumulation and structural development in the long term. After accounting for stem density and stand age, annual rainfall had a positive effect on biomass accumulation and structural development. Other climate and soil variables were retained in the various models but had weaker effects. Spatial extrapolations of the HBMs indicated that the central and eastern parts of the study region are most suitable for biomass accumulation; however, these may not correspond to the areas that historically supported the highest biomass Brigalow forests. We conclude that carbon and restoration goals are largely congruent within areas of similar climate. At the regional scale, however, spatial prioritization of restoration and carbon projects may only be aligned where carbon benefits will be high.

Responses to conspecific density in an arborescent bryozoan

Sessile colonial invertebrates that must tolerate environmental variation without the luxury of mobility may be expected to cope with such variation through phenotypic plasticity. While plastic responses to a range of biotic factors (e.g. predation) are increasingly documented, the details of responses to competition (mediated by the densities of conspecifics or heterospecifics vying for the same resources) remain unclear, despite a rich literature for terrestrial plants on which to draw. We examined phenotypic responses to conspecific density in a colonial invertebrate (the arborescent bryozoan, Bugula neritina) under field conditions. We found that colonies at higher densities were generally less fecund, lower in biomass and more elongate than those growing virtually alone. We also found such responses to vary only subtly with the life history stage of neighbours, with colonies exposed to higher densities of contemporary recruits achieving elongation via increased budding between bifurcation points and the elongation of individual zooids, while colonies exposed to established adults achieved elongation via increased budding only. These responses are broadly consistent with those of terrestrial plants competing for light and are probably due to the combined effects of resource limitation and altered flow in high density stands. Future studies are needed to disentangle these effects and the extent to which such responses to density in branching colonial invertebrates are a passive result of food limitation, or reflect active (and potentially adaptive) plasticity to other aspects of the local density environment.

A coupled view of above and below-ground resource capture explains different weed impacts on soil water depletion and crop water productivity in maize

Conventional wisdom suggests that soil water lost through transpiration by weeds can intensify crop drought stress, and that this is an important mechanism by which weed competition decreases yields and crop water productivity (CWP). To assess the impact of weed competition on soil water uptake and crop drought stress, a 3-year field experiment was conducted with maize ( Zea mays L.) grown in monoculture and in combination with high-density stands of three troublesome weed species ( Setaria faberi Herrm., Abutilon theophrasti Medic., and Amaranthus retroflexus L.). Independent of weed species type, when canopy light interception was largely unchanged by the presence of weeds, maize–weed mixtures did not deplete more soil water than maize grown in monoculture. Conversely, when early weed growth was vigorous, maize–weed mixtures intercepted more light early in the growing season and this resulted in higher initial rates of water depletion and drier soils. In both cases, measurements of stomatal conductance suggested that the internal water status of the maize was not significantly impacted by the presence of weeds as the soil dried. CWP was influenced by weed competition and in the most extreme case declined by almost 50% relative to monoculture maize (2.6 kg m −3 vs. 5.0 kg m −3). However, the impact of weeds on CWP was not uniform. Under low yield loss scenarios (ca. 10%), weeds had little impact on the soil water balance and CWP did not markedly decline relative to monoculture maize. Our results suggest that if weeds do not increase canopy light interception and are subordinate to lower parts of the canopy, high-density infestations can be retained in the field without increasing the probability of drought or substantially reducing CWP.