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1. The Roles of the Moss Layer in Mediating Tree Seedling Environmental Stress, Mercury Exposure, and Regeneration in High-Elevation Conifer Forests

   https://doi.org/10.1007/s10021-022-00806-0  

   Berdugo, Monica B.; Dovciak, Martin; Kimmerer, Robin W.; Driscoll, Charles T. (November 2022, Ecosystems)

   The persistence of future forests depends on the success of tree seedlings which are experiencing increasing physiological stress from changing climate and air pollution. Although the moss layer can serve as an important substrate for tree seedlings, its potential for reducing environmental stress and enhancing the establishment of seedlings remains poorly understood. We tested if the moss layer decreased environmental stress and increased the abundance of balsam fir seedlings dominant in high-elevation forests of northeastern United States that are sensitive to changing climate and mercury deposition. We surveyed balsam fir seedling density by substrate (moss, litter, other) on 120 quadrats (1 × 1 m) in two contrasting canopy environments (in gaps and under canopies), measured seedling stress, and quantified mercury content in seedlings and substrates. We observed that, in both canopy environments, tree seedlings established on moss exhibited (i) increased density, (ii) decreased physiological stress, and (iii) higher potential to recruit into larger size classes, compared to seedlings established in litter. Regardless of canopy environment, seedling foliar mercury levels did not correspond to substrate mercury despite large differences in substrate mercury concentrations (relative to moss, litter concentrations were ~ 4-times greater and soil concentrations were ~ 6-times greater), likely reflecting the dominance of foliar over root uptake of mercury. Because the moss layer appeared to mitigate seedling drought stress, and to increase seedling establishment and recruitment compared to other substrates, these microsite effects should be considered in models predicting forest regeneration and dynamics under increased drought stress associated with the ongoing climate warming. 

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2. Conspecific Adult Dominance Interacts With Microsite Factors to Influence Forest Regeneration Patterns Across Elevation

   https://doi.org/10.1111/jbi.70153  

   Kiel, Nathan G; Tourville, Jordon C; Wason, Jay W; Dovciak, Martin (January 2026, Journal of Biogeography)

   Aim: As climate warms, contemporary tree regeneration patterns will affect future forest composition. Yet the factors influencing forest regeneration across environmental gradients remain difficult to disentangle. We studied how conspecific canopy tree dominance (as a proxy for seed availability) interacts with microsite factors along elevational climatic gradients to affect regeneration patterns and recruitment limitations across temperate deciduous and montane boreal forests. Location: Adirondack and Appalachian Mountains (NY, VT, NH, and ME), USA. Taxa: Abies balsamea, Acer saccharum, Betula alleghaniensis, B. papyrifera var. cordifolia, Fagus grandifolia, and Picea rubens. Methods: We measured overstory tree dominance (via species-specific basal area), understory seedling abundance, and microsite factors (e.g., understory vegetation cover, canopy openness) across elevation on ten mountains. Generalised linear regression models were fit to quantify the importance of conspecific tree basal area (representing seed availability), microsite factors, and their interaction on recruitment for each species along elevational climatic gradients. Results: Seedling abundance increased with greater conspecific adult basal area (i.e., overstory-understory relationships were positive) for all species except the shade-intolerant and wind-dispersed B. papyrifera var. cordifolia. Sparse understory vegetation amplified positive overstory-understory relationships for A. saccharum and B. alleghaniensis, while closed canopies dampened them for the shade-intolerant B. alleghaniensis and B. papyrifera var. cordifolia. The positive overstory-understory relationship for F. grandifolia was weaker in warmer microclimates, while that for the shade-tolerant A. balsamea was greater under closed canopies, likely owing to seedling banks. Main Conclusions: Nearly ubiquitous positive overstory-understory relationships indicated that local conspecific adult tree dominance drives recruitment (likely through seed dispersal) as expected. However, microsite conditions interacted with overstory basal area and modified these overstory-understory relationships differently for each species to either inhibit or facilitate tree recruitment. This framework integrates broad-scale density-dependent processes (i.e., overstory-understory conspecific relationships) with fine-scale drivers to better understand tree species recruitment across biome boundaries. 

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3. Shifts in light availability driven by dieback across a marsh‐forest gradient

   https://doi.org/10.1002/ecs2.70021  

   Nordio, Giovanna; Gedan, Keryn; Fagherazzi, Sergio (October 2024, Ecosphere)

   Abstract Ecological zonation in coastal forests is driven by sea level rise and storm‐surge events. Mature trees that can survive moderately saline conditions show signs of stress when soil salinity increases above its tolerance levels. As leaf burn, foliar damage, and defoliation reduce tree canopy cover, light gaps form within the crown. At the forest‐marsh edge, canopy cover loss is most severe; trunks of dead trees without canopies form “ghost forests.” Canopy thinning and light from the edge alter conditions for understory vegetation, promoting the growth of shrubs and facilitating establishment and spread of invasive species that were previously limited by light competition. In this research, we present an analysis of illuminance and temperature in a coastal forest transitioning to a salt marsh. Light sensors above the ground surface were used to measure light attenuation of trees and understory vegetation and to observe the effect of reduced canopies at the forest‐marsh edge. Farther from the marsh, where salinity is lower and trees are healthy, dense canopies attenuate light. We estimate that during the growing season, tree canopies intercept 50% of illuminance on average. Closer to the marsh, canopy thinning, and tree death allow greater light penetration from above, as well as from the adjacent marsh. These illuminance values are further increased by light penetration from the forest‐marsh edge (edge effect). Here, higher illuminance may permitPhragmites australisexpansion. At intermediate locations, trees intercept between 32% and 49% of light and the understory shrubMorella ceriferaintercepts a further 45% of penetrating light based on comparisons of illuminance above and below shrub canopies. Light penetration from the edge can also be felt. The presence ofM. ceriferareduces the air temperature close to the soil surface, creating a cooler summer microclimate. The tree health state is reflected in the canopy size. The canopy patterns and the edge effect are responsible for light availability distribution along forest‐marsh gradients, consequently affecting the understory vegetation biomass. We conclude that during forest retreat driven by sea level rise, tree dieback increases light availability favoring the temporary encroachment ofPh. australisandM. ceriferain the understory. 

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4. Multi‐cohort survival of northern red oak seedlings at a northern hardwood forest transition

   https://doi.org/10.1002/ecs2.4949  

   Cleavitt, Natalie L; Battles, John J; Matthes, Jaclyn Hatala; Wentzell‐Brehme, Sage (August 2024, Ecosphere)

   Van_Bael, Sunshine; Anand, Madhur (Ed.)

   Abstract Global change has created less stable forest systems and given urgency to understanding limitations to the establishment of tree seedlings beyond current range boundaries. We quantified trends in 13 years of annual northern red oak (QURU) seedling survival data for 1733 marked individuals at a local species distribution boundary within the northern hardwood forest in New Hampshire, USA. Over the study period, the median distance of seedlings into the valley did not change, although there was a net gain of 89 plots (5 m²) occupied. For a subset of seedlings that were marked in their year of birth (N = 937), we examined relationships among terrain, vegetation community, and initial individual seedling traits, and evaluated their effects on time to seedling mortality using a parametric accelerated failure time model. The year of seedling germination had the largest effect on survival with increasing mortality rates for seedlings from more recent cohorts. Seedlings had longer survival times where oak seedling densities were lower, shrub cover was higher, and when the acorn remained attached. Additionally, survival time was increased in higher elevation plots, which were also located further into the valley. Interannual seedling survival (N = 1580) was strongly impacted by seedling condition in the previous year, particularly leaf number and amount of leaf damage. Most seedling deaths occurred over winter, and seedlings failed to break bud the following spring. Interannual variation in seasonal climate, particularly deep, heavy snowpack in 2019 followed by drought conditions in 2020, coincided with recent elevated mortality. Overall, the median survival time of 3–4 years and the rapid turnover of the oak seedling population currently limit ability for expansion, although the net gain of occupied plots and increase in survival at higher elevation plots with lower QURU densities present some mechanisms that could promote expansion if the current suboptimal understory conditions shift to favor QURU. 

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5. Active restoration increases tree species richness and recruitment of large‐seeded taxa after 16–18 years

   https://doi.org/10.1002/eap.3053  

   Schubert, Spencer C; Zahawi, Rakan A; Oviedo‐Brenes, Federico; Rosales, Juan Abel; Holl, Karen D (November 2024, Ecological Applications)

   Tropical forest restoration presents a potential lifeline to mitigate climate change and biodiversity crises in the Anthropocene. Yet, the extent to which human interventions, such as tree planting, accelerate the recovery of mature functioning ecosystems or redirect successional trajectories toward novel states remains uncertain due to a lack of long‐term experiments. In 2004–2006, we established three 0.25‐ha plots at 10 sites in southern Costa Rica to test three forest restoration approaches: natural regeneration (no planting), applied nucleation (planting in patches), and plantation (full planting). In a comprehensive survey after 16–18 years of recovery, we censused >80,000 seedlings, saplings, and trees from at least 255 species across 26 restoration plots (nine natural regeneration, nine applied nucleation, eight plantation) and six adjacent reference forests to evaluate treatment effects on recruitment patterns and community composition. Both applied nucleation and plantation treatments resulted in significantly elevated seedling and sapling establishment and more predictable community composition compared with natural regeneration. Similarity of vegetation composition to reference forest tended to scale positively with treatment planting intensity. Later‐successional species with seeds ≥5 mm had significantly greater seedling and sapling abundance in the two planted treatments, and plantation showed similar recruitment densities of large‐seeded (≥10 mm) species to reference forest. Plantation tended toward a lower abundance of early‐successional recruits than applied nucleation. Trees (≥5 cm dbh) in all restoration treatments continued to be dominated by a few early‐successional species and originally transplanted individuals. Seedling recruits of planted taxa were more abundant in applied nucleation than the other treatments though few transitioned into the sapling layer. Overall, our findings show that active tree planting accelerates the establishment of later‐successional trees compared with natural regeneration after nearly two decades. While the apparent advantages of higher density tree planting on dispersal and understory establishment of larger seeded, later‐successional species recruitment is notable, more time is needed to assess whether these differences will persist and transition to the more rapid development of a mature later‐successional canopy. Our results underscore the need for ecological restoration planning and monitoring that targets biodiversity recovery over multiple decades. 

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