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Using physical models to predict patterns of plant growth has been a long-standing goal for biologists. Most approaches invoke either thermodynamic, biomechanical or hydraulic principles and assume the mechanism of interest applies similarly throughout the plant branching architecture. A recent effort, the flow similarity model, predicts numerous aspects of branching physiology and morphology and argues that the physiological constraints experienced by plants change as a function of branch order and size, with more basal portions satisfying more biomechanical constraints, and more distal portions, hydraulic ones. Distal branches are expected to have a strong influence on allometric relationships within plants due to their numerical abundance. Here we evaluate the predictions of the flow similarity model and a well-known alternative fractal branching model, using data on the dimensions of 3,484 individual stem internodes across four individual Acer platanoides trees. Overall, we find strong agreement between model predictions and the allometric exponents describing tree branch allometry. Further the predicted curvature in allometric relationships is found in all 24 cases examined and the frequency distributions of branch lengths and diameters are consistent with model expectations in 6/8 cases. We also find the area ratios are consistent with the model assumption of area-preserving branching. Collectively, our data and analysis provide strong support for the flow similarity model, and identifies several areas in need of subsequent inquiry.

Temperate deciduous forest understoreys are experiencing widespread changes in community composition, concurrent with increases in rates of nitrogen supply. These shifts in plant abundance may be driven by interspecific differences in nutrient foraging (i.e. conservative vs. acquisitive strategies) and, thus, adaptation to contemporary nutrient loading conditions. This study sought to determine if interspecific differences in nutrient foraging could help explain patterns of shrub success and decline in eastern North American forests. Using plants grown in a common garden, fine root traits associated with nutrient foraging were measured for six shrub species. Traits included the mean and skewness of the root diameter distribution, specific root length (SRL), C:N ratio, root tissue density, arbuscular mycorrhizal colonization and foraging precision. Above- and below-ground productivity were also determined for the same plants, and population growth rates were estimated using data from a long-term study of community dynamics. Root traits were compared among species and associations among root traits, measures of productivity and rates of population growth were evaluated. Species fell into groups having thick or thin root forms, which correspond to conservative vs. acquisitive nutrient foraging strategies. Interspecific variation in root morphology and tissue construction correlated with measures of productivity and rates of cover expansion. Of the four species with acquisitive traits, three were introduced species that have become invasive in recent decades, and the fourth was a weedy native. In contrast, the two species with conservative traits were historically dominant shrubs that have declined in abundance in eastern North American forests. In forest understoreys of eastern North America, elevated nutrient availability may impose a filter on species success in addition to above-ground processes such as herbivory and overstorey canopy conditions. Shrubs that have root traits associated with rapid uptake of soil nutrients may be more likely to increase in abundance, while species without such traits may be less likely to keep pace with more productive species.

Traits associated with root morphology and nutrient uptake rate may contribute to the competitive ability of invasive species by determining their access to soil nutrients and their ability to extract those resources. Here, we tested the hypotheses that (a) exotic woody shrubs would be superior belowground competitors for nitrogen in heterogeneous soil resulting from key aspects of root architecture and (b) larger plants would be superior belowground competitors. We tested these hypotheses using two native shrubs, Rubus allegheniensis and Viburnum dentatum, and two invasive exotic shrubs, Rubus phoenicolasius and Berberis thunbergii, all four of which can become abundant in plant communities in the eastern United States. We grew replicate plants from each species with interspecific competitors, with intraspecific competitors, and individually in a randomized layout in a greenhouse in two temporal blocks. Each experimental container had a central soil patch amended with ¹⁵N-labeled litter. We measured above- and belowground growth, root morphology, and nitrogen uptake to assess the effects of intra- and interspecific competition on plant growth and nitrogen uptake. All species grew better in the second temporal block, but we did not detect any differences in the competitive ability or root traits for exotic versus native species; rather, plant size was the key trait that predicted competitive effects. Both Rubus species, which capitalized on the extended growing season offered by our greenhouse conditions, were stronger competitors and typically larger plants than B. thunbergii and V. dentatum. Both Rubus species exerted measurable competitive effects on other species, resulting in decreased aboveground size of competitors by 50% or more relative to control plants, but did not routinely decrease ¹⁵N uptake or root biomass of competitors. When competing with Rubus, leaf C:N ratios of all species except R. phoenicolasius were greater than when grown alone, suggesting that large Rubus plants did decrease the total nitrogen available to competitors. Our data suggest that belowground competitive ability in shrubs may be more closely associated with plant size and growth rate than plant origin. In addition, plant species that exhibit plastic growth phenology, such as those in the genus Rubus, may gain a competitive advantage during years with warmer autumn months by extending their growing seasons, facilitating their invasion and establishment in new habitats.

Traits associated with root morphology and nutrient uptake rate may contribute to the competitive ability of invasive species by determining their access to soil nutrients and their ability to extract those resources. Here, we tested the hypotheses that (a) exotic woody shrubs would be superior belowground competitors for nitrogen in heterogeneous soil resulting from key aspects of root architecture and (b) larger plants would be superior belowground competitors. We tested these hypotheses using two native shrubs, Rubus allegheniensis and Viburnum dentatum, and two invasive exotic shrubs, Rubus phoenicolasius and Berberis thunbergii, all four of which can become abundant in plant communities in the eastern United States. We grew replicate plants from each species with interspecific competitors, with intraspecific competitors, and individually in a randomized layout in a greenhouse in two temporal blocks. Each experimental container had a central soil patch amended with 15N-labeled litter. We measured above- and belowground growth, root morphology, and nitrogen uptake to assess the effects of intra- and interspecific competition on plant growth and nitrogen uptake. All species grew better in the second temporal block, but we did not detect any differences in the competitive ability or root traits for exotic versus native species; rather, plant size was the key trait that predicted competitive effects. Both Rubus species, which capitalized on the extended growing season offered by our greenhouse conditions, were stronger competitors and typically larger plants than B. thunbergii and V. dentatum. Both Rubus species exerted measurable competitive effects on other species, resulting in decreased aboveground size of competitors by 50% or more relative to control plants, but did not routinely decrease 15N uptake or root biomass of competitors. When competing with Rubus, leaf C:N ratios of all species except R. phoenicolasius were greater than when grown alone, suggesting that large Rubus plants did decrease the total nitrogen available to competitors. Our data suggest that belowground competitive ability in shrubs may be more closely associated with plant size and growth rate than plant origin. In addition, plant species that exhibit plastic growth phenology, such as those in the genus Rubus, may gain a competitive advantage during years with warmer autumn months by extending their growing seasons, facilitating their invasion and establishment in new habitats.

Acer platanoides L. individuals were dissected to determine if branch allometry changed as branches increased in length. Branches were found to transition from a log-log curvilinear relationship to a linear relationship when above 3,000 mm in length. The log-log linear relationship was best modeled with the elastic similarity model. The total number of subordinate lateral branches was found to increase rapidly after the primary branch length surpassed 3,000 mm, suggesting that branches are transitioning to a structural role as size increases. The shift in allometry appears to correspond to a shift from increasing slenderness ratio (length/radius) with increasing branch length to decreasing ratio, and is likely due to a transition from flexible sun branches to stiffer structural branches.

Aboveground forest carbon sequestration is known to be a function of allometric relationships, stand history, and edaphic conditions. We investigate how the heterogeneous edaphic conditions of a naturally assembled urban brownfield influence the allometric relationships of the dominant species Betula populifolia Marsh. We measured diameter at breast height (DBH), height, mass and age on four sites that exhibited considerable edaphic differences. Site conditions did not appear to impact total tree mass to diameter relationships. However, mean DBH at the various sites ranged from 6.7 to 9.8 cm and the mean height from 637.4 to 911.8 cm. In addition, above ground woody biomass ranged from 40017 to 71935 kg ha⁻¹. Apparently resource allocation between growth and maintenance within the heterogeneous edaphic conditions of the urban context clearly results in considerably different growth rates and stocking densities. These results help to establish accurate metrics for the development of inventory-based forest carbon allocation models within the difficult environs of the urban context.

Using physical models to predict patterns of plant growth has been a long-standing goal for biologists. Most approaches invoke either biomechanical or hydraulic principles and assume the mechanism of interest applies similarly throughout the plant branching architecture. A recent effort, the flow similarity model, predicts numerous aspects of branching physiology and morphology and argues that the physiological constraints experienced by plants change as a function of branch order and size, with more basal portions satisfying more biomechanical constraints, and more distal portions, hydraulic ones. Distal branches are expected to have a strong influence on allometric relationships within plants due to their numerical abundance. Here we evaluate the predictions of the flow similarity model and a well-known alternative fractal branching model, using data on the dimensions of 3,484 individual stem internodes across four individual Acer platanoides trees. Overall, we find strong agreement between model predictions and the allometric exponents describing tree branch allometry. Further the predicted curvature in allometric relationships is found in all 24 cases examined and the frequency distributions of branch lengths and diameters are consistent with model expectations in 6/8 cases. We also find the area ratios are consistent with the model assumption of area-preserving branching. Collectively, our data and analysis provides strong support for the flow similarity model, and identifies several areas in need of subsequent inquiry.Competing Interest StatementThe authors have declared no competing interest.

Branch diameter relative to the trunk diameter (aspect ratio) affected the extent of discolored and decayed wood in the trunk of seedling-propagated red maple (Acer rubrum L.) after branch removal. More discoloration resulted from removing codominant stems than removing branches that were small compared to the trunk. Removing limbs that originated from lateral buds resulted in the same amount of discoloration and decay as removing suppressed limbs that were once the leader. This result provides indirect evidence that a small codominant stem suppressed by pruning techniques designed to slow its growth rate can result in a branch protection zone at the union. There was no relation between the presence of a bark inclusion and decay 4 years after making pruning cuts.

Invasive wood-boring insects are a major economic and ecological concern worldwide as they impact native woody plant populations. These pest species are increasing in prevalence, with devastating impact, as global trade leads to higher rates of introduction and establishment. The emerald ash borer ( Agrilus planipennis ; EAB) is one such species, which has caused widespread damage across much of the United States and is now spreading across Europe. Non-indigenous woodborers such as EAB are difficult to detect at early stages of invasion, which is when management and eradication efforts are most effective and cost efficient. Environmental DNA (eDNA) surveys have demonstrated power in detecting invasive species when rare in the landscape due to their ability to detect trace amounts of DNA and identify to species. Here, we trialled a novel eDNA method for collecting environmental samples within host trees where invasive pest larvae are feeding, using EAB as a case study. We extracted tree cores approximately 1 cm in length using an increment hammer to assess detectability of eDNA from larvae feeding under the bark. In trees visibly infested with EAB, we observed a seasonal peak in EAB DNA detection probability (~ 64%; towards the end of the growing season), indicating a potential impact of ash tree phenology or EAB phenology on detection. When we trialled the method in a site with ash trees of low or uncertain EAB abundance, we did not record positive EAB eDNA detections. This outcome may have resulted from differing EAB phenology at the northern latitude of this survey site or because larval galleries were less numerous causing EAB DNA to be scarcer within the tree. Our results, however, provide preliminary evidence that increment hammer tree cores can be used to detect eDNA of EAB and, perhaps, other wood-boring pests. Further work is needed to clarify false negative survey detections at ash trees showing little to no signs or symptoms of infestation, as well as investigating the deposition, transport and persistence dynamics of EAB eDNA within trees.