Semi-arid Mountainous Area Research Articles

Estimation of Picea Schrenkiana Canopy Density at Sub-Compartment Scale by Integration of Optical and Radar Satellite Images

This study proposes a novel approach to estimate canopy density in Picea Schrenkiana var. Tianschanica forest sub-compartments by integrating optical and radar satellite data. This effort is aimed at enhancing methodologies for forest resource surveys and monitoring, particularly vital for the sustainable development of semi-arid mountainous areas with fragile ecological environments. The study area is the West Tianshan Mountain Nature Reserve in Xinjiang, which is characterized by its unique dominant tree species, Picea Schrenkiana. A total of 411 characteristic factors were extracted from Gaofen-2 (GF-2) sub-meter optical satellite imagery, Gaofen-3 (GF-3) multi-polarization synthetic aperture radar satellite imagery, and digital elevation model (DEM) data. Consequently, 17 characteristic parameters were selected based on their correlation with canopy density data to construct an estimation model. Three distinct models were developed, including a multiple stepwise regression model (a linear approach), a Back Propagation (BP) neural network model (a neural network-based method), and a Cubist model (a decision tree-based technique). The results indicate that combining optical and radar image characteristics significantly enhances accuracy, with an Average Absolute Percentage Precision (AAPP) value improvement in estimation accuracy from 76.50% (with optical image) and 78.50% (with radar image) to 78.66% (with both). Of the three models, the BP neural network model achieved the highest overall accuracy (79.19%). At the sub-component scale, the BP neural network model demonstrated superior accuracy in low canopy density estimation (75.37%), whereas the Cubist model, leveraging radar image characteristics, excelled in medium density estimations (87.46%). Notably, the integrated Cubist model combining optical and radar data achieved the highest accuracy for high canopy density estimation (89.17%). This study highlights the effectiveness of integrating optical and radar data for precise canopy density assessment, contributing significantly to ecological resource monitoring methodologies and environmental assessments.

Analysis of extreme precipitation variation characteristics in mountain grasslands of arid and semi-arid regions in China

Introduction: With global warming, the disaster losses caused by extreme precipitation events are increasing. The poor natural conditions and climate change make the arid and semi-arid mountainous grassland area a sensitive region of climate change. The study on the spatio-temporal variation characteristics of extreme precipitation events in this region is helpful to improve the ability of climate prediction and disaster prevention and reduction in grassland.Methods: Based on the daily precipitation data of four meteorological monitoring stations in the Yinshanbeilu from 1970 to 2020, the trend analysis, M-K test and wavelet analysis were used to select seven typical extreme precipitation indicators to analyze the temporal and spatial characteristics of extreme precipitation.Results and Discussion: The results showed that the precipitation in the Yinshanbeilu increased in the past 51a, and the number of heavy rain days increased significantly. The significance test of CDD and CWD showed that the number of continuous dry days and continuous wet days decreased abruptly. The spatial analysis showed that the high value areas of R95p, R95d and PRCRTOT were all located in Siziwang Banner, and it could be concluded that the extreme precipitation risk was the highest in Siziwang Banner, while the low value areas of SDII, Rx1day, R95p and PRCRTOT were all located in Sonid Right Banner, which could be inferred that the Sonid Right Banner was relatively dry. The first main cycle of the seven indexes of extreme precipitation almost runs through the whole time series, and the starting point of the minimum main cycle changes inconsistent. In addition to the number of consecutive dry days, the other indices have a good correlation with annual precipitation, flood season and monthly precipitation from June to September, and July is the peak period of extreme precipitation events.

The climate and climatological water balance of Brazilian semi-arid mountainous areas and inland depression

The climate and climatological water balance of Brazilian semi-arid mountainous areas and inland depression

Can soil health in degraded woodlands of a semi-arid environment improve after thirty years?

In natural habitats, especially in arid and semi-arid areas that are fragile ecosystems, vegetation degradation is one of the most important factors affecting the variability of soil health. Studying physicochemical and biological parameters that serve as indicators of soil health offers important information on the potential risk of land degradation and the progression of changes in soil performance and health during recovery periods. This study specifically examines the impact of vegetation degradation on soil health indicators and the duration needed to improve the physical, chemical, and biological parameters in a semi-arid mountainous area site types with the dominance of Quercus macranthera Fisch & C.A. Mey and Carpinus orientalis Miller in northern Iran. In different years (2003, 2013, and 2023), litter and soil samples (at depths of 0–10, 10–20, and 20–30 cm) were collected in different types of degraded sites. Additionally, in 2023, a non-degraded site was chosen as a control and similar samples were collected. A total of 48 litter (12 samples for each of the study site types) and 144 soil (4 study site types × 3 depths × 12 samples) samples were collected. In order to investigate the spatial changes of soil basal respiration (or CO2 emission), which is involved in global warming, from each site type, 50 soil samples were taken along two 250-meter transects. The findings showed that litter P and Mg contents in the non-degraded site were 1.6 times higher than in degraded site types (2003). Following vegetation degradation, soil fertility indicators decreased by 2–4 times. The biota population was lower by about 80 % under the degraded site types (2003) than in the non-degraded site, and the density of fungi and bacteria in the degraded site types was almost half that of the non-degraded site types. Geostatistics showed the high variance (linear model) of CO2 emissions in areas without degradation. In addition, vegetation degradation significantly reduced soil carbon and nitrogen mineralization. Although soil health indicators under the degraded vegetation have improved over time (30 years), results showed that even thirty years is not enough for the full recovery of a degraded ecosystem, and more time is needed for the degraded area to reach the same conditions as the non-degraded site. Considering the time required for natural restoration in degraded site types, it is necessary to prioritize the conservation of vegetation and improve the ecosystem restoration process with adequate interventions.

Capabilities of using UAVs and close range photogrammetry to determine short-term soil losses in forest road cut slopes in semi-arid mountainous areas.

The soil losses that are caused should be examined by monitoring the forest roads in the periods after they are built. In traditional soil loss determination studies that require intensive labour, due to difficulties in parcel setup, leaks from the connection points of the system, overflow of the material in the storage units and regular follow-up of the precipitation, etc., the system has its disadvantages. In this study, studies were carried out to evaluate the possibilities of using UAV and close range photogrammetry (CRP) methods, which are remote sensing techniques, and to determine the soil losses in forest road cut slopes in semi-arid mountainous areas. In addition, the advantages and disadvantages of the methods are discussed. A 100-m section of the secondary forest road was chosen as the study area. Data acquisition was carried out by UAV and CRP methods in the period of May 2020-November 2020. In the results of the study, the volumetric deformation per unit area results, a - 0.0060 m3m-2 erosion amount and a 0.0046 m3m-2 accumulation amount were determined by UAV. In addition, in the photogrammetric method, a - 0.0050 m3m-2 erosion amount and a 0.0031 m3m-2 accumulation amount were found. When both methods were compared temporally, the processes took approximately 2 times longer in the CRP method. In addition, while the ground sampling distance of DEMs and orthophotos produced with the UAV was 2cm, it was obtained as 1cm in the CRP method and terrestrial receptions were found to be 2 times higher resolution. According to the results obtained, the CRP method gives results that are more accurate in such studies.

Projection of landscape ecological risk and exploration of terrain effects in the Qilian Mountains, China

AbstractThe development of a reasonable landscape ecological risk (LER) assessment system is the key means to establish an eco‐safety early warning mechanism, promote ecological protection, and achieve sustainable management of regional ecosystems. Based on the Cellular Automaton‐Markov (CA‐Markov) model, we predicted the land use/cover changes (LUCC) in 2026 and 2034 in the typical arid and semiarid mountainous area of the Qilian Mountains. We constructed an LER model to analyze the spatial–temporal changes in LER, quantified the driving mechanism, and synthesized the constraining effect of terrain features. The results showed that the study area was dominated by ecologically stable areas in 1980–2000 and 2010–2018. Ecologically improved and deteriorated areas in the central and northwest expanded in 2000–2010 and 2018–2036, which was attributed to ecological restoration policies (returning farmland to forest and grassland, closing mountains to forest and grassland, etc.) and economic development (livestock and mineral development, etc.). Due to the frequent interference of human activities in flat and complex terrain areas, the degree of landscape fragmentation, separation, and vulnerability is enhanced, and the ecological conditions of bare land and snow cover are relatively fragile, resulting in high ecological risk. Therefore, we could focus on zoning management and rational land use planning. This study provides a new framework for controlling LER in the Qilian Mountains, and will provide a scientific reference for the formulation of risk mitigation strategies in other arid and semiarid mountains worldwide.

Spatial and temporal effects on the value of ecosystem services in arid and semi-arid mountain areas—A case study from Helan Mountain in Ningxia, China

In the context of global warming and the continued increase in greenhouse gas emissions, the expansion of the global arid and semi-arid zones will accelerate, and there will be a risk of further land degradation. Changes in land use are one of the human activities that cause the increase in CO2 concentration in the global atmosphere. This article takes Helan Mountain as the research object which is located in the arid and semi-arid regions. It analyzes the spatial and temporal effects of land use changes, ecosystem service value, and ecological risk of land use in Helan Mountain by using GIS technology and five periods of land use data in 2000, 2005, 2010, 2015, and 2020. The study results show the following: First, due to land use changes in the arid and semi-arid zones, grasslands lost the largest area (in total 50,407.11 ha) during the study period. Construction land is the highest dynamic land type that increased by 22,364.73 ha. Land use generally shows a trend of increasing before decreasing, with an inevitable increase in the degree of utilization. Second, the total ecosystem service value decreased by 0.1148 million yuan from 5,674.5490 to 5,674.6638 million yuan, which shows a slight change in the overall ecosystem service value. Third, the ecological risk of land use could be divided into five classes, with a transformation characteristic from high and low ecological risk to comparatively high, medium, and comparatively low ecological risk. Fourth, the western side of Helan Mountain is the principal supply area for ecosystem services in the entire study area, which is an area with high-grade ecological risk at the same time. The vulnerability and importance of this area need to be given high priority. This study will provide spatial guidelines for the protection and restoration of ecological security issues such as environmental damage and land degradation in the arid and semi-arid regions of the mountains.

Artificial recharge by means of careo channels versus natural aquifer recharge in a semi-arid, high-mountain watershed (Sierra Nevada, Spain)

The acequias de careo are ancestral water channels excavated during the early Al-Andalus period (8th–10th centuries), which are used to recharge aquifers in the watersheds of the Sierra Nevada mountain range (Southeastern Spain). The water channels are maintained by local communities, and their main function is collecting snowmelt, but also runoff from rainfall from the headwaters of river basins and distributing it throughout the upper parts of the slopes. This method of aquifer artificial recharge extends the availability of water resources in the lowlands of the river basins during the dry season when there is almost no precipitation and water demand is higher. This study investigates the contribution of the careo channels in the watershed of Bérchules concerning the total aquifer recharge during the 2014–2015 hydrological year. Several channels were gauged, and the runoff data were compared with those obtained from a semi-distributed hydrological model applied to the same hydrological basin. The natural infiltration of meteoric waters accounted for 52% of the total recharge, while the remaining 48% corresponded to water transported and infiltrated by the careo channels. In other words, the careo recharge system enhances by 92% the natural recharge to the aquifer. Our results demonstrate the importance of this ancestral and efficient channel system for recharging slope aquifers developed in hard rocks. The acequias de careo are nature-based solutions for increasing water resources availability that have contributed to a prosperous life in the Sierra Nevada. Its long history (>1200 years) suggests that the system has remarkable resilience properties, which have allowed adaptation and permance for centuries in drastically changing climatic and socioeconomic conditions. This recharge system could also be applied to —or inspire similar adaptation measures in— semi-arid mountain areas around the world where it may help in mitigating climate change effects.

Hydrological modelling using the distributed hydrological model CEQUEAU in a semi-arid mountainous area: a case study of Ourika watershed, Marrakech Atlas, Morocco

Hydrological modelling using the distributed hydrological model CEQUEAU in a semi-arid mountainous area: a case study of Ourika watershed, Marrakech Atlas, Morocco

The impact of geomorphology on groundwater recharge in a semi-arid mountainous area

The impact of geomorphological structures on rainwater infiltration and groundwater recharge in the mountainous terrain of a semi-arid region was studied in the southern part of the Judean Western Mountain Aquifer (WMA) in Israel and the Palestinian Authority. The study was conducted in two main typical geomorphological landscape units which include exposed rock outcrops on the hillslope, and human-made agricultural soil terraces in a first-order stream channel. We measured rainwater infiltration through the unsaturated zone in the two geomorphological setups using vadose zone monitoring systems (VMS), that enabled continuous tracking of rainwater percolation and tracer transport through the unsaturated zone with respect to the rain patterns.The results from three years of continuous monitoring of water percolation through the unsaturated zone indicated a quick and intense percolation pattern in the rocky terrain compared to that measured in the deep soil of the terrace. The precipitation patterns, as reflected from the rain distribution and intensities, appeared to have a significant impact on deep water percolation and ultimately on groundwater recharge. In the rocky terrain, rain events of high intensity, even during drought years of low cumulative precipitation, resulted in deep water infiltration, which ultimately turned into groundwater recharge. On the other hand, infiltration through deep soils, as prevailed in terraces, valleys, and high plains, was found to be very limited, subjected to intense evaporation and ultimately result by reduced groundwater recharge. It appears that the value of annual cumulative precipitation is of a lower importance to groundwater recharge while high rain intensity is the most dominant factor responsible for deep water infiltration.Tracer migration patterns in the unsaturated zone revealed that precipitation on the rock outcrops promotes the generation of local runoff that rapidly drains into the subsurface through infiltration hot spots. It has been observed that these hot spots are often marked by prickly burnet shrubs (Sarcopoterium spinosum), which thrive year-round on the water abundancy in the soil pockets beneath the impervious rock on land surface. An important outcome of this study is the suggested strong coupling between geomorphological features and groundwater recharge. Application of variable infiltration functions to the different geomorphological characteristics of each terrain may improve groundwater recharge models. Climate changes in recent years, expected to lead to decreased annual precipitations and increased evaporation in parallel with an increase in rainfall intensities in the semi-arid mountainous areas of the eastern Mediterranean. Yet according to our results, the expected increase in rain intensity may actually result in an increase in groundwater recharge.

Geophysics and remote sensing applications for groundwater exploration in fractured basement: A case study from Abha area, Saudi Arabia

Groundwater can be the only salvation in arid or semi-arid mountainous areas, especially for those covered by basement rocks. The fractured groundwater aquifers are characterized by a lack of extension in all directions and the distribution of the main fractured and shear zones. An integrated approach of satellite remote sensing and geophysical techniques was used to investigate groundwater occurrence in the fractured aquifers of the mountainous area north of Abha city. A digital elevation model (DEM) was used to extract the streamlines and determine the eastward direction of the surface water flow. The average annual precipitation over the study area shows a positive increase trend estimated at 156 mm yr-1 based on Tropical rainfall measuring mission (TRMM) data during the 2002–2018 period. Seven vertical electrical soundings (VES) using Schlumberger array and twelve time-domain electromagnetic soundings (TEM) using an induction loop with size 50 by 50 m were conducted to detect accumulated groundwater in irregular shape pockets within the fractured basement rocks of the area. VES's and TEM indicate the occurrence of water saturation zones at depths varying from 45 to 60 m. The thickness of the saturated zone exceeds 100 m in some parts, extending toward the northeast-southwest direction parallel to the fractures and faults. The integrated approach provides a reliable study to better understand the groundwater occurrence in fractured basement aquifers.

Heritage conservation hazard in archaeological sites in Santa María Valley (NW Argentina): A geoarchaeological approach

AbstractAlluvial fans are typical geomorphic features of arid and semiarid mountain areas. Most archaeological sites in Santa María valley (NW Argentina) are located on this kind of landform. The aims of this paper are to describe the geomorphological context and assess the state of conservation of four archaeological sites on the piedmont of Sierra de Quilmes (NW Argentina) from a geoarchaeological perspective; to diagnose the geomorphological processes affecting the sites over the last 50 years; to assess their vulnerability and conservation hazard; and to propose some corrective measures. By mapping with remote sensors (e.g., drones, aerial photographs, and satellite images) and conducting field surveys, we found that most archaeological sites are set on the H1 and H2 units of the alluvial fans. and that their geomorphological dynamic has increased in the last 50 years. The main active processes are debris flows, overflows, and mudflows, accompanied by the development of rills, sheetflood, and aeolian deflation. Human impact is also severe. The four sites need mitigation measures and a structured management plan. To date, no studies of this kind have been done rin the region, although one of the sites has been partially reconstructed for touristic purposes.

Pollen reconstructed Holocene vegetation response to climate change and human activity in a semi-arid mountain area of north China

Understanding Holocene vegetation response regimes will improve predictions of future vegetation dynamics in mountain areas of semi-arid north China. In this paper, we statistically analyzed the relationship between a pollen record in a mountain area of semi-arid north China and influencing variables independent of pollen data, in the hope that potential vegetation forcing and response regimes can be explored. Our result suggests that the cool deciduous forest (CDF) at high elevation and the temperate deciduous forest (TDF) at middle and lower elevation were sensitive in responding to temperature before 6000 cal. yr BP, but were not sensitive to the summer monsoon (SM). Since 6000 cal. yr BP, the CDF and TDF were sensitive in responding to the SM instead of temperature. Hence, a shift of response regimes can be placed at ∼6000 cal. yr BP, which happened to be consistent with the time that forest expansion changed to deforestation. The insensitive response to the SM during the early Holocene might be attributed to the vegetation elevational movement, the topographic barrier for forest migration from refugia, and/or the topographically related high resistance and resilience of mountain forests. The sensitive response of CDF and TDF to the weakening SM since 6000 cal. yr BP was mainly caused by the cumulative moisture deficit caused by the continual weakening of the SM. Human activity had little impact on mountain forests until several centuries ago. Mountain forest expansion in response to climate warming during the early to middle Holocene can provide analogues for the prediction of forest dynamics of future warming.

Fine-scale detection of vegetation in semi-arid mountainous areas with focus on riparian landscapes using Sentinel-2 and UAV data

Sparse vegetation such as riparian forests and trees outside forests (TOF) cover only small areas but present various ecological advantages. The detection of these vegetation types in semi-arid mountainous areas is challenging as trees are heavily mixed with other land cover types. Their mapping requires therefore high-resolution imagery. We propose to leverage the advantages and synergies of freely available Sentinel-2 data and a light-weight consumer-grade unmanned aerial vehicle (UAV) with a simple red–greenblue (RGB) camera to detect these vegetation types. In our approach, an object-based random forest land cover classification is first developed over smaller sites using very high-resolution UAV data. The resulting maps are afterwards used as training data for multi-temporal Sentinel-2 based classifications at regional scale. We tested the approach in five different riparian landscapes of a semi-arid mountainous area in Iran. For comparison, mono- and multi-temporal Sentinel-2 data were also used alone – without support from UAV data – to build pixel-based random forest classification models at regional scale. Our results show that compared to the best mono-temporal results, the multi-temporal classification approach improved the overall accuracy and Kappa values of Sentinel-2 classifications from 77.0% to 83.9% and 0.72 to 0.81, respectively. The producer’s and user’s accuracy of the riparian forest class were also improved from 64.0% to 70.0% and 57.1% to 73.7%, respectively. Combining UAV and Sentinel-2 data improved the overall accuracy only slightly, but enabled a much better detection of Persian oak stands – for this class, the producer’s accuracy increased by 13.0 percentage points. Overall, we recommend the combined use of UAV and multi-temporal Sentinel-2 data to detect Persian oak forest stands.

Impacts of Vegetation and Topography on Land Surface Temperature Variability over the Semi-Arid Mountain Cities of Saudi Arabia

Land surface temperature (LST) can fully reflect the water–heat exchange cycle of the earth surface that is important for the study of environmental change. There is little research on LST in the semi-arid region of Abha-Khamis-Mushyet, which has a complex topography. The study used LST data, retrieved from ASTER data in semi-arid mountain areas and discussed its relationship with land use/land cover (LULC), topography and the normalized difference vegetation index (NDVI). The results showed that the LST was significantly influenced by altitude and corresponding LULC type. In the study area, during the summer season, extreme high-temperature zones were observed, possibly due to dense concrete surfaces. LST among different types of land use differed significantly, being the highest in exposed rocky areas and built-up land, and the lowest in dense vegetation. NDVI and LST spatial distributions showed opposite trends. The LST–NDVI feature space showed a unique ABC obtuse-angled triangle shape and showed an overall negative linear correlation. In brief, the LST could be retrieved well by the emissivity derived NDVI TES method, which relied on upwelling, downwelling, and transmittance. In addition, the LST of the semi-arid mountain areas was influenced by elevation, slope zenith angle, aspect and LULC, among which vegetation and elevation played a key role in the overall LST. This research provides a roadmap for land-use planning and environmental conservation in mountainous urban areas.

Comparison of spectral and spatial-based approaches for mapping the local variation of soil moisture in a semi-arid mountainous area

Accurate information on soil moisture (SM) is critical in various applications including agriculture, climate, hydrology, soil and drought. In this paper, various predictive relationships including regression (Multiple Linear Regression, MLR), machine learning (Random Forest, RF; Triangular regression, Tr) and spatial modeling (Inverse Distance Weighing, IDW and Ordinary kriging, OK) approaches were compared to estimate SM in a semi-arid mountainous watershed. In developing predictive relationship, Remote Sensing datasets including Landsat 8 satellite imagery derived surface biophysical characteristic, ASTER digital elevation model (DEM) derived surface topographical characteristic, climatic data recorded at the synoptic station and in situ SM data measured at Landsat 8 overpass time were utilized, while in spatial modeling, point-based SM measurements were interpolated. While 70%(calibration set) of the measured SM data were used for modeling, 30%(validation set) were used to evaluate modeling accuracy. Finally, the SM uncertainty maps were created for different models based on a bootstrapping approach. Among the environmental parameter sets, land surface temperature (LST) showed the highest impact on the spatial distribution of SM in the region at all dates. Mean R2(RMSE) between measured and modeled SM on three dates obtained from the MLR, RF, IDW, OK, and Tr models were 0.70(1.97%), 0.72(1.92%), 0.59(2.38%), 0.59(2.27%) and 0.71(1.99%), respectively. The results showed that RF and IDW produced the highest and lowest performance in SM modeling, respectively. Generally, the performance of RS-based models was higher than interpolation models for estimating SM due to the influence from combination of topographic parameters and surface biophysical characteristics. Modeled SM uncertainty with different models varies in the study area. The highest uncertainty in SM modeling was observed at the north part of the study area where the surface heterogeneity is high. Using RS data increased the accuracy of SM modeling because they can capture the surface biophysical characteristics and topographical properties heterogeneity.

Biophysical controls on nocturnal sap flow in plantation forests in a semi-arid region of northern China

Recent studies have recognized the importance of nocturnal sap flow (Qn) in affecting forest carbon and water budgets and responses to climate change at stand, regional, and global scales. However, biophysical controls on Qn are not fully understood, and their implications for land surface and vegetation models are unclear. We measured growing season sap flow of two widely distributed afforestation species, Pinus tabuliformis and Acer truncatum, in a middle-aged and a young monoculture forest stand, respectively, in a semi-arid mountainous area of northern China. We found a convergence in Qn between the two species and in the proportion of Qn to the total sap flow (12.2–15.0%) across species and ages. Total growing season Qn was higher for middle-aged stands than for young stands because of larger diameters at breast height of older stands. Nighttime vapor pressure deficit (VPDn) influence on Qn of young stands was soil moisture dependent. Nighttime wind speed indirectly controlled Qn through enhancing VPDn in young stands and directly promoted Qn in middle-aged stands with relatively low tree densities. For each species, both increased and decreased soil water content were able to promote Qn in stands with relatively dry soils, which might be due to enhanced nighttime water recharge for capacitance refilling and for avoiding hydraulic failure under prolonged water stress, respectively. Total effects of these three environmental factors explained less than 55% of the Qn variations. This study highlights uncertain physiological influences of VPDn on nighttime stomatal water loss, the nighttime water loss induced by wind, region-specific patterns of nighttime water recharge, and the importance of biotic controls on Qn. Our findings help to improve the existing VPD-based method for partitioning nighttime transpiration and water recharge at tree and stand levels, and suggest the importance of incorporating nocturnal sap flow into large-scale models.

Sparse Pinus Tabuliformis Stands Have Higher Canopy Transpiration Than Dense Stands Three Decades After Thinning

Hydrological effects of forest thinning have been studied at small watershed scales using the paired watershed approach since the 1920s. However, how forest transpiration, a critical component of evapotranspiration, changes decades after thinning is not well understood despite its importance for modifying drought resilience of forest ecosystem under climate change. In a semi-arid mountainous area of northern China, we measured growing season sap flow of Pinus tabuliformis, a widely planted afforestation species, in 44-year-old monoculture plantation stands with low (983 stems ha−1), medium (1688 stems ha−1), and high (2160 stems ha−1) density. Three decades after thinning, diameters at breast height (DBH) were larger in sparse stands than in dense stands. While its relation with sapwood area was density independent, the accompanying high sapwood area at the tree level for sparse stands resulted the highest stand sapwood area in the medium density stand (33.26 m2 ha−1), rather than in the high density stand (29.84 m2 ha−1). Similar to short-term studies, sparse stands demonstrated higher sensitivity to climatic fluctuations and drought depressions than dense ones. Nevertheless, stand density had no effect on the isohydric strategy of Pinus tabuliformis. Contrary to the positive relation between stand density and stand canopy transpiration soon after thinning, sparse stands exhibited higher growing season canopy transpiration than dense stands three decades later. In the dry year 2014, these density differences were relatively most pronounced, with July-September transpiration totals of 56.7 mm, 31.1 mm, and 22.1 mm in the low, medium, and high density stands, respectively. Our findings highlighted that stand density was not an appropriate indicator of thinned forest transpiration over long time scales. Interactions between soil droughts and thinning on forest transpiration need to be further clarified, especially in longer periods of time.

Differential Trends of Qinghai Spruce Growth with Elevation in Northwestern China during the Recent Warming Hiatus

Tree growth strongly responds to climate change, especially in semiarid mountainous areas. In recent decades, China has experienced dramatic climate warming; however, after 2000 the warming trend substantially slowed (indicative of a warming hiatus) in the semiarid areas of China. The responses of tree growth in respect to elevation during this warming hiatus are poorly understood. Here, we present the responses of Qinghai spruce (Picea crassifolia Kom.) growth to warming using a stand-total sampling strategy along an elevational gradient spanning seven plots in the Qilian Mountains. The results indicate that tree growth experienced a decreasing trend from 1980 to 2000 at all elevations, and the decreasing trend slowed with increasing elevation (i.e., a downward trend from −10.73 mm2 year−1 of the basal area increment (BAI) at 2800 m to −3.48 mm2 year−1 of BAI at 3300 m), with an overall standard deviation (STD) of 2.48 mm2 year−1. However, this trend reversed to an increasing trend after 2000, and the increasing trends at the low (2550–2900 m, 0.27–5.07 mm2 year−1 of BAI, p > 0.23) and middle (3000–3180 m, 2.08–2.46 mm2 year−1 of BAI, p > 0.2) elevations were much weaker than at high elevations (3300 m, 23.56 mm2 year−1 of BAI, p < 0.01). From 2000–2013, the difference in tree growth with elevation was much greater than in other sub-periods, with an overall STD of 7.69 mm2 year−1. The stronger drought conditions caused by dramatic climate warming dominated the decreased tree growth during 1980–2000, and the water deficit in the 2550–3180 m range was stronger than at 3300 m, which explained the serious negative trend in tree growth at low and middle elevations. After 2000, the warming hiatus was accompanied by increases in precipitation, which formed a wetting–warming climate. Although moisture availability was still a dominant limiting factor of tree growth, the relieved drought pressure might be the main reason for the recent recovery in the tree growth at middle and low elevations. Moreover, the increasing temperature significantly promoted tree growth at 3300 m, with a correlation coefficient between the temperature and BAI of 0.77 (p < 0.01). Our results implied that climate change drove different growth patterns at different elevations, which sheds light into forest management under the estimated future climate warming: those trees in low and middle elevations should be paid more attention with respect to maintaining tree growth, while high elevations could be a more suitable habitat for this species.

A modified Logit model for assessment and validation of debris-flow susceptibility

The accuracy of a debris-flow susceptibility assessment model can not be achieved to an acceptable level without rigorously selecting/processing model input parameters and reliably validating the results with ground truth. In this study, a modified logistic regression model was applied to examine the debris-flow susceptibility in the semi-arid mountainous areas of the southeast Tibetan Plateau, taking the case of the Benzilan-Changbo segment of the Jinsha River. The logistic regression model validation was supported by a field investigation and interpreting aerial photographs. The frequency ratio method was adapted for the sensitivity analysis of ten predisposing factors, including a factor of human activities that has never been considered in previous studies for the selected study area. The model with the highest prediction accuracy (83%) was obtained by comparing 55 possible combinations of the ten factors evaluated using a receiver operating characteristic curve technique. Composite analysis and random sample testing were also conducted to verify the reliability of the susceptibility assessment. The prediction results showed that the actual debris-flow area accounted for 92% of the total predicted debris-flow area. In summary, this study reveals that: (1) the highest susceptibility area was mainly distributed in the northeast and middle riparian zones, partially scattered in the southwest. Human activity was found to be closely related to the probability of debris flow occurrences; (2) under the two situations of independent and index combination participation evaluation, the performances of the same related factors showed largely different effects on the assessment results, which reflected the interrelation and interaction of the factors causing the debris flow; (3) the accuracy of the modified logistic regression model increased by 15%, when compared with the index entropy model in a previous study, and the reasons for this were discussed in the current study. The modified model, along with its enhanced validation process, could be easily extended to other regional mass movement susceptibility analyses.