Non-growing Season Research Articles

Temporal Stability of Salmonella enterica and Listeria monocytogenes in Surface Waters Used for Irrigation in the Mid-Atlantic United States

Enteric bacterial pathogen levels can influence the suitability of irrigation water sources for fruits and vegetables. We hypothesize that stable spatial patterns of Salmonella enterica and Listeria monocytogenes levels may exist across surface water sources in the Mid-Atlantic U.S. Water samples were collected at four streams and two pond sites in the mid-Atlantic U.S. over 2 years, biweekly during the fruit and vegetable growing seasons, and once a month during nongrowing seasons. Two stream sites and one pond site had significantly different mean concentrations in growing and nongrowing seasons. Stable spatial patterns were determined for relative differences between the site concentrations and average concentration of both pathogens across the study area. Mean relative differences were significantly different from zero at four of the six sites for S. enterica and three of six sites for L. monocytogenes. There was a similarity between the mean relative difference distribution between sites over growing season, nongrowing season, and the entire observation period. Mean relative differences were determined for temperature, oxidation-reduction potential, specific electrical conductance, pH, dissolved oxygen, turbidity, and cumulative rainfall. A moderate-to-strong Spearman correlation (rs > 0.657) was found between spatial patterns of S. enterica and 7-day rainfall, and between relative difference patterns of L. monocytogenes and temperature (rs = 0.885) and dissolved oxygen (rs = -0.885). Persistence in ranking sampling sites by the concentrations of the two pathogens was also observed. Finding spatially stable patterns in pathogen concentrations highlights spatiotemporal dynamics of these microorganisms across the study area can facilitate the design of an effective microbial water quality monitoring program for surface irrigation water.

Divergent seasonal responses of above- and below-ground to environmental factors in alpine grassland.

Under current global warming, the relationship between season changes of plants and environmental factors is focused on high-elevation and latitude regions. Due to the desynchronized growth of above- and below-ground and the buffering of soil, the driving factors in leaf and root show seasonal dynamics. We measured above- and below-ground intensity in the alpine steppe over the non-growing season (October-April) and growing season (May-September). Air temperature, precipitation, soil moisture, and soil temperature were used to analyze the correlation based on the growth rhythm. Results showed that an earlier growth in spring and a delayed dormancy in autumn of root than leaf was observed. Our results strongly suggest that soil moisture plays a more important role in leaf unfolding while temperature is consistent with the withering of the shoots. Soil moisture comes from soil melt driving the spring phenology of roots, which derived from the storage of the subsoil layer in the last autumn. Climate change will affect the strong seasonal patterns that characterized these precipitation-limited systems, especially in the spring and fall shoulder seasons. As seasonality changes in the alpine steppe, divergent responses of leaf and fine root would be explored.

Extreme temperature events reduced carbon uptake of a boreal forest ecosystem in Northeast China: Evidence from an 11-year eddy covariance observation.

Boreal forests, the second continental biome on Earth, are known for their massive carbon storage capacity and important role in the global carbon cycle. Comprehending the temporal dynamics and controlling factors of net ecosystem CO2 exchange (NEE) is critical for predicting how the carbon exchange in boreal forests will change in response to climate change. Therefore, based on long-term eddy covariance observations from 2008 to 2018, we evaluated the diurnal, seasonal, and interannual variations in the boreal forest ecosystem NEE in Northeast China and explored its environmental regulation. It was found that the boreal forest was a minor CO2 sink with an annual average NEE of -64.01 (± 24.23) g CO2 m-2 yr-1. The diurnal variation in the NEE of boreal forest during the growing season was considerably larger than that during the non-growing season, and carbon uptake peaked between 8:30 and 9:30 in the morning. The seasonal variation in NEE demonstrated a "U" shaped curve, and the carbon uptake peaked in July. On a half-hourly scale, photosynthetically active radiation and vapor pressure deficit had larger impacts on daytime NEE during the growing season. However, temperature had major control on NEE during the growing season at night and during the non-growing season. On a daily scale, temperature was the dominant factor controlling seasonal variation in NEE. Occurrence of extreme temperature days, especially extreme temperature events, would reduce boreal forest carbon uptake; interannual variation in NEE was substantially associated with the maximum CO2 uptake rate during the growing season. This study deepens our understanding of environmental controls on NEE at multiple timescales and provides a data basis for evaluating the global carbon budget.

Methane dynamics from a mixed plantation of north China: Observation using closed-path eddy covariance method.

Although an important greenhouse gas, methane flux in hilly forest ecosystems remains unclear. By using closed-path eddy covariance systems, methane flux was measured continuously from 2017 to 2019 in a mixed plantation in the Xiaolangdi area of the Yellow River in North China. The methane flux footprint and its diurnal and monthly variations were analysed, and its characteristics on rainy days are discussed. The results showed that: (a) the observation data were reliable with good spatial representation (b) The methane flux in the mixed plantation ecosystem had obvious diurnal and seasonal variations: the monthly average diurnal variation of the methane flux had a single-peak; the methane flux value was source in the daytime and sink at night. The daily mean maximum value of methane flux in growing season was lower than that in non-growing season with the maximum value appearing in March, and the minimum value in October. (c) The forest is an atmospheric CH4 source with the annual emission in 2017 of (3.31g C·m-2·year -1) >2019 (2.94g C·m-2·year-1) >2018 (2.81g C·m-2·year -1), and the main influencing factor was precipitation. Rainfall affected CH4 flux with a lag period of approximately three days. Rainfall also changed the balance of CH4 flux between sink or source according to precipitation intensity and frequency.

Seasonal differences in nitrogen deposition affect nitrogen content in mosses: Implications for biomonitoring

Nitrogen (N) pollution is a global threat to biodiversity. Moss biomonitoring is used to assess the levels of nitrogen pollution due to the high correlation between the nitrogen content in moss tissues (%N) and atmospheric N deposition. However, this relationship is controversial as %N can be affected by seasonal changes in climate and N depositions. This study aimed to investigate the effects of seasonal climate and N deposition patterns on %N in moss (Calohypnum plumiforme). Ten study sites in Japan were classified as “summer” and “winter” N deposition areas based on the season which received the highest N deposition (summer and winter, respectively). The %N showed a stronger positive correlation with N concentration than N amount in the wet deposition in both areas. These correlations exhibited seasonal differences and were not significant during the growing season with high precipitation and warm temperatures, or during the snowy season. Regression analyses revealed that winter and autumn N concentrations were the most important factors for determining the %N in the summer and winter N deposition areas, respectively. This may be due to: (1) the large contributions from high N concentrations in the initial rain on N uptake by mosses, (2) limitations in N uptake by mosses per rainfall event, (3) dilution of %N during growing seasons, and (4) low retention efficiency for snow N in mosses. The influence of the first and second factors on %N was noticeable in the summer group with large differences in seasonal precipitation, whereas the influence of the fourth factor was strongly reflected in the winter group with frequent snowfall. Based on these results, the %N can be determined not by the total N amount but by the N concentration in the wet deposition during the non-growing and snow-free seasons.

Impacts of summer-seeded legume cover crops and termination-tillage method on nitrogen availability to corn in rotation

Summer-seeding legume cover crops can reduce erosion, mitigate nitrate leaching losses during the non-growing season, and provide bio-accumulated nitrogen (N) to the following crop. Very little information is available, however, on the most effective methods for terminating summer-seeded cover crops, or on the amount of N they can provide to subsequent crops. We therefore evaluated the impacts of selected legume cover crops and cover crop termination-tillage methods on corn (Zea mays L.) N credit and grain yield in a soybean (Glycine max Merr.) - winter wheat (Triticum aestivum L.)/cover crop - corn rotation over 3 consecutive years on a clay loam soil in southwestern Ontario. The cover crops were summer-seeded after wheat harvest, and included hairy vetch (Vicia villosa L. Roth), red clover (Trifolium pratense L.), white clover plus crimson clover mix (incarnatum and Trifolium alba L.), and a no cover crop control (CK). The cover crops were nested within termination-tillage method, including moldboard plow-down in fall, and herbicide spray-down in spring under strip-tillage or no-tillage then corn planting. Nitrogen fertilizer to corn was applied via side-dress at 200 kg N ha-1 to the CKs, and at 100 kg N ha-1 to the cover crops. Averaged over 3 years, above-ground biomass N level followed the pattern: hairy vetch > white clover plus crimson clover mix > red clover for fall plow-down termination. There were no significant differences in cover crop biomass N levels for spring termination. Corn response to cover crop was significantly affected by termination-tillage method and cover crop species, with poor corn stands after no-tillage, red clover and white plus crimson clover. Fall plow-down termination provided a range in N credit of 80-85 kg N ha-1, which was significantly greater than the N credit under herbicide spray-down in spring. Under spring strip-tillage and no-tillage, hairy vetch produced significantly greater corn grain yields than red clover and white plus crimson clover mix.

Quantifying Temperate Forest Diversity by Integrating GEDI LiDAR and Multi-Temporal Sentinel-2 Imagery

Remotely sensed estimates of forest diversity have become increasingly important in assessing anthropogenic and natural disturbances and their effects on biodiversity under limited resources. Whereas field inventories and optical images are generally used to estimate forest diversity, studies that combine vertical structure information and multi-temporal phenological characteristics to accurately quantify diversity in large, heterogeneous forest areas are still lacking. In this study, combined with regression models, three different diversity indices, namely Simpson (λ), Shannon (H′), and Pielou (J′), were applied to characterize forest tree species diversity by using GEDI LiDAR data and Sentinel-2 imagery in temperate natural forest, northeast China. We used Mean Decrease Gini (MDG) and Boosted Regression Tree (BRT) to assess the importance of certain variables including monthly spectral bands, vegetation indices, foliage height diversity (FHD), and plant area index (PAI) of growing season and non-growing seasons (68 variables in total). We produced 12 forest diversity maps on three different diversity indices using four regression algorithms: Support Vector Machines (SVM), Random Forest (RF), K-Nearest Neighbors (KNN), and Lasso Regression (LR). Our study concluded that the most important variables are FHD, NDVI, NDWI, EVI, short-wave infrared (SWIR) and red-edge (RE) bands, especially in the growing season (May and June). In terms of algorithms, the estimation accuracies of the RF (averaged R2 = 0.79) and SVM (averaged R2 = 0.76) models outperformed the other models (R2 of KNN and LR are 0.68 and 0.57, respectively). The study demonstrates the accuracy of GEDI LiDAR data and multi-temporal Sentinel-2 images in estimating forest diversity over large areas, advancing the capacity to monitor and manage forest ecosystems.

A Trade-Off between the Growing Performance and Sowing Density of Sunflower (Helianthus annuus L.) under Fertigation in an Arid Saline Area

Sunflower is one of the pioneer crops cultivated in salt-affected arid areas. The influences of sowing density on the growth performance of this crop under fertigation conditions have not been well studied. This study arranged four sowing density treatments, 41,667, 35,714, 31,250, and 27,778 plants ha−1, marked as D30, D35, D40, and D45, respectively, to reveal the relationships between soil salinity, growth performance, and sowing density under drip fertigation conditions. The results showed that the electrical conductivity of saturated paste extracts (ECe) decreased during the growing seasons but increased on the topsoil during the non-growing seasons in all of the treatments. The sowing densities had remarkable influences on the ECe in the 0–40 cm soil layer (ECe-40). The average ECe-40 during the two seasons for treatments D30–D45 correspondingly decreased by 7.0%, 33.9%, 11.1%, and 15.8% when compared to the original value. The soil pH in the 0–40 cm soil layer during the two seasons for treatments D30–D45 correspondingly decreased by −0.03, 0.20, 0.20, and 0.27 when compared to the original value. Increasing the spacing in the rows could promote the stem diameter, plant biomass, and proportion of biomass allocated underground. The yield and related yield components in this experiment under fertigation were significantly higher than those under surface irrigation. A sowing density between 31,250 and 35,714 plants ha−1 could ensure both the high yield and high morphological quality of the seeds, which could be recommended for sunflower cultivation under drip fertigation conditions.

Effects of Grazing, Extreme Drought, Extreme Rainfall and Nitrogen Addition on Vegetation Characteristics and Productivity of Semiarid Grassland.

Grassland use patterns, water and nutrients are the main determinants of ecosystem structure and function in semiarid grasslands. However, few studies have reported how the interactive effects of rainfall changes and nitrogen deposition influence the recovery of semiarid grasslands degraded by grazing. In this study, a simulated grazing, increasing and decreasing rainfall, nitrogen deposition test platform was constructed, and the regulation mechanism of vegetation characteristics and productivity were studied. We found that grazing decreased plant community height (CWMheight) and litter and increased plant density. Increasing rainfall by 60% from May to August (+60%) increased CWMheight; decreasing rainfall by 60% from May to August (-60%) and by 100% from May to June (-60 d) decreased CWMheight and coverage; -60 d, +60% and increasing rainfall by 100% from May to June (+60 d) increased plant density; -60% increased the Simpson dominance index (D index) but decreased the Shannon-Wiener diversity index (H index); -60 d decreased the aboveground biomass (ABG), and -60% increased the underground biomass (BGB) in the 10-60 cm layer. Nitrogen addition decreased species richness and the D index and increased the H index and AGB. Rainfall and soil nitrogen directly affect AGB; grazing and rainfall can also indirectly affect AGB by inducing changes in CWMheight; grazing indirectly affects BGB by affecting plant density and soil nitrogen. The results of this study showed that in the semiarid grassland of Inner Mongolia, grazing in the nongrowing season and grazing prohibition in the growing season can promote grassland recovery, continuous drought in the early growing season will have dramatic impacts on productivity, nitrogen addition has a certain impact on the species composition of vegetation, and the impact on productivity will not appear in the short term.

Asymmetric responses of leaf litter decomposition to precipitation changes in global terrestrial ecosystem

Climate change-induced alterations in precipitation regimes can substantially impact litter decomposition and consequently affect carbon (C) and nutrient cycling in terrestrial ecosystems. However, the response patterns of litter decomposition to precipitation changes and their controlling factors remain ambiguous. We conducted a global meta-analysis to examine the responses of leaf litter decomposition and their sensitivities to increased precipitation (IP) and decreased precipitation (DP) based on 68 publications with 2940 paired observations in field litterbag experiments. We further explored how responses of litter decomposition varied with the intensity and timing of precipitation changes, ecosystem type, climatic conditions, litter traits, decomposition duration, and soil fauna. Litter decomposition showed double-asymmetric responses to precipitation changes. On a global scale, IP enhanced litter decomposition on average by 16%, whereas DP reduced litter decomposition by 21%. Litter decomposition was more sensitive to IP than DP at low precipitation changes but exhibited a greater response to DP than IP at extreme precipitation changes. In addition, litter decomposition in forests showed a stronger response to precipitation changes than that in other ecosystems, and litter decomposition was more sensitive to precipitation change under warm and wet conditions. Compared to precipitation changes in the growing season, litter decomposition showed a stronger response to IP but exhibited no response to DP in the non-growing season. The impacts of precipitation change gradually decreased with ongoing litter decomposition. Moreover, the initial lignin concentration in litter primarily determined the responses of litter decomposition to precipitation changes, with litter mixture and soil fauna partially mitigating the precipitation change effects. Our meta-analysis extends the double asymmetry model regarding the responses of litter decomposition to precipitation changes, which vary with ecosystem type, climatic conditions, litter chemical traits, decomposition duration, and soil fauna. These findings improve our ability to project the responses of terrestrial C and nutrient cycling to global precipitation changes.

Habitat heterogeneity and biotic interactions mediate climate influences on seedling survival in a temperate forest

Seedling stage has long been recognized as the bottleneck of forest regeneration, and the biotic and abiotic processes that dominate at seedling stage largely affect the dynamics of forest. Seedlings might be particularly vulnerable to climate stress, so elucidating the role of interannual climate variation in fostering community dynamics is crucial to understanding the response of forest to climate change. Using seedling survival data of 69 woody species collected for five consecutive years from a 25-ha permanent plot in a temperate deciduous forest, we identified the effects of biotic interactions and habitat factors on seedling survival, and examined how those effects changed over time. We found that interannual climate variations, followed by biotic interactions and habitat conditions, were the most significant predictors of seedling survival. Understory light showed a positive impact on seedling mortality, and seedling survival responded differently to soil and air temperature. Effects of conspecific neighbor density were significantly strengthened with the increase of maximum air temperature and vapor pressure deficits in the growing season, but were weakened by increased maximum soil temperature and precipitation in the non-growing season. Surprisingly, seedling survival was strongly correlated with interannual climate variability at all life stages, and the strength of the correlation increased with seedling age. In addition, the importance of biotic and abiotic factors on seedling survival differed significantly among species-trait groups. Thus, the neighborhood-mediated effects on mortality might be significantly contributing or even inverting the direct effects of varying abiotic conditions on seedling survival, and density-dependent effects could not be the only important factor influencing seedling survival at an early stage.

Relationship between climate and land cover change in Aral Sea Basin

In the past several decades, substantial changes were observed in Central Asia’s land cover. Water-sensitive Central Asia has experienced an increase in farming and expansion of urban areas. These are considered the main reasons for water level reduction in the Aral Sea. The disappearance of the Aral Sea has not only affected the climate of the region but also caused regional land cover changes. In this article, we analyse the temporal variation of the Normalized Difference Vegetation Index (NDVI) and its correlation with climatic variables in the territory of the Aral Sea Basin from 1982 to 2015 using Global Inventory Modelling and Mapping Studies (GIMMS) and Moderate Resolution Imaging Spectroradiometer (MODIS). The results indicate that the mean annual NDVI value recorded a weak positive trend of 0.0023/10a over the last 34 years. The Hurst index is used to test whether the tendency observed in past can be extrapolated in the future or not. Our results showed, the Hurst exponent indicates that the vegetation dynamic trend was consistent, which means that NDVI values will continue to rise in the future. During the study period, precipitation and surface soil moisture increased in the growing season, which affected the temperature during the non-growing season.

Responses to Climate Factors in the Radial Growth of Schrenk Spruce (Picea Schrenkiana) in the Yili Region, Northwest China

Abstract To examine the associations existing between elevation and tree growth, tree-ring width chronologies of Schrenk spruce (Picea schrenkiana) were developed from sampling sites at different elevations in the Wusun Mountain, Yili region, Xinjiang, northwest China. The radial growth of high-elevation trees was observed to be primarily influenced by meteorological factors in the non-growth season of the previous year. The radial growth of mid-elevation trees was mainly affected by temperature. The radial growth of low-elevation trees was mainly affected by both temperature and relative humidity. About climate change, the radial growth of Schrenk spruce at different elevations showed a non-uniform response to climate variables. Under the condition of intensified drought stress in the future, the forest ecosystem is likely to be greatly affected, and the radial growth of Schrenk spruce at low elevations will be especially inhibited.

Seasonal nitrogen and phosphorus leaching in urban agriculture: Dominance of non-growing season losses in a Southern Swedish case study

Urban agriculture, as most agriculture, can potentially contribute to eutrophication via losses to ground and surface water. Few published studies have empirically measured nitrogen and phosphorus losses (including leaching) from urban agriculture, and even fewer have examined losses in real-world settings throughout the year. Here we investigated year-round (May 2020–2021) weekly nitrogen and phosphorus leaching from allotment gardens in Linköping, southern Sweden. We installed eight lysimeters (8 plots) and collected water 0.3 m below the soil surface in four gardens (2 plots per garden), each with their own gardening practices (organic fertilizers, irrigation, and crops). The gardens exhibited large nutrient leaching per area cultivated compared to observed nutrient leachate in rural agriculture in similar climates. There was a large variability among studied plots, where nitrogen leaching reached 39–191 kg ha−1 y−1 and phosphorus 0.9–2.4 kg ha−2 y−1. Importantly, the non-growing season, especially snowmelt, was a key period for leaching. Most of the nitrogen (78–91 %) and phosphorus (45–97 %) leaching occurred from November to April when the soil was bare, suggesting that mineralization of organic matter was important. Three of the gardens received high amounts of organic fertilizers, though no clear relation between inputs and leaching could be discerned. One plot deviated from the pattern, with less than 40 % of the nutrient leaching occurring in the non-growing season. This gardener had a fine net covering the plot to deter insects. This protected from precipitation as the water volume collected was the lowest, with only 26 % collected in the non-growing season, and nitrogen leaching was also the lowest. Our results illustrate that additional monitoring studies should occur year-round and in several gardens to account for high temporal and spatial heterogeneity and avoid under-estimating leaching losses from urban agriculture. Providing guidance on fertilization, irrigation, and soil covering may be a way to minimize leaching.

Combined effects of tree canopy composition, landscape location, and growing season on Nothofagus forest seeding patterns in Southern Patagonia

The ecological mechanisms that regulate seeding processes in temperate forests are poorly understood especially in the Southern Hemisphere. We studied the effect of different growing season metrics determined by the Normalized Difference Vegetation Index (NDVI) across eight annual periods between 2012 and 2020. We analyzed the seed production and biomass of two Nothofagus species in Tierra del Fuego (Argentina) to develop new tools for ecological forest management strategies. We focused on three forest types in coast and mountain landscapes: (i) pure deciduous N. pumilio, (ii) pure evergreen N. betuloides, and (iii) mixed forests with a similar proportion of both species. We performed generalized linear models for each species to analyze the influence of the growing season metrics and regional climate patterns (temperature and rainfall) considering forest types on coasts and mountains as main factors and analyzing seed production and biomass. Seed production and biomass annually varied with tree species growing at different landscape locations, with marked synchrony for each forest type, but not between different species or landscape locations (e.g. deciduous and evergreen species presented different synchrony among years). Seed production and biomass were higher when NDVI-peak was high in N. pumilio and when the growing season end was late in N. betuloides. For N. pumilio, the seed production and biomass were significantly leaded by growing season minimum temperature and growing season rainfall in coastal and mountain forests. For N. betuloides, the higher seed production and biomass were significantly influenced by non-growing season maximum temperature, growing season rainfall and non-growing season rainfall in coastal and mountain forests. These outputs highlight the importance of considering the fluctuation of the growing season in the modeling of seed production and biomass. Forest management guidelines should forecast rainy years that may benefit seedling recruitment after stand management or conservation efforts of target species (e.g. simulations of population dynamics at stand-level, decisions about passive vs active restoration in degraded forests for recover of the natural ecosystem functions).

Ratio of non-growing season to growing season N2O emissions in Canadian croplands: an update to national inventory methodology

National inventory reporting of agricultural nitrous oxide (N2O) emissions in Canada is based primarily on measurements obtained using static chambers. In regions with cold winters and an accumulated snowpack (including Canada), these measurements tend to focus on the growing season (typically May–October). However, research has shown that emissions continue throughout the non-growing season (NGS) and that these account for a significant proportion of annual emissions. In the Canadian National Inventory NGS emissions currently are assumed to be adequately captured in western Canada, while they are accounted for in eastern Canada by multiplying the growing season emissions by a correction factor of 1.4, a value that was derived based on a limited number of measurements. Here we use recent Canadian studies to validate this correction factor. We collected data from available Canadian studies that measured soil N2O emissions from agricultural systems for the entire year and determined the proportion of these emissions that occurred during the NGS. The proportion of annual N2O emissions that occurred during the NGS varied widely, ranging from −4% to 119% with a mean of 35.5%, compared to the previous estimate of 30%. Due to high variability, few differences were observed between means associated with climatic, soil, and management variables. To correct for NGS N2O emissions from Canadian agricultural soils, we suggest that the current correction factor for converting growing season to total annual emissions be changed from 1.4 to 1.55 and that this be used for all agricultural soils in Canada rather than just eastern Canada.

Do cover crops on the Canadian prairies affect soil nitrogen cycling?

For one of Canada’s most important regions of crop production—the prairies—it’s uncertain if cover crops can be successfully integrated into rotations; if so, will soil nitrogen (N) cycling be influenced to benefit main crops? To address these gaps, we compared a crop rotation with cover crops (CC) vs. without cover crops (LR) from 2018 to 2021 in Saskatoon, SK. The main crops were grown in sequence of wheat–canola–potato–pea; the cover crops included red clover, berseem clover/oat mix, fall rye, and a brassica cover. Yield and aboveground biomass were collected each year and analyzed to determine crop yield and N use efficiency (NUE). Soil N availability was monitored in various ways, that is, by assessing pre-plant soil nitrate, soil inorganic N (SIN) supply rate, and potentially mineralizable N (PMN). We found that the influence on soil N dynamics was restricted to the non-growing season where cover crops reduced SIN supply rate and nitrate content compared to the conventional practice without cover crops. Yet, rotations with vs. without cover crop did not differ in crop NUEs, yields, or in-season N dynamics. We found some evidence that diversifying rotations with cover crops may help the system to function more like perennial systems in terms of regulating N in the long run; but had limited impact during the three years studied. To ensure that cover crops are effective and functional on the prairies, innovative design approaches are needed to adapt cover crops to reach soil health goals under prairie conditions.

Estimation of Heat Released from Fire Based on Combustible Load in Inner Mongolian Grasslands

The grasslands of Inner Mongolia are prone to wildfires, which can endanger the grassland ecosystem, as well as people’s lives and property. The amount of heat released by grassland fires must be determined for the quantitative evaluation of grassland fires. On the basis of a field survey of combustible load and an indoor heat release experiment, together with the acquisition of NDVI and fire area data, this study evaluated the amount of combustible load, plant heat release, potential heat release, and fire-caused heat release in Inner Mongolia grasslands. The following results were obtained: (1) The models for determining Inner Mongolia’s combustible load (Y) per unit area throughout the growing and nongrowing seasons were Yi = 412.74NDVIi1.5917 and Yj = −7.21tj + Y10 (i represents May–October of the growing season; j represents November–April of the following year). In the northern temperate zones, grasslands and meadows showed a decline in combustible load per unit area. The interannual combustible load variance increased between 2001 and 2016; (2) the per square meter average heat release of Stipa capillata, Cleistogenes squarrosa, Carex doniana, Leymus chinensis, and other plants was 0.51, 0.18, 0.17, 0.3, and 1.42 MJ/g, respectively. Unit weights were released at 2.13, 1.77, 2.06, 1.9, and 3.99 MJ/m2; (3) from 2001 to 2016, Inner Mongolia’s grassland fires predominantly occurred in northern temperate grasslands and meadows. Over the 16 year period, the total heat emission was 1.01 × 1012 MJ, with variable decreasing trends noted in spring and fall. The main practical objectives of this paper were to provide basic data for fire spread modeling and suggest more scientific and effective fire management methods for the future.

Dynamic changes in soil moisture in three typical landscapes of the Heihe River Basin

The water diversion scheme of Heihe River Basin was implemented in 2000. Herein, we investigated the dynamic changes in soil moisture content and analyzed the fundamental reasons supporting the water diversion plan in this typical inland river basin in northwest China. Accordingly, we selected three typical landscape gradients—a mountain water conservation forest belt, an artificial sand-fixing forest belt at the edge of a desert oasis, and a desert riparian forest belt in the upper, middle, and lower reaches of the Heihe River Basin, respectively. In these diverse landscapes, an environmental measuring system was used to continuously monitor the dynamic and differentiation regularity of soil moisture in the 0–160 cm layer for 5 years. The results revealed that 1) the soil moisture content in each landscape increased and varied significantly across seasons. In the upper and middle reaches, the soil moisture content was significantly higher during the growing season than in the non-growing season, whereas the lower reach displayed a converse pattern. 2) The distribution of soil moisture at various depths of soil profiles varied significantly for each landscape. As the soil depth increased, the soil moisture in the upper reach decreased. Although the deep layers (120–160 cm) could store water in the non-growing season, the stored water was consumed during the growing season. The soil moisture content in the mid-reach initially increased, later decreased, and ultimately attained its highest level at 40–60 cm; however, the soil moisture content in the lower reach increased, and reached its highest level at 120–160 cm. 3) In the upper and middle reaches, the coefficient of variation of soil moisture decreased with an increasing soil depth, whereas the lower reach exhibited a converse trend. Similarly, the coefficient of variation of soil water storage were higher during the non-growing season than during the growing season within the upper and middle reaches, whereas an opposite trend was observed in the lower reach. 4) Since the implementation of the Heihe river water diversion plan, the soil moisture content in both the upper and middle reaches have increased but that of the lower reach has fluctuated and declined, especially in the shallow depths during the growing season. The present findings imply that the lower reaches of the Heihe River may require additional water transfers during the growing season.

Quantifying understory vegetation density using multi-temporal Sentinel-2 and GEDI LiDAR data

ABSTRACT Understory vegetation contributes considerably to biodiversity and total aboveground biomass of forest ecosystems. Whereas field inventories and LiDAR data are generally used to estimate understory vegetation density, methods for large-scale and spatially continuous estimation of understory vegetation density are still lacking. For an evergreen coniferous forest area in southern China, we developed and tested an effective and practical remote sensing-driven approach for mapping understory vegetation, based on phenological differences between over and understory vegetation. Specifically, we used plant area volume density (PAVD) calculations based on GEDI data to train a support vector regression model and subsequently estimated the understory vegetation density from Sentinel-2 derived metrics. We produced maps of PAVD for the growing and non-growing season respectively, both performing well compared against independent GEDI samples (R2 = 0.89 and 0.93, p < 0.01). Understory vegetation density was derived from the differences in PAVD between the growing and non-growing season. The understory vegetation density map was validated against field samples from 86 plots showing an overall R2 of 0.52 (p < 0.01), rRMSE = 21%. Our study developed a tangible approach to map spatially continuous understory vegetation density with the combination of GEDI LiDAR data and Sentinel-2 imagery, showing the potential to improve the estimation of terrestrial carbon storage and better understand forest ecosystem processes across larger areas.