Subtropical Karst Area Research Articles

The restoration of karst rocky desertification has enhanced the carbon sequestration capacity of the ecosystem in southern China

The control of rocky desertification is the largest ecological restoration project in southwestern China, but its impact on the carbon sequestration capacity of karst ecosystems is not clear. Therefore, in this paper selects typical subtropical karst areas in Guangxi are selected as the research object, the carbon sequestration potential of the terrestrial ecosystems are quantified, including karst inorganic carbon sinks, and the response of the terrestrial ecosystem carbon sink to rocky desertification restorationis discussed. The results show that (1) the karst inorganic carbon sink flux (CCSF) is 42.75 t CO2/km2/yr, with a total CCS of 491.12 × 104 t CO2, accounting for only 2.5 % of the country's land area and contributing 7.6 % of the karst inorganic carbon sink. (2) The flux of the vegetation organic carbon sink is 380.44 t CO2/km2/yr, and the total amount is 4403.55 × 104 t CO2/yr., Overall, the spatial distribution exhibits a pattern of high in the northwest and low in the northeast. (3) With decreasing rocky desertification area, the magnitude of the terrestrial ecosystem carbon sink has exhibited a corresponding increasing trend. In particular, during 2010–2020, the rocky desertification area decreased by about 0.868 × 104 km2, and the terrestrial ecosystem carbon sink increased by 114.04 t CO2/km2/yr. This article provides a systematic spatial diagnosis of the carbon sequestration potential of terrestrial ecosystems, including karst inorganic carbon sinks, in the karst areas of Guangxi, and reveals their responses to the restoration of karst rocky desertification. This work has strong reference value and significance for the diagnosis and analysis of the carbon neutrality capacity at the national and global levels.

Stimulation of organic N mineralization by N‒acquiring enzyme activity alleviates soil microbial N limitation following afforestation in subtropical karst areas

Stimulation of organic N mineralization by N‒acquiring enzyme activity alleviates soil microbial N limitation following afforestation in subtropical karst areas

Afforestation enhances glomalin-related soil protein content but decreases its contribution to soil organic carbon in a subtropical karst area

Afforestation on degraded croplands has been proposed as an effective measure to promote ecosystem functions including soil organic carbon (SOC) sequestration. Glomalin-related soil protein (GRSP) plays a crucial role in promoting the accumulation and stability of SOC. Nevertheless, mechanisms underlying the effects of afforestation on GRSP accumulation have not been well elucidated. In the present study, 14 pairs of maize fields and plantation forests were selected using a paired-site approach in a karst region of southwest China. By measuring soil GRSP and a variety of soil biotic and abiotic variables, the pattern of and controls on GRSP accumulation in response to afforestation were explored. The average content of total GRSP (T-GRSP) and its contribution to SOC in the maize field were 5.22 ± 0.29 mg g−1 and 42.33 ± 2.25%, and those in the plantation forest were 6.59 ± 0.32 mg g−1 and 25.77 ± 1.17%, respectively. T-GRSP content was increased by 26.4% on average, but its contribution to SOC was decreased by 39.1% following afforestation. T-GRSP content decreased as soil depth increased regardless of afforestation or not. Afforestation increased T-GRSP indirectly via its positive effects on arbuscular mycorrhizal fungi biomass, which was stimulated by afforestation through elevating fine root biomass or increasing the availability of labile C and N. The suppressed contribution of T-GRSP to SOC following afforestation was due to the relatively higher increase in other SOC components than T-GRSP and the significant increase of soil C:N ratio. Our study reveals the mechanisms underlying the effects of afforestation on T-GRSP accumulation, and is conducive to improving the mechanistic understanding of microbial control on SOC sequestration following afforestation.

Afforestation increases soil inorganic N supply capacity and lowers plant N limitation in subtropical karst areas

The positive effects of afforestation in karst rocky desertification areas are often limited by soil nitrogen (N) availability. However, how plant N limitation evolves in response to prolonged afforestation as well as the mechanisms underlying plant N limitation and soil N availability are poorly understood. In this study, plant leaves and soil samples were collected from cropland, Dodonaea viscosa plantations 10, 20 and 40 years after their establishment on abandoned cropland in a karst rocky desertification area. The natural succession of shrubland served as the control. Foliar N/phosphorus (P) stoichiometry was used to evaluate plant N limitation, and the N concentration and δ15N value of foliar and soil were measured to evaluate the changes in soil N availability. Soil N transformation rates were also determined, using 15N tracing technique to elucidate the mechanism underlying soil inorganic N availability. The plant community-level foliar N/P ratio (13.5) was lower than 14 following 10-y afforestation, indicating that plant growth is predominantly N limited. With the prolonged afforestation, the plant community-level foliar N/P ratio increased to 14.0–15.5, suggesting decreased plant N limitation. This could be attributed to increased soil N availability confirmed by the raises in the N concentration and δ15N of plant community-level foliar and soil δ15N with prolonged afforestation. Compared to cropland, D. viscosa afforestation increased the soil inorganic N supply capacity, by stimulating organic N mineralization, heterotrophic nitrification and the release of adsorbed ammonium, more obviously with prolonged afforestation. The rates of mineralization, heterotrophic nitrification and the release of adsorbed ammonium correlated significantly and positively with the plant community-level foliar N concentration, N/P ratio, and δ15N, indicating that a larger soil inorganic N supply alleviates plant N limitation. This effect can be attributed to the increases in organic matter content, bacterial and fungal abundances and soil structure improvement following afforestation, all of which facilitate mineralization and heterotrophic nitrification. However, plant N limitation after 40 years of afforestation still exists, suggesting that monoculture afforestation may need to be adapted to maximize soil N availability and to alleviate plant N limitation in karst rocky desertification areas.

Amplified Production and Export of Dissolved Inorganic Carbon During Hot and Wet Subtropical Monsoon

AbstractUnderstanding the origins and processes of riverine dissolved inorganic carbon (DIC) is crucial for predicting the global carbon cycle with projected, more frequent climate extremes yet our knowledge has remained fragmented. Here we ask: How and how much do DIC production and export vary across space (shallow vs. deep, uphill vs. depression) and time (daily, seasonal, and annual)? How do the relative contributions of biogenic (soil respiration) and geogenic (carbonate weathering) sources differ under different temperature and hydrological conditions? We answer these questions using a catchment‐scale reactive transport model constrained by stream flow, stable water isotopes, stream DIC, and carbon isotope data from a headwater karstic catchment in southwest China in a subtropical monsoon climate. Results show climate seasonality regulates the timing of DIC production and export. In hot‐wet seasons, high temperature accelerates soil respiration and carbonate weathering (up to a factor of three) via elevating soil CO2 and carbonate solubility, whereas high discharge enhances export by two orders of magnitude compared to cold‐dry seasons. Carbonate weathering is driven more by soil CO2 than water flow. At the annual scale, 92.9% and 7.1% of DIC was produced in shallow and deep zone, respectively, whereas 64.5% and 35.5% of DIC was exported from shallow and deep zone, respectively. These results highlight the uniqueness of subtropical karst areas as synchronous reactors and transporters of DIC during the hot‐wet monsoon, contrasting the asynchronous production and export in other climate regions. A future hotter and wetter climate with more intensive storms in the region may further intensify DIC production and export, accentuating the potential of subtropical karst regions as global hot spots for carbon cycling.

Responses of plant sap flow rate to epikarst water availability under different karst water-bearing media

The water supply capacities of epikarst zones play an essential role in the restoration of vegetation and sustainable utilization of groundwater resources in karst areas. The permeability and water storage capacity of karst water-bearing media directly determines prospective water acquisition for plants. However, the degree to which karst water-bearing systems influence plant transpiration remains unclear. Consequently, this study selected three common karst water-bearing structures (pore and fissure (L1), fissure and conduit (L2), dissolution seam and pore (L3), to examine the sap flow rate (SF), infiltration rate, meteorological factors, soil water, and groundwater depth (GD) associated with the growth of Toona sinensis. The results revealed that: (1) Marked differences in the SF based on long term monitoring (2016/1–2022/6) were observed in three sample plots with the order of L2 > L1 > L3. Similar trends in the rock permeability coefficient were also found. (2) Groundwater depth and soil water are the major factors that determine the SF. (3) The threshold of GD (2.54 m for L1, 0.96 m for L2, and 3.56 m for L3) was identified, wherein the sensitivity of SF to GD (λn) was weakened with greater GD. (4) The fissure and conduit sample plot with a shallower GD and higher rock permeability provided more plant-available water, resulting in a larger SF. In contrast, the dissolution seam and pore sample plot with a more stable GD and media rock permeability hindered the absorption of water by plants, which translated to a lower SF. These results highlighted that the structures of karst water-bearing media determined the overall SF. This study provides useful information toward guiding the restoration of vegetation in subtropical karst areas with abundant rainfall and shallow groundwater reservoirs.

Ecological evaluation of the development of Neanderthal niche exploitation

Understanding the ecological niche occupied by Neanderthals and their ancestors is at the forefront of many Palaeolithic investigations. In this study, a complex characterization of the environments once occupied by these hominins in Western Eurasia were performed. It included the determination of the biome and climatic zones occupied, the potential wind exposures, the classification of the karst regions where they lived, the direction of the entrance of caves inhabited and the average daily total solar irradiation values. In addition, the climatic suitability of the Mediterranean mosquito fauna, tick-borne encephalitis and four large mammal species as potential mammal hosts were also studied. It was found that Western Eurasian hominins occupied a wide ecological niche range from the semi-arid to the subarctic climates. Between the MIS20-MIS4 interval, the biomes occupied by Neanderthals and their ancestors shifted from the woodland-shrubland to the boreal biomes, indicating the increasing general biocultural adaptation to the more continental environments. These populations inhabited territories with weak to moderate wind exposure (Beaufort 2–5) and usually occupied caves with a south or southwest-oriented entrance. This habitat selection indicates the recognition of local microclimatic factors and a conscious choice of shelters. In the glacial periods, the increasing extension of temperate-subpolar karst regions at the expense of the more habitable subtropical karst areas might have adversely affected the overwintering chances of ancient western Eurasian populations in West, Central Europe, and Central Asia. The supraorbital area of ancient hominins with low receding foreheads and prominent supraorbital ridges may have been exposed to a greater risk of the development of frontal sinusitis and its complications than modern humans with more gracile skulls. Neanderthals and their predecessors were exposed to moderate average daily solar radiation values, although these values exhibit heterogenic picture in the comparison of Central and Southern Europe. While the southern European Neanderthal populations were dominantly exposed to the risk of mosquito-borne diseases, the northern and central populations could suffer from tick-borne diseases. However, both tick-borne encephalitis virus and its medium-large mammal hosts were present in most sites dwelled by Neanderthals and their ancestors.

Variation of soil physicochemical properties of different vegetation restoration types on subtropical karst area in southern China.

To carry out differentiated ecological restoration activities and formulate appropriate environmental conservation strategies for karst regions, it is essential to investigate the impact of ecological restoration and forest management strategy differences on soil properties. The karst region in Xiangxi, Hunan province, China was selected as the study site. Here, we determined soil physical and chemical differences in soil profiles of karst areas with ecological restoration activities. The results showed that (1) the soil properties showed a significant difference between the restoration vegetation and uncultivated land, especially in soil physical properties. The soil moisture conversion coefficient (83.0%) and soil bulk density (1.37g/cm3) of Liriodendron chinense (Hemsl.) Sarg reached the highest value among 12 vegetations. 2) The topsoil was more sensitive to ecological restoration. Soil physical properties in the topsoil samples from the forest management areas were significantly higher than uncultivated lands (P < 0.05). (3) Redundancy analysis showed that the soil chemical content differed significantly among the types of forest vegetation restoration and different soil layers. Among the nutrients analysis, Mg, Zn and K were the main factors affecting soil properties in the rocky desertification areas. Therefore, our results recommend planting the broadleaved deciduous forest as the preferred forest among three different forest types to enhance soil fertility and water conservation functions, especially in subtropical karst areas ecosystems, which provided for making scientific forest restoration management in the karst region.

Vegetation Restoration with Mixed N2-Fixer Tree Species Alleviates Microbial C and N Limitation in Surface Soil Aggregates in South Subtropical Karst Area, China

Soil extracellular enzyme stoichiometry (EES) is the essential predictor in nutrient status and resource limitation of soil microorganisms, whose metabolism has a vital role in biogeochemical cycling and ecosystem function. However, little is known about how N2-fixer tree species with different planting patterns affect soil nutrient resources in terms of extracellular enzyme activity (EEA) or EES within aggregates in degraded karst ecosystems. In this study, we evaluated soil EEA and EES related to carbon (C), nitrogen (N), and phosphorus (P) cycles across two eight-year-old pure plantations of legume species [Dalbergia odorifera T. Chen (PD) and Acrocarpus fraxinifolius Wight ex Arn. (PA)] and a mixed plantation of the two tree species listed above (MP). Meanwhile, a nearby undisturbed shrubland was used as a control (CK). We concluded that the activities of C-, N-, and P-acquiring enzyme increased to different degrees in the N2-fixer tree species stands (particularly in MP) compared to CK in all aggregates. Compared to CK, MP significantly increased by 39.0%, 54.0%, 39.3%, and 24.8% in total C-acquiring EEA, 41.1%, 60.5%, 47.8%, and 12.5% in total N-acquiring EEA, and 100.4%, 79.7%, 69.2%, and 56.4% in total P-acquiring EEA within &gt;2 mm, 1–2 mm, 0.25–1 mm, and &lt;0.25 mm aggregates, respectively. Furthermore, the logarithmic transformed ratio of C-, N-, and P-acquiring enzyme activities was 1.20:1.08:1, which deviated from the global ratio (1:1:1). Vector analysis of EEA showed that the vector length (VL) within aggregates was significantly lower than that of CK in all stands of N2-fixer species except PD; while in all treatments, vector angle (VA) was &lt;45° for all aggregate sizes, except in MP, where VA reached 45° for &lt;0.25 mm aggregate. These indicated soil microbes were limited by C and N together. However, MP significantly alleviated microbial C and N limitation than CK (p &lt; 0.05). There were obvious positive relationships between enzyme C:N, C:P, and N:P ratios. VL was markedly negatively linked to VA. EES was markedly related to most soil nutrients and microbial biomass stoichiometry ratios. Changes in soil EEA and EES were primarily driven by available phosphorus (AP), microbial biomass carbon (MBC), soil C:N and MBN:MBP ratios. Together, our results demonstrate the influences after introducing N2-fixer tree species (particularly MP) for vegetation recovery on soil microbial nutrient limitation and ecological processes in aggregate level and will contribute to the development of ecological restoration practices and fertility management in degraded karst ecosystems of southwest China.

Plankton distribution patterns and the indicative significance of diverse cave wetlands in subtropical karst basin

In subtropical karst area, small wetlands are widely distributed owing to the well-developed karst and the high degree of groundwater-surface water interaction. However, detailed plankton community composition, spatial-temporal dynamics, and its assembly mechanism were rarely discussed. To address these, plankton distribution patterns and its relationship with hydrological conditions and environment gradients were analyzed to discover the biological characteristics of wetlands in a typical karst basin in South China. The results showed that the wetlands were diverse in types due to the hydrogeological background. Water quality evaluation according to major ions analyses showed good results, with significant differences among samples and a poor status when measured by biotic indicator such as Escherichia coli. During high and low-flow condition, a total of 163 and 162 species of phytoplankton, and a total of 62 and 36 species of zooplankton, respectively were observed. The dominant species of phytoplankton communities in low-flow condition metamorphosed from Cyclotella sp. to Navicula sp. and Cocconeis placentula; and Pediastrum simplex to Ankistrodesmus spiralis. The metamorphosis in zooplankton community was from Difflugia globulosa to Strobilidium sp. Comparing with other karst water in different climate regimes, the number of plankton species in the subtropical karst wetland was the highest, which is related to the high and stable water temperature, large karst underground spaces, and frequent interaction between groundwater and surface water. The abundance, biomass and community semblance of plankton were increased with an increase in nutrients, precipitating improvement in hydrodynamics and light intensity. This study also suggested that the plankton in karst cave wetlands in subtropical area were characterized by low diversity, attributable to the calcium-rich and weakly alkaline, but poor nutrient status of the water environment, as well as drew attention to the possible functions of wetlands in relation to the quality of connected river.

Slash-and-burn in karst regions lowers soil gross nitrogen (N) transformation rates and N-turnover

Slash-and-burn is a common practice in the subtropical karst areas of southwest China, as well as in other places around the world, to prepare forest land for agricultural plantations. Understanding the effect of slash-and-burn practice on soil nitrogen (N) supply capacity is critical for the preservation of ecosystem services including long-term plant productivity. This is of particular relevance in mountainous karst regions with thin calcareous soils characterized by high pH, high calcium (Ca), and high soil loss risk. In this study, a 15N tracing approach was employed to quantify the effect of slash-and-burn on gross N-transformation rates and the mineralization-immobilization turnover (MIT) in soils from subtropical karst regions half-year after the burning, right before the agricultural cultivation began. Soil samples were collected from burned and nearby control sites (woodland). Compared to the control sites, the contents of soil organic carbon, total N, and Ca were lower in the burned sites, while no significant difference was observed in soil enzyme activities. The rates of mineralization of organic N to ammonium (NH4+), NH4+ adsorption and release, NH4+ oxidation to nitrate (NO3–) (i.e., autotrophic nitrification, ONH4), and microbial NO3– immobilization were significantly lower in burned sites, suggesting that slash-and-burn has a negative effect on MIT in karst regions. The mean residence times of inorganic N were 4.33 ± 0.53 d-1 for NH4+ and 6.83 ± 1.76 d-1 for NO3– in burned soil, which is remarkably higher than the control soil (0.77 ± 0.08 d-1 for NH4+ and 2.47 ± 0.83 d-1 for NO3–), indicating decreased N turnover after slash-and-burn. Abundances of bacteria and fungi, reflected by 16 s rRNA and ITS gene analyses, were not different between burned and control sites, suggesting that microbial community composition and activity, rather than the population size, controls the microbial-modulated N transformations. Owning to the slower NH4+ and NO3– turnover and reduced soil N transformation rates, we conclude slash-and-burn practice significantly lowers soil inorganic N supply capacity, leading to remarkably reduced soil inorganic N content, especially for NO3– (from 59.9 ± 8.67 mg N kg−1 to 44.1 ± 1.86 mg N kg−1), which may further restrict the efficiency of agricultural exploitation. Thus, we recommend that the follow-up agricultural activities after slash-and-burn practice need to take this into account in addition to erosion control and nutrient conservation measures.

Transformation process of five water in epikarst zone: a case study in subtropical karst area

Five water stands for five form existence models of water. In karst area, five water means precipitation, groundwater, evapotranspiration water, soil water, and overland flow. The complicated water-bearing hydrogeological media and the inhomogeneous water storage structure lead to low efficiency of water utilization. To reveal intricate water resources transformation in karst areas, a typical epikarst zone was selected. The five water and their conversion processes were studied and the transformation models were built based on the long-term positioning observations. The results show that (1) overland flow can be generated when precipitation reaches 6 mm and lasts for 6 h. Under light and moderate rainfall (LMR) conditions, less than 6% of the precipitation is converted to overland flow. Under heavy rainfall and rainstorm (HRR) conditions, the conversion of precipitation into overland flow in about 3.5–6%. (2) Under LMR conditions, the conversion rate of precipitation to vegetation water, soil water and groundwater was 2–3.5%, 40–60% and 25–35%, and the conversion rate under HHR conditions was 1.5–2.2%, 25–30% and 32–50%. (3) The proportion of different levels precipitation was transformed to soil water is 20–70%. (4) The conversion rate of groundwater and karst fissure water for LMR conditions are 8–15% and 10–15%, and that for HRR is 15–20% and 40–50%. (5) The proportions of different degrees of precipitation transformed into vegetation transpiration and evaporation water are 1.5–3.5% and 6–9%, respectively. (6) Generally, <4% of the precipitation is converted into overland flow, 20–70% into soil water, 25–50% into karst groundwater, and 1–10% into evaporative water.

Spatiotemporal Evolution Analysis and Future Scenario Prediction of Rocky Desertification in a Subtropical Karst Region

Landscape change is a dynamic feature of landscape structure and function over time which is usually affected by natural and human factors. The evolution of rocky desertification is a typical landscape change that directly affects ecological environment governance and sustainable development. Guizhou is one of the most typical subtropical karst landform areas in the world. Its special karst rocky desertification phenomenon is an important factor affecting the ecological environment and limiting sustainable development. In this paper, remote sensing imagery and machine learning methods are utilized to model and analyze the spatiotemporal variation of rocky desertification in Guizhou. Based on an improved CA-Markov model, rocky desertification scenarios in the next 30 years are predicted, providing data support for exploration of the evolution rule of rocky desertification in subtropical karst areas and for effective management. The specific results are as follows: (1) Based on the dynamic degree, transfer matrix, evolution intensity, and speed, the temporal and spatial evolution of rocky desertification in Guizhou from 2001 to 2020 was analyzed. It was found that the proportion of no rocky desertification (NRD) areas increased from 48.86% to 63.53% over this period. Potential rocky desertification (PRD), light rocky desertification (LRD), middle rocky desertification (MRD), and severe rocky desertification (SRD) continued to improve, with the improvement showing an accelerating trend after 2010. (2) An improved CA-Markov model was used to predict the future rocky desertification scenario; compared to the traditional CA-Markov model, the Lee–Sallee index increased from 0.681 to 0.723, and figure of merit (FOM) increased from 0.459 to 0.530. The conclusions of this paper are as follows: (1) From 2001 to 2020, the evolution speed of PRD was the fastest, while that of SRD was the slowest. Rocky desertification control should not only focus on areas with serious rocky desertification, but also prevent transformation from NRD to PRD. (2) Rocky desertification will continue to improve over the next 30 years. Possible deterioration areas are concentrated in high-altitude areas, such as the south of Bijie and the east of Liupanshui.

亚热带岩溶区典型常绿和落叶树种的蒸腾特征及其对环境因子的响应

植物蒸腾是水循环的重要组成部分，为了解亚热带岩溶区树木的蒸腾耗水情况，探究气候和水文地质条件对植物蒸腾的影响，运用Granier热耗散探针技术，对亚热带岩溶区次生林内的常绿树种女贞（L.lucidum）和落叶树种刺槐（R.pseudoacacia）的树干液流进行了连续监测，并同步监测了气象因子及土壤含水率（SMC），探讨在不同时间尺度下两种生活型树种的蒸腾特征及其对环境因子的响应。结果表明：（1）在季节尺度下，影响两树种整树蒸腾量（ET）的主要因子为太阳辐射强度（Rs）、气温（T）和水汽压亏缺（VPD）；女贞蒸腾量（ET_L）表现为夏季（1.29 kg/h） > 春季（0.57 kg/h） > 冬季（0.15 kg/h） > 秋季（0.13 kg/h），刺槐蒸腾量（ET_R）表现为夏季（0.90 kg/h） > 春季（0.31 kg/h） > 秋季（0.16 kg/h） > 冬季（0.04 kg/h）。（2）在日尺度下，晴天两树种ET呈现出明显的单峰日变化，且主要影响因子均为T、VPD和Rs；但由于常绿和落叶树种的生理特征差异，降雨时ET_L受到抑制，而ET_R则显著提升。（3）从昼夜层面来看，两树种夜间蒸腾量不足日蒸腾总量的35%。在夜雨现象和树木生理特征的影响下，秋冬季夜间蒸腾量占比明显高于春夏季，刺槐的平均夜间蒸腾量及其占比（1.56 kg，24.1%）高于女贞（1.08 kg，13.9%）。;Plant transpiration is an important part of water cycle in ecosystem, that is, the process of water transfer from soil to atmosphere through plants. It is driven by solar radiation, air temperature, air humidity and wind speed, and also affected by soil water storage capacity. Understanding and quantifying the influence mechanism of transpiration is an important basis to explore the water consumption of vegetation and the water balance of ecosystem. However, although the climate conditions in the subtropical karst area are good, there is a drought period of 4-5 months. At the same time, plants are often affected by water stress because of the thin soil layer and low water holding capacity of soil in karst area. The study of plant transpiration characteristics helps us understand the water consumption of trees in subtropical karst areas, and is also of great significance for exploring the impact of climate and hydro-geological conditions on plant transpiration. Granier's thermal dissipation probe method was used to continuously monitor the trunk sap flow of the evergreen tree species (L.lucidum) and the deciduous tree species (R.pseudoacacia) in the secondary forest in the subtropical karst area. The meteorological factors and soil moisture content (SMC) are also monitored simultaneously. Our purpose is to analyze the transpiration characteristics of two life-form tree species and their responses to environmental factors on different time scales. The results show that:(1) at the seasonal scale, the main factors affecting the whole-tree transpiration (ET) are solar radiation intensity (Rs), air temperature (T) and vapor pressure deficiency (VPD). The transpiration of L.lucidum (ET_L) is shown as summer (1.29 kg/h) > spring (0.57 kg/h) > winter (0.15 kg/h) > autumn (0.13 kg/h), transpiration of R.pseudoacacia (ET_R) is shown as summer (0.90 kg/h) > spring (0.31 kg/h) > autumn (0.16 kg/h) > winter (0.04 kg/h). (2) On the daily scale, the ET of the two tree species shown obvious single peak curve on sunny day, and the main factors affecting transpiration are T, VPD and Rs. However, due to the differences in the physiological characteristics of evergreen and deciduous tree species, ET_L is weakened during rainfall, while ET_R is significantly increased. (3) in terms of day and night, the nocturnal transpiration of the two tree species accounts for less than 35% of the total daily transpiration on average. Affected by the phenomenon of night precipitation and the physiological characteristics of trees, the proportion of nocturnal transpiration in the autumn and winter is significantly higher than that in the spring and summer. At the same time, the average daily nocturnal transpiration and its proportion of R.pseudoacacia (1.56 kg, 24.1%) is higher than that of L.lucidum (1.08 kg, 13.9%).

Plectania lutea (Sarcosomataceae), a new species from southwestern karst areas of China

Plectania lutea, is described as a new species from subtropical karst areas of China. The main distinguishing characteristics of P. lutea are the yellow hymenium, the external surface with ridges, the gelatinous but not semifluid medullary excipulum, and the large finely warted ascospores. The phylogenetic analyses based on internal transcribed spacer (ITS) and the large subunit (LSU) regions of rDNA sequences using Bayesian (BA) and Maximum Likelihood (ML) methods showed that the new taxon can be distinguished from phenotypically similar and phylogenetically related species.

Seasonal transpiration dynamics of evergreen Ligustrum lucidum linked with water source and water-use strategy in a limestone karst area, southwest China

Transpiration dynamics of karst ecosystems are not conditioned only by the shallow-soil water status, mainly because of the presence of deep-water source pools formed within the underlying bedrock if trees can develop their deep roots to the pools. However, the strength of these relationships and how as well as why they can vary from an area to another and from season to season are poorly understood, due primarily to high heterogeneity of karst ecosystems. Therefore, the present study was conducted during 2018–2019 rainy and dry seasons in a subtropical (limestone) karst area on southwest China to explore the functional responses to reduced water availability of evergreen Ligustrum lucidum (Chinese glossy privet) growing on the shallow soils. Seasonal variations in the transpiration, water-use patterns, and water sources were investigated for Ligustrum lucidum through high-resolution monitoring of the micrometeorology, sap flow, and soil moisture data, in combination with the carbon stable isotope composition of the tree leaves, as well as oxygen and hydrogen stable isotope composition of the tree stem, soil, and deep water. Our findings showed that high transpiration rates of Ligustrum lucidum were mainly associated with the soil moisture during rainy seasons (87% in total) but the low rates were associated primarily with the deep-water sources during dry seasons (13% in total). During the rainy season, when plenty of water was available to the plant, Ligustrum lucidum tended to obtain water inexpensively through the soil layers (on average, 59%) and consumed it profligately through reduction of its water use efficiency (WUEi) (mean WUEi rainy-season = 82.6 μmol/mol). In contrast, during the dry season, when limited water was available to the plant resulting from the significant reduction in rainfall, Ligustrum lucidum had to obtain the water expensively (albeit at a small amount) through deep-water source pools (on average, 62%), and consumed it conservatively through increasing its WUEi (mean WUEi rainy-season = 108.6 μmol/mol). Our findings can provide the insights into temporal transpiration dynamics as well as the strategies and mechanisms adopted by the plants to counteract the seasonal drought stress associated with the shallow soils in karst ecosystems.

Difference in hydraulic resistance between planted forest and naturally regenerated forest and its implications for ecosystem restoration in subtropical karst landscapes

Drought-induced plant hydraulic failure is expected to increase under climate change, and this negatively affects reforestation under the fragile karst environment in subtropical China. Compared with naturally regenerated forests, planted forests can rapidly increase vegetation coverage and have considerable economic benefits; however, their degree of resistance to drought-induced hydraulic failure is still unclear. Our study aimed to fill this knowledge gap by examining the differences in hydraulic characteristics between planted forests and naturally regenerated forests. Sample sites for both types of forest were selected in the humid subtropical karst region, and the hydraulic safety margin, stomatal conductance, and species-specific growing season length were measured for statistical analyses. The planted forest had a narrower hydraulic safety margin and presented a greater vulnerability to hydraulic failure than the naturally regenerated forest; however, it can bud later and abscise leaves early to avoid hydraulic failure. This study is the first to provide evidence of coordination between plant safety margin and species-specific growing season length in fragile karst ecosystems. To increase vegetation coverage and reduce soil erosion, the combination of both forest types could be an optimal ecosystem restoration strategy. Our study provides critical information for ecosystem restoration and management in subtropical karst areas.

Responses of soil gross nitrogen transformations to three vegetation restoration strategies in a subtropical karst region

AbstractInvestigation of soil internal nitrogen cycles is useful for unravelling the mechanisms responsible for soil nitrogen availability change. Here, gross nitrogen transformations were investigated in a subtropical karst area after 16 years of vegetation restoration. The experiment included four treatments with one control and three restoration strategies, that is: (i) maize‐soybean rotation field (CR, control); (ii) restoration with Toona sinensis (TS); (iii) restoration with Guimu‐1 hybrid elephant grass (GG, Pennisetum americanum (L.) Leeke × Pennisetum purpureum Schumach); and (iv) restoration with Guimu‐1 hybrid elephant grass and Zenia insignis as a mixture (ZG). Soil NH4+ and NO3− responded differentially to three restoration strategies, resulting in no significant change of total inorganic N content following vegetation restoration. Gross nitrogen mineralization (GNM) rate increased by 124.0%, 96.8%, and 60.3% in TS, GG, and ZG, respectively, relative to CR (2.88 ± 0.05 mg N kg−1 d−1). The strongest explanatory variable was microbial biomass carbon for both GNM (R2 = 0.82) and gross nitrification (R2 = 0.66). Dissimilatory NO3− reduction to NH4+ (DNRA) rate in TS or ZG, which was significantly greater than that in CR, was significantly correlated with SOC: NO3− ratio (R2 = 0.61), implying that DNRA was limited by SOC availability. Gross NH4+ immobilization rate, which was highest in GG, and lowest in CR, was best predicted by carbon to nitrogen ratio (R2 = 0.65). However, gross NO3− immobilization rate could not be well predicted by the measured soil properties. Our results suggest that vegetation restoration especially TS significantly enhanced soil N availability as reflected by gross N mineralization rate.

Developing Growth Models of Stand Volume for Subtropical Forests in Karst Areas: A Case Study in the Guizhou Plateau

Forest stand volume is one of the key forest structural attributes in estimating and forecasting ecosystem productivity and carbon stock. However, studies on growth modeling and environmental influences on stand volume are still rare to date, especially in subtropical forests in karst areas, which are characterized by a complex species composition and are important in the global carbon budget. In this paper, we developed growth models of stand volume for all the dominant tree species (groups) (DTSG) in a subtropical karst area, the Guizhou Plateau based on an investigation of the effects of various environmental factors on stand volume. The Richards growth function, space-for-time substitution and zonal-hierarchical modeling method were applied in the model fitting, and multiple indices were used in the model evaluation. The results showed that the climatic factors of annual temperature and precipitation, as well as the site factors of stand origin, elevation, slope gradient, topsoil thickness, site quality degree, rocky desertification type and rocky desertification degree, have significant influences on stand volume, and the topsoil thickness and site quality degree have the strongest positive effect. A total of 959 growth equations of stand volume were fitted with a five-level stand classifier (DTSG–climatic zone–site quality degree–stand origin–rocky desertification type). All the growth equations were qualified, because all passed the TRE test (≤30%), and the majority of the R2 ≥ 0.50, above 70% of the RMSE were between 5.0 and 20.0, and above 80% of the P ≥ 75%. These findings provide updated knowledge about the environmental effect on the stand volume growth of subtropical forests in karst areas, and the developed stand volume growth models are convenient for forest management and planning, further contributing to the study of forest carbon storage assessments and global carbon cycling.

Comparative analysis of bacterioplankton assemblages from two subtropical karst reservoirs of southwestern China with contrasting trophic status

Although bacterioplankton play an important role in aquatic ecosystems, less is known about bacterioplankton assemblages from subtropical karst reservoirs of southwestern China with contrasting trophic status. Here, 16S rRNA gene next-generation sequencing coupled with water chemistry analysis was applied to compare the bacterioplankton communities from a light eutrophic reservoir, DL Reservoir, and a mesotrophic reservoir, WL Reservoir, in subtropical karst area of southwestern China. Our findings indicated that Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes, Cyanobacteria and Verrucomicrobia dominated bacterioplankton community with contrasting relative frequency in the two subtropical karst reservoirs. Proteobacteria and Bacteroidetes were the core communities, which played important roles in karst biogeochemical cycles. Though WT, TN and DOC play the decisive role in assembling karst aquatic bacterioplankton, trophic status exerted significantly negative direct effects on bacterioplankton community composition and alpha diversity. Due to contrasting trophic status in the two reservoirs, the dominant taxa such as Enterobacter, Clostridium sensu stricto, Candidatus Methylacidiphilum and Flavobacteriia, that harbor potential functions as valuable and natural indicators of karst water health status, differed in DL Reservoir and WL Reservoir.