Overstory Trees Research Articles

Developing a Structural Complexity Index for Oriental Beech Forests in Northern Iran

Abstract The structural complexity index (SCI) has become an important metric for forest managers to monitor ecosystem services and conservation value in a wide variety of forest types. In this study, we developed an SCI for an unmanaged mixed Fagus orientalis Lipsky forest in northern Iran, which incorporated structural information specific to mature and old-growth forests. Our results showed that we were able to develop an SCI for this forest that would assist managers to make conservation decisions in a forest where large overstory trees and small understory trees are equally important. The SCI was significantly positively correlated with the density of five minor tree species (Acer velutinum Boiss., Acer cappadocicum Gled., Tilia begoniifolia Chun & H.D. Wong, Quercus castaneifolia C.A. Mey., and Prunus avium L.) and the density of large ([50 cm < diameter at breast height {DBH} < 75 cm]) and very large (DBH > 75 cm) overstory trees. The SCI remains a highly flexible tool for forest conservation and decision making and may assist with decisions about forest management in response to climate change and shifting disturbance regimes. Study Implications: Forest managers have begun to use the structural complexity index (SCI) to assess the successful achievement of conservation management objectives. In many mature, mixed-species forests, large trees, minor species, and understory tree species are important structural components. In this study, we demonstrate that reduction of the minimum diameter used for sampling trees and careful selection of the variables used to calculate SCI results in a valuable metric for making conservation decisions. The advantage of SCI as a forest decision tool is that forest managers are able to adjust the inputs used to calculate SCI to reflect specific management objectives or monitoring goals.

Microbial controls over soil priming effects under chronic nitrogen and phosphorus additions in subtropical forests.

The soil priming effect (PE), defined as the modification of soil organic matter decomposition by labile carbon (C) inputs, is known to influence C storage in terrestrial ecosystems. However, how chronic nutrient addition, particularly in leguminous and non-leguminous forests, will affect PE through interaction with nutrient (e.g., nitrogenandphosphorus) availability is still unclear. Therefore, we collected soils from leguminous and non-leguminous subtropical plantations across a suite of historical nutrient addition regimes. We added 13C-labeled glucose to investigate how background soil nutrient conditions and microbial communities affect priming and its potential microbial mechanisms. Glucose addition increased soil organic matter decomposition and prompted positive priming in all soils, regardless of dominant overstory tree species or fertilizer treatment. In non-leguminous soil, only combined nitrogen and phosphorus addition led to a higher positive priming than the control. Conversely, soils beneath N-fixing leguminous plants responded positively to P addition alone, as well as to joint NP addition compared to control. Using DNA stable-isotope probing, high-throughput quantitative PCR, enzyme assays and microbial C substrate utilization, we found that positive PE was associated with increased microbial C utilization, accompanied by an increase in microbial community activity, nutrient-related gene abundance, and enzyme activities. Our findings suggest that the balance between soil available N and P effects onthe PE, wasdependent on rhizosphere microbial community composition. Furthermore, these findings highlight the roles of the interaction between plants and their symbiotic microbial communities in affecting soil priming and improve our understanding of the potential microbial pathways underlying soil PEs.

Silvicultural treatments improve pest and disease conditions of white pine (Pinus strobus) residual trees and regeneration

Managing multiple forest insect pests and diseases is challenging. For example, in eastern white pine (Pinus strobus) stands whereas partial shading and high seedling density is encouraged to reduce damage by white pine blister rust (Cronartium ribicola) and white pine weevil (Pissodes strobi), dense conditions in the understory may increase damage by foliar diseases such as brown spot needle blight (Lecanosticta acicola) and Caliciopsis canker (Caliciopsis pinea). We evaluated the effect of silvicultural treatments, shelterwoods (residual basal area < 18 m2 ha−1), low density thinnings (residual basal area ≤ 14 m2 ha−1), patch cuts (1.2 ha openings), and untreated controls on damage by these insect pest and diseases in residual overstory trees and regeneration. Shelterwoods and low density thinnings provided a good balance of some shading and reduced stem density, which resulted in less weevil damage and foliar disease severity. Crown condition and quality of regeneration was better in all treatments compared to unmanaged controls. Shelterwoods, low density thinnings and patch cuts have the added benefit on increasing seral habitat, resulting in greater songbird diversity.

Patterns of deep fine root and water utilization amongst trees, shrubs and herbs in subtropical pine plantations with seasonal droughts.

Seasonal droughts will become more severe and frequent under the context of global climate change, this would result in significant variations in the root distribution and water utilization patterns of plants. However, research on the determining factors of deep fine root and water utilization is limited. We measured the fine root biomass and water utilization of trees, shrubs and herbs, and soil properties, light transmission, and community structure parameters in subtropical pine plantations with seasonal droughts. We found that the proportion of deep fine roots (below 1 m depth) is only 0.2-5.1%, but that of deep soil water utilization can reach 20.9-38.6% during the dry season. Trees improve deep soil water capture capacity by enhancing their dominance in occupying deep soil volume, and enhance their deep resource foraging by increasing their branching capacity of absorptive roots. Shrubs and herbs showed different strategies for deep water competition: shrubs tend to exhibit a "conservative" strategy and tend to increase individual competitiveness, while herbs exhibited an "opportunistic" strategy and tend to increase variety and quantity to adapt to competitions. Our results improve our understanding of different deep fine root distribution and water use strategies between overstory trees and understory vegetations, and emphasize the importance of deep fine root in drought resistance as well as the roles of deep soil water utilization in shaping community assembly.

Tree Stem Detection and Crown Delineation in a Structurally Diverse Deciduous Forest Combining Leaf-On and Leaf-Off UAV-SfM Data

Accurate detection and delineation of individual trees and their crowns in dense forest environments are essential for forest management and ecological applications. This study explores the potential of combining leaf-off and leaf-on structure from motion (SfM) data products from unoccupied aerial vehicles (UAVs) equipped with RGB cameras. The main objective was to develop a reliable method for precise tree stem detection and crown delineation in dense deciduous forests, demonstrated at a structurally diverse old-growth forest in the Hainich National Park, Germany. Stem positions were extracted from the leaf-off point cloud by a clustering algorithm. The accuracy of the derived stem co-ordinates and the overall UAV-SfM point cloud were assessed separately, considering different tree types. Extracted tree stems were used as markers for individual tree crown delineation (ITCD) through a region growing algorithm on the leaf-on data. Stem positioning showed high precision values (0.867). Including leaf-off stem positions enhanced the crown delineation, but crown delineations in dense forest canopies remain challenging. Both the number of stems and crowns were underestimated, suggesting that the number of overstory trees in dense forests tends to be higher than commonly estimated in remote sensing approaches. In general, UAV-SfM point clouds prove to be a cost-effective and accurate alternative to LiDAR data for tree stem detection. The combined datasets provide valuable insights into forest structure, enabling a more comprehensive understanding of the canopy, stems, and forest floor, thus facilitating more reliable forest parameter extraction.

Mechanical thinning restores ecological functions in a seasonally dry ponderosa pine forest in the inland Pacific Northwest, USA

An increasingly important goal of federal land managers in seasonally dry forests of the western US is restoring forest resilience. In this study, we quantified the degree to which a thinning treatment in a dry forest of eastern Oregon restored aspects of forest resilience by focusing on key functional attributes of our study system. First, we measured several physiological responses of overstory trees that are associated with resilience, including radial growth, resin production, abundance of non-structural carbohydrates (NSC), and leaf area. Second, we investigated understory vegetation responses including species diversity, composition, and cover by growth form that influence fire behavior and resilience to disturbance. We found that tree radial growth was greater in trees in thinned stands beginning three years post-treatment. The abundance of key chemical compounds found in trees, including resin, starch, total NSC and sucrose did not differ between treatments; however, abundance of glucose plus fructose was lower in treated stands, suggesting mobilization and use of carbon reserves for foliar and wood production following thinning. We observed an increase in species richness and diversity within thinned stands three to four years after thinning, primarily due to the release of forbs and reestablishment of graminoids following ground disturbance. Here, we demonstrate that elements of forest resilience can be restored in dry forest systems via selective thinning to promote historical forest structure. In forests where thinning reduces stand density, vigorous overstory trees and increased herbaceous cover can help facilitate the re-establishment of low intensity surface fire regimes that maintain stable and persistent vegetative states. Understanding the ecological effects of fuel reduction treatments allows land managers to assess potential forest resiliency and adapt future treatments based on the observed results of previous activities.

Effects of tree canopy on herbaceous understorey throughout the developmental cycle of a temperate mountain primary forest

Temporal dynamics of temperate montane primary forests were described based on data from Central-European nature reserves in the 20th century as a long-term cycle involving tree regeneration, growth, maturation, and disintegration. However, the lack of chronosequences focusing on the herbaceous understory has limited our understanding of plant community dynamics within this developmental cycle. To address questions regarding whether the herbaceous understory follows the cyclic trajectory of the tree layer and if the properties of the tree layer drive the dynamics of the herbaceous understory, we investigated a 19-year chronosequence in ten plots (with five sampling periods) within an unmanaged primary beech-fir forest in the Western Carpathians. The vegetation data from the resampled plots, including herb species frequencies and dendrometric variables, covered a significant portion of the developmental cycle of a single plant community type. The results confirmed the cyclical nature of herbaceous understory development in terms of species composition and diversity trajectories, demonstrating transitions between developmental stages. Diversity was highest during the disintegration stage, with a rapid decline towards the growth stage, followed by a slow recovery as it progressed towards the optimum stage. Temporal responses of individual species, along with ordination analyses, allowed for the separation of taxa associated with the disintegration, growth, and optimum stages, as well as the most stable ones throughout the entire cycle. The proportion of therophytes slightly increased after gap-opening events during disintegration, geophytes were most represented during the growth stage, and hemicryptophytes culminated during the optimum stage. The composition of herb species was significantly affected by the above-ground tree biomass (AGTB), which explained 21.4% of the temporal variability. The sum of the diameter at breast height (Sum of DBH) described an additional 7.2%. The relationship between the frequency of the herbaceous understory and AGTB followed a “U” shape, while the relationship with Sum of DBH was negative and linear. The relationship between the herbaceous understory and the tree overstory is likely driven by competition for light, water, and nutrient resources. These findings provide new insights into the temporal relationships between the herbaceous understory and tree layer development, and they may inspire future studies and management strategies aimed at conserving or enhancing biodiversity in temperate forests.

Vertical and Horizontal Crown Fuel Continuity Influences Group-Scale Ignition and Fuel Consumption

A deeper understanding of the influence of fine-scale fuel patterns on fire behavior is essential to the design of forest treatments that aim to reduce fire hazard, enhance structural complexity, and increase ecosystem function and resilience. Of particular relevance is the impact of horizontal and vertical forest structure on potential tree torching and large-tree mortality. It may be the case that fire behavior in spatially complex stands differs from predictions based on stand-level descriptors of the fuel distribution and structure. In this work, we used a spatially explicit fire behavior model to evaluate how the vertical and horizontal distribution of fuels influences the potential for fire to travel from the surface into overstory tree crowns. Our results support the understanding that crown fuels (e.g., needles and small-diameter branchwood) close to the surface can aid in this transition; however, we add important nuance by showing the interactive effect of overstory horizontal fuel connectivity. The influence of fuels low in the canopy space was overridden by the effect of horizontal connectivity at surface fire-line intensities greater than 1415 kW/m. For example, tree groups with vertically continuous fuels and limited horizontal connectivity sustained less large-tree consumption than tree groups with a significant vertical gap between the surface and canopy but high-canopy horizontal connectivity. This effect was likely the result of reduced net vertical heat transfer as well as decreased horizontal heat transfer, or crown-to-crown spread, in the upper canopy. These results suggest that the crown fire hazard represented by vertically complex tree groups is strongly mediated by the density, or horizontal connectivity, of the tree crowns within the group, and therefore, managers may be able to mitigate some of the torching hazard associated with vertically heterogenous tree groups.

Preliminary vegetational changes in frequently burned and unburned upland pine-hardwood forests at Cook’s Branch Conservancy in Montgomery County, Texas, U.S.A.

Vegetational changes over a 6-year period (2012 to 2017) were recorded in upland pine – hardwood forests on Cook’s Branch Conservancy (CBC) in Montgomery County, Texas, using permanent vegetation monitoring plots as a basis for following future vegetational changes. Cook’s Branch Conservancy is a 2,160-hectare preserve purchased by George and Cynthia Mitchell in 1964 and is now part of a conservation program operated by the Cynthia and George Mitchell Foundation. Plots sampled for this study include eight Fire Monitoring Handbook vegetation plots located in upland forests over sandy soils including four plots located in frequently burned stands (11 times since 2002) and four plots located in stands that have not been burned in decades (20+ years). Data collected confirm significant changes to vegetational structure in frequently burned forests compared to similar habitats that are unburned. Prior to initiation of prescribed burning in 2001, habitat conditions and species composition was similar in stands surrounding all eight plots sampled. The overstory tree structure of all eight plots sampled is similar in basal area, canopy cover, and in species composition. The number of midstory trees is less in frequently burned plots, however, frequently burned plots possess a much greater number of seedlings. At the shrub level, Callicarpa americana and Ilex vomitoria represented 97.7% of total stems in the frequently burned plots, with Callicarpa americana stems count increasing in frequently burned plots, suggesting the species is pyrophytic. Additionally, diversity of herbaceous species was higher in frequently burned plots vs unburned plots. Visually, the frequently burned forest is open in the understory, while the unburned forest is dense with woody understory vegetation making it difficult to traverse on foot.

Tree species growth response to climate warming varies by forest canopy position in boreal and temperate forests.

Reports of forest sensitivity to climate change are based largely on the study of overstory trees, which contribute significantly to forest growth and wood supply. However, juveniles in the understory are also critical to predict future forest dynamics and demographics, but their sensitivity to climate remains less known. In this study, we applied boosted regression tree analysis to compare the sensitivity of understory and overstory trees for the 10 most common tree species in eastern North America using growth information from an unprecedented network of nearly 1.5 million tree records from 20,174 widely distributed, permanent sample plots across Canada and the United States. Fitted models were then used to project the near-term (2041-2070) growth for each canopy and tree species. We observed an overall positive effect of warming on tree growth for both canopies and most species, leading to an average of 7.8%-12.2% projected growth gains with climate change under RCP 4.5 and 8.5. The magnitude of these gains peaked in colder, northern areas for both canopies, while growth declines are projected for overstory trees in warmer, southern regions. Relative to overstory trees, understory tree growth was less positively affected by warming in northern regions, while displaying more positive responses in southern areas, likely driven by the buffering effect of the canopy from warming and climate extremes. Observed differences in climatic sensitivity between canopy positions underscore the importance of accounting for differential growth responses to climate between forest strata in future studies to improve ecological forecasts. Furthermore, latitudinal variation in the differential sensitivity of forest strata to climate reported here may help refine our comprehension of species range shift and changes in suitable habitat under climate change.

Which demographic processes control competitive equilibria? Bayesian calibration of a size‐structured forest population model

AbstractIn forest communities, light competition is a key process for community assembly. Species' differences in seedling and sapling tolerance to shade cast by overstory trees is thought to determine species composition at late‐successional stages. Most forests are distant from these late‐successional equilibria, impeding a formal evaluation of their potential species composition. To extrapolate competitive equilibria from short‐term data, we therefore introduce the JAB model, a parsimonious dynamic model with interacting size‐structured populations, which focuses on sapling demography including the tolerance to overstory competition. We apply the JAB model to a two‐“species” system from temperate European forests, that is, the shade‐tolerant species Fagus sylvatica L. and the group of all other competing species. Using Bayesian calibration with prior information from external Slovakian national forest inventory (NFI) data, we fit the JAB model to short time series from the German NFI. We use the posterior estimates of demographic rates to extrapolate that F. sylvatica will be the predominant species in 94% of the competitive equilibria, despite only predominating in 24% of the initial states. We further simulate counterfactual equilibria with parameters switched between species to assess the role of different demographic processes for competitive equilibria. These simulations confirm the hypothesis that the higher shade tolerance of F. sylvatica saplings is key for its long‐term predominance. Our results highlight the importance of demographic differences in early life stages for tree species assembly in forest communities.

Partial cutting favours northern white-cedar regeneration but does not ensure recruitment to canopy: does browsing matter?

Abstract Gap dynamics facilitate recruitment of late-successional species such as northern white-cedar (Thuja occidentalis L.). For this reason, harvests that result in partial rather than complete canopy removal have been suggested for cedar. However, success of regenerating cedar following partial harvests is uncertain, especially where there is heavy browsing by white-tailed deer (Odocoileus virginianus Zimmerman) or snowshoe hare (Lepus americanus). Efforts to understand how partial harvests and browsing interact to affect natural regeneration of cedar have been limited. We inventoried partially harvested stands along a gradient of 1–11 years since harvest in New Brunswick, Canada, in an area where deer frequently overwinter. Cedar regeneration was negatively influenced by browsing, but positively influenced by years since harvest and sapling basal area. Cedar seedling survival was assessed over a 2-year period and found to be primarily a function of height at initial measurement, with little influence of browsing. Annual vertical gain of cedar seedlings (a surrogate for height growth) decreased with increasing years since harvest and was influenced by distance from gap edge. In gaps, vertical gain increased with distance to gap edge, while distance to gap edge had a negative influence on vertical gain of seedlings in the matrix (between-gap areas). Many tagged cedar seedlings disappeared prior to remeasurement. These were likely completely consumed by browsers, limiting our ability to determine relative influences of partial harvest and browsing on cedar regeneration. We conclude that partial harvests, as applied in this study, increase cedar abundance and vertical gain, but browsing may necessitate seedling protection. We suggest monitoring to confirm sufficient cedar regeneration in partially harvested stands, using exclosures where browsing is a concern, and retaining or at least delaying removal of seed-bearing cedar overstory trees until regeneration has reached heights safe from browsing.

Interplay between tree genetic variation, plant community composition and environment in forest communities dominated by black alder (Alnus glutinosa (L.) Gaertn.)

Studies in community genetics have often revealed a relationship between genetic diversity of the focal species and species diversity of the associated biotic communities. This relationship was studied in forest communities dominated by black alder (Alnus glutinosa Gaertn.), one of the few tree species tolerating an anoxic environment of waterlogged soils. It is a dominant species of tree overstory in two types of communities: alder carrs, forest swamps with stagnating water, and riparian forests occurring along smaller waterflows, periodically flooded with a considerable water level fluctuation during the vegetation period. Plant community composition and genetic variation of alder populations were studied in 218 black alder communities of both types distributed along a broad latitudinal transect from the Pannonian lowland to the Western Carpathians (Hungary, Slovakia, Poland). Species diversity was significantly higher in riparian stands than in alder carrs, while no difference was observed in the genetic diversity. The analysis of population structure revealed differentiation between Pannonian and Carpathian populations, which may be attributed to different migration pathways during the Holocene. No correlation was observed between genetic diversity of alder and species diversity of the associated vascular plant communities. On the other hand, using the ddRAD-sequencing approach applied to 96 trees, we identified 19 single-nucleotide polymorphisms significantly associated with climatic and soil variables. However, the hypothesized bioindication function of the plant community composition on the genetic variation of black alder as a focal species was not confirmed.

Modeling Juvenile Stand Development and Fire Risk of Post-Fire Planted Forests under Variations in Thinning and Fuel Treatments Using FVS–FFE

In the past, the dry mixed conifer forests of California’s Sierra Nevada mountains experienced frequent low to mixed severity fires. However, due to fire suppression and past management, forest structure has changed, and the new fire regimes are characterized by large, high severity fires which kill a majority of the overstory trees. These new disturbance patterns require novel approaches to regenerate the forest as they are not adapted to large, high severity fires. We forecasted growth and fire behavior of young plantations for 100 years into the future using the Forest Vegetation Simulator (FVS) and its Fire and Fuels Extension (FFE). In these simulations, we tested combinations of different fuel treatments (mastication only, mastication with prescribed burning, and no fuels treatments) with different overstory thinning intensities (residual densities of 370 SDI (stand density index), 495 SDI, 618 SDI (TPH), and no overstory thinning) on stand growth and potential fire behavior using analysis of variance. We compared growth and crowning index at the end of the simulation and the simulation age when the flame length, basal area mortality, and fire type reached low severity between fuel treatment, thinning intensity, and original management of stands (plantation with PCT [precommercial thinning], plantation without PCT, and natural regenerating stands). These comparisons are essential to identify which fuel treatment categories reduce fire risk. We found an overall pattern of decreasing crown fire occurrence and fire induced mortality across all simulations due to increasing canopy base height and decreasing canopy bulk density. In particular, stands with mastication and prescribed burning transitioned from crown fire types to surface fires 10 years earlier compared to mastication only or no fuel treatment. Furthermore, pre-commercially thinned stands transitioned from crown fire states to surface fires 10 years earlier in the simulations compared to un-thinned and naturally regenerating stands. Stands with mastication and burning went below 25% reference threshold of basal area mortality 11 and 17 years earlier before the mastication only and no fuel treatment, respectively. In addition, pre-commercially thinned stands went below 25% basal area mortality 9 and 5 years earlier in the simulation compared to un-thinned or naturally regenerated stands, respectively. Mastication with prescribed burning (MB) was the most effective treatment for quickly reducing fire behavior by consuming surface fuels, thus drastically lowing flame length (e.g., surface flame length of MB was 0.6 m compared to mastication only [1.3 m] and no treatment [1.4 m]). Furthermore, intensive thinning reduced risk of active crown fires spreading through the stand. Prioritizing prescribed burning, when possible, and thinning (both pre-commercially and from below) are the most effective ways to quickly improve fire resistance in mixed conifer plantations. Our results highlight the different stressors that post-fire planted forests experience and how different silvicultural treatments interact over time to reduce fire risk, which demonstrates the importance of treating stands early and the effectiveness of surface fuel treatments.

Drone remote sensing in urban forest management: A case study

We applied drone remote sensing to identify relationships between key forest health indicators collected in the field and four Vegetative Indices (VI) to improve conservation management of urban forests. Key indicators of urban forest health revealed several areas of conservation concern including a majority of overstory trees in moderate to severe decline, canopy gaps, anthropogenic dumping, vines overtaking the forest canopy, and invasion by non-native plant species. We found plot-level vegetation index (VI) values of NDVI, NDRE, GNDVI, and GRVI calculated from drone imagery are significantly related to the impact of several of these ecological concerns as well as metrics of forest composition and equitability. Despite the small number of plots, too few to provide a general predictive framework, these findings indicate a substantial potential for drone remote sensing as a low-cost, efficient tool for urban forest management. We discuss how our findings can advance urban forest management and discuss challenges and opportunities for future drone VI research in urban natural areas.

DUET - Distribution of Understory using Elliptical Transport: A mechanistic model of leaf litter and herbaceous spatial distribution based on tree canopy structure

Heterogeneity in surface fuels produced by overstory trees and understory vegetation is a major driver of fire behavior and ecosystem dynamics. Previous attempts at predicting tree leaf and needle litter accumulation over time have been constrained in scope to probabilistic models that consider a limited number of key factors influencing tree litter dispersal patterns and decomposition processes. We present a mechanistic model for estimating variation in surface fuels called the Distribution of Understory using Elliptical Transport (DUET). DUET uses a pre-generated voxelated canopy array and distributes the leaf litter from the trees in an elliptical shape based on tree species characteristics, wind data, and location-specific features. DUET then calculates grass growth between trees and around the litter patterns and sets moisture levels based on relative humidity. In this study, we use DUET to simulate spatial distributions of leaf litter and illustrate model sensitivity to several parameters, providing inputs for testing fuel spatial heterogeneity impacts on fire behavior using process-based models.

Reproductive characteristics in an understory bamboo and gradual environmental changes after its dieback provide an extended opportunity for overstory tree regeneration in a mixed cool-temperate forest in central Japan.

The reproductive characteristics of understory bamboo and the effects of dieback on overstory tree seedlings through temporal changes in the environment at the forest floor have only been examined in a few bamboo species, due to the unpredictable occurrence of flowering events and long intervals between them but provide valuable information on tree regeneration and succession in a forest with dense dwarf bamboo cover. We investigated environmental conditions and assessed seedlings (< 30-cm tall) of the dwarf bamboo Sasa borealis and overstory tree species at 44-50 measurement points during 2016-2021, which included a S. borealis mass flowering event in 2017. We also conducted seed germination tests to determine germination rates and patterns in S. borealis. Environmental factors affecting seedling recruitment of S. borealis and of overstory trees were analysed using spatiotemporal generalized linear mixed models in the Bayesian framework. We observed gradual temporal changes in the environment, including increasing canopy openness and decreasing maximum height of dead S. borealis culms. The seeds germinated slowly and the emergence of current-year S. borealis seedlings peaked in spring-summer in 2019. The tree seedling density after 2019 increased significantly compared to that before the dieback. The model results suggest that tree seedling establishment was enhanced by increased light availability. Continuous field observation beginning before S. borealis dieback revealed gradually enhanced tree recruitment in response to slow decay of the remaining dead culms and slow recovery of S. borealis. The seedling regeneration pattern of understory bamboo partly contributes to a prolonged opportunity for overstory tree regeneration.

The century-long shadow of fire exclusion: Historical data reveal early and lasting effects of fire regime change on contemporary forest composition

Historical logging practices and fire exclusion have reduced the proportion of pine in mixed-conifer forests of the western United States. To better understand pine’s decline, we investigate the impact of historical logging on the tree regeneration layer and subsequent stand development over almost a century of fire exclusion. We use a unique dataset derived from contemporary (∼2016) remeasurement of 440 historical quadrats (∼4m2) in the central Sierra Nevada, California, in which overstory trees, tree regeneration, and microsite conditions were measured and mapped both before and after logging in 1928–1929. We found that pine made up only 12% of the prefire regeneration layer despite comprising 32% of nearby adult basal area. Pine relative abundance changed little with logging and declined to 5% of the contemporary regeneration layer. In contrast, the relative abundance of incense-cedar regeneration (32%) already outpaced its representation in the overstory (17% by basal area) before logging and proceeded to dominate the contemporary understory (49%). We did not find strong evidence for the positive influence of gaps on pine regeneration in any time period. However, across species, post-logging skid trails were positively associated with regeneration and woody debris was negatively associated with regeneration in at least one time period. We discovered that the occurrence of advance regeneration (regeneration that preceded and survived logging) best predicted new contemporary trees across all species. For shade-tolerant species, post-logging regeneration that established up to ten years after logging was also associated with new contemporary trees. In contrast, the few pine that transitioned into the contemporary canopy during the study period all established prior to logging. Our work provides evidence that low pine abundance in the regeneration layer as early as 1928 contributed to low rates of pine in the overstory in 2016, showcasing that the decline of pine likely began before logging and official federal fire suppression policies. We suggest that fire exclusion before logging perpetuated shifts towards shade-tolerant and fire-intolerant species in the regeneration layer that were early and lasting.

Seasonal water source patterns in a northern Arizona pine forest

The relationships among stand density and seasonal plant water source patterns in ponderosa pine (Pinus ponderosa) forests are important for informed management decisions in a bimodal climate with increasing variability in winter and monsoon precipitation inputs. Winter precipitation recharges soil moisture, yet it has declined over the past 20 years in the southwestern United States, and monsoon precipitation is becoming more variable in both magnitude and timing. Near Flagstaff, Arizona in August 2013 (monsoon), October 2013 (post-monsoon), and May 2014 (post winter snow melt), we measured soil moisture, soil water δD at five depths, and xylem water δD in Muhlenbergia montana (a perennial C4 grass), Festuca arizonica (a perennial C3 grass), and P. ponderosa seedlings (&amp;lt; 2 years old), saplings (2–5 cm basal diameter), and mature trees (&amp;gt; 60 cm diameter at breast height) in treated (thinned and burned) and untreated (no thinning, no burning) stands. We found that soil moisture was higher at all soil depths in treated stands in May, after snow melt, and this pattern persisted through August in the deepest soil (60 cm). We also found that, in all sampling months, δD in xylem water of grasses and pine seedlings indicated use of shallower soil water than for pine saplings and mature trees, presumably due to differences in rooting depths. Additionally, in August, δD in xylem water of pine saplings and mature trees indicated greater reliance on a deeper water source in untreated stands than in treated stands, likely due to greater competition for shallow water in untreated stands. Our isotopic data indicate that grasses and seedlings used predominantly monsoon water in August and October, while pine saplings and mature trees used predominantly winter water during all sampling months. Importantly, our data indicate that regenerating trees (seedlings and saplings) used both winter and monsoon seasonal water sources, suggesting an increasingly important role for monsoon precipitation if winter precipitation inputs continue to decline. Collectively, these findings demonstrate that management actions can benefit forests via increased soil moisture, that overstory trees rely predominantly on winter precipitation, and that monsoon precipitation is important for herbaceous species and younger, regenerating overstory trees.

Responses of understory vascular plant communities up to 6 years after retention harvesting in planted Abies sachalinensis forests

Planted forests managed to supply timber are usually harvested by clearcutting but are also expected to contribute to biodiversity conservation. The retention forestry approach that retains important forest structures, such as trees and decayed logs at harvest, can be applied in planted forests to mitigate the negative impacts of clearcutting. Planted forests are disturbed by harvesting and postharvest silvicultural management activities, such as site preparation, planting, and weeding. The responses of understory vascular plant communities to various retention harvesting and postharvest silvicultural management were studied by comparing the composition of understory plant species before and after harvesting at the Retention Experiment for plantation FoREstry in Sorachi, Hokkaido, Japan (REFRESH). Dispersed and aggregated retention sites were established in planted Abies sachalinensis forests. In the dispersed retention sites, broad-leaved trees established in conifer plantations were retained. Clearcut and unharvested stands were used as controls and patch-cutting sites (one third of the area was clearcut by patches of 1-ha) were also established. Harvesting took place the year after the preharvest survey, and postharvest surveys were conducted in the first, third, and sixth years thereafter. The effects on species richness, the persistence of species occurrence in the preharvest survey, and the species composition of understory plants were then examined. Colonization by early seral species resulted in changes to the understory plant communities after harvesting and site preparation, irrespective of overstory tree retention. The species composition within aggregated retention patches was not altered, even though most overstory trees were blown down and are expected to provide a refuge for forest species. The presence of retained trees and steep slopes enhanced preharvest species persistence, which was presumed to be due to hampered soil disturbance. Harvesting and site preparation methods that avoid soil disturbance may help to maintain habitats for more understory plants in planted forests. The effects of weeding on the persistence of understory species differed from that of harvesting and site preparation. The trajectories for each site in the Non-metric multidimensional scaling ordination space were almost parallel from the preharvest to postharvest surveys, suggesting that the preharvest vegetation left a legacy. The direction of movement was reversed in the survey six years after harvesting. It is necessary to determine the resistance and resilience of understory plants within the management cycle (planting through harvesting) to establish management practices that maintain the diversity of understory plants in planted forests.