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Frequently burned low‐latitude coniferous forests maintain a high‐diversity understory. Longleaf pine (Pinus palustris Mill.) forests and woodlands have exceptionally high diversity at fine scales and very frequent fire return intervals (1–3 yr). Furthermore, the positive association between high‐frequency, low‐intensity surface fires and high species richness in longleaf pine ecosystems is well documented but poorly understood. Recent studies have demonstrated additional linkages between specific fuel assemblages and fire intensity at small spatial scales. In this study, we build upon both patterns by using long‐term datasets to examine the relationship between fire and specific fuel types, and how the combination of these two elements contributes to ground cover species diversity. We used 11 yr of monitoring data from longleaf pine forests at Eglin Air Force Base, Florida (USA), to parameterize a structural equation model that examines causal relationships between fuels and fire history on ground cover plant diversity. Overstory‐derived fuels, including pine needle litter, pine cones, and other 10 and 100‐h woody fuels, had the greatest positive impact on diversity in relatively open‐canopied, frequently burned reference stands. A second model examined surface fuel components originating from the forest overstory as characterized by airborne light detection and ranging and found that pine needle litter was positively associated with canopy density. Our parameter estimates for causal relationships between easily measured variables and plant diversity will allow for the development of management models at the stand scale while being informed by fuels measured at the plot scale.

Background and aims Ecological stoichiometry plays important roles in ecosystem dynamics and functioning, but relationships between above- and belowground stoichiometry and stoichiometric effects on the growth of different plant functional groups in forests remain poorly understood. Methods In an age sequence of 2-, 4- and 6-year-old Eucalyptus plantations in subtropical China, we examined C, N and P concentrations and their ratios in the soil and leaves. Each plantation was divided into overstory and understory plant functional groups. The relationships between stoichiometric characteristics and forest growth were analyzed. Results Soil C and P decreased in the Eucalyptus age sequence, which led to changes in soil stoichiometric characteristics. Leaf C:P and N:P ratios were higher for Eucalyptus trees than for understory plants because of the low P concentrations in Eucalyptus leaves. Soil and plant N:P ratios were strongly related. Understory biomass was positively related to N:P ratios, while overstory growth was negatively related to N:P ratios. Conclusions Our results suggest that nutrient concentrations in soil and plants are tightly linked in Eucalyptus plantations and that P limitation increases with stand age. Stoichiometric characteristics appear to mediate forest properties and functions under nutrient limitation in subtropical regions.

Vegetation cover estimation for overstory and understory layers provides valuable information for modeling forest carbon and water cycles and refining forest ecosystem function assessment. Although previous studies demonstrated the capability of light detection and ranging (LiDAR) in the three-dimensional (3D) characterization of forest overstory and understory communities, the high cost inhibits its application in frequent and successive survey tasks. Low-cost commercial red–green–blue (RGB) cameras mounted on unmanned aerial vehicles (UAVs), as LiDAR alternatives, provide operational systems for simultaneously quantifying overstory crown cover (OCC) and understory vegetation cover (UVC). We developed an effective method named back-projection of 3D point cloud onto superpixel-segmented image (BAPS) to extract overstory and forest floor pixels using 3D structure-from-motion (SfM) point clouds and two-dimensional (2D) superpixel segmentation. The OCC was estimated from the extracted overstory crown pixels. A reported method, called half-Gaussian fitting (HAGFVC), was used to segement green vegetation and non-vegetation pixels from the extracted forest floor pixels and derive UVC. The UAV-based RGB imagery and field validation data were collected from eight forest plots in Saihanba National Forest Park (SNFP) plantation in northern China. The consistency of the OCC estimates between BAPS and canopy height model (CHM)-based methods (coefficient of determination: 0.7171) demonstrated the capability of the BAPS method in the estimation of OCC. The segmentation of understory vegetation was verified by the supervised classification (SC) method. The validation results showed that the OCC and UVC estimates were in good agreement with reference values, where the root-mean-square error (RMSE) of OCC (unitless) and UVC (unitless) reached 0.0704 and 0.1144, respectively. The low-cost UAV-based observation system and the newly developed method are expected to improve the understanding of ecosystem functioning and facilitate ecological process modeling.

Summary There is growing concern over rates of global species diversity loss and its implications on healthy ecosystem functioning. While positive relationships between tree species diversity and forest biomass production have been observed, forests are structurally complex, consisting of understorey vegetation layers that also contribute to ecosystem functioning as they often account for the majority of species richness. However, relationships between understorey vegetation diversity and function are largely unexplored. Further, few studies have simultaneously assessed how both overstorey and understory vegetation interact and contribute to overall ecosystem function. By analysing Canada's National Forest Inventory data base using structural equation modelling, we explored the relationships between species richness and above‐ground biomass production across forest vegetation strata while accounting for potentially confounding factors, including climate, physical site characteristics and forest ageing. We found positive relationships between species richness and biomass production across all forest vegetation layers, but the relationship was strongest for the overstorey layer. Species richness of the understorey tree, shrub and herb layers was positively related to overstorey species richness. However, overstorey biomass had a negative effect on the biomass production of all understorey layers. Our results suggest that resource filtering by overstorey trees might have reduced the strength of the positive diversity–productivity relationships in the forest understorey, supporting previous hypotheses that the magnitude and direction of diversity–productivity relationships is context specific and dependent on the conditions of the surrounding environment. Further, heterogeneity in understory resources, as affected by the overstorey, may promote niche complementarity as the main mechanism driving diversity–productivity relationships in understorey vegetation. Lay Summary

Although Powers writes primarily as a novelist and Thoreau primarily as a nonfiction essayist, they overlap and reflect each other as each writer finds new ways to call attention to the intelligence, benevolence, longevity and necessity of trees. Both show us ways to open our eyes to trees, to listen to their stories and to understand them as connective, communicative beings.

This short essay identifies the treatment of temporality in The Overstory as a significant formal innovation and contribution to the genre of climate fiction. Glossing the Hoel photographs of the novel’s first chapter and Nick Hoel’s land art in its final chapter, the essay argues that Powers’ novel seeks to increase human awareness of ecological processes and arboreal life by training us to think in terms of arboreal time. The essay draws comparisons between this decentering of human temporality and the aims of Thoreau’s Journal practice, arguing that while Powers’ plot turns on acts of civil disobedience, it is his interest in expanding our temporal imaginations that is this novel’s most important Thoreauvian feature.

This essay uses an eco-theological lens to explore the relationship between Richard Powers’ novel The Overstory and various instances of Henry David Thoreau’s sometimes animistic characterizations of the natural world. It places Powers and Thoreau within an emerging conversation in the Religion and Ecology field. This conversation seeks to take animism more seriously as a form of religious expression and interpretation, despite a long history of Euro-American resistance to animism as a legitimate worldview. This essay interprets the current popularity of The Overstory within a broad historical context wherein the idea of the “personhood” of non-human beings—once largely rejected in the West—is slowly gaining traction in the realm of religious studies. In light of recent eco-theological writing that embraces animism, Henry David Thoreau was clearly prescient in representing the natural world as kin who are both animate and relational.

Thoreau’s Walden might be the most famous first-person narrative in American letters, but the book also tells a story of a remarkable community that emerges at the edge of Concord, where marginalized peoples fashion rich lives closely interwoven with the more-than-human inhabitants of the woods and waters that eluded the ax, the plow, and the railroad. On this cultural edge, Thoreau forges a new ontology and a new ethics, an understory that serves as a useful touchstone for Richard Powers’ novel The Overstory. In both, people leave behind the quiet desperation of society to find themselves transformed by the world that still flourishes at the edges of the capitalist world.

The response of understory trees to climate variability is key to understanding current and future forest dynamics. However, analyses of climatic effects on tree growth have primarily focused on the upper canopy, leaving understory dynamics unresolved. We analyzed differences in climate sensitivity based on canopy position of four common tree species (Acer rubrum, Fagus grandifolia, Quercus rubra, and Tsuga canadensis) using growth information from 1,084 trees across eight sites in the northeastern United States. Effects of canopy position on climate response varied, but were significant and often nonlinear, for all four species. Compared to overstory trees, understory trees showed stronger reductions in growth at high temperatures and varied shifts in precipitation response. This contradicts the prevailing assumption that climate responses, particularly to temperature, of understory trees are buffered by the overstory. Forest growth trajectories are uncertain in compositionally and structurally complex forests, and future demography and regeneration dynamics may be misinferred if not all canopy levels are represented in future forecasts.