Subalpine Conifer Research Articles

Altitudinal Variation in Soil Acid Phosphomonoesterase Activity in Subalpine Coniferous Forests in China

Studying the altitudinal variation and driving factors of soil acid phosphomonoesterase (ACP) activity in subalpine regions is crucial for understanding nutrient cycling processes within mountainous ecosystems. This study focused on fir (Abies fabri (Mast.) Craib) forests located at three altitudes (2781 m, 3044 m, and 3210 m) on the eastern slope of Mt. Gongga in southwest China. We measured soil ACP activity alongside soil climate, nutrients, and microorganisms at various depths and elevations to investigate how these factors influence ACP activity. The results indicated that in the organic matter horizons (Oe and Oa horizons), ACP activity gradually decreased with elevation. However, the surface mineral horizon (A horizon) did not show a decline in ACP activity with increasing elevation, which could be attributed to significantly lower ACP activity recorded at the 2781 m sample site compared to the 3044 m site. Variance partitioning analysis revealed that among soil climate, nutrients, and microorganisms, soil nutrients had the most substantial impact on ACP activity across all horizons, with a particularly high contribution of 89.4% observed in the A horizon. Random forest model analysis further demonstrated that soil total carbon (TC) played a crucial role in determining ACP activity in the Oe and Oa horizons, with importance values of 8.5% and 7.3%, respectively. Additionally, soil total nitrogen (TN) was identified as the primary factor influencing ACP activity in the A horizon, with an importance value of 12.6%. Furthermore, soil ACP activity was positively regulated by the soil TC:TP and TN:TP ratios, indicating a stoichiometric control of ACP activity in the Abies fabri (Mast.) Craib forests on Mt. Gongga.

The Growth-Climate Relationships of Three Dominant Subalpine Conifers on the Baima Snow Mountain in the Southeastern Tibetan Plateau.

The impact of climates on the radial growth of muti-species remains insufficiently understood in the climate-sensitive southeastern Tibetan Plateau, and this hampers an effective assessment of forest growth under the background of global warming. Here, we studied the growth-climate relationships of three major species (Abies georgei, Larix potaninii, and Picea likiangensis) on the Baima Snow Mountain (BSM) by using dendrochronology methods. We constructed basal area increment (BAI) residual chronologies based on the dated ring-width measurements and correlated the chronologies with four climate factors. We also calculated the contributions of each climate factor to species growth. We found that photothermal conditions played a more important role than moisture in modulating radial growth, and P. likiangensi presented the strongest sensitivity to climate change among the three species. The growing season (June and July) temperature positively affected the radial growth of three species. Winter (previous December and current January) SD negatively impacted the tree growth of A. georgei and P. likiangensis. Significant correlations between growth and precipitation were detected only in A. georgei (January and May). Warming since the beginning of the 1950s promoted the growth of A. georgei and P. likiangensis, while the same effect on L. potaninii growth was found in the recent 50 years.

Plant–plant interactions affect seasonal nitrogen uptake of subalpine conifer seedlings by altering root traits and soil nitrogen availabilities

AbstractAlthough it is known that plant–plant interaction is an important factor influencing plant nitrogen (N) uptake and biomass productivity, its effects on seasonal inorganic N uptake, preference, and allocation remain unclear. In this study, two conifer species (Picea asperata and Abies faxoniana) were planted in three different planting modes (i.e., single, intraspecific, and interspecific interaction). Using 15N stable isotope tracer, we quantified plant biomass, ammonium (NH4+), and nitrate (NO3−) uptake rate (mass) and allocation in the middle (July) and the end (September) of the growing season, respectively, followed by analyses of root traits and soil properties so as to explore the underlying mechanism. Across the two seasons, intraspecific interaction decreased plant biomass and inorganic N‐uptake rate, which triggered intense competition for both species. Intraspecific competition of P. asperata was stronger than that of A. faxoniana. In contrast, interspecific interaction revealed significant facilitative effects on A. faxoniana, particularly in September. From the middle to the late growing season, the inorganic N‐uptake rate of P. asperata reduced, whereas that of A. faxoniana increased under interspecific interaction. The seasonal variation in plant N uptake was regulated by changes in root traits (such as root nitrogen concentration, specific root length, and branching intensity) and soil N availabilities. Both species indicated a preference for NO3− across seasons. Furthermore, we observed that 15N allocation to shoots of A. faxoniana under interspecific interaction was higher than that of P. asperata and declined from July to September. These findings on how plant–plant interactions affect plant N uptake seasonally can facilitate our understanding of species co‐existence and community assembly in forest ecosystems.

지리산국립공원 구상나무의 수직적 분포 현황

This study was conducted to provide raw data for predicting, preserving, and managing distribution changes due to climate change in the future by investigating the vertical distribution of Abies Koreana in Jirisan National Park. A. Koreana are a major tree species that make up subalpine vegetation in republic of Korea and are particularly sensitive to climate change, so vertical distribution changes can serve as an indicator for these changes. To this end, the vertical distribution status of A. Koreana in major trails in Jirisan National Park was investigated in 2014 and 2021, and the distribution change according to the time series was confirmed. As a result, A. Koreana trees showed a lower distribution limit in the range of 800-890m in the northern part of the main ridge in the valley and 900-1080m in the southern part. In the ridge, the distribution lower limit was shown in the range of 1,220-1,420m in the northern area of the main ridge and 1,030-1,200m in the southern area. This vertical distribution difference was seen as a result of microclimate effects caused by the location environment between the northern and southern regions, starting from the valley, ridge, and main ridge. The vertical distribution change according to the time series has not yet appeared, and generally, A. Koreana trees located on the trail showed a healthy state. Through this study, the vertical distribution of A. Koreana trees in Jirisan National Park was identified, and it is believed that future changes in subalpine conifers due to climate change should be recorded and conservation plans should be prepared.

Spatial Distribution and Determinants of Aboveground Biomass in a Subalpine Coniferous Forest in Southwestern China

Aboveground biomass (AGB) is the most dynamic carbon pool in forest ecosystems and is sensitive to biotic and abiotic factors. Previous studies on AGB have mostly focused on tropical and temperate forests, while studies on AGB and its determinants in subalpine coniferous forests are lacking and the mechanisms are not yet clear. Here, we systematically investigated all woody plants in 630 subplots (20 m × 20 m) in the Wanglang Plot (25.2 ha) to explore the spatial distribution of AGB and the effects of topography, soil, and stand structure on AGB. The results showed that AGB varied remarkably among different subplots with an average of 184.42 Mg/ha. AGB increased significantly with aspect, soil organic matter, maximum DBH, and important value of spruce–fir, while it decreased significantly with slope, total phosphorus, and stem density. Stand structure exerted greater influences than topography and soil factors, and especially maximum DBH determines the variation of AGB. Our results are of great significance to accurately estimate and predict the productivity of this forest type, and can provide insights into the diversity maintenance of subalpine coniferous forests as well as the conservation and management of forest ecosystems.

Effect of Gap Size and Elevation on the Regeneration and Coexistence of Abies, Betula, and Acer Tree Species in a Subalpine Coniferous Forest

Forest gaps play an important role in species regeneration and forest succession. Gap size has a primary influence on tree species coexistence and community assembly along an elevation gradient. In this study, we evaluated the regeneration and coexistence of Abies faxoniana, Betula utilis, and Acer maximowiczii at different life history stages in varied gap sizes along an elevation gradient (between 3000 and 3500 m a.s.l). We found that gap size can positively enlarge the effects of elevation on the regeneration density of the three species. In the process of regeneration from seedling to sapling, Abies had stronger regeneration capability, with regeneration niche breadths of more than 0.6 in different gap sizes. A factor analysis of mixed data indicated that regeneration density, soil nutrient contents, and air humidity were mainly related to gap size, but habitat temperature was largely determined by elevation. The connection between the species regeneration density and habitat conditions was due to differences in their regeneration niches, reflecting the selective preemption of environmental resources of different species in different life stages. Microhabitat heterogeneity, controlled by the characteristics of forest gaps along an elevation, affected the regeneration niche differences of the tree species, which contributed to the species coexistence and community assembly processes.

Fire‐regime variability and ecosystem resilience over four millennia in a Rocky Mountain subalpine watershed

Abstract Wildfires strongly influence forest ecosystem processes, including carbon and nutrient cycling, and vegetation dynamics. As fire activity increases under changing climate conditions, the ecological and biogeochemical resilience of many forest ecosystems remains unknown. To investigate the resilience of forest ecosystems to changing climate and wildfire activity over decades to millennia, we developed a 4800‐year high‐resolution lake‐sediment record from Silver Lake, Montana, USA (47.360° N, 115.566° W). Charcoal particles, pollen grains, element concentrations and stable isotopes of C and N serve as proxies of past changes in fire, vegetation and ecosystem processes such as nitrogen cycling and soil erosion, within a small subalpine forest watershed. A published lake‐level history from Silver Lake provides a local record of palaeohydrology. A trend towards increased effective moisture over the late Holocene coincided with a distinct shift in the pollen assemblage c. 1900 yr BP, resulting from increased subalpine conifer abundance. Fire activity, inferred from peaks in macroscopic charcoal, decreased significantly after 1900 yr BP, from one fire event every 126 yr (83–184 yr, 95% CI) from 4800 to 1900 yr BP, to one event every 223 yr (175–280 yr) from 1900 yr BP to present. Across the record, individual fire events were followed by two distinct decadal‐scale biogeochemical responses, reflecting differences in ecosystem impacts of fires on watershed processes. These distinct biogeochemical responses were interpreted as reflecting fire severity, highlighting (i) erosion, likely from large or high‐severity fires, and (ii) nutrient transfers and enhanced within‐lake productivity, likely from lower severity or patchier fires. Biogeochemical and vegetation proxies returned to pre‐fire values within decades regardless of the nature of fire effects. Synthesis. Palaeorecords of fire and ecosystem responses provide a novel view revealing past variability in fire effects, analogous to spatial variability in fire severity observed within contemporary wildfires. Overall, the palaeorecord highlights ecosystem resilience to fire across long‐term variability in climate and fire activity. Higher fire frequencies in past millennia relative to the 20th and 21st century suggest that northern Rocky Mountain subalpine ecosystems could remain resilient to future increases in fire activity, provided continued ecosystem recovery within decades.

Isotopic Characterization of Mercury Atmosphere-Foliage and Atmosphere-Soil Exchange in a Swiss Subalpine Coniferous Forest.

To understand the role of vegetation and soil in regulating atmospheric Hg0, exchange fluxes and isotope signatures of Hg were characterized using a dynamic flux bag/chamber at the atmosphere-foliage/soil interfaces at the Davos-Seehornwald forest, Switzerland. The foliage was a net Hg0 sink and took up preferentially the light Hg isotopes, consequently resulting in large shifts (-3.27‰) in δ202Hg values. The soil served mostly as net sources of atmospheric Hg0 with higher Hg0 emission from the moss-covered soils than from bare soils. The negative shift of δ202Hg and Δ199Hg values of the efflux air relative to ambient air and the Δ199Hg/Δ201Hg ratio among ambient air, efflux air, and soil pore gas highlight that Hg0 re-emission was strongly constrained by soil pore gas evasion together with microbial reduction. The isotopic mass balance model indicates 8.4 times higher Hg0 emission caused by pore gas evasion than surface soil photoreduction. Deposition of atmospheric Hg0 to soil was noticeably 3.2 times higher than that to foliage, reflecting the high significance of the soil to influence atmospheric Hg0 isotope signatures. This study improves our understanding of Hg atmosphere-foliage/soil exchange in subalpine coniferous forests, which is indispensable in the model assessment of forest Hg biogeochemical cycling.

Elevational variations in stem hydraulic efficiency and safety of Abies fabri

Abstract The structural and functional traits of coniferous trees can reflect their growth states and adaptive strategies to a harsh environment. However, it is still unclear if and how hydraulic traits of subalpine conifers change with altitude. Therefore, the changes in stem hydraulic characteristics of Abies fabri along an elevational gradient (2700–3700 m a.s.l.) were identified in a subalpine ecosystem in southwest China and linked to anatomical properties. Xylem hydraulic efficiency decreased with increasing elevation. Surprisingly, higher hydraulic dysfunction and vulnerability to embolism occurred at higher altitudes. The trade‐off between hydraulic efficiency and safety was weak in A. fabri at higher elevations. Low temperature and superfluous precipitation may be the main constraints for hydraulic function and mechanical strength ((t/b)2)) in plants at high elevations (p < 0.05). The strongly limited hydraulic transport system reflected the severe growth constraint of A. fabri at high elevations. Although series of anatomical traits varied with elevation (e.g. smaller mean diameter of tracheid, mean hydraulic conduit diameter and mean pit aperture diameter at higher altitude) and revealed the adaptive strategies to enhance embolism resistance, thickness‐to‐span ratio ((t/b)2) played a dominant role in the trade‐off between hydraulic efficiency and safety. Thickness‐to‐span ratio was positively correlated with stem hydraulic conductivity but negatively correlated with percent loss of conductivity and water potential at 50% loss of conductivity, (t/b)2 and specific leaf area. Therefore, plants with better hydraulic and growth states at low elevations could allocate more resources to building up solid mechanical supporting systems to cope with high wind load, while those at high elevations with impaired growth states and limited hydraulic functions had to invest more resources in leaves under the harsh environment. The weak trade‐off between hydraulic efficiency and safety (lower hydraulic efficiency and high risk of embolism) could limit the growth and distribution of A. fabri at timberlines; however, global warming trends may facilitate hydraulic transport and benefit plants' growth in the future. Read the free Plain Language Summary for this article on the Journal blog.

Soil Bacteria and Soil Fungi Respond Differently to the Changes in Aboveground Plants along Slope Aspect in a Subalpine Coniferous Forest

In a locale-scale mountainous forest, changes in slope aspect can cause microclimate heterogeneity, which can impact both soil environment and plant community, and influence the soil microbial communities as well. However, the relationship between the aboveground plant community and belowground soil microbial communities and the change in slope aspect is not well understood. A case study was carried out in a subalpine coniferous forest in western China to investigate the above- and belowground relationship of three slope aspects, the north-facing slope, the flat site and the south-facing slope. The plant community attributes were evaluated by the community survey, considering species diversity and the plant total basal area of trees and shrubs to represent the plant productivity. Soil bacteria were determined based on 16S rRNA gene and soil fungi on a nuclear ribosomal internal transcribed spacer (ITS) using high-throughput Illumina sequencing. The results showed that slope aspects significantly affect the aboveground plant productivity and diversity, and the community composition and structure of both aboveground plants and soil bacteria and soil fungi. Soil bacteria and soil fungi correlated differently with aboveground plant community changes in the slope aspects in terms of diversity and community composition and structure. A structural equation model (SEM) revealed that slope aspects caused by aboveground plant productivity changes correlated significantly with the soil fungal community composition and structure, while the soil bacterial community was rather weakly correlated with the plant community, and its changes in community composition and structure were mainly affected by the soil properties and soil fungal community. Further studies considering plant functional traits, soil microbial functional groups, and seasonal changes may reveal a deeper correlation between the aboveground plants and belowground soil microbials at the local scale in subalpine coniferous forests.

Sensitivity of radial growth of subalpine conifer trees to climate warming on the southeastern Tibetan Plateau

Sensitivity of radial growth of subalpine conifer trees to climate warming on the southeastern Tibetan Plateau

Impact of freeze-thaw-induced pit aspiration on stem water transport in the subalpine conifer Abies veitchii.

During winter, subalpine conifers experience frequent freeze-thaw cycles in stem xylem that may cause embolism and pit aspiration due to increased water volume during the sap to ice transition. This study examined the occurrence and ecological impacts of a combination of freeze-thaw-induced pit aspiration and embolism triggered by natural and artificial stem freezing. In subalpine Veitch's fir (Abies veitchii) trees, the fraction of closed pits and embolized tracheids as well as conductivity losses were measured to examine pit aspiration and its effects. When trees incurred mild drought stress in February and early March, 70%-80% of stem conductivity was lost. Cryo-scanning electron microscopy indicated <20% embolized tracheids but ∼90% closed pits. Severe drought stress in late March caused 96% ± 1.2% (mean ± standard error) loss of stem conductivity, while the fraction of embolized tracheids increased to 64% ± 6.6%, and aspirated pit fraction decreased to 23% ± 5.6%. Experimental freeze-thaw cycles also increased pit aspiration from 7.1% ± 0.89% to 49% ± 10%, and the fraction of closed pits was positively correlated to the percent loss of stem hydraulic conductivity. The results indicated that freezing-induced pit aspiration is an important factor for stem xylem dysfunction under mild drought, and upon severe drought in winter; stem water transport is predominantly inhibited by xylem embolism.

Interactive effects of citric acid and mineral fertilization on soil microbial carbon use efficiency in the rhizosphere of two coniferous species

Soil microbial carbon use efficiency (CUE) has been identified as a critical parameter in regulating soil organic carbon (SOC) sequestration. Climate change has been shown in numerous studies to increase the input of root exudates into the soil, thereby affecting soil communities and soil carbon (C) balance. However, it remains uncertain how citric acid (a major component of root exudates) impacts the microbial CUE in forest soil under different soil fertility conditions. Hence, we investigated the responses of soil microbial community composition, enzyme activity, and microbial CUE to a 2-year citric acid addition and mineral fertilization experiment in the rhizospheres of Abies fargesii var. faxoniana and Picea purpurea. Citric acid addition had no significant effects on the C: nutrient imbalance between microbial biomass and their resources in fertilized soils, but it increased the microbial C: nutrient imbalance in non-fertilized soils. The effects of citric acid on microbial CUE were influenced by soil fertility. Citric acid addition had a significant negative effect on microbial CUE in non-fertilized soils in both species tested, whereas no significant change in this parameter was observed in soil treated with NPK fertilizers. NPK fertilization can reduce citric acid-induced soil C loss by increasing microbial CUE. The microbial CUE of P. purpurea was lower than that of A. faxoniana. The differences in root C allocation, the C:N ratio of fine root and root exudates, and N uptakes may contribute to tree species variation in microbial CUE. Our findings contribute to a better understanding of the processes governing the impacts of C and nutrient inputs on microbial C cycles in subalpine conifer forest soils.

A Hierarchical Ecosystem Approach to Evaluate Global Warming Impacts in Three Global Ecoregions

Global warming has substantial effects on terrestrial ecosystems in the different ecoregions. A hierarchical ecosystem approach was conducted to analyze global warming influences with global warming impacts on the three distinct global ecoregions. The ecosystem classification of land (ECL) has been developed and integrated into a hierarchical system. Recently, the hierarchical ecosystem classifications in the 300 Dry Domain of the United States, 100 Polar Domain of Canada, and 500 Plateau Domain of China were demonstrated and explored in studying the environmental system changes and global warming impacts. This article presents the distinctive dissimilarity in each ecoregion and demonstrates the ecosystem responses linked to the hierarchical ecosystem structure and ecological function level. 1) In the Dry Domain, the warmer and wetter climate of Utah gave rise to Rocky Mountain subalpine conifer forests and Great Basin pinyon and juniper woodlands suitable for growth, which corresponds to Utah’s Climate life zone and is affiliated with the middle levels of ECL, scale ranging from U7 up to U4. Conversely, in a warmer and drier climate in Utah, annual plant species and invaded species shifted and expanded at the lower levels of ECL, scale ranging from U10 up to U9. 2) In the Polar Domain, a warmer and wetter winter of the Yukon climate influences the Spruce treeline moving northward and to higher elevations, as well as for the Arctic tundra and alpine tundra. Arboreal species grow fast to reach fructification. These typically appeared in the middle levels of ECL with ranging from Y8 up to Y5, and changed the carbon budget to a carbon sink, with a scale ranging from Y4 up to Y2. With a warmer and drier summer, shrubification in Yukon occurs rapidly, in a range from Y6 to Y5. Potentilla shrub and Salix shrub expand to the Arctic tundra region. 3) In the Plateau Domain, the annual air temperature Increased by 0.5o C/10 y over the last 45 years, and the temperature fluctuations have significantly affected the essential changes in the global energy balance and carbon budget in the upper levels of ECL, scale ranging from Q4 up to Q1. However, the precipitation showed no noticeable difference. The alpine tundra vegetation was simulated by the Vegetation Dynamic Simulation Model (VDSM) integrated with scenarios of a global temperature increase of 1 to 3°C. This illustrated the vegetation biomass changes in the lower levels to middle levels of ECL with ranging from Q8 up to Q6, and the vegetation distribution dynamics appeared in upper levels of ECL with ranging from Q4 up to Q1.

Gross primary production (GPP) and red solar induced fluorescence (SIF) respond differently to light and seasonal environmental conditions in a subalpine conifer forest

The phenology of montane conifer forests is likely to shift in response to climate change and altered seasonal dynamics of light, temperature, and moisture. Solar-induced fluorescence (SIF) is expected to provide substantial improvement for mapping temporal changes in evergreen gross primary production (GPP) over greenness-based remote sensing indices. The utility of SIF to monitor seasonal changes in the phenology of conifer photosynthesis depends on the degree to which GPP and SIF respond in synchrony to key environmental drivers. However, to what extent SIF and GPP become decoupled by responding differently to the combined effects of light and other environmental conditions remains unknown. The goal of this study was to characterize the responses of GPP and SIFred to a suite of environmental drivers at the half-hour time scale and determine how these relationships change across seasons. We analyzed one year of tower-based SIFred and eddy covariance-derived GPP data from a conifer forest at Niwot Ridge, Colorado. We compared the light responses of GPP and SIFred across the year, finding that SIFred increased in response to light earlier in the year than did GPP. The light response of GPP had a positive temperature dependence in spring, and this dependency reversed in summer due to increased evaporative demand, while the light response of SIFred was less temperature dependent. Using artificial neural network ensemble analysis, we found that from spring to summer, SIFred did not exhibit a parallel response to the seasonally dynamic temperature and moisture controls on GPP. In summer SIFred was not correlated with canopy conductance, suggesting that SIF is less sensitive to stomatal control than GPP. Our results suggest that, in conifers, photosystems begin to activate in spring prior to when water becomes available for photosynthesis, presenting a challenge for the use of SIF as a phenological indicator in conifer forests.

Topographic Controls on Stomatal and Mesophyll Limitations to Photosynthesis in Two Subalpine Conifers

Premise of research. Leaf stomatal and mesophyll conductances limit photosynthesis and influence water use efficiency. Few studies have quantified the relative limitations imposed by these CO2 diffusion pathways on photosynthesis in mature conifer trees under natural conditions. Here, we report observations of stomatal and mesophyll conductance changes during seasonal drying across contrasting topographic positions in two Rocky Mountain conifers. We predicted that topographic controls on soil water availability and energy balance would determine limitations to photosynthesis by mesophyll conductance across conifer species with contrasting patterns of stomatal and hydraulic traits. Methodology. Concurrent measurements of leaf gas exchange and carbon isotope discrimination were used to estimate stomatal (gs) and mesophyll (gm) conductance in branches of an isohydric species, lodgepole pine (Pinus contorta), and an anisohydric species, Engelmann spruce (Picea engelmannii), in the central Rocky Mountains. Quantitative limitation analysis of photosynthesis (A) was then performed using data from CO2 response curves. Pivotal results. Stomatal conductance imposed greater limitations on photosynthesis (42%–67%) than mesophyll conductance (5%–17%), but no significant differences in gm were observed between the two conifer species. At the mesic lower hillslope position, A, gs, and gm increased during the growing season despite declines in soil moisture. In contrast, at the drier upper hillslope position, declines in soil moisture and increases in air temperature during the growing season are correlated with reductions in gs but not with A or gm. Conclusions. Adjustments in gm played a potentially important role in sustaining photosynthesis and improving plant water use efficiency when stomatal conductance decreased with water limitation during the growing season at the research site. Sustained gm with seasonal drought may be an important mechanism allowing conifers to survive and maintain competitive dominance in low-resource habitats.

Seasonal Variations in Alpha-Diversity and Composition of Soil Fungal Communities in Subalpine Coniferous and Broadleaved Forests

Seasonal Variations in Alpha-Diversity and Composition of Soil Fungal Communities in Subalpine Coniferous and Broadleaved Forests

Poor acclimation to experimental field drought in subalpine forest tree seedlings.

The ability of tree species to acclimate and tolerate projected increases in drought frequency and intensity has fundamental implications for future forest dynamics with climate change. Inquiries to date on the drought tolerance capacities of tree species, however, have focused almost exclusively on mature trees with scant in situ work on seedlings, despite the central role that regeneration dynamics play in forest responses to changing conditions. We subjected naturally established seedlings of co-dominant subalpine conifer species (Abies lasiocarpa and Picea engelmannii) in the southern Rocky Mountains to 2 years of in situ summer precipitation exclusion, simulating summer drought conditions similar to a failure of the North American monsoon. We compared the morphological and physiological responses of seedlings growing in drought vs. ambient conditions to assess the relative changes in drought tolerance traits as a function of seedling size. Drought treatments had a marked impact on soil moisture: volumetric water content averaged ≈5–8 % in drought treatments and ≈8–12 % in ambient controls. We detected no significant shifts in morphology (e.g. root biomass, leaf:stem area ratio) in response to drought for either species, but net photosynthesis in drought treatments was 78 % lower for spruce and 37 % lower for fir. Greater stomatal control associated with increasing stem diameter conferred greater water use efficiencies in larger seedlings in both species but was not significantly different between drought and ambient conditions, suggesting an overall lack of responsivity to water stress and a prioritization of carbon gain over investment in drought mitigation traits. These results indicate a canonization of traits that, while useful for early seedling establishment, may portend substantial vulnerability of subalpine seedling populations to prolonged or recurrent droughts, especially for spruce.

Temporal evolution of the hydromechanical properties of soil-root systems in a forest fire in China

Plant roots generally enhance soil strength and stabilize slopes through hydro-mechanical effects, especially in forested areas prone to shallow slope failure. Forest fires can severely weaken the hydro-mechanical contribution of roots to slopes, however, the hydro-mechanical characteristics of soil–root systems (SRS) affected by wildfire remain poorly understood. To obtain insight into the post-fire hydro-mechanical characteristics of SRS, a subalpine conifer forested area in Sichuan Province, China that suffered a wildfire on March 30, 2019 was continuously monitored over two consecutive years. Samples from zones with different degrees of burn severity were collected and tests both for roots and SRS were performed. The results revealed a substantial decline in root number, which decreased by 46%–58% two years after the wildfire in the medium- and high-severity areas. The tensile strength tests indicated a reduction of root tensile strength by 36%–47% for roots with diameters less than 2 mm. The shear strength of the SRS determined from saturated direct shear tests strongly and had degraded by 55%–82% two years after the wildfire because of root death and reduced root reinforcement. The results of hydraulic conductivity tests over the same time period indicated an abrupt reduction of SRS hydraulic conductivity within several months after the fire owing to ash clogging and the formation of a hydrophobic layer. After more time had elapsed, however, hydraulic conductivity had increased unexpectedly by a factor of 2.2–3.2 greater than that of unburned soil. We attribute this observation to the formation of macropore flow pathways from decayed roots, which was observed by scanning electron microscopy. The findings presented here provide important insight into the temporal changes of the hydro-mechanical characteristics of SRS in burned areas and their associated mechanisms and could be a useful reference to better evaluate post-wildfire stability of subalpine conifer forest in similar environmental conditions.

Adaptation of Abies fargesii var. faxoniana (Rehder et E.H. Wilson) Tang S Liu seedlings to high altitude in a subalpine forest in southwestern China with special reference to phloem and xylem traits

Key messageAbies fargesii var. faxoniana (Rehder et E.H. Wilson) Tang S Liu seedlings at high elevations compensate for the low efficiency of their water conducting system and high phloem hydraulic resistance by the enhancement in xylem:leaf area, phloem:leaf area, and phloem:xylem area.ContextMaintenance of xylem and phloem transport is particularly important for the survival and growth of trees at the treeline. How plants modify the allocation to leaf, xylem, and phloem structures to adapt to the treeline environment is an important issue.AimsThe purpose of this study was to estimate how xylem and phloem anatomy and volume as well as leaf functional traits of A. fargesii seedlings vary with elevation.MethodsWe examined elevation-related differences in a variety of phloem and xylem functional areas and hydraulic conduit diameters of A. fargesii seedlings growing at elevations between 2600 and 3200 m in the subalpine conifer forest of southwest China.ResultsXylem area, last xylem ring area, and leaf:sapwood area significantly decreased, while xylem:leaf area, phloem:leaf area, and non-collapsed phloem:xylem area significantly increased with elevation. Principal components analysis showed that xylem area, non-collapsed phloem area, and xylem:phloem area were positively correlated with growth rates.ConclusionOur results showed that A. fargesii tree seedlings at the treeline tend to facilitate growth and maintain functional water and sugar balance between stem and leaves by the enhancement in xylem:leaf area, phloem:leaf area, and phloem:xylem area, but not through differences in vessel lumen diameter.