Asimina triloba is replacing shade-tolerant Acer saccharum as the most abundant seedling in many Ac. saccharum-dominated forests in the midwestern USA. Selective browsing of Ac. saccharum by Odocoileus virginianus and avoidance of As. triloba contributes to this increased As. triloba abundance. However, shade tolerance may also be a factor contributing to increasing As. triloba. We tested our hypothesis that As. triloba is more shade tolerant than Ac. saccharum. Fraxinus quadrangulata is less shade tolerant than As. triloba and Ac. saccharum; however, other than As. triloba, it was the only species that increased stems per hectare in the seedling stratum from 2003 to 2008. We evaluated leaf photosynthetic traits to assess shade tolerance of these three species. Light response curves (LRCs) of As. triloba, Ac. saccharum, and F. quadrangulata saplings were measured under a dense Ac. saccharum canopy in situ using a portable infrared gas analyzer and were tested for fit to rectangular hyperbola, nonrectangular hyperbola, and exponential LRC models. Multivariate analysis of covariance, with estimated ambient irradiance and leaf temperature as covariates, tested interspecific differences in gas exchange properties: (R_d [dark respiration], α [LRC initial slope], and A_MAX [maximum photosynthetic rate]). Multivariate contrasts indicated that F. quadrangulata had significantly higher rates of photosynthesis than the two other species, indicating lesser shade tolerance, whereas As. triloba had greater initial light response (α values) than Ac. saccharum, which confers an advantage to As. triloba under low light conditions. These results implicate greater shade tolerance by As. triloba as another contributor to its increasing dominance in the understory.

Forest understory shrub and small tree taxa vary in their shoot architectures in response to their physical and biotic environments and in turn alter the conditions around them, particularly patterns of light. Clonal shrubs establish patches that expand three dimensionally over time as new ramets are produced and the ramet population responds to environmental heterogeneity, competition, and disturbance. Ramet leaf-area scaling and patch foliar structure were examined for the shade-tolerant clonal species Asimina triloba L. Dunal (pawpaw) on upland sites in a mixed-deciduous, old-growth forest preserve in west-central Indiana, USA to address three questions: (a) How strong are predictive relationships for ramet foliar organization across individual leaves, leaf clusters, and whole ramets of differing size? (b) Is it possible to estimate patch foliar area nondestructively and accurately using scaling factors derived from the analysis of ramet foliar organization? (c) How does foliar structure differ within and across patches, including total leaf area estimated with scaling factors? A sample of 48 ramets spanning a wide size range of 0.25 to 2.5 m in height was extracted from the field and analyzed for whole-ramet relationships between main-stem length and several foliar variables. Forty-one long and short leaf clusters from these ramets were used to quantify relationships among cluster size, individual leaf area, and total leaf area and to estimate total ramet leaf area. Eight patches varying in areal size, stature, and ramet density were measured for numerous structural variables and a single cluster-based scaling factor was used to estimate total leaf area, leaf-area index (LAI), and leaf-area density in 2 ｘ 2 m quadrats along continuous transects through patches. The results showed strong curve fits among foliar variables for both the ramet-level and cluster-level relationships and suggest that leaf clusters may be an important focus of foliar organization and dynamic adjustment for A. triloba. The eight patches showed substantial within-patch structural variation and their means differed significantly for all structural variables including LAI, which ranged from 0.42 to 1.28 m² m^–2. The results also suggest that total leaf area for A. triloba patches in undisturbed, mature upland forests like those in this study may be estimated nondestructively by counting the number of clusters projected over a sample plot base and multiplying by a weighted mean total cluster leaf area derived from the cluster-scaling relationships. This promising method should be tested across other physiographic conditions and forest types and where nontrivial canopy disturbance has affected A. triloba patch history.

Pawpaw (Asimina triloba L. Dunal) occurs across a wide geographic range in eastern North America, forms clonal patches that can dominate the shrub/small-tree stratum, and has been associated with lower tree seedling density, survival, and growth, potentially due to light interception. However, relatively little research has been done on alteration of light regimes by shrubs in mesic forests compared with the impacts of overstory and understory trees, and there are no data on light attenuation by A. triloba on the basis of direct irradiance measurements. Photosynthetic photon flux density (PPFD) patterns were measured simultaneously above (3 m) and below (0.25 m) the A. triloba canopy in summer 2016 at eight locations along transects through each of eight patches of varying size and ramet/ foliar density in an old-growth deciduous forest in west-central Indiana. Six-second readings across eight 14-hr sample days per patch were analyzed in numerous ways to quantify light regimes above and below the A. triloba canopies, to relate transmission to foliar density, and to compare PPFD levels inside patches with previous measurements in the surrounding understory. A. triloba patches in this system received < 1.5% full sun (mean 22.0 lmol m^–2 sec^–1) on the upper canopy and transmitted 40–50% overall. The result was very low mean daily PPFD irradiance near the ground on both sunny (9.3 lmol m^–2 sec^–1) and cloudy (3.5 lmol m^–2 sec^–1) days. Foliar density indices (leaf area index, leaf area volume) were mostly not correlated with percent transmission on the basis of mean or median PPFD on selected days with comparable open-site PPFD and wind conditions. Sunny days produced higher mean PPFD both above and below the canopy and more variable percent transmission than cloudy days, but overall mean percent transmission (41.5 versus 42.9%, respectively) did not differ with sky conditions. Sunfleck regimes near the ground were significantly reduced in terms of total daily fleck number (58% lower), total daily duration (67% lower), and total daily fluence (65% lower) compared with above the canopy. Mean daily below-canopy PPFD (9.0 lmol m^–2 sec^–1) across all patches and sample days was approximately 45% lower than in the surrounding understory outside A. triloba patches (13.1 lmol m^–2 sec^–1) measured in 2012. The results confirm that light attenuation by patches in this upland forest poses substantial challenges for leaf-level carbon gain by seedlings of most tree species, especially on cloudy days, and suggest that sunfleck utilization may be critical for seedling survival and growth overall.

Males and females of the dioecious woody shrub Lindera benzoin (spicebush) have been shown to differ in stem volume (as estimated from basal area and stem heights taken from living plants). More specifically, prior to significant seed production, female plants may attain a larger prereproductive size. Thereafter, growth rates of males may be higher leading to larger average size as reproductive adults, presumably due to differential costs of reproduction. These previous studies have assumed that the sexes are similar in allometry, wood density, and degree of branching; therefore, stem volume could be used as a surrogate for aboveground biomass. This study addressed these assumptions using offspring derived from 15 maternal plants collected from a site in Maryland. Starting in 2013, plants were grown in pots to eliminate root competition and in a loose array to minimize shoot competition. By 2020, 229 of the plants that could be sexed (125 female and 104 male) were randomly selected for stem dry mass and branch number analysis. In agreement with previous studies, the largest stems of female plants had significantly higher volume than those of males. Stem mass/volume and basal area/height relationships did not, however, differ between males and females based upon regression models that did or did not contain sex as an independent variable. Therefore, stem mass was also higher for female plants. Males and females did not differ in the number of branches in the terminal meter of the stem or in stem density. These results suggest that prior studies utilizing stem volume as an indicator of stem mass were not likely biased by allometric differences between the sexes.

We examined the effects of soil chemistry on the herbaceous forest wildflower Podophyllum peltatum (Mayapple) in an existing large-scale ecosystem level experiment that raised soil pH by two units (from ∼ 4 to ∼ 6) and increased available phosphorus (P) at least sevenfold. Plant leaf tissue was monitored for four years, and we began detailed plant measurements after we observed changes in leaf P content. Plant nutrient content, reproductive allocation, and the extent of root colonization by arbuscular mycorrhizal fungi were determined. We found no effect of changing soil chemistry on P. peltatum reproductive allocation. Addition of P resulted in significant increases in leaf P content, but also marginally reduced leaf N and C content. Increases in pH had no effect on leaf N or P, but there was a significant interaction between P addition and pH elevation on leaf C content. Root colonization by arbuscular mycorrhizal fungi was significantly affected by soil treatment; our data suggest that pH elevation and P addition may have opposite effects on mycorrhizal colonization in forest herbs, with pH increasing and P addition decreasing root colonization. Our study has important implications for how forest wildflowers may respond to anthropogenic changes in soil nutrient content and chemistry.

Shrubs are dominant plants in many hot deserts with limited moisture throughout the world. Shrubs are woody plants of relatively low height, distinguished from trees by having several stems rather than a single trunk. Soon after seedling establishment, main stems of shrubs produce a large number of growth centers that produce independent stems. The purpose of this study is to describe the characteristics of growth centers in stems responsible for the production of independent stems in three desert shrub species. Samples were processed from the transition zone to about 30 mm above the transition zone. Axis splits and lobes were common in the three species. Growth centers were identified as areas of concentric circles of xylem cells. Concentric circles of xylem cells can only be produced by cambial cells. In some cases, axis splits directly fractured the vascular cambium to produce fragmented cambia to produce growth centers. In other tissues, axis splits were not visible but growth centers developed. Lobes may move cambial cells that may have temporarily ceased activity but may become active at a later date and begin producing cells to become growth centers. These cambial cells are from a fragmented cambium that have moved to various locations by axis splits and lobes. To our knowledge, this is the first study to describe the process of producing fragmented cambia in shrub species in tandem with axis splits and eccentric growth (lobes) of stems. Moreover, this is the first study to document the production of individual stems from individual fragmented cambia.

Dirca palustris L. (eastern leatherwood) is an understory shrub found throughout much of eastern North America. Dirca palustris wood has a low concentration of lignin, which is a molecule that grants structural rigidity to cell walls. The branches of D. palustris are thus remarkably flexible, but their low lignin content could cause greater vulnerability to water stress. We examined the conductivity, vessel anatomy, and field water potential of D. palustris and conducted ecological surveys. The data showed that D. palustris was not experiencing substantial drought-induced dysfunction in the field, even during an exceptionally hot and dry summer. Its water potentials in the field were less negative than water potentials that caused substantial embolism in the lab, and it continued to photosynthesize and conduct water throughout the summer. We also found that D. palustris had small vessels, which may help it resist water stress. Ecological surveys showed that 15% of surveyed plants survived litterfall from canopy trees, almost all without damage, demonstrating a possible advantage of the low lignin content and flexible branches seen in D. palustris. These characteristics may allow D. palustris to continue to thrive in increasingly challenging environments brought on by climate change.

The flora at Riverdale Park was sampled during the 2021 growing season, April through October, to create an inventory of the native and nonnative vascular plant species. One hundred ninety species in 151 genera in 85 families were identified. Maples, Acer spp., and oaks, Quercus spp., were the most diverse genera in the flora with six species, while Rubus was represented by four species. Seventy-six taxa, 40% of the flora, were not native to the region. Many nonnative vascular plant species may be relicts of the past land use history, as Riverdale Park is situated on old estates created in the 1800s. The high percentage of nonnative taxa in the Riverdale Park study is related to the urban environment and past land use history.