Overstory Canopy Research Articles

Understory Environmental Conditions Drive Leaf Level-Lipid Biosynthesis in a Deciduous and Evergreen Tree Species.

Plants in the understory experience climatic conditions affected by the overstory canopy that influence physiological and biochemical processes. Here, we investigate the relationships of leaf lipid molecular abundances to leaf water content, transmitted irradiance, and free-air temperature (Tair) from deciduous angiosperm (Quercus buckleyi) and evergreen gymnosperm (Juniperus ashei) understory trees across an elevation gradient in a central Texas (USA) woodland. Monthly sampling from 04/2019 to 01/2020 revealed that long-chain leaf waxes (≥ C27) accumulated with leaf water deficit over the growing season for both tree species. Higher transmitted light during the hottest, driest months was due to a decreased leaf area index (LAI) in the canopy as leaf shedding is a common drought response. Isoprenoids (sesqui-, di-terpenoids, phytosterols) in leaves changed by month with changing LAI and transmittance associated with monthly Tair changes. The chain length of n-alkanols in Q. buckleyi shifted with seasonal LAI at different topographic positions. The unsaturation of fatty acids in both tree species decreased with increased seasonal Tair but showed topography sensitivity. Leaf-level metabolites responded to understory microclimatic variables that were influenced by seasonality and topography.

Pre-drought effects on northern temperate trees and vine invasion in forest gaps hindering regeneration

Northern temperate coniferous forests serve as crucial connectors between boreal and temperate forests, yet they are vulnerable to various stressors such as climate change and human activities. Severe drought poses a significant threat to plant species within these forests, prompting recent research into its impacts. However, many studies lack explicit definitions of post-disturbance vegetation processes and fail to identify potential interactions with disturbance factors, necessitating comprehensive discussions. This study examines the effects of drought on tree growth patterns of the main dominant species in northern temperate regions: Abies nephrolepis and Picea jezoensis, along with two commonly associated Betula ermanii, and Quercus mongolica. Additionally, new disturbance factors in forests inhabited by these species (A. nephrolepis and P. jezoensis) were evaluated based on community classification. The study sites were located in the Mt. Baekdu (Changbai) and South Korea regions, which are positioned at the southern limit of the phytogeographical patterns of target species. Results indicate that A. nephrolepis and P. jezoensis exhibit high levels of recovery and resilience, while B. ermanii and Q. mongolica demonstrate high resistance. Species-specific responses align with drought intensity, with resistance, recovery, and resilience decreasing notably with increasing pre-drought radial growth. South Korean forests, the invasion of the vine species Tripterygium regelii after the death of A. nephrolepis in the overstory vegetation threatens the regeneration of new trees. However, certain environmental factors, such as high rock exposure and dense overstory canopy, limit vine invasion. Based on the results, pre-drought radial growth emerges as a key determinant in how trees respond to drought. Additionally, the results suggest the potential for new disturbances to emerge in forest gaps due to overstory vegetation mortality induced by global warming. These findings contribute to a deeper understanding of increasing drought stress, aid in identifying climate refugia, and inform conservation priorities based on habitat characteristics.

Overstory and understory resource competition variably influences conversion of introduced conifer plantations to native hardwoods

Restoration of native hardwood forests through tree planting may provide significant ecological and economic benefits, but reforestation in natural forests and afforestation on open field sites is challenging. Conversion of existing plantations of introduced conifers to hardwoods may provide an alternative opportunity for restoration. In the Midwest US, large areas of mature, introduced pine (Pinus spp.) plantations exist that have little economic and ecological value. These stands may provide ideal sites for planting of native hardwood species that have similar site preferences to pine. We sought to determine optimal management strategies for converting pine plantations by manipulating overstory canopy density and understory competition. We underplanted American chestnut (Castanea dentata (Marsh.) Borkh.) and northern red oak (Quercus rubra L.) seedlings in three overstory canopy treatments: control, shelterwood, and clearcut. Understory competition was either controlled or not through two growing seasons. After three growing seasons, oak performed best in the clearcut and chestnut in the shelterwood, reflecting variation in the species’ shade tolerance. Chestnut height and root collar diameter (RCD) were double that of oak, and chestnut leaf N content increased with light availability while oak did not, which can be explained by the species’ different adaptive strategies in biomass allocation. Both species had highest photosynthesis in the clearcut. Chestnut seedlings had significantly higher RCD in weeded clearcut and shelterwood plots, and oak in weeded clearcut plots. Weeding in the uncut control plots was ineffective because shade limited competition. Our results indicate that pine plantations offer suitable habitat for these hardwood species and provide insight regarding their growth strategies. Pine shelterwoods and clearcuts are each viable silvicultural conversion options for chestnut and oak if understory competition is controlled.

Early treatment effects on plantation growth and biodiversity in mature ponderosa pine forest

We address the concerns that managed forest plantations may cause reductions of diversity of understory vegetation. We performed inventories of two mature ponderosa pine plantations in which multiple treatments were applied during plantation establishment. At stand age 35, we measured tree heights and diameters, understory plant cover and diversity, soil nutrients and chemical properties, and soil microbial biomass and diversity with phospholipid fatty acid. We found a significant, positive effect from both herbicide (H) and fertilizer (F) applications on subsequent overstory tree growth and development ( P < 0.05); insecticide (I) effect was minimal. We observed negative effects on understory plant diversity decades later when herbicide was applied during stand establishment. However, lower plant diversity and ground cover appeared to have been caused primarily from overstory canopy closure, supported by the increased understory cover and diversity observed in the HI and HFI plots that had been thinned at age 12. Similarly, while fertilizer increased tree growth at both sites, it only negatively affected understory plants at the higher quality site. We did not find significant influence of treatment effects on soil nutrients and microbial communities. Therefore, to mitigate the potential loss of understory biodiversity in plantations, foresters can manage overstory trees with traditional pre-commercial thinning techniques and early tending.

Bark and crown morphology drive differences in rainwater distribution in an upland oak forest

Throughout the central and eastern US, the composition and structure of upland oak (Quercus) forests are changing. These changes are in part due to fire exclusion, whereby shade-tolerant, often fire-sensitive tree species are gaining a competitive edge over relatively shade-intolerant, fire-adapted oaks. This shift has been hypothesized to occur via a positive feedback termed mesophication, whereby encroaching tree species (‘mesophytes’) create dense, shaded understory conditions that limit oak regeneration and create cooler and wetter microclimates that decrease forest flammability. The change in canopy composition and structure may also impact hydrologic inputs by redistributing rainfall via throughfall and stemflow. To assess the extent of this redistribution by mesophytes compared to oaks and the net impact to forest floor moisture availability, we monitored throughfall, stemflow, and surface soil moisture at depths of 7.6 cm, 12.0 cm, and 20.0 cm near the bole, mid-crown, and crown edge monthly over a 14-month period. Measurements were made across seasons in both the midstory and overstory canopy strata, including oak (Q. alba, Q. falcata) and non-oak species that are increasing in relative abundance in midstory canopy strata in northern Mississippi (Carya tomentosa, Acer rubrum, Ulmus alata). Non-oak species in the midstory (A. rubrum, U. alata) had smoother bark that led to more than three-times larger stemflow generation and more than two-times larger throughfall generation per basal area compared to oak species. However, there was mixed response to soil moisture, with both non-oak (A. rubrum) and oak (Q. falcata) species having wetter than average volumetric water content compared to all other midstory species. Overstory C. tomentosa generated 2.7-times larger stemflow volume than Q. alba while Q. alba generated the largest throughfall volume. Although overstory A. rubrum generated intermediate volumes of throughfall and stemflow, the cumulative effect led to wetter than average soils compared to all other overstory species. These findings suggest that the hydrologic partitioning by mesophyte crowns may serve as an additional mechanism by which encroaching non-oak species act to reduce forest flammability and promote conditions favorable for their own regeneration. This differential rainwater redistribution mechanism may ultimately compromise long-term oak regeneration and our ability to restore fire to fire-dependent forest ecosystems.

The dependence of evaporative efficiency of vegetated surfaces on ground cover mass fractions in vegetated soils in mesic ecosystems

AbstractBare soil evaporation has been studied extensively, but less is certain regarding how site‐specific features, especially the overstory tree canopy and ground covers, mediate evaporation processes. Inspired by recent advances on modelling bare soil evaporative efficiency (SEE), this study explored SEE over a range of soil substrates and ground cover types, with and without the presence of an overstory canopy in three mesic ecosystems in Canadian Rocky Mountains. A significant relationship was found between the critical soil water content and ground cover mass fractions across various ground cover types, both with and without the presence of an overstory canopy. This relationship is expected to be prevalent across various ecosystems. Moreover, a simple approach for modelling SEE of vegetated surfaces and a correction method to account for below‐canopy SEE is also proposed. The model yields satisfactory simulations, and the approach is expected to be widely applicable, given the strength that its parameters are easily acquired, and its formulations are simple and straightforward. While the model may be particularly suited to mesic ecosystems, the underlying mechanism of SEE suggests that this model can also be applied in dryer conditions. This approach will greatly improve ET parameterization in land‐surface models (LSMs) and increase our knowledge of the global water cycle and ecosystem responses under climate change impacts.

Enrichment-planting with pines alters fuel amount and structure in endangered Araucaria araucana forests in northwestern Patagonia, Argentina

The introduction of non-native tree species for large-scale afforestation may alter the fire regime of native ecosystems by modifying fuel proprieties. We quantified changes in fuel abundance and structure resulting from the establishment of commercial Pinus spp. plantations in Araucaria araucana ecosystems in northwestern Patagonia, Argentina. Specifically, we assessed the amount, distribution, and condition (live/dead) of surface and standing fine fuel in A. araucana stands with mature pine plantations (i.e. > 20 cm dbh) and in stands dominated only by A. araucana (control). Our study shows that both types of stands are prone to wildfires, but pine plantations have fuel characteristics that imply greater flammability due to higher fuel load and vertical continuity in the understory and in the overstory canopy. In the absence of fuel mitigation practices, A. araucana stands with plantations exhibit greater flammability than the control A. araucana stands, potentially promoting the occurrence and spread of fires of greater severity. This study contributes to understanding the effects of enrichment planting of pines, and possibly pine invasions, on A. araucaria ecosystem flammability and their potential consequences on fire behavior.

Differential impacts of spruce beetle outbreaks on snowshoe hares and red squirrels in the southern Rocky Mountains

Spruce beetles (Dendroctonus rufipennis) have impacted millions of acres of Engelmann spruce (Picea engelmannii) – subalpine fir (Abies lasiocarpa) forest in North America over the past decade, resulting in the most extensive outbreak in recorded history. This dramatic alteration of forest composition and structure has precipitated numerous changes to forest ecology and ecosystem services. Among the least studied of these changes are impacts to wild mammals, including snowshoe hares (Lepus americanus) and red squirrels (Tamiasciurus hudsonicus). We sampled a chronosequence of spruce-fir stands along a gradient of ‘years elapsed since spruce beetle outbreak’ (YSO) in order to estimate impacts to abundance of these two species in the southern Rocky Mountains. Snowshoe hare abundance was not related to YSO, at least in the first decade post-outbreak. Instead, hare abundance during this period was positively related to horizontal cover, especially that due to stem density of small diameter subalpine fir. Notably, snowshoe hare abundance was negatively related to stem density of small diameter Engelmann spruce, suggesting that elements of horizontal cover may not be uniformly beneficial to hares. Hare abundance was also negatively related to ground cover, which could help explain the lack of relationship to YSO, assuming reduction in overstory canopy would lead to increases in ground cover. Red squirrel abundance was negatively related to YSO and outbreak severity (i.e., basal area of large diameter dead trees). This was likely due to diminished cone crops in impacted areas, which red squirrels cache and rely on heavily to sustain them through the winter. Basal area of remaining large live fir trees was not related to squirrel abundance, suggesting that regeneration of spruce and associated cone crops may be necessary for recovery of red squirrels, which may take several decades.

Seedling drought responses governed by root traits, site-soil moisture regimes and overstory competition-facilitation

Periodic drought may interact with spatially patterned soil moisture regimes, canopy disturbance and species seedling traits to influence tree regeneration processes in northern temperate forests. In northern Michigan, USA, we assessed the effects of regional upland soil moisture regimes (six sites classified from dry to mesic) and tree canopy shading (% light) on planted-seedling dry mass, probability of mortality and functional traits for eight tree species varying in shade tolerance and natural distribution over soil moisture gradients (i.e., drought tolerance): Quercus velutina, Quercus alba, Quercus rubra, Prunus serotina, Acer rubrum, Acer saccharum, Fraxinus americana, and Betula alleghaniensis. The assessment period included droughts during the seedlings’ 2nd and 3rd growing seasons. High mortality (especially on drier-classified sites and in more open, high-light environments) and negative associations of soil water minima with mortality collectively suggest drought-driven mortality across sites. In contrast, canopy shade driven mortality occurred only on mesic sites at <10 % light. Overstory canopies facilitated seedling survival relative to more open environments, with survival maxima ranging from 3 to 20 % light among sites. Among species, probability of survival was 1) highest for Quercus species (∼0.70) and lowest for B. alleghaniensis (0.10), 2) positively and generally associated with drought tolerance, but not associated with shade tolerance, and 3) positively and strongly associated with species seed-size related differences in total dry mass, root dry mass and rooting depth. In turn, rooting depth (range 3–68 cm) was positively related to seedling water status, and water status to survival during drought. In summary, site soil moisture regimes, facilitation vs. competition via variation in canopy density, and size-related species traits drove patterns of young seedling mortality during an interval that included droughts. Results suggests current drought regimes likely have strong influence on canopy composition across the region’s spatially patterned dry to mesic soil gradients, and may have larger impacts if drought intensifies with climate change. Our results can aid development of planting strategies for assisted migration and other emerging needs, including planting 1) species more drought adapted than those dominating current overstories, 2) large stock with well-developed root systems, and 3) in microsites that maximize canopy facilitation.

Midstory removal of encroaching species has minimal impacts on fuels and fire behavior regardless of burn season in a degraded pine-oak mixture

Anthropogenic fire exclusion has contributed to forest structural and compositional shifts, resulting in the encroachment of shade-tolerant, often fire-sensitive and/or opportunistic species throughout forest ecosystems. In the central and eastern U.S., encroaching species often exhibit fire-suppressing leaf litter and crown traits, reducing prescribed fire efficacy to restore and maintain fire-dependent mixed pine (Pinus spp.) and oak (Quercus spp.) forests. Despite fire’s natural role in these forests, management using prescribed fire alone is unlikely to reverse encroachment impacts due to the degree of forest change since fire exclusion. Consequently, additional management techniques (e.g., chemical, mechanical) are often combined with fire to ensure persistence of pine-oak mixtures and their associated ecosystem services. In current-day degraded pine-oak mixtures in the southeastern U.S., where no one species represents > 75% dominance by stand basal area, encroaching species (e.g., Liquidambar styraciflua, Quercus nigra) commonly form a dense midstory and occupy overstory positions. To quantify how encroaching species in degraded mixed pine-oak forests impact fuels, canopy cover, and seasonal fire behavior, we thinned the midstory (<20 cm DBH) of all non-pyrophytic species in 12 0.10-ha blocks within a mixed pine-oak stand at the Mary Olive Thomas Demonstration Forest (Auburn, Alabama, USA) in July 2021. Midstory thinning reduced basal area by 6.7 m2 ha−1 and canopy cover by 9.1%. In general, midstory thinning reduced shrub and fine woody debris fuel loads while having no impact on herbaceous, leaf litter, and duff fuel loads. When compared to pre-treatment levels, midstory thinning increased the relative proportion of pine leaf litter contribution to the fuelbed by 10.9% while reducing that of encroaching species by 11.4%. Despite changes to residual basal area, canopy cover, and leaf litter contribution, midstory thinning had no statistical impact on fire behavior, regardless of burn season. Early (January) and late dormant season (April) experimental fires had higher fireline intensities, consumed more surface fuels, had faster rates of spread, and higher average maximum temperatures compared to growing season (September) fires. We also noted an increase in these fire parameters with increased pine litter contribution to the fuelbed. In degraded pine-oak mixtures where encroaching species now occupy overstory canopy positions, we recommend investigating the efficacy of more intense thinning treatments that remove overstory individuals with known fire-suppressing traits to have a larger impact on canopy openness, fuels, and fire behavior.

STAY-GREEN Accelerates Chlorophyll Degradation in Magnolia sinostellata under the Condition of Light Deficiency.

Species of the Magnoliaceae family are valued for their ornamental qualities and are widely used in landscaping worldwide. However, many of these species are endangered in their natural environments, often due to being overshadowed by overstory canopies. The molecular mechanisms of Magnolia's sensitivity to shade have remained hitherto obscure. Our study sheds light on this conundrum by identifying critical genes involved in governing the plant's response to a light deficiency (LD) environment. In response to LD stress, Magnolia sinostellata leaves were endowed with a drastic dwindling in chlorophyll content, which was concomitant to the downregulation of the chlorophyll biosynthesis pathway and upregulation in the chlorophyll degradation pathway. The STAY-GREEN (MsSGR) gene was one of the most up-regulated genes, which was specifically localized in chloroplasts, and its overexpression in Arabidopsis and tobacco accelerated chlorophyll degradation. Sequence analysis of the MsSGR promoter revealed that it contains multiple phytohormone-responsive and light-responsive cis-acting elements and was activated by LD stress. A yeast two-hybrid analysis resulted in the identification of 24 proteins that putatively interact with MsSGR, among which eight were chloroplast-localized proteins that were significantly responsive to LD. Our findings demonstrate that light deficiency increases the expression of MsSGR, which in turn regulates chlorophyll degradation and interacts with multiple proteins to form a molecular cascade. Overall, our work has uncovered the mechanism by which MsSGR mediates chlorophyll degradation under LD stress conditions, providing insight into the molecular interactions network of MsSGR and contributing to a theoretical framework for understanding the endangerment of wild Magnoliaceae species.

Mapping understory plant communities in deciduous forests from Sentinel-2 time series

Understory plant communities are an integral component of deciduous forests, playing a vital role in the overall health of the ecosystem. However, remote sensing of understory plant communities is challenging due to the obstruction by the forest canopy. In this study, we proposed an automated dense Sentinel-2 time series-based approach for understory plant communities and created maps of four understory classes (i.e., native shrubs of greenbrier and mountain laurel, and invasive shrubs of barberry and the assemblage of mixed invasive) at 10 m resolution in Connecticut's deciduous forests in 2020. A harmonic time series model and three years of Sentinel-2 time series from 2019 to 2021 were used to classify understory species based on their unique, intra-annual phenology characteristics. The time series model coefficients captured the subtle phenology differences and created synthetic cloud-free images within a short temporal window in the spring prior to canopy leaf-on (hereafter called “observation window”). During the observation window, Sentinel-2 data penetrated the deciduous overstory canopy and observed the unique trajectories of different understory species due to their phenology differences. We also calculated spatial texture features (i.e., mean, second moment, and contrast from gray level co-occurrence matrix) based on the synthetic images created within the observation window to capture the different conditions of leaf growth and distinct spatial patterns within deciduous forests. By using the spectral, temporal, and spatial features as input variables from dense Sentinel-2 data, auxiliary data (i.e., LiDAR and soil drainage layer), a random forest classifier, and a new strategy to iteratively select representative sample (namely ISRS), understory species maps were created with an overall accuracy of approximately 93%, and the user's and producer's accuracies varied from 39% to 99% for the three mapped understory species and one assemblage of species. The proposed method created an accurate binary map of understory presence with an overall accuracy of 95%, a producer's accuracy of 84%, and a user's accuracy of 68%. Additionally, we separated the invasive (i.e. barberry and mixed invasive of multi-flora rose, oriental bittersweet, honeysuckle, winged euonymus, and autumn olive) and native (greenbrier and mountain laurel) species with an overall accuracy of 94%. We estimated that the invasive species cover an area of 649.33 ± 140.59 km2, which occupied a large proportion (∼53%) of the shrub understory in Connecticut's deciduous forests.

Canopy and surface fuel estimations using RPAS and ground-based point clouds

Abstract Forest management activities intended to reduce wildfire risk rely on accurate characterizations of the amount and arrangement of canopy and surface fuels. Metrics that describe these fuels are typically estimated with various systems that transform plot-level field data into metrics that can be used within fire behaviour models. Remote sensing data have long been used to estimate these metrics across large spatial scales, but more advanced, high-density point clouds have the potential to estimate these metrics with higher accuracy. This study collected LiDAR and digital aerial photogrammetric (DAP) point clouds from a remotely piloted aerial system (RPAS), as well as mobile laser scanning (MLS) point clouds from a mobile ground-based system, and compared their ability to estimate fuel metrics. This involved the extraction of predictor variables from each point cloud, of which small subsets were used to estimate various fuel metrics. These included six overstory canopy metrics (stand height, canopy cover, tree density, canopy fuel load, canopy bulk density and canopy base height), three diameter at breast height (DBH)–related metrics (stand density index, basal area and quadratic mean diameter) and three surface fuel metrics (total woody debris (TWD), coarse woody debris (CWD) and fine woody debris (FWD)). Overall, canopy metrics were estimated most accurately by the RPAS LiDAR models, although none of the point clouds were able to accurately estimate DBH-related metrics. For the other six canopy metrics, RPAS LiDAR models had an average R2 value of 0.70; DAP – 0.63 and MLS – 0.63. CWD (&amp;gt;7 cm) and TWD loads were estimated most accurately by the MLS models (average R2 values – 0.70), followed by the RPAS LiDAR – 0.38 and DAP – 0.13. None of these models were able to accurately estimate FWD loads (≤7 cm in diameter), with the three types of point clouds having a maximum R2 value of 0.08. Overall, this research shows the relative ability of three types of high-density point clouds to estimate metrics relevant for fire behaviour modeling.

Twenty Years of Urban Reforestation: Overstory Development Structures Understory Plant Communities in Lexington, KY, USA

Urban forests provide critical ecosystem services in an increasingly urbanized global landscape. The reforestation of undeveloped parcels and serially mowed grasslands can increase urban forest cover, but plant community development in planted urban forests is poorly understood. We conducted a study to elucidate the roles of time since tree planting, invasive species abundance, and other abiotic and biotic site-level factors in structuring understory plant communities within a 20-year chronosequence of planted urban forests in Lexington, KY, USA. We assessed the percent of groundcover of all understory species in fixed-radius plots on the site. Understory herbaceous plant communities demonstrated shifts from graminoid dominance to forb dominance over time, and plant communities in successively younger sites were increasingly dissimilar from that of the 20-year-old site. Invasive plant species were abundant, representing 21% of total groundcover across all surveyed plots, and became increasingly prevalent over time. Understory plant diversity was negatively associated with invasive species abundance. Overall, site factors, including time since planting, forest canopy closure, density of tree and shrub reproduction, and soil pH, accounted for much of the variability among understory communities. Understory plant communities across the chronosequence of planted sites demonstrated apparent structural shifts with overstory canopy development, but the increasing prevalence of invasive species and their negative impacts to plant diversity warrant future management to ensure the continuation of the desired successional trajectories.

Remote mapping of leafy spurge (Euphorbia esula, L.) in Northwestern Colorado

Leafy spurge (Euphorbia esula L.) has been introduced to the Yampa River in Northwestern Colorado for over 40 years and flood and runoff events transport leafy spurge propagules onto adjacent landscapes. The spread of leafy spurge beyond the river channels has yet to be mapped and recorded, and this research was conducted to map leafy spurge occurrence in the Yampa River Valley. Significant stakeholder mapping efforts took place in the summer of 2019–2021, leading to excellent spatial data on leafy spurge presence and absence along the main channel. In summer 2019, multispectral SPOT seven satellite imagery, stakeholder ground mapping efforts, and bright yellow-green leafy spurge bracts were used to interpret imagery, identify dense, unobscured patches of leafy spurge, and digitize them. Spectral signatures from training samples for leafy spurge and other land cover classes (generalized as “not leafy spurge”) were then used to train a Random Forest machine learning classification. In the summer of 2021, generated classification maps were compared to multispectral satellite imagery and stakeholder ground mapped leafy spurge presence. Mismatches were identified, and 271 validation locations were identified, navigated to, and evaluated for leafy spurge presence. Leafy spurge training samples were classified with 96% accuracy. Correctly classified leafy spurge locations had higher leafy spurge coverage and lower overstory canopy than missed leafy spurge locations. Leafy spurge growing beneath shrub canopy or growing as individual plants along the riverbanks were more likely to be missed. A frequency analysis for other plant species found at validation locations determined that smooth brome (Bromus inermis Leyss.), dandelion (Taraxacum officinale L.), and willow (Salix sp.) were most frequently misclassified as leafy spurge. In conclusion, multispectral satellite imagery was useful at remote detection of leafy spurge in open areas with dense leafy spurge coverage, but more work must be done for identification of sparse and diffuse leafy spurge infestations.

20-year effects of white-tailed deer (Odocoileus virginianus) herbivory on vegetation composition and structure

Throughout the southeastern U.S., white-tailed deer (Odocoileus virginianus; hereafter deer) are the dominant herbivores and have the potential to impact vegetation communities through selective browse pressure. Previous studies regarding deer herbivory impacts focused on northeastern and midwestern U.S. forests. Additionally, most studies have observed herbivory effects over short timescales (i.e., <10 years). Therefore, we sought to determine deer herbivory effects on vegetation structure and composition among three different ecoregions in Mississippi at 0, 5, and ∼ 20 years after exclusion. In 2000, 3 pairs of experimental units were identified in each of the Upper Coastal Plain, Lower Coastal Plain, and Mississippi Alluvial Valley ecoregions. An exclosure treatment was randomly assigned within each pair (averaging 2.7 ha) with the other experimental unit a control. From May-June, vegetation structure and composition were measured among 10 systematic sampling points per plot during 2000, 2005, and 2021. While no treatment effects on vegetation composition and structure were observed, among-year differences suggested a shift in overstory canopy conditions between 2005 and 2021 towards greater basal area and canopy coverage resulting in a decrease in forbs, legumes, shade intolerant species, and high year-round deer-preferred forage species. Deer herbivory effects could have been obscured by overstory conditions, low deer densities, and similar baseline conditions reflecting past herbivory effects. Regional differences in forest structure and composition and exclosure breaches may also be responsible for the lack of treatment effects. Future studies should consider utilizing disturbance to offset prior herbivory impacts and to standardize overstory conditions.

Growth and Morphological Patterns of Norway Spruce (Picea abies (L.) Karst.) Juveniles in Response to Light Intensities

(1) Background: The growth and morphology of Norway spruce (Picea abies (L.) Karst.) juveniles were observed under different light conditions due to overstory canopy openness. The aim was to investigate the photo-morphological responses of juveniles for the development of a desirable forest structure. The research was located in a higher altitude zone in central Europe. (2) Methods: Light was estimated using hemispherical photographs. Eight different parameters of natural regeneration were measured on individuals within 1 × 1 m around each point on a 3 × 3 m grid. A total of 1214 sapling measurements (from 10 to 431 cm in height) were taken. (3) Results: Light affected the growth and morphological development of juveniles, resulting in variation in the lateral crown growth and crown length. Acclimations manifested as trade-offs between height and lateral crown growth. Similar shifts were found within relative height growth (the terminal length to the total height) and apical dominance ratio (the terminal length to the branch length). The crown length was proven to be highly capable to acclimation. Light influenced the density weakly, however, when a regeneration index (density × median height) was considered, significant relations with light conditions were discovered.

Seeing past the green: Structure, composition, and biomass differences in high graded and silviculture-managed forests of similar stand density

Forests of the eastern United States (US) mostly comprise a mix of stands managed following silvicultural principles and stands managed with exploitative timber harvesting practices. These stands can have similar stand densities (e.g., basal area per hectare) but vary vastly in structure, composition, and biomass and carbon storage. High grading, a prevalent exploitative timber harvesting practice in the eastern US, is of particular concern because it can negatively affect future forest health and productivity. This study quantifies differences in forest structure, composition, and biomass and carbon storage between high graded stands and stands that received a seed/establishment cut of a uniform shelterwood regeneration sequence treatment, which is a comparable and well-established silvicultural method used to regenerate mixed-oak forests. It focuses on mixed-oak forests (mixed-Quercus), where the effects of high grading have been understudied, and uses a sample with broader spatial coverage than previous studies. The sample comprised nine stands that were known to have been high graded 8–15 years ago and nine stands that received the seed/establishment cut of a uniform shelterwood regeneration sequence. Stand were systematically sampled using fixed-area plots. Field measurements were collected and used to calculate metrics describing forest structure and function. The structure of high graded stands was characterized by a higher proportion of trees with poor health and/or form compared to shelterwood stands, with 18.3 % less acceptable growing stock and trees with lower crown compaction. Diameter distributions of high graded stands were characterized by numerous small trees and few large-diameter trees. Spatial variability of overstory trees was contingent on the tree size range evaluated, with a larger variability of sawtimber-sized trees (trees ≥ 29.2 cm in diameter at breast height) in high graded stands. High graded stands also had 2.2 times fewer oak trees (Quercus spp.) in the overstory canopy, 17,897 fewer seedlings per hectare (ha), and 45 Mg/ha less biomass than shelterwood stands. These results indicate that high grading generally degrades mixed-oak forests and impairs their long-term capacity to supply vital ecosystem services such as habitat for specific wildlife species, carbon storage, and high-quality wood products.

Second-Entry Burns Reduce Mid-Canopy Fuels and Create Resilient Forest Structure in Yosemite National Park, California

Understanding the patterns and underlying drivers of forest structure is critical for managing landscape processes and multiple resource management. Merging several landscape-scale datasets, including long-term fire histories, airborne LiDAR, and downscaled topo-climatic data, we assessed complex ecological questions regarding the interactions of forest structure, climate, and fire in the Yosemite National Park, a protected area historically dominated by frequent fire and largely free of the impacts of commercial industrial logging. We found that forest structure broadly corresponded with forest types arranged across elevation-driven climatic gradients and that repeated burning shifts forest structure towards conditions that are consistent with increased resilience, biodiversity, and ecosystem health and function. Specifically, across all forest types, tree density and mid-canopy strata cover was significantly reduced compared to overstory canopy and the indices of forest health improved after two fires, but no additional change occurred with subsequent burns. This study provides valuable information for managers who seek to refine prescriptions based on an enhanced understanding of fire-mediated changes in ladder fuels and tree density and those seeking to define the number of treatments needed to mitigate severe fire risk and enhance resiliency to repeated fires. In addition, our study highlights the utility of large-landscape LiDAR acquisitions for supporting fire, forest, and wildlife management prioritization and wildfire risk assessments for numerous valued resources.

Effects of canopy cover on fruiting intensity and fruit removal of a tropical invasive weed

Lantana camara (hereafter, Lantana) is among the worst invasive alien plants spread extensively across Africa, Australia and Asia at an alarming rate, posing significant challenges to conservation of native biodiversity. While, Lantana invasion is widely recognised to be more pronounced in open-canopy habitats (including deciduous forests, forest edges and gaps), the potential role of variation in seed dispersal across habitats varying in overstory canopy cover is poorly understood.Avian frugivores are among primary seed dispersers of the fleshy-fruited Lantana. We monitored 45 Lantana shrubs across a gradient of overstory canopy cover to determine the relationship between fruiting intensity and canopy cover. We watched 80 Lantana shrubs (240 h) across a canopy cover gradient to determine 1) differences in frugivore assemblage visiting Lantana across open- and closed-canopy habitats, 2) drivers of frugivore visitation on Lantana, and 3) relationship between seed disperser visitation rate and overstory canopy cover. We found that Lantana shrubs under low overstory canopy cover had higher fruit abundance than those in high canopy cover. Frugivore assemblage differed between Lantana shrubs in open- versus closed-canopy cover habitats. Drivers of frugivore visitation on Lantana varied across different frugivore species with a greater probability of occurrence of bulbuls (the primary seed dispersers of Lantana) on shrubs under low overstory canopy cover.Visitation rates of the effective seed dispersers were higher on shrubs under low overstory canopy cover. Thus, there was greater chances of dispersal of seeds in habitats with low overstory canopy cover.The study demonstrates variable fruiting intensity and fruit removal rate as a driver of differences in dispersal of seeds across habitats. It highlights greater vulnerability of open habitats to invasion and the need to prioritise Lantana management efforts in open habitats. Anthropogenic activities that lead to canopy openings (e.g. tree lopping and logging) likely facilitate Lantana invasion through greater fruit production and seed dispersal.