Subalpine Coniferous Forest Research Articles

Soil Microbial Biomass Carbon and Freeze‐Thaw Cycles Drive Seasonal Changes in Soil Microbial Quotient Along a Steep Altitudinal Gradient

AbstractUnderstanding seasonal changes in soil microbial quotient (qMB, i.e., the MBC‐to‐SOC ratio) along altitudinal gradients and their dominant drivers is essential for determining the effects of environmental changes on the soil organic carbon pool. A 3‐year in situ soil core incubation experiment was thus conducted in a dry valley shrub, a valley‐mountain ecotone forest, a subalpine coniferous forest, an alpine coniferous forest, and an alpine meadow across a 2,431‐m altitudinal gradient on eastern the Qinghai‐Tibetan Plateau. Soil qMB in the soil organic layer (OL) and mineral soil layer (ML) was measured at five critical periods. The average soil qMB was significantly higher in the alpine meadow than at other sites, with no significant difference among the other sites. The average soil qMB in this region was less than the global average value (1.2%), indicating that the soil organic carbon pool of the local ecosystem has higher stability and a lower mineralization potential than that of the other ecosystems globally. Seasonal variations in soil qMB differed along the altitudinal gradient. Significant increases in soil qMB during the 3‐year incubation were mainly observed in the OL at higher altitudes, implying increased sensitivity of the soil organic carbon pool to environmental changes. Microbial biomass carbon and soil freeze‐thaw cycles were critical factors determining the altitudinal pattern and seasonal changes in soil qMB. Soil microbial biomass and freeze‐thaw regime changes under global climate change scenarios will alter the balance between SOC microbial assimilation and maintenance respiration demands, affecting soil carbon biogeochemical cycling in mountainous ecosystems.

Differential seasonal changes in soil enzyme activity along an altitudinal gradient in an alpine-gorge region

Information about the seasonal changes in soil enzyme activity along altitudinal gradients is key for understanding the responses of soil biochemical processes to ongoing climate change. Therefore, a 3-year in situ soil core incubation experiment was conducted along a 2431-m altitudinal gradient in an alpine-gorge region on the eastern Qinghai-Tibet Plateau, which includes dry valley shrubland, valley-mountain ecotone forest, subalpine coniferous forest, alpine coniferous forest and alpine meadow. The invertase, urease, and acid phosphatase activities in the soil organic layer (OL) and mineral layer (ML) were measured at five critical periods from 2013 to 2017. Higher soil enzyme activity was observed in the subalpine coniferous forest and alpine meadow, whereas lower activity was observed in the dry valley shrubland and valley-mountain ecotone forest. Overall, the seasonal sensitivity of soil enzyme activity decreased with increases in altitude. In the subalpine and alpine coniferous forests, the seasonal sensitivity of soil enzyme activity was significantly higher in the ML than in the OL. The key drivers of the seasonal variation in soil enzyme activity exhibited marked variation with changes in altitude, and these variations depended on the enzyme type and soil layer. Significant effects of temperature and precipitation on seasonal fluctuations in enzyme activities were observed at lower altitudes, whereas soil freeze-thaw cycles significantly affected the seasonal changes in acid phosphatase activity at higher altitudes. In addition, the seasonal variations in specific enzyme activities differed from those found for enzyme activities, which indicated that microorganisms might have mediated the effects of environmental factors on soil enzyme activity by altering the enzyme production efficiency. Briefly, the seasonal fluctuations in soil enzyme activity and their dominant drivers varied greatly among different altitudes in the alpine-gorge region, implying that soil biochemical processes will exhibit differential responses to ongoing climate change at different altitudes.

Carbon isotope variations of modern soils in the eastern margin of the Tibetan Plateau and their controlling factors

The marginal areas of the Tibetan Plateau have great vertical altitude gradient and abundant vegetation, they are therefore the ideal places for investigating the relationships among carbon isotope composition (δ13C) of modern soils, vegetation and environmental factors, which would be very useful for the reconstructions of both paleovegetation and paleoclimate. In this paper, modern soil samples collected in different vegetation vertical zones along 4 km elevation gradient in the eastern margin of the Tibetan Plateau were analyzed for their carbon isotope composition. The results show that the modern soils in different vegetation vertical zones show apparent difference of δ13C values, which get heavier in the sequence of mixed evergreen and deciduous broad-leaved forest (−27.28‰ on average), evergreen broad-leaved forest (−27.25‰), subalpine shrub-meadow (−25.81‰), subalpine coniferous forest (−25.81‰), alpine bush-meadow (−25.16‰), and drought-enduring shrub (−24.07‰). 1800 m and 3500 m are two critical points for the δ13C values with respect to altitude. Specifically, the δ13C values decrease with increasing altitude below both points while increasing with increasing altitude above both points. Further analyses indicate that the declining δ13C values are mainly controlled by the decreasing proportion of C4 plants with elevation and the increasing δ13C values are attributed to the plant physic-morphological adaptation to the alpine environment. In the absence of drought stress, temperature is the main controlling factor for the carbon isotopic variations with altitude gradient.

Bird Community Heterogeneity Along Four Gradients of Different Orientations on a Temperate Mountain

Mountains harbour high biodiversity that is facing a crisis due to recent climate change. Bird communities shift along mountain gradients. Some previous studies have suggested that the effect of vegetation is prevalent on temperate mountains. In this study, we investigated the limiting effects of vegetation on the altitudinal distribution of birds on Mount Norikura in central Japan, by comparing bird communities along four gradients of different orientations. We recorded birds using the line-transect method and classified vegetation into six types: montane deciduous broadleaved forest, subalpine coniferous forest, subalpine birch forest, Japanese Stone Pine scrub forest, alpine tundra and bare ground below the alpine area.We identified the treelines and ecotones between the predominant vegetation types along each gradient. We found that bird species composition changed drastically at the treeline along three gradients. A non-metric multidimensional scaling (NMDS) ordination revealed: 1) the similarity between bird community structures in the subalpine coniferous forest and the Japanese Stone Pine scrub forest differed among the gradients, 2) despite birch being broadleaved, the bird community in the subalpine birch forest was a subset of that in the subalpine coniferous forest and was more similar to that in the subalpine coniferous forest than that in the montane deciduous broadleaved forest, and 3) in upper parts of the subalpine zone, the bird community structures in the subalpine coniferous and birch forests were similar to each other.Our findings suggest that although the treeline limits the distribution of forest-dwelling bird species, this effect differs among gradients with different orientations, probably depending on the scrub height of the Japanese Stone Pine scrub forests. Furthermore, the heterogeneity of coniferous and birch forests in the upper parts of the subalpine zone may have little effect on the altitudinal distribution of birds.

Amanita chuformis, a new Amanita species with a marginate basal bulb.

Amanita chuformis, a new species in the A. pseudogemmata-A. ballerina subclade of Amanita section Phalloideae, is described from China with both multi-gene phylogenetic and morphological data. This species occurs in subalpine coniferous forests in southwestern China and is characterized by its brownish pileus decorated with conical to patch-like volval remnants, a slightly striate pileal margin, a marginate basal bulb, and weakly amyloid to amyloid, subglobose to broadly ellipsoid basidiospores measuring 9.5-11 × 8-9.5 μm. Phylogenetic analyses based on internal transcribed spacer (ITS) region, the nuclear ribosomal RNA large subunit (nrLSU) and the genes for the polymerase II second largest subunit (RPB2) and for translation elongation factor 1-α (TEF1α) indicate A. chuformis is close to A. pseudogemmata and A. levistriata. The new species is described, illustrated and compared with closely related and similar species.

Soil moisture regime and canopy closure structure subalpine understory development during the first three decades following fire

Subalpine coniferous forests are adapted to cycles of fire and successional development, but increasing fire frequency and severity are altering historical stand structure, composition, and plant diversity. For instance, conifer regeneration has become increasingly variable as a result of prolonged aridity following fire, but the potential cascading effects on understory community development remain virtually unexplored. We used a natural experiment to investigate the relationships among understory plant succession, soil moisture regime, and variable tree canopy development over 30 years following the 1988 fires in the Greater Yellowstone Ecosystem, U.S.A. In 1990, at each of two study areas, plots were established in different site types defined by burn status (burned or unburned) and soil moisture regime (mesic or xeric), determined using biophysical indicators. We asked: (1) Are differences in soil moisture regime associated with differences in understory community composition (species richness, diversity, and functional group representation) during the first three decades after fire? (2) Does the relationship between soil moisture regime and community composition change over time as subcanopy (height >137 cm) tree densities increase? We confirmed our original designations of soil moisture regime using in situ measurements of soil moisture. Over the first decade of succession, species richness and diversity were lower in xeric-burned than in mesic-burned plots. Nearly 30 years after fire at the south-aspect study area, where subcanopy tree densities were low, seral understories diverged by soil moisture regime. There, graminoid cover was 23-fold higher, and forb cover was 3-fold lower, in xeric-burned than in mesic-burned plots. In contrast, in the mixed aspect study area, subcanopy tree densities were markedly higher. There, the understory did not vary by soil moisture regime, converging successionally and becoming more similar to unburned forest communities. Our results suggest that site-specific soil moisture regimes structure the early trajectory of post-fire understory recovery, but the relationship diminishes as tree canopies develop over time. However, under a warming climate, variable tree canopy development may compound increasing aridity, particularly on xeric sites, resulting in decreases in plant diversity and forb cover, increases in graminoid cover, and the potential for altered successional dynamics.

Growth rates and crown morphology of Abies amabilis in the seedling bank of an ancient subalpine conifer forest

The population dynamics of shade-tolerant tree species often includes a seedling bank — small trees persisting in the understory until canopy opening allows increased height growth. We studied the growth and morphology of 0.1–1.3 m tall Abies amabilis Douglas ex J. Forbes trees in the seedling bank of an ancient (>1300 years old) subalpine conifer forest on Vancouver Island, western Canada. We determined rates of height growth by counting growth rings at 10 cm intervals along the main stem and assessed crown morphology by measuring the length and location of each branch along the main stem. These small trees were very old (mean basal ring count, 112 years) and grew very slowly, taking on average 170 years to reach a height of 1 m. Instead of the typical conical form of open-grown Abies Mill., seedling bank individuals had flat crowns. Most branches were near the top of the main stem. The maximum distance along the main stem plus a branch was 1.6 times the main stem length, indicating that resource allocation is focused on lateral growth. The ability to grow slowly and adjust crown morphology allows seedling bank trees to persist for decades to centuries and to retain the possibility of release and growth into the canopy.

Physiological response of Swiss ecosystems to 2018 drought across plant types and elevation.

Using five eddy covariance flux sites (two forests and three grasslands), we investigated ecosystem physiological responses to the 2018 drought across elevational gradients in Switzerland. Flux measurements showed that at lower elevation sites (below 1000 m.a.s.l.; grassland and mixed forest) annual ecosystem productivity (GPP) declined by approximately 20% compared to the previous 2 years (2016 and 2017), which led to a reduced annual net ecosystem productivity (NEP). At the high elevation sites, however, GPP increased by approximately 14% and as a result NEP increased in the alpine and montane grasslands, but not in the subalpine coniferous forest. There, increased ecosystem respiration led to a reduced annual NEP, despite increased GPP and lengthening of the growing period. Among all ecosystems, the coniferous forest showed the most pronounced negative stomatal response to atmospheric dryness (i.e. vapour pressure deficit, VPD) that resulted in a decline in surface conductance and an increased water-use efficiency during drought. While increased temperature enhanced the water-use efficiency of both forests, de-coupling of GPP from evapotranspiration at the low-elevation grassland site negatively affected water-use efficiency due to non-stomatal reductions in photosynthesis. Our results show that hot droughts (such as in 2018) lead to different responses across plants types, and thus ecosystems. Particularly grasslands at lower elevations are the most vulnerable ecosystems to negative impacts of future drought in Switzerland. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

Topographic position amplifies consequences of short-interval stand-replacing fires on postfire tree establishment in subalpine conifer forests

Stand-replacing fires burned at 100 to 300-year intervals for millennia in subalpine conifer forests of western North America, but forests are burning more frequently as climate warms. Postfire tree regeneration is reduced when young forests reburn before recovering from previous fires or when drought occurs during postfire years. However, whether seedling vulnerabilities to harsh microclimate conditions may be amplified in short-interval (<30 years) fires is unclear. We conducted a field experiment to answer three questions: (1) How do germination, survival, and establishment of lodgepole pine (Pinus contorta var. latifolia) and Douglas-fir (Pseudotsuga menziesii var. glauca) vary by aspect following high-severity, short-interval fires? (2) What environmental factors control germination, survival, and establishment of both species? (3) Based on our experimental evidence, what proportion of available seed would be expected to establish across landscapes that burned in these short-interval fires? One year postfire, we planted seeds of both species in north-facing, south-facing and flat plots at four sites across the Greater Yellowstone Ecosystem (Wyoming, USA). Soil microclimate was monitored continuously. Seed germination and seedling survival were measured every two weeks during the following growing season and at the beginning and end of the second growing season. Germination did not vary with aspect but increased with early-season soil moisture and temperature. Survival and establishment were low on south-facing aspects (<1% of seeds established for both species) and declined with warmer soil temperatures and drier soils. For lodgepole pine, we predicted establishment rates of <1% of available seed over 25% of the reburned landscape. Soil temperatures in short-interval fires were 2˚C warmer than similar areas of long-interval fire, with maximum temperatures frequently exceeding 40 °C. Topographic variation will mediate the consequences of short-interval fire for seedling establishment, leading to patchier tree regeneration as climate warming raises the likelihood of short-interval fires.

High‐severity and short‐interval wildfires limit forest recovery in the Central Cascade Range

AbstractIncreasing forest fuel aridity with climate change may be expanding mid‐to‐high‐elevation forests' vulnerability to large, severe, and frequent wildfire. Long‐lasting changes in forests' structure and composition may occur if dominant tree species are poorly adapted to shifting wildfire patterns. We hypothesized that altered fire activity may lower existing forest resilience and disrupt the recovery of upper‐montane and subalpine conifer forest types. We empirically tested this hypothesis by quantifying post‐fire forest structure and conifer tree regeneration after spatially large, severe, and rapidly repeated wildfires (&lt;12‐yr interval) in the Central Cascade Range in the U.S. Pacific Northwest. Post‐fire conifer regeneration was generally very poor among plots that experienced either a single high‐severity fire or rapid reburn, driven primarily by lack of proximate seed source. Pre‐fire dominant, shade‐tolerant species' abundance was highly negatively correlated with increasing seed source distances and dry, exposed post‐fire environmental conditions. In rapidly reburned plots, the order of burn severity was critical and promoted establishment of all conifer species, if low‐then‐high severity, or primarily fire‐adapted pines, if high‐then‐low severity. Our findings suggest that these forests, affected by expansive high‐severity and/or short‐interval wildfire, may transition into a patchy, low‐density, pine‐dominated forest state under future warming trends. These emerging, early seral ecosystems will incorporate more fire‐adapted tree species, lower tree densities, and more non‐forest patches than prior forests, likely expanding their resilience to anticipated increases in fire frequency. If future larger, more severe, and more frequent wildfire patterns manifest as expected in the Cascade Range, previously denser, moist mid‐to‐high‐elevation forests may begin resembling their drier, lower‐elevation mixed‐conifer counterparts in structure and composition.

The formation of protected SOM facilitated by labile C input via artificial roots

Plant root-derived labile carbon (C) delivered to soils can regulate the dynamics and turnover of soil organic matter (SOM); however, it remains largely unclear how inputs of individual labile components (e.g., glucose) affect the formation of physicochemical-protected SOM. In a firmly controlled rhizosphere system, we added a glucose solution through artificial roots to soils collected from two subalpine coniferous forests (an approximately 200-year-old spruce-fir forest and an approximately 70-year-old spruce plantation) with the soils subsequently incubated over 25 days. The results showed that the addition of glucose significantly increased the concentrations of aluminum (Al) and iron (Fe) bound in both metal-organic complexes (MOCs) and short-range order phases (SROs) by 45%, 68%, 31% and 38%, respectively (soil-averaged), which indicated that glucose addition enhanced the formation of physicochemical-protected SOM. The induced protection of SOM mainly resulted from the stimulation of interactions of the microbial residues with soil minerals and/or metal cations, as indicated by the concurrently increased microbial communities and zeta potential after glucose addition. Moreover, the glucose-induced absolute changes in MOCs at the spruce-fir site were higher than those at the spruce plantation site, whereas the changes in SROs exhibited an opposite trend to that observed for MOCs. This discrepancy is presumably due to the higher organic matter content with more extractable metals at the spruce-fir site being more beneficial for supporting microbial stabilization of glucose and subsequent necromass sequestration as MOCs. In addition, more clay minerals at the spruce plantation site had larger surface areas and more binding sites to bind microbial-derived organic molecules as SROs. Collectively, our findings provide robust evidence that the input of labile C to soils could facilitate the formation of physicochemical-protected SOM, which is potentially involved in transformation into microbial residues and subsequent interaction with metal cations and/or minerals and may also be mediated by soil properties. The formation of protected SOM may potentially offset native SOM decomposition and thus has ecologically important implications for long-term terrestrial C storage.

Bryophytes impact the fluxes of soil non-carbon dioxide greenhouse gases in a subalpine coniferous forest

Terrestrial bryophytes substantially regulate ecosystem processes in high latitude and high elevation ecosystems; however, the role of these plants in mediating soil emissions of non-CO2 greenhouse gases (GHGs) (N2O and CH4) in these ecosystems remains poorly understood. A removal experiment was conducted to investigate the influence of bryophytes on soil non-CO2 GHGs emission in a subalpine coniferous forest on the eastern edge of the Tibetan Plateau. Bryophyte removal decreased the average N2O emission rate, but did not significantly alter the CH4 flux rate. The cumulative amount of N2O emission and CH4 uptake was decreased by bryophyte removal. Compared to bryophyte-removal plots, soil in the control plots had higher concentrations of soil organic C (SOC), total N (TN), dissolved organic C (DOC), dissolved N (DN), and exchangeable NH4+. The control plots also had higher cumulative N2O emission and CH4 uptake compared to plots with bryophytes removed. Bryophyte-removal significantly changed the relationship between soil non-CO2 GHGs flux rates and soil water content, induced a linear decreasing relationship between the N2O emission rate and soil water content and a quadratic relationship between CH4 flux rate and soil water content in the bryophyte-removal plots. Results from this study indicate that bryophytes may regulate non-GHG emissions from subalpine coniferous soils and demonstrate that in the presence of bryophytes, these soils may act as a greater N2O source and CH4 sink in southwestern China.

A Taxonomic Revision of Leccinum rubrum in Subalpine Coniferous Forests, Southwestern China

A Taxonomic Revision of Leccinum rubrum in Subalpine Coniferous Forests, Southwestern China

Elevational variation in abundance of coarse woody debris in subalpine forests, central Japan

In recent years, coarse woody debris (CWD) of deadwood has attracted attention as a long-term carbon pool for global warming problems. Environmental factors such as temperature and precipitation vary greatly with elevation. Hence, it is expected that the carbon storage by CWD varies with elevation. However, few studies have examined the abundance of CWD along elevations. Therefore, we investigated the abundance and decay classes of CWD, aboveground biomass (AGB) and aboveground net primary production (ANPP) of living trees along an elevational gradient (1600–2800 m above sea level) in subalpine coniferous forests, central Japan. AGB, ANPP and CWD decreased with increasing elevation. AGB at 2800 m was about 10% of that at 1600 m a.s.l. On the other hand, CWD at 2800 m was only 2.2% of that at 1600 m a.s.l. Thus, the ratio of CWD to AGB was lower at higher elevations. The relative frequency of small CWD increased with elevation. In addition, smaller CWD tended to decay less. The lower ratio of CWD mass to AGB at higher elevations was probably caused by smaller CWD because small CWD rapidly disappears via the high decomposition rate due to its high ratio of surface area to volume. Therefore, this study suggests that the importance of CWD relative to AGB for carbon storage decreases with elevation in subalpine forests. Climate change may alter the balance between CWD and AGB by changing ANPP and decomposition rates along elevation gradients.

Evidence for climate instability during the Younger Dryas interval in the Hengduan Mountains, Yunnan, southwestern China

The Younger Dryas (YD) is the most recent abrupt climatic event recognized during the transition from the last glaciation to the Holocene. Growing evidence from the middle and high latitudes of Europe and North America shows that the climate within the YD event was highly unstable. Until recently, however, climate instability during this event has received little attention elsewhere in the world. Here we present evidence of a climatically unstable YD based on pollen-climate transfer function and REDFIT spectral analysis of pollen data from Haligu in the Hengduan Mountains of southwestern China, a low latitude temperate biodiversity hotspot. Our data show that the climate fluctuated significantly during the YD, with three warm peaks recognized at 11.5, 11.7 and 12.3 cal kyr BP. In particular, we estimate a gradual rise in mean annual temperature by 3.7 °C between the end of the YD and the early Holocene (11.2–10.9 cal kyr BP). In response to this, the subalpine conifer forest dominated by Picea and Abies may have migrated about 610 m upwards. We also detected that the climate fluctuations during the YD-Holocene transition show significant centennial periodicities of 76 yr, 84 yr, 91 yr, 95 yr, and 638 yr. The former four cycles are possibly related to the Gleissberg (around 80 yr) solar cycle, which was also detected in the Hongyuan peat, the PC-6 core of the East China Sea and the GISP2 ice core. These new findings enhance our understanding of millennial-scale abrupt climate events which may be of great significance in predicting the impact of future climate change on mountainous ecosystems in southwestern China. The warming at the end of the YD may provide a potential analog for global warming in the near future, during which the upward migration of vegetation zones will likely lead to the loss of habitat for threatened species. We therefore should take action to restore habitats and provide protected areas for species at risk.

Bark Stripping by Deer Was More Intensive on New Recruits than on Advanced Regenerants in a Subalpine Forest

Research Highlights: To ensure sustainable forest regeneration, it is important to clarify whether new recruits or advanced regenerants are more likely to be stripped. Therefore, the effects of bark stripping on saplings in subalpine forests with abundant saplings should be analyzed by regeneration mode, but there have been no such studies until now. Background and Objectives: I investigated the effects of bark stripping by Cervus nippon on saplings in a subalpine coniferous forest in central Japan to (1) reveal differences in bark stripping between new recruits and advanced regenerants and (2) clarify the factors affecting survivorship. Materials and Methods: A 50 m × 140 m (0.7 ha) plot was set in the old-growth subalpine coniferous forest. All trees in the plot that were ≥2 m in height were tagged, identified to species, measured diameter at breast height and recorded bark stripping by deer. These trees and new recruits were counted and measured in 2005, 2007, 2012, and 2017. I compared saplings recruited in 2007, 2012, and 2017 (“new recruits”) with existing saplings of the same size (“advanced regenerants”). Results: The density of new recruits of Abies mariesii and Tsuga diversifolia increased, whereas that of Abies veitchii decreased. The proportion of stripped saplings was greater in new recruits than in advanced regenerants, significantly so in A. veitchii, which also had the highest maximum bark stripping ratio. Factors affecting the survivorships applied by the regression tree analysis were the maximum stripping ratio of stems for the two Abies species and the initial size for the T. diversifolia. Conclusions: Bark stripping by deer was more intensive on new recruits than on advanced regenerants in a subalpine forest, and regeneration in canopy gaps might fail because of intensive bark stripping in areas overabundant in deer.

Drought vulnerability among China's ungulates and mitigation offered by protected areas

AbstractOngoing perturbations in the global climate have triggered changes in the frequency or magnitude of extreme climatic events, including drought. Increasingly common or intense droughts have threatened ungulates. Intensifying trend of drought has been observed in China since the 1980s. We assessed drought vulnerability of 60 ungulate taxa distributed in China by synthesizing information on drought exposure and intrinsic vulnerability related to biological traits. In total, 27 taxa were identified as vulnerable to drought, which represent over half of the taxa assessed as threatened in the IUCN Red List and China's National Red List. We identified hotspots where a high number of drought‐vulnerable taxa are concentrated, including Northeast Himalayan subalpine conifer forests, alpine conifer and mixed forests of Nujiang‐Lancang Gorge, and Qionglai‐Minshan conifer forests, which are all located in Southwest China. We also assessed conservation efforts that China has allocated to ungulate taxa vulnerable to drought. Drought‐vulnerable taxa that are endemic to China have significantly lower coverage in China's National Nature Reserve system compared with nonvulnerable taxa. These findings reveal the gaps in existing conservation efforts and indicate possible improvements that might be needed to maintain species resistance in the face of increasing and intensifying drought impacts.

A framework for harmonizing multiple satellite instruments to generate a long-term global high spatial-resolution solar-induced chlorophyll fluorescence (SIF)

Several decade-long satellite retrievals of solar-induced chlorophyll fluorescence (SIF) have become available during the past few years, but understanding the long-term dynamics of SIF and elucidating its co-variation with historical gross primary production (GPP) remains a challenge. Part of the challenge is due to the lack of direct comparability among these SIF products as they are derived from various satellite platforms with different retrieval methods, instruments characteristics, overpass time, and viewing-illumination geometries. This study presents a framework that circumvents these discrepancies and allows the harmonization of SIF products from multiple instruments to achieve long-term coverage. We demonstrate this framework by fusing SIF retrievals from SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) and Global Ozone Monitoring Experiment 2 (GOME-2) onboard MetOp-A developed at German Research Center for Geosciences (GFZ). We first downscale both original SIF datasets from their native resolutions to 0.05° (SIF¯GOME2_005 and SIF¯SCIA_005 respectively) using machine learning (ML) algorithms imposed with regionalization constraints to account for the varying relationships between predictors and SIF in space and time. We then apply the cumulative distribution function (CDF) matching technique to correct the offset between SIF¯GOME2_005 and SIF¯SCIA_005 inherited from the original instrumental discrepancies to generate a harmonized SIF time series from 2002 to present (SIF¯005). Finally, we quantify the uncertainty of SIF¯005. SIF¯005 is validated with 1) the original retrievals to ensure the spatial and temporal variabilities are preserved, 2) airborne SIF derived from the Chlorophyll Fluorescence Imaging Spectrometer (CFIS, R2 = 0.73), and 3) ground-based SIF measurements at a subalpine coniferous forest (R2 = 0.91). The SIFyield derived from SIF¯005 has high seasonal consistency with the ground measurements (R2 = 0.93), suggesting that the harmonized product SIF¯005 carries physiological information beyond the absorbed photosynthetically active radiation. Additionally, SIF¯005 has a good capability for large-scale stress monitoring as demonstrated with several major historical drought and heatwave events. The framework developed in this study sets the stage for future development of even more advanced SIF products from all SIF-capable satellite platforms once issues related to inter-sensor calibration are resolved and SIF physiology is better understood.

Effects of plant interspecific interaction and warming on soil microbial community in root zone soil of two dominant tree species in the subalpine coniferous forest in southwestern China

Effects of plant interspecific interaction and warming on soil microbial community in root zone soil of two dominant tree species in the subalpine coniferous forest in southwestern China

Nitrogen Addition Alleviates Microbial Nitrogen Limitations and Promotes Soil Respiration in a Subalpine Coniferous Forest

Soil microbes are an important component of soil ecosystems that influence material circulation and are involved in the energy flow of ecosystems. The increase in atmospheric nitrogen (N) deposition affects all types of terrestrial ecosystems, including subalpine forests. In general, alpine and high-latitude ecosystems are N limited. Increased N deposition could therefore affect microbial activity and soil respiration. In this study, four levels of N addition, including CK (no N added), N1 (2 g m−2 a−1), N2 (5 g m−2 a−1), and N3 (10 g m−2 a−1), were carried out in a Sichuan redwood forest at the eastern edge of the Tibetan Plateau. The dynamics of soil respiration, major microbial groups, ecoenzymatic stoichiometry, and microbial biomass carbon and nitrogen (MBC and MBN, respectively) were investigated over a year. The results showed that N application significantly increased soil respiration (11%–15%), MBC (5%–9%), MBN (23%–34%), N-acetylglucosidase (56.40%–204.78%), and peroxidase (42.28%–54.87%) activities. The promotion of soil respiration, N-acetylglucosidase, and peroxidase was highest under the N2 treatment. The carbon, nitrogen, and phosphorus metabolism of soil microbes in subalpine forests significantly responded to N application. In the latter stages of N application, microbial metabolism changed from being N restricted to phosphorus restricted, especially under the N2 treatment. Soil bacteria (B) and gram-positive (G+) bacteria were the dominant microbial groups affecting soil respiration. Structural equation modelling indicated that N application significantly promoted soil respiration and microbial biomass, whereas the main microbial groups did not significantly respond to N application. Therefore, we conclude that short-term N addition alleviates microbial nitrogen limitation and promotes soil respiration in the subalpine forest ecosystem that accelerates soil carbon (C) and N cycling. Continuous monitoring is needed to elucidate the underlying mechanisms under long-term N deposition, which may help in forecasting C, N, and P cycling in the alpine region under global climate change.