Coniferous Research Articles

Trees have similar growth responses to first-entry fires and reburns following long-term fire exclusion

Managing fire ignitions for resource benefit decreases fuel loads and reduces the risk of high-severity fire in fire-suppressed dry conifer forests. However, the reintroduction of low-severity wildfire can injure trees, which may decrease their growth after fire. Post-fire growth responses could change from first-entry fires to reburns, as first-entry fires reduce fuel loads and the vulnerability among trees to fire effects, which may result in trees sustaining less damage during reburns. To determine whether trees had growth responses that varied from first-entry fires to reburns, we cored 87 ponderosa pine trees in the Gila Wilderness, New Mexico, USA that experienced 3–5 fires between 1950 and 2012 following long-term fire-exclusion and 67 unburned control trees from the Gila and Apache-Sitgreaves National Forests. We assessed tree growth response to fire by comparing tree-ring growth among burned and unburned trees from two years before to two years after fires. We compared growth between burned and unburned trees using a bootstrapping procedure to calculate annual median tree-ring width index values with 95 % confidence intervals. We compared post-fire growth after first-entry fires and reburns following long-term fire-exclusion. Burned trees had similar growth responses following first-entry fires and reburns, with lower growth during the fire year through two years post-fire compared to unburned controls. Burned tree growth returned to expected rates following these immediate post-fire growth reductions. Interestingly, trees had lower growth during the year before and the year of reburns compared to the first-entry fire, reflecting greater aridity before reburns. Greater aridity may have contributed to larger-than-expected growth reductions following reburns, which could explain similar growth responses to first-entry fires and reburns. Our results indicate that trees had consistent short-term growth responses to low-severity fires following long-term fire-exclusion. As trees retained vigor after multiple fires, managing fires for resource benefit is an effective approach to reduce the likelihood of high-severity fire without long-term negative effects on tree growth.

Multi-scale geographically weighted regression estimation of carbon storage on coniferous forests considering residual distribution using remote sensing data

The Above-Ground Carbon (AGC) is an important indicator reflecting the carbon sink function of forest ecosystems. The area of coniferous forests in China accounts for about 50 % of the national forest area. Accurately estimating the AGC of coniferous forests plays an important role in evaluating forest carbon sink function. This study takes coniferous forests in Chun’an, Zhejiang Province as an example, using Landsat 8 OLI as the remote sensing data source, and innovatively proposes a multi-scale Geographically Weighted Regression model (MGWR) combined with Co-Kriging (COK), namely MGWR-COK and Ordinary Kriging (OK), namely MGWR-OK, for AGC estimation. This method first processes Landsat8 OLI remote sensing data, extracts and screens variables, and then constructs an MGWR model with AGC survey samples to estimate AGC and calculate residuals; Afterwards, two models, COK and OK, were used to spatially estimate the residuals; Finally, the AGC estimation results are overlaid with the residual estimation results to obtain the spatial distribution of AGC in coniferous forests. The study indicates that: (1) The selected remote sensing feature variables such as Optimized Soil-Adjusted Vegetation Index (OSAVI), Normalized Difference Vegetation Index (NDVI), Entropy (B3_ent_1 and B2_ent_2), Correlation (B4_corr and B5_corr_2) of texture information, B754 with band combination and elevation have a significant impact on AGC estimation. (2) The AGC accuracy R2 estimated by MGWR-OK and MGWR-COK are 0.837 and 0.857, respectively, which are 6 % and 8 % higher than the MGWR model, and the root mean square error is also reduced by 10 % and 12 %, respectively. This indicates that the combination of MGWR and Kriging interpolation can effectively reduce its spatial estimation error. (3) The range of AGC values estimated by the MGWR-COK model is 0.189–62.591 Mg ∙ hm−2, with a mean of 28.795 Mg ∙ hm−2. The spatial distribution shows a characteristic of high in the southeast and low in the northwest, which is consistent with the actual situation in the study area.

High-precision estimation of plant alpha diversity in different ecosystems based on Sentinel-2 data

At present, the accuracy of remote sensing estimation models of plant alpha diversity is generally low, and high-precision estimation models in deciduous broadleaved forest (DBF), deciduous coniferous forest (DCF) and evergreen coniferous forest (ECF) are still lacking. The main purpose of this study is to construct high-precision remote sensing models for plant alpha diversity in multiple ecosystems at global scale. Normalized Difference Vegetation Index (NDVI) were derived from Sentinel-2 data. NDVI and NDVI based spectral diversity/heterogeneity indices were selected as predictive variables, and alpha diversity indices were selected as response variables. Simple linear regression (SLR), partial linear regression (PLSR), and random forest (RF) models were used to evaluate the predictive ability of the predictive variables against the response variables under six ecosystems (evergreen broadleaved forest (EBF), DBF, ECF, DCF, shrub, and grassland), and to compare the estimated robustness of various spectral diversity indices. In terms of prediction accuracy, the SLR models were the worst, and the PLSR model were average. RF performed best, outperforming most current models. Especially in DBF, ECF, shrub and grassland, the determination coefficient R2 of RF models can be as high as 0.9. In terms of the prediction of α-diversity, the prediction effect of species richness was better than that of Shannon index, Simpson index and Pielou index. The higher the vegetation complexity, the more accurate the assessment of vegetation α-diversity tends to be, especially in DBF, shrub and grassland. According to the importance of predictive variables and model stability evaluation results, NDVI, standard deviation of NDVI (SD), and NDVI derived Shannon’s diversity index (Sha), Cumulative Residual Entropy (CRE), Pielou’s evenness index (Pie), Hill’s numbers (Hill), Berger-Parker’s diversity index (Ber), Parametric Rao’s index of quadratic entropy(paRao) are all powerful indicators for predicting plant alpha diversity. Among them, the prediction performance of NDVI and SD is better. This study is not only an exploration of the practicability of R package “rasterdiv”, but also an attempt to construct high-precision remote sensing estimation models of plant alpha diversity at global scale.

Fine Roots in Hemiboreal Forest Stands and Clearcut Areas with Nutrient-Rich Organic Soils in Latvia: Morphological Traits, Production and Carbon Input

Within this study, we evaluated the fine root (trees and understory vegetation combined) morphological traits, fine root production (FRP), and carbon (C) input with fine root litter in forest stands (dominated by either coniferous or deciduous trees) and clearcut areas (previously dominated by coniferous trees) with nutrient-rich organic soils. The study was conducted in 26 sites in hemiboreal forest land in Latvia and summarizes the results obtained in a two-year study (2020–2022) using the root ingrowth method. Traits and production of fine roots varied significantly depending on forest development stage (stand or clearcut area), dominant tree species type (coniferous or deciduous), and soil drainage status (drained or naturally wet). According to the results of the second study year, mean FRP among groups of study sites varied from 0.58 ± 0.13 to 1.38 ± 0.28 t ha−1 yr−1, while C input with fine root litter ranged from 0.28 ± 0.06 to 0.68 ± 0.14 t C ha−1 yr−1. More than half (59 ± 4%) of the total FRP occurred in the upper 0–20 cm soil layer. FRP tended to correlate positively with soil C/N ratio and negatively with soil pH and soil nutrient concentration. Incubating ingrowth cores for at least two years is strongly recommended to accurately estimate annual FRP and C input. This helps to avoid potential underestimation that may occur when using results of only one incubation year (12 months after ingrowth core installation). This study provided new insights into the dynamics and traits of fine roots and will help to improve the accuracy of C flow estimation in hemiboreal forests with nutrient-rich organic soils in Latvia.

Nitrogen deposition in the middle-aged and mature coniferous forest: Impacts on soil microbial community structure and function

The impact of nitrogen (N) deposition on soil microbial community structure and function has been a focal point of research; however, the influence of forest age and N form on the microbial response to N deposition has yet not to be fully understood. To address this gap, metagenomic sequencing was utilized to explore the responses of soil microbial community structure and functional genes to five years of N addition in middle-aged and mature coniferous forests in southwest China. Adopting a randomized block design, the experiment included two N forms ((NH4)2SO4 and KNO3) across four levels of addition (0, 10, 20 and 40 kg N ha−1 yr−1). Our findings reveal that the composition of the microbial community structure and functional genes involved in the carbon (C), N, phosphorus (P), and sulphur (S) cycling varied significantly between middle-aged and mature forest (P < 0.001). At the phylum level, the soil microbial community in the mature forest were dominated by Proteobacteria (30.4–38.8 %), Acidobacteriota (8.5–24.1 %), and Chloroflexota (8.1–21.5 %), and the soil microbial community in the middle-aged forest showed a greater presence of Acidobacteriota (28.9–34.0 %) and Actinobacteriota (13.3–19.2 %) with a lower proportion of Chloroflexota (0.4–1.2 %). In both middle-aged and mature forests, functional gene abundance, and the composition of microbial community and functional genes were unaffected by N addition rate (P = 0.793) and N form (P = 0.725). The taxonomic composition of soil microbial community in the mature forest showed a significant positive correlation with soil pH, soil organic carbon (SOC) and total nitrogen (TN) (P < 0.01), while the microbial community composition in the middle-aged forests was not significant correlated with soil pH, SOC and TN (P > 0.05). These findings underscore that N addition in the middle-aged and mature coniferous forest does not significantly impact the soil microbial community structure and function across different N forms and N addition rate, suggesting that soil microorganisms of the forest ecosystem exhibit strong resilience to increased N supply.

A Multi-Baseline Forest Height Estimation Method Combining Analytic and Geometric Expression of the RVoG Model

As an important parameter of forest biomass, forest height is of great significance for the calculation of forest carbon stock and the study of the carbon cycle in large-scale regions. The main idea of the current forest height inversion methods using multi-baseline P-band polarimetric interferometric synthetic aperture radar (PolInSAR) data is to select the best baseline for forest height inversion. However, the approach of selecting the optimal baseline for forest height inversion results in the process of forest height inversion being unable to fully utilize the abundant observation data. In this paper, to solve the problem, we propose a multi-baseline forest height inversion method combining analytic and geometric expression of the random volume over ground (RVoG) model, which takes into account the advantages of the selection of the optimal observation baseline and the utilization of multi-baseline information. In this approach, for any related pixel, an optimal baseline is selected according to the geometric structure of the coherence region shape and the functional model for forest height inversion is established by the RVoG model’s analytic expression. In this way, the other baseline observations are transformed into a constraint condition according to the RVoG model’s geometric expression and are also involved in the forest height inversion. PolInSAR data were used to validate the proposed multi-baseline forest height inversion method. The results show that the accuracy of the forest height inversion with the algorithm proposed in this paper in a coniferous forest area and tropical rainforest area was improved by 17% and 39%, respectively. The method proposed in this paper provides a multi-baseline PolInSAR forest height inversion scheme for exploring regional high-precision forest height distribution. The scheme is an applicable method for large-scale, high-precision forest height inversion tasks.

Long-term post-fire recovery of an oribatid mite assemblage: A case study from a temperate coniferous forest

Wildfire represents a significant natural disturbance factor in forest ecosystems expected to further increase in importance due to global climate change. It has a detrimental short-term impact on soil biota, but much less is known about its long-term effects, especially on soil mesofauna. Our study compared oribatid assemblages of the forest floor in moderately-burned forest sites along a post-fire chronosequence (8 fire history classes covering 0–110 years since fire) with near-by reference sites without fire history. All sites were situated on acidic soils in the Central European Elbe Sandstone Mountains (Bohemian Switzerland National Park, NW Czechia), mostly covered by pine and spruce forests. Data were analysed using linear mixed-effect models. We found a substantial impact of fire on oribatid assemblages. Whereas lower densities were observed for the first few years after a fire only, changes in assemblage feeding guilds persisted over at least four decades. Shifts towards smaller body size, parthenogenesis and fungivory at burned sites compared to larger body size, sexual reproduction and detritivory in unburned controls implied changes in assemblage functioning. The changes in functional traits, which correspond to previous research findings on the recovery of oribatid mites after clear-cutting, underscore a more universal pattern of post-disturbance development.

A Mechanistic Prediction Model of Resistance to Uprooting of Coniferous Trees in Heilongjiang Province, China.

Coarse roots and the root plate play an important role in tree resistance to uprooting. In this study, a qualitative mechanistic model was developed to analyze coniferous tree resistance to uprooting in relation to tree morphological characteristics. The sizes of the crown, stem, and root plate of twenty sample spruces and twenty sample Korean pines were individually measured for this purpose. Using Ground Penetrating Radar (GPR), the coarse root distribution and root plate size were detected. In the qualitative mechanistic model, a larger crown area increased the overturning moment, while higher DBH and root plate mass increased the resistance moment. The resistance coefficient (Rm) was calculated by comparing resistive and overturning moments, classifying samples into three uprooting hazard levels. Trees with smaller crown areas, larger stems, and root plates tend to have higher resistance to uprooting, as indicated by higher Rm values. This qualitative mechanistic model provides a useful tool for assessing coniferous standing tree uprooting resistance.

Tree richness increased biomass carbon sequestration and ecosystem stability of temperate forests in China: Interacted factors and implications

Biodiversity loss and forest degradation have received increasing attention worldwide, and their effects on forest biomass carbon storage and stability have not yet been well defined. This study examined 1275 tree plots using the field survey method to quantify the effects of tree diversity, tree sizes, and mycorrhizal symbiont abundance on biomass carbon storages (Cs) and NDVI (Normalized Difference Vegetation Index)-based ecosystem stability (standard deviation/mean NDVI = NDVI_S) during the field survey period from 2008 to 2018. Our data showed Cs and NDVI_S averaged at 31–108 t ha−1 and 32.04–49.28, respectively, and positive relations between Cs and NDVI_S were observed (p < 0.05). Large forest-type and regional variations were found in these two parameters. Broadleaf forests had 74% of Cs (p < 0.05) of the conifer forests, but no differences were in NDVI_S. Cold regions at high latitudes had 71% of NDVI_S in the warm regions at low latitudes, while no differences were in Cs. Moist regions at high longitudes had 2.04 and 1.28-fold higher Cs and NDVI_S (p < 0.05). The >700 m a.s.l. regions had 1.24-fold higher Cs (p < 0.01) than the <700 m a.s.l. regions, but similar NDVI_S (p > 0.05). Nature Reserves had 1.94-fold higher Cs but 30% lower NDVI_S than outside Reserves (p < 0.001). > 40-year-old forests had 1.3- and 2-fold higher Cs and NDVI_S than the young forests. Structural equation modeling and hierarchical partitioning revealed the driving paths responsible for these variations. Tree richness was positively associated with Cs and ecosystem stability, contributing 21.6%–30.6% to the total effects on them; tree sizes significantly promoted the Cs, but had negligible impacts on NDVI_S. MAT's total effects on NDVI_S of conifer forests were 40% higher than that of broadleaf forests, MAP's total effects on Cs varied with forest types; arbuscular mycorrhizal tree dominance exhibited a smaller positive impact on Cs and ecosystem stability in comparison to other factors. Our findings underscore that the significance of climatic-adapted forest management, diversity conservation, and big-sized tree protections can support the achievement of carbon neutrality in China from biomass carbon sequestration and ecosystem stability.

Contrasted NCED gene expression across conifers with rising and peaking abscisic acid responses to drought

Conifer trees have diverse strategies to cope with drought. They accumulate the plant hormone abscisic acid (ABA) following a range of profiles from constantly rising to peaking and falling (R- and P-type) with direct effect on foliar transpiration. The molecular basis of this adaptive diversification among species is largely unknown. Here, we analysed the sequences of candidate ABA biosynthesis and catabolism genes and monitored their expression in response to intensifying drought. We studied young trees from Cupressaceae, Pinaceae, and Taxaceae under controlled drought conditions and compared changes in water status, ABA profiles and gene-specific transcript levels. Our data indicate that R-type and P-type ABA profiles may be controlled by divergent expression of genes involved in the biosynthetic and catabolic pathways of ABA, respectively, and emphasize a key role of nine-cis-epoxycarotenoid dioxygenases (NCED) genes. Our results open the doors to understanding the molecular basis of contrasted drought response strategies across conifer taxa, which we expect will help foresters grow more drought-resilient trees.

Indicators of woody plants and soil in the urbanised environment of the Southern Urals (green zone of Ufa, Russia)

ABSTRACT The study investigates biometric indicators of assimilation and reproductive organs of coniferous and deciduous trees in urban environments: leaf area, inflorescence flower count, stipule length of Tilia cordata Mill. and needle length and weight of Scots pine (Pinus sylvestris L.). Samples of wood, needles, leaves and soil from the planting sites were analysed for 17 chemical elements, including Hazard Class I elements (Arsenic, Lead, Cadmium, Mercury, Selenium) and Hazard Class II elements (Chromium, Cobalt, Nickel, Copper, Molybdenum, Antimony). Tree plantations decreased airborne dust by 30–40% compared to open areas. Concentrations of PM2.5 and PM10 particles showed a direct correlation with busy roads (p < 0.05). In instances of environmental degradation, healthy trees exhibited in needle an increase in biomass and a reduction in weight and length. Consequently, forestry measurements and the impact of the urban environment require specific and individual monitoring of trees, rather than a general assessment of plantations.

Volatile Terpene Profiles of Needle and Phloem Tissues of Healthy and Tomicus destruens-Infested Pinus brutia Trees.

Coniferous trees produce secondary or defense chemicals, such as terpenes, against pest insects. Terpenes could serve as constitutive or induced defensive mechanisms, defending the tree from invasive herbivores. The Mediterranean pine shoot beetle Tomicus destruens colonizes stems and branches of Pinus brutia trees and even can kill mature trees during periodic outbreaks. We investigated whether terpene profiles of needle and stem of P. brutia trees differ between health and those infested by T. destruens. We selected 20 healthy and T. destruens-infested trees and analyzed the monoterpenes and sesquiterpenes of their needles and phloem. We found higher concentrations of tricyclene, camphene and p-cymene in the phloem of infested trees. Similarly, the needles of infested trees had higher concentrations of α-pinene, β-pinene, myrcene, limonene, trans-β-caryophyllene and α-humulene than healthy trees. These results show that the monoterpene and sesquiterpene profiles of P. brutia trees differed between healthy and infested trees, suggesting that volatile terpenes may be an important part of plant-induced responses against T. destruens.

Habitat preferences of breeding Eurasian Sparrowhawks Accipiter nisus in Central Europe

ABSTRACT Capsule A description of habitats used for breeding by Eurasian Sparrowhawks, along with variables affecting their breeding habitat selection. Aims To describe the habitat preferences of breeding Eurasian Sparrowhawks, and to identify variables affecting their breeding habitat selection at different spatial scales, in a rural area of Central Europe (Czech Republic). Methods Eurasian Sparrowhawk nests were found and the habitat characteristics of breeding sites were measured at three spatial scales, and then compared with randomly selected locations. Results The nests were regularly distributed within the forests of the study area, avoiding small forest patches. The territories were somewhat associated with dense forest stands of a distinctive structure. All nests were built in coniferous trees, in the lower parts of their green crowns and close to the trunks. In the areas surrounding the nests, we found a greater abundance of some bird (prey) species and a higher diversity of birds, compared to the random areas. Conclusion The Eurasian Sparrowhawks preferred to breed in forests with a specific structure that provided sufficient concealment and access to their nests, and they located their breeding territories as far from each other as possible. The birds also seemed to prefer breeding in localities with a greater food supply. The variables affecting Eurasian Sparrowhawk breeding habitat selection may include intraspecific competition, size and structure of available forests, availability of coniferous trees for nest placement, and the composition of the food supply in the vicinity of their nests.

Physically based modelling of spectral transmittance through forest canopies

Abstract Physically based models simulating the spectral transmittance of solar radiation through forest canopies are useful tools for examining the connections between the shortwave radiation environment and the productivity and biodiversity of the forest floor. We report a comprehensive evaluation of two approaches simulating forest canopy spectral transmittance. The approaches were (i) three‐dimensional radiative transfer modelling in canopies composed of individual trees filled with turbid media and (ii) photon recollision probability theory (p‐theory), and were implemented using DART‐FT and PARAS models, respectively. The simulations were evaluated against mean and standard deviation of canopy transmittance spectra measured under clear‐sky conditions in forest plots across central and Northern Europe. In general, both models agreed well with the in situ measurements. They performed equally in conifer forests, while PARAS had a slightly lower accuracy than DART‐FT in broadleaved forests. We conclude that both approaches produce realistic simulations of canopy spectral transmittance at the spatial scale tested in this study, and that p‐theory constitutes a computationally efficient and easy‐to‐parameterize alternative to three‐dimensional radiative transfer.

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.

Vegetation dynamics 1946–2018 in an old‐growth conifer forest

We analysed ground vegetation in 250 plots in 2018 in the old‐growth Picea abies‐dominated forest Säby Västerskog, south‐east Sweden as a follow‐up of studies in 1946 and 1998 with the same layout of plots. The vegetation changes were not clearly directional: the species composition in 2018 was intermediate between that of 1946 and 1998, whereas in 1998 species richness was higher and with the unique presence of a number of species indicating small‐scale disturbances. Vaccinium myrtillus increased in cover since 1946 and Avenella flexuosa decreased. This goes against regional trends that have been attributed to nitrogen deposition. Regional changes are overshadowed by local dynamics.

Characterization of switchgrass (Panicum virgatum L.) PvKSL1 as a levopimaradiene/abietadiene-type diterpene synthase.

The diverse class of plant diterpenoid metabolites serves important functions in mediating growth, chemical defence, and ecological adaptation. In major monocot crops, such as maize (Zea mays), rice (Oryza sativa), and barley (Hordeum vulgare), diterpenoids function as core components of biotic and abiotic stress resilience. Switchgrass (Panicum virgatum) is a perennial grass valued as a stress-resilient biofuel model crop. Previously we identified an unusually large diterpene synthase family that produces both common and species-specific diterpenoids, several of which accumulate in response to abiotic stress. Here, we report discovery and functional characterization of a previously unrecognized monofunctional class I diterpene synthase (PvKSL1) via in vivo co-expression assays with different copalyl pyrophosphate (CPP) isomers, structural and mutagenesis studies, as well as genomic and transcriptomic analyses. In particular, PvKSL1 converts ent-CPP into ent-abietadiene, ent-palustradiene, ent-levopimaradiene, and ent-neoabietadiene via a 13-hydroxy-8(14)-ent-abietene intermediate. Notably, although featuring a distinct ent-stereochemistry, this product profile is near-identical to bifunctional (+)-levopimaradiene/abietadiene synthases occurring in conifer trees. PvKSL1 has three of four active site residues previously shown to control (+)-levopimaradiene/abietadiene synthase catalytic specificity. However, mutagenesis studies suggest a distinct catalytic mechanism in PvKSL1. Genome localization of PvKSL1 distant from other diterpene synthases, and its phylogenetic distinctiveness from known abietane-forming diterpene synthases, support an independent evolution of PvKSL1 activity. Albeit at low levels, PvKSL1 gene expression predominantly in roots suggests a role of diterpenoid formation in belowground tissue. Together, these findings expand the known chemical and functional space of diterpenoid metabolism in monocot crops.

Unveiling the threshold in forest patch shapes to soil retention ecosystem services

Soil erosion is a critical process that leads to landscape degradation, compromising soil fertility and ecosystem functions. Forest ecosystems, with their intricate characteristics, play a pivotal role in mitigating soil erosion and providing soil retention ecosystem services (SRES). This study explores the impact of forest patch thresholds and critical points on soil erosion rates, focusing on 401 catchments in Poland using generalised additive models to identify thresholds and critical points in forest patches.Landscape metrics were applied to measure landscape structure, including shape, fractal dimension, contiguity, related circumscribing circles, and perimeter-area ratio indexes. These metrics, along with slope, rainfall, organic carbon content, water content, and clay ratio variables, were considered dependent variables in the models. The developed models have demonstrated reliable performance in estimating soil erosion rates, with a significant deviation explained from 80.5 to 81.1 for coniferous forest patches, 79.1 to 80.1 for broad-leave forest patches, and 80.9 to 81.4 for mixed forest patches at p < 0.05.In broad-leaved forests, three thresholds are identified in the shape index, which influence soil erosion rates in a complex manner. For coniferous forests, thresholds in the perimeter area ratio, related circumscribing circles, and contiguity indexes exhibit nonlinear relationships with soil erosion rates. Mixed forests show two thresholds in the related circumscribing circle and one in the fractal dimension index, affecting soil erosion rates differently.This research contributes significantly to understanding the interplay between forest patch shapes and soil erosion rates, providing decision support for land use planning. The identified thresholds and critical points offer valuable tools to enhance sustainable landscape functionality, emphasizing the importance of considering forest landscape structure in preserving soil retention ecosystem services.

The nonlinear rainfall–quick flow relationships in a humid mountainous area: Roles of soil thickness and forest type

The nonlinear behavior of hillslope quick flow (QF) in response to variations in soil thickness and forest types remains unclear due to uncertainties or unknown features in field studies, hindering accurate interpretation of their impact on QF. To this end, three artificial physical models with different mineral soil thickness (0 cm, shallow 50 cm, and thick 150 cm) beneath a 50 cm root zone for each forest (i.e., the broadleaf and coniferous forests) were constructed and intensively monitored. Results showed that the threshold-based QF was a function of the sum of rainfall amount and antecedent soil wetness, which was isolated by two breakpoints—the generation and rising thresholds—across all artificial physical models. These breakpoints indicated the storage thresholds for QF generation and the transition from slow to fast QF in humid forests. The broadleaf forest required more rainfall to reach the generation threshold due to higher litter interception capacity and soil water deficit by plant water uptake compared to the coniferous forest. Above the rising threshold phase, QF increased rapidly and linearly due to the establishment of flow path connectivity driven by soil water content, with the broadleaf forest showing a faster increase in QF than the coniferous forest. The dominant preferential/unsaturated flow in humid mountainous areas was also confirmed. Preferential flow paths were well-connected in the root zone and extended to the shallow mineral soil, resulting in an insignificant difference in mean QF between forest types. The thick mineral soil disrupted preferential flow path connectivity, leading to a greater amount of water being laterally diverted and retained within the soil. Consequently, a sharp decrease in QF was observed in hillslope with the root zone capping thick mineral soil for both forests. Our results highlighted the application of threshold-based theory and controlled experiments in enhancing the comprehension of the fundamental physical mechanism governing runoff processes in humid forests.

Phylotranscriptomic and ecological analyses reveal the evolution and morphological adaptation of Abies.

Coniferous forests are under severe threat of the rapid anthropogenic climate warming. Abies (firs), the fourth-largest conifer genus, is a keystone component of the boreal and temperate dark-coniferous forests and harbors a remarkably large number of relict taxa. However, the uncertainty of the phylogenetic and biogeographic history of Abies significantly impedes our prediction of future dynamics and efficient conservation of firs. In this study, using 1,533 nuclear genes generated from transcriptome sequencing and a complete sampling of all widely recognized species, we have successfully reconstructed a robust phylogeny of global firs, in which four clades are strongly supported and all intersectional relationships are resolved, although phylogenetic discordance caused mainly by incomplete lineage sorting and hybridization was detected. Molecular dating and ancestral area reconstruction suggest a Northern Hemisphere high-latitude origin of Abies during the Late Cretaceous, but all extant firs diversified during the Miocene to the Pleistocene, and multiple continental and intercontinental dispersals took place in response to the late Neogene climate cooling and orogenic movements. Notably, four critically endangered firs endemic to subtropical mountains of China, including A. beshanzuensis, A. ziyuanensis, A. fanjingshanensis and A. yuanbaoshanensis from east to west, have different origins and evolutionary histories. Moreover, three hotspots of species richness, including western North America, central Japan, and the Hengduan Mountains, were identified in Abies. Elevation and precipitation, particularly precipitation of the coldest quarter, are the most significant environmental factors driving the global distribution pattern of fir species diversity. Some morphological traits are evolutionarily constrained, and those linked to elevational variation (e.g., purple cone) and cold resistance (e.g., pubescent branch and resinous bud) may have contributed to the diversification of global firs. Our study sheds new light on the spatiotemporal evolution of global firs, which will be of great help to forest management and species conservation in a warming world.