Neighbor Tree Research Articles

DETECTION OF SILVER BIRCH GROWTH DYNAMICS AND TIMING WITH DENSE SPATIO-TEMPORAL LIDAR TIME-SERIES

Abstract. Silver birch (Betula pendula Roth) is a deciduous pioneer tree species with significant economic and ecological importance due to its rapid growth, high genetic variability and adaptability to diverse climates and environments. In this regard, understanding the factors that influence silver birch tree growth variability and its seasonal patterns has been a subject of research interests, which aim at effective forest management and ecological analyses. Tree size, competition, light availability, and topography has been considered significant factors affecting tree growth patterns. However, their relative contributions are not well understood. This is because, to study the interactions between neighbor trees and their competitive responses requires complex measurements. Accurately measuring tree attributes, such as 3D canopy shape and arrangement, is challenging but has been made possible through advancements in high-resolution 3D remote sensing, specifically laser scanning technology. This study shows the potential of high-spatial and temporal resolution LiDAR time-series from a permanent laser scanning setup to detect detailed structural changes and timing in individual tree canopies, focusing on assessment of the structural canopy growth characteristics of silver birch trees. We first investigate how silver birch trees respond to competition and neighboring species. Our results focusing on canopy height increment show that tree size, competition, neighboring species, and water availability affect the rate of vertical (height) growth of the studied silver birch trees. Further, we detect the timing in canopy vertical and horizontal growth using LiDAR time-series. Significant variations of up to one week were detected among trees, providing insights for future studies on growth dynamics of silver birch in coniferous-dominant forests.

It takes a village to grow a tree: Most tree species benefit from dissimilar neighbors.

Scientific consensus is that diverse tree species positively impact forest productivity, especially when species are functionally dissimilar. Under the complementarity hypothesis, differences in species traits reduce competition among neighboring tree species. However, while this relationship has been extensively studied at the community level, there is a lack of understanding regarding how individuals of different species specifically respond to a functionally dissimilar neighborhood. In this study, we used permanent plots from Quebec, Canada, and 19 focal tree species to test whether: (1) tree growth response to neighborhood dissimilarity varies with their identity and competition intensity, and (2) focal tree species' traits explain their response to neighborhood dissimilarity. We demonstrate that: tree growth is primarily influenced by competition, species identity, and their interactions, but that dissimilarity, alone and in interaction with the main drivers of tree growth, explains an additional 1.8% of the variation in species growth. Within this context, (1) most species' respond positively to neighborhood dissimilarity, with magnitude being species and competition dependent, and (2) focal tree traits partly explain these dependencies, with shade-intolerant species benefiting most from dissimilar neighbors under high competition. Our study provides empirical support for the complementarity hypothesis, emphasizing the small but consistent positive effect of functional dissimilarity on tree growth in local neighborhoods. Our findings identify the species with the highest potential of benefiting from dissimilar neighbors but also demonstrate that the positive effect of neighborhood dissimilarity is not limited to a select few species with specific traits; rather, it is observed across a diverse range of species. The cumulative growth responses of individuals to functionally dissimilar neighbors may help explain the commonly observed higher productivity in more diverse communities.

Changes in forest structure over 23 years under disturbances in a warm temperate rain forest on Yakushima Island, Japan

AbstractClarifying the influence of disturbances by biotic and abiotic agents on long‐term changes in forest structure and demographic rates is an important task of forest ecology. Here, we investigated changes in forest structure and demographic rates and the effects of biotic and abiotic factors (sika deer, ambrosia beetle, neighborhood trees, and indices of strong wind and wetness) in a 4‐ha long‐term ecological study plot in a lowland warm temperate rain forest on Yakushima Island, Japan, from 1996 to 2019 for three regeneration groups related to gap dynamics (gap demander: GD, light shade tolerant: LS, and strong shade tolerant: SS). GD had relatively lower recruitment and survival than LS and/or SS. Changes in demographic rates were not significantly correlated with those in the index of strong wind in any regeneration group. The effects of sika deer on demographic rates did not differ among groups. Stems of Fagaceae species belonging to LS attacked by ambrosia beetles recorded in 2013 did not significantly differ in RGR and survival rates. Our results indicate that despite the relatively long monitoring period of 23 years, we did not detect significant effects of strong winds on demographic rates among regeneration groups, and shade‐tolerant species (LS and SS) showed better demographic performance than light‐demanding species (GD) and an increase in total basal area over time. We need longer monitoring of forest dynamics to understand forest responses to disturbances considering several factors, including the time lag of demographic responses to disturbances and the long‐term effects of disturbances.

Tree demographic and neighbourhood responses to regional environmental gradients of the northwestern United States

Abstract Studies of drivers shaping forest communities frequently include abiotic or biotic factors, while their interactive effects remain understudied. Here, we combined data on two prominent abiotic gradients, climatic moisture deficit and wildfire probability, along with tree functional neighbourhoods (i.e. trait differences of close neighbours) to assess variation in survival and growth of 56 tree species in the northwestern US. We asked two questions: (1) How does functional neighbourhood dissimilarity vary with environmental gradients? and (2) How do demographic rates of tree species in the northwestern United States vary with the interactive effects of environmental gradients and functional neighbourhood? We expected functional neighbourhoods to become more similar as environmental stress increased, due to a convergence of species towards an optimum stress tolerance strategy. We also predicted the interactive effects of abiotic and biotic factors on tree demography and high variation in species‐specific responses to these interactive effects due to divergent species life history strategies. Functional neighbourhoods defined by dissimilarities in stem conductivity, litter decomposition, resprouting ability and specific leaf area changed with climate, shifting to more diverse neighbourhoods as climatic moisture deficit and wildfire probability increased. Results supported interactive effects of the functional neighbourhood and climatic moisture deficit or wildfire probability on tree demography, but only when the identity of dominant species was considered. Species‐specific responses were highly variable in their direction and magnitude and often demonstrated opposing effects of climate and the functional neighbourhood and climatic moisture deficit on tree demography. Synthesis. Our findings show that climate and tree neighbourhood functional dissimilarity jointly impact tree demography; however, the effects are species‐specific. Results of this study highlight the need to consider the interactive effects of abiotic and biotic contexts and individual species responses to their environment to adequately understand tree persistence under current and future climate conditions.

Successful forest restoration using plantation at high deer density: How neighboring vegetation drives browsing pressure and tree growth

Browsing can be an environmental stress to forest ecosystems, where the composition and structure of plant communities depend on the balance between ungulate browsing and the ability of plants to tolerate and acclimate to this stressor. Tree planting is a management tool for restoring forest whose integrity has been compromised by the intensity and repetition of browsing. This tool may be insufficient when the stress leading to forest degradation is ongoing. Balsam fir (Abies balsamea (L.) Mill.) plantations are used on Anticosti Island (Quebec, Canada) to restore winter habitat for white-tailed deer (Odocoileus virginianus Zimmermann, 1780). To promote natural regeneration and growth of planted firs that are the staple winter forage of deer, enclosures were temporarily erected and deer densities reduced to decrease browsing pressure. The stress induced by heavy browsing at the time of fence removal and the density of competitors in regenerating cutblocks, however, could compromise fir growth and forest recruitment. Our objective was to identify factors influencing browsing intensity and balsam fir’s growth following fence removal. We aimed to identify factors influencing lateral browsing intensity and number of trunk reiterations on fir at the landscape scale, including distance and size of the surrounding forests, and at the local scale through intraspecific density-dependence and interspecific associative effects. We then sought to identify the stressors most influencing balsam fir growth among browsing variables and a neighborhood crowding index (NCI). We measured browsing intensity and growth of 114 balsam firs 8 years following planting and again 2 and 4 years after dismantling two management enclosures. At the landscape scale, our results showed that when balsam fir was more than 150 m from a forest edge, the probability of browsing was close to zero and consistently a low percentage of shelter area surrounding a focal fir led to a decrease in browsing pressure. At the local scale, intensity of lateral browsing increased with the NCI of white spruce (Picea glauca (Moench) Voss). Fir growth was negatively influenced by the additive effects of browsing and NCI. Finally, our results showed that 19.7% (95 %CI: 4.5%, 42.6%) of the effect of the surrounding vegetation on growth was mediated by browsing. The success of forest ecosystem restoration through planting depends on density of neighboring tree species that can generate environmental stress by influencing growth indirectly through browsing as well as directly through competition for resources.

Modification of Density Dependence and Habitat Filtering on Seedling Survival of Different Mycorrhizal-Type Tree Species in Temperate Forests

Conspecific negative density dependence (CNDD) and habitat filtering are critical to seedling survival. However, the relative importance of the two processes in affecting survival of seedlings with different types of mycorrhizae remains unclear. In this study, the effects of CNDD and habitat filtering on the survival of tree seedlings with different mycorrhizal types were investigated at different successional stages of a temperate forest in the Changbai Mountain Natural Reserve, Northeast China. Conspecific negative density dependence and habitat filtering significantly affected seedling survival. In the early successional stage, the interactions between conspecific neighbor tree density and light availability and soil properties significantly negatively affected survival of all species and arbuscular mycorrhizal (AM) seedlings in the community, but not that of ectomycorrhizal (EcM) seedlings, and the CNDD effect was stronger on AM seedlings than on EcM seedlings. In the mid-successional stage, CNDD effects were stronger on EcM seedlings. Therefore, different types of mycorrhizal seedlings responded differently to CNDD and habitat filtering mechanisms during community succession, and thus, tree mycorrhizal association could determine the effects of CNDD and habitat filtering on seedling survival in temperate forests.

Effects of tree density and herbaceous plants on tree seedling survival across the growing and non-growing season in a temperate forest

Tree seedling survival is critical for estimating forest recruitment, but it remain elusive about how neighboring tree’s density and herbaceous plants affect it. To assess the relative importance of tree density and herbaceous plants on tree seedling survival during the growing season and non-growing season, we established a two-year experiment with seedling census and removal of herbaceous plants in both secondary and old-growth temperate forest sites in the temperate forests of northeastern China. A total of 3,443 seedlings in 17 tree species were surveyed in this study. We found that both tree density and herbaceous plants significantly affected tree seedling survival, but the specific effects depended on shade tolerance, seedling growth stage and growing season. Conspecific adult neighbors had significant negative effects on the survival of shade-tolerant seedlings during the growing season, while conspecific seedling densities positively correlated with shade-intolerant seedlings. Herb removal significantly improved the survival of newly germinated and shade-intolerant seedlings during the growing season. Furthermore, we found that the survivals of both newly germinated and established seedlings differed seasonally in response to adult, seedling neighbors and herb removal. Herb removal enhances seedling survival during the non-growing season by offsetting the negative effects of conspecific adult neighbors during the growing season. Our findings provide new evidence for seedlings’ shade tolerance, growth stage and seasonal dependence of density dependence in temperate forests, and highlight the significance of herbaceous plants for seedling survival, an important factor that has been neglected in many previous studies.

Association between urban greenspace, tree canopy cover and intentional deaths: An exploratory geospatial analysis

Greenspaces can provide restorative experiences, offer opportunities for outdoor recreation, and reduce mental fatigue; all of which may improve community health and safety. Yet few studies have examined the neighborhood-level benefits of greenspace in reducing violent deaths. This study explored the association between three distinct greenspace metrics: public greenspace quantity, public greenspace accessibility, neighborhood tree canopy cover, and intentional deaths (i.e., homicides and suicides). Generalized linear models and spatial error models investigated the association between greenspace, tree canopy and intentional deaths in three geographically distinct cities in North Carolina, USA. Results revealed that increased neighborhood greenspace accessibility and tree canopy cover were associated with reduced intentional deaths in all three urban areas. Neighborhood greenspace accessibility was the most protective factor across all study areas. The association between neighborhood greenspace accessibility and intentional deaths was more significant for non-firearm deaths as compared to firearm deaths, indicating that weapon type may be an important consideration for neighborhood greenspace interventions. Compared to predominantly White neighborhoods, predominantly Black neighborhoods had higher rates of homicide in Asheville and Durham and higher rates of suicide in Charlotte. Future policy and research should focus on improving equitable access to existing and future greenspaces, especially in primarily Black neighborhoods.

Sap flow as a function of variables within nested scales: ordinary least squares vs. spatial regression models

Understanding scale-dependent influential drivers of sap flow variability can help managers and policymakers to allocate resources within a particular scale to improve forest health and resiliency against water-stress stimuli such as drought and insects, e.g. bark beetle infestations. We defined a daily measure of sap flow as a function of variables within nested scales of landscape, stand, and tree, using ordinary least squares (OLS), spatial lag and error regression models. Model covariates were elevation, latitude (Y-coordinate), longitude (X-coordinate), neighborhood tree density, tree diameter at breast height, and bark temperature for 40-surveyed Norway spruce (Picea abies) in the Czech Republic, Central Europe. Trees were spatially distributed within 19-established subplots across five plots, with distances ranging 2–9 km, at which variations in soil water potential and temperature were limited. The daily measure of sap flow within the regional scale allowed us to avoid the temporal and spatial variability of climate effects on sap flow. A relatively flat terrain across subplots also allowed us to control the effects of slope, aspect, and topography-related solar incidence angle on sap flow. Sap flow was strongly spatially autocorrelated, so OLS models failed to take spatial autocorrelation into account unless to some extent, depending on the spatial distribution of samples, by including latitude and/or longitude in the models. Among spatial regression models, spatial error models performed better than lag models, allowing to capture the effects of unmeasured independent variables. Sap flow variability for the most part (∼70%) was explained by the landscape-level variable of elevation followed by the stand-level variable of tree density, and the remaining part by variables related to tree characteristics; a nested down-scaling function, defined and visualized for the first time. Therefore, thinning forest stands and future plantations with optimum distances, based on the elevation gradients, may be required to counterbalance the allocation of resources, e.g. water, nutrients, and light, among trees, leading to enhance forest health and resiliency against water-stress stimuli.

Tree species richness increases spatial variation but not overall wood decomposition

Empirical evidence for the positive relationship between tree species richness and wood decomposition is weak, despite the greater numbers of decomposers in diverse stands. Tree species identity affects decomposition rates strongly by altering soil communities and micro-environments. It is therefore likely that tree species richness influences spatial variation rather than the mean rate of decomposition. We used a neighbourhood approach to test this hypothesis by quantifying decomposition rates and decomposer activities within deadwood, whilst measuring the surrounding micro-environments.Using the BEF-China experimental platform, we placed three fine branches from seven tree species across richness gradients for one and two years. As expected, plot species richness increased the spatial variation of wood decomposition of three species but decreased the mean rate of wood decomposition of two species. Neighbourhood trees altered wood decomposition in two ways. First, increasing amounts of leaf tannins reduced termite activities and the number of fungal fruitbodies. Second, higher species richness increased tree volumes and thus cooled the understory, which reduced the occurrence and feeding intensity of ectothermic termites. Neighbourhood species richness showed positive effects on decomposer activities (i.e. termite occurrence, fruitbody number and mycelium cover) after two years, probably by weakening resource limitation of deadwood biomass and increasing the stages of decay. Neighbourhood metrics had comparable (after 1 year) and even stronger effects (after 2 years) on decomposer activities than wood traits. Our study reveals that tree species richness promotes spatial variation in wood decomposition rates by enhancing environmental heterogeneity. Our results explain how young regenerating species-rich forests, where termites are the dominant decomposers, can maintain deadwood stocks and decomposer activities over relatively longer time spans.

Dear neighbor: Trees with extrafloral nectaries facilitate defense and growth of adjacent undefended trees.

Plant diversity can increase productivity. One mechanism behind this biodiversity effect is facilitation, which is when one species increases the performance of another species. Plants with extrafloral nectaries (EFNs) establish defense mutualisms with ants. However, whether EFN plants facilitate defense of neighboring non-EFN plants is unknown. Synthesizing data on ants, herbivores, leaf damage, and defense traits from a forest biodiversity experiment, we show that trees growing adjacent to EFN trees had higher ant biomass and species richness and lower caterpillar biomass than conspecific controls without EFN-bearing neighbors. Concurrently, the composition of defense traits in non-EFN trees changed. Thus, when non-EFN trees benefit from lower herbivore loads as a result of ants spilling over from EFN tree neighbors, this may allow relatively reduced resource allocation to defense in the former, potentially explaining the higher growth of those trees. Via this mutualist-mediated facilitation, promoting EFN trees in tropical reforestation could foster carbon capture and multiple other ecosystem functions.

Competition intensity affects growing season nutrient dynamics in Korean pine trees and their microhabitat soil in mixed forest

Exploring the response mechanisms of the understorey replanted target trees to competitive pressure from upper neighbouring trees in mixed forests is the scientific basis for growth promotion of the target trees. Taking a mixed forest of artificial planted Korean pine trees (target species) and natural broadleaved trees as the studying case, six competition intensity gradients to Korean pine trees were established according to the Hegyi competition index and the responses of growth, hormones and nutrition status of the target Korean pine trees to different competition intensities were analysed. Competition intensities had significant effect on the growth of Korean pine trees, and the more significant the competition intensity was, the more pronounced the inhibition effect. When the Hegyi competition index was greater than 15, the neighbourhood trees had a significant inhibition effect on the growth of Korean pine trees. With the increase of the competition intensity, the content of gibberellin and indoleacetic acid in Korean pine trees decreased; the content of abscisic acid increased; the moisture content of needles, the soluble sugar content and C/N ratio of needles and roots decreased; the chlorophyll, P and K and starch content, and total non-structure carbohydrates (NSC) of needles and roots increased at first and then decreased; the P and K content and C/N ratio in Korean pine microhabitat soil decreased. Excessive adjacent tree competition reducing the living space, the endogenous hormone content, nutrient content, and NSC of the target Korean pine trees, resulting in the growth of the target trees and subsequent nutrient synthesis to be inhibited. Appropriate adjacent tree competition increased the target tree chlorophyll, NSC, and nutrient content and had no excessive inhibitory effect on the growth of the target Korean pine trees. There was certain trait correlationship between Korean pine needles and fine roots, but there was an endostatin equilibrium mechanism for changes in the surrounding soil of the target trees. The above results indicated that competition intensity of the upper neighbourhood trees significantly influenced the growth and internal nutrient situation of the replanted target trees and their microhabitat soil conditions. In this case, when the Hegyi competition index of neighbouring trees in the surrounding area of the understorey planted target Korean pine trees was greater than 15, the growth space of target trees should be regulated to ensure their healthy development.

Seed dispersal and tree legacies influence spatial patterns of plant invasion dynamics

Invasive plant species alter community dynamics and ecosystem properties, potentially leading to regime shifts. Here, the invasion of a non-native tree species into a stand of native tree species is simulated using an agent-based model. The model describes an invasive tree with fast growth and high seed production that produces litter with a suppressive effect on native seedlings, based loosely on Melaleuca quinquenervia, invasive to southern Florida. The effect of a biocontrol agent, which reduces the invasive tree's growth and reproductive rates, is included to study how effective biocontrol is in facilitating the recovery of native trees. Even under biocontrol, the invader has some advantages over native tree species, such as the ability to tolerate higher stem densities than the invaded species and its litter's seedling suppression effect. We also include a standing dead component of both species, where light interception from dead canopy trees influences neighboring tree demographics. The model is applied to two questions. The first is how the mean seedling dispersal rate affects the spread of the invading species into a pure stand of natives, assuming the same mean dispersal distance for both species. For assumed litter seedling suppression that roughly balances the fitness levels of the two species, which species dominates depends on the mean dispersal distance. The invader dominates at both very high and very low mean seedling dispersal distances, while the native tree dominates for dispersal distances in the intermediate range. The second question is how standing dead trees affect either the rate of spread of the invader or the rate of recovery of the native species. The legacy of standing dead invasive trees may delay the recovery of native vegetation. The results here are novel and show that agent-based modeling is essential in illustrating how the fine-scale modeling of local interactions of trees leads to effects at the population level.

Green space type and healthy ageing in place: An Australian longitudinal study

Research on the health benefits of people’s long-term exposure to green space is lacking. Addressing this emerging topic, this study uncovers the association of green space types with a person’s healthy ageing score (HAS). We investigated the association with HAS by continuous exposure to various green space types for two years. Green space types include tree canopy, low-lying vegetation and open grass percentage within 1.6-km road network distance buffers at baseline and second follow-up. Healthy ageing outcomes included functional capacity, resilience and HAS while accounting for relocation. This is a longitudinal study of a cohort of 22,715 New South Wales residents aged over 45 who participated in the Sax Institute’s 45 and Up Study living in Sydney, Wollongong and Newcastle. Longitudinal models of healthy ageing on green space types were fit with controls for socioeconomic confounders. Women who did not relocate were associated with a lower functional capacity (β; 95%CI: −0.10; −0.15,−0.05) and higher resilience (0.11; 0.08, 0.14) compared to those who relocated. Apart from age, personal characteristics did not explain the variation in healthy ageing outcomes for participants who moved. For participants who did not relocate, 30% grass cover was associated with decreased functional capacity (−0.22; −0.41,−0.04) and HAS (−0.31; −0.56,−0.05). Also, 5–9% low-lying vegetation was associated with a decline in functional capacity (−0.09; −0.15, −0.03) and HAS (−0.09; −0.17, −0.01) of participants who stayed but improved resilience (0.28; 0.01, 0.55) of participants who relocated. Green space comprising over 30% tree cover appeared most beneficial for functional capacity (0.42; 0.31, 0.53), resilience (0.19; 0.13, 0.25) and HAS (0.60; 0.45, 0.75). For participants who relocated, over 30% tree canopy was associated with improved functional capacity (0.33; 0.54, 0.62) but not resilience or their HAS. Increased neighbourhood tree canopy supports healthy ageing. Older people should be supported to live in-home or move into nearby residential care and maintain regular contact with green spaces and trees, to maximise potential benefits for health and wellbeing.

Fire-caused mortality within tree neighborhoods increases growth of Pinus lambertiana more than growth of Abies concolor

Global increases in large-tree mortality and multiple disturbances such as fire, drought, and pestilence increase the importance of understanding how large-tree growth responds to changes in competitive neighborhoods. We used 34,175 spatially mapped trees within 25.6 ha that burned at low to moderate severity in 2013 to investigate how changes to tree neighborhoods affected growth of 3,652 surviving trees of two species, Abies concolor (white fir) and Pinus lambertiana (sugar pine). Using diameter measurements taken in 2014 and 2019, we estimated post-fire radial growth of each surviving tree. We modeled annual basal area increment as a function of diameter and extracted residuals to yield relative growth rate (RGR). We used a multi-model approach and AIC to compare the species-specific effects on RGR of all post-fire tree mortality within a neighborhood radius versus mortality broken down by cause. To represent density reduction around each surviving tree, we computed crowding metrics for neighborhood radii up to 20 m for all tree mortality combined and for each mortality cause. The best-fit model included terms for density reduction within 10 m of the surviving tree due to 1) fire or mechanical damage and 2) bark beetle mortality, as well as their interactions with species and diameter. Density reduction due to fire or mechanical damage had a positive effect on RGR that was nearly twice as strong for P. lambertiana as A. concolor. Tree diameter interacted with density reduction for just P. lambertiana. Small-diameter (10-cm) P. lambertiana had a fourfold larger increase in basal area increment than large-diameter (60-cm) P. lambertiana as density reduction due to fire or mechanical damage ranged from 0 to 100 %. The strong growth response of medium-diameter P. lambertiana to density reductions could eventually increase the abundance of large-diameter (≥60 cm) P. lambertiana. In conjunction with mortality rates, which were lower for large-diameter P. lambertiana (0.97 %) than A. concolor (3.31 %), this could increase the relative abundance of large-diameter P. lambertiana from 51.2 % in 2020 to 62.6 % by 2050. Changes to competitive neighborhoods can have species-specific effects on large-tree growth, an important consideration for long-term management of fire-prone forests.

Widespread stem snapping but limited mortality caused by a category 5 hurricane on the Caribbean Island of Dominica

Hurricanes are projected to increase in frequency and severity over the coming century, and predicting the effects of strong hurricanes on forest communities requires an understanding of the damage patterns caused to individual trees and what drives those damage patterns. To better understand damage patterns caused by a category 5 hurricane, we censused nine permanent vegetation plots across the island of Dominica, West Indies, nine months after Hurricane Maria took a direct path over the island. We measured size, damage, and mortality for all adult trees, and measured wood density and carbon content for the 44 most common species to calculate biomass and carbon relocation from the hurricane. Despite widespread major tree damage (76% of trees), mortality nine months after the hurricane was relatively low (10% of trees). The most common damage types were stem snapping (40% of trees) and major branch damage (26% of trees), but the damage types with the highest rates of mortality were uprooting and being crushed by a neighboring tree (33% and 47% of uprooted and crushed trees died, respectively). Larger individual trees and species with lower wood density were more susceptible to snapping, uprooting and mortality, and trees on steeper slopes were more prone to being crushed by neighboring trees. Hurricane Maria caused an average loss of 44 Mg ha−1 of aboveground carbon and 90.3 Mg ha−1 of aboveground biomass. Synthesis – Trees in these hurricane-prone forests show a strong ability to survive extremely strong storms even when severely damaged, but substantial amounts of forest carbon are redistributed to the forest floor even when mortality rates are relatively low. Our data suggest that these hurricane-prone forests will continue to shift toward smaller, high wood-density species with increasing hurricane frequency and intensity.

Nitrogen fixation responds to soil nitrogen at low but not high light in two invasive understory species

Abstract Light and soil nitrogen availability can be strong controls of plant nitrogen (N) fixation, but data on how understory N‐fixing plants respond to these drivers are limited despite their important role in ecosystem N cycling. Furthermore, ecosystem N cycling can be altered by the introduction of species with nutrient use patterns that differ from natives. We assessed how N fixation of two exotic, understory species responded to varying light and soil N environments. We sampled leaf tissue from Mimosa pudica L., Desmodium triflorum (L.) DC., and a nonfixing reference plant (Axonopus) growing in control and two N fertilization treatments under either N‐fixing or non‐N‐fixing trees, which may alter local soil nutrient cycling, across a range of light conditions. We measured N fixation with 15N isotope dilution, and ensured that N‐fixing neighbour trees were in fact fixing N. All understory plants were wild‐growing species not native to the study location. Desmodium and Mimosa acquired 82.6% and 71.6% of their nitrogen from fixation (%Ndfa) in the control, compared to 66.8% and 58.1% in the +10 g N m−2 year−1 treatment and 73.1% and 64.7% in the +15 g N m−2 year−1 treatment. These subtle %Ndfa differences across fertilization treatments were more apparent at low light availability and disappeared at high light availability. The amount of N fixed by neighbouring trees did not influence %Ndfa in the understory species. Synthesis. Our study shows some differences in N fixation across different nutrient environments at low light for two N‐fixing species, though the changes were small, and both species derived most of their N from fixation. These findings imply that introduced N‐fixing species could exacerbate ecosystem N enrichment, particularly under high soil N conditions.

Impact of plant community structure and its diversity on richness and abundance of arthropod aphidophagous natural enemy community

Arthropod natural enemies (ANEs) play an indispensable role in maintaining the balance for plant communities that also significantly affects the ANE diversities not only by supporting herbivorous prey, but also by providing habitats and floral food resources. Studying the diversity of ANEs is thus vital to develop an understanding of sustainable pest control. Relationships of the vegetation diversity (including richness, coverage, and Shannon–Wiener diversity) with associated arthropod aphidophagous natural enemies (AANEs) and their groups under Pinus tabuliformis of different distances were analyzed by non-metric multidimensional scaling (metaMDS). Our results indicated that the vegetation richness affects AANEs at a small scale, while the vegetation structure affects it at a large scale. The richness and abundance of AANEs and the abundance of green lacewings (GLs) and aphid parasitoids (APs) were positively related to neighboring tree richness. But the richness of AANEs and aphidophagous ladybirds (ALs) were negatively associated with nearby tree coverage, as well as AANE richness with close-up shrub coverage, while the abundance of AANEs, ALs, and GLs possessed a negative and hump-shaped relationship with nearby tree coverage, as well as the abundance of AANEs and GLs with close-up shrub coverage. When tree and shrub layers each had approximately half coverage within a vegetation structure, the richness and abundance of AANEs were highest. Similarly, the richness and abundance of AANEs were positively related to neighboring blooming plant richness. However, the richness of AANEs and ALs, and the abundance of AANEs, AL, GL, and APs had a positive association either with the coverage or with the Shannon–Wiener diversity of close-up blooming plants. Half coverage of the tree and shrub layer combined with higher diversities (richness, coverage, and Shannon–Wiener diversity) of blooming plants resulted in low aphid density. The results provide a basis for effectively improving AANE diversity.

Local-scale soil nematode diversity in a subtropical forest depends on the phylogenetic and functional diversity of neighbor trees

Local-scale soil nematode diversity in a subtropical forest depends on the phylogenetic and functional diversity of neighbor trees

Gina Franco, The Accidental

Reviewed by: The Accidental by Gina Franco John Rubio (bio) Gina Franco, The Accidental. University of Arkansas Press, 2019. Pp. 88. Upon first reading, one might jump to the conclusion that Gina Franco’s The Accidental is a rather “inaccessible” book of poetry. I find such critiques less than useful in that they presume a standard of accessibility and determine any work that exceeds this boundary to be obscure or esoteric, with “accessible” now referring less to a standard than to a subjective scale with which the critic feels comfortable. In this way, the critique become less accessible than the subject of its criticism. These are no doubt challenging poems. Franco weaves references to Heidegger, Hegel, and Simone Weil with biblical scripture and personal narrative to form a dialectic of the spirit as it struggles through the confines and exposures of being. The dynamics of life and death and the impasse of the spirit remain prime articulations of this dialectic throughout The Accidental. If readers are so inclined (or already have a background in theology and continental philosophy), they could spend much time with Franco’s notes, poring over her conceptual associations, inspirations, and references. Indeed, Franco does not simply drop names; she elevates discourse. She engages not only with the legacy of these thinkers, but with the very substance of their thought, linking that substance to Franco’s own stories and ideas, especially those concerning death, family, and the soul’s grief. Simultaneously, readers could approach these poems with no formal training, without consulting the notes at all. This is not to say that Franco’s writing is unfocused, but rather that multiple approaches to her poems can be taken without having to sacrifice (or expect) intellectual rigor. Franco’s poetic dimension does not require an academic passport, and entering without one does not mandate that the philosophical musings of her poems be abandoned. I note a similar experience in reading the work of Alice Notley. In this sense, The Accidental is not inaccessible so much as it reworks the parameters of accessibility. Franco does not shrink or expand these parameters, but rather allows them a more flexible and porous contemplation. As Franco is reinterpreting ontological theory for her own purposes, so too are readers encouraged to do the same. Therefore, whether or not we have experience with the actual theories is unimportant; we are still welcomed by Franco to play in their imaginative fervor, to mix it with our own. Reading (and rereading) the poems is all the research we need. Also, as stated above, The Accidental is a dialectic process, meaning that it frequently dwells in a liminal space, in a space between two extremities, a space that cleaves from two determinant ends. Thus, to inhabit its pages is to go along with that process, and it is frequently a disquieting and discomforting habitation. Even when not dealing with tragedy—such as the death of Franco’s maternal grandmother, Carmen Rios, who drowned in a flood and whose body was caught by a neighboring tree, which becomes a thematic presence [End Page 134] throughout the collection—The Accidental can still feel unsettling because its dialectical process in many ways eschews experiences of stability and settlement. We are placed in between determinations, just out of reach of their stable footings, but not so separated that we are unable to clutch at their presence: to cleave (as these poems do) is both to divide and to draw together. We are placed in a poetic flood; we sense momentum and perpetuation—an ongoing thrust that moves us along, ever in view of the sidelined dry land that we can never quite grasp. To be swept up in The Accidental is to be washed by aporias of being. But rather than trying to solve these paradoxes, Franco encourages us to instead realize that only from the view in the flood, only from the space between, can we see the connection between these two shorelines—a connection that is now personified by the writer and readers themselves, thus generating another dialectic to engage. As the speaker of “America” (dedicated to Maribel Alejandra Cortinas Segura, who was murdered in 2009) states while looking...