Early Spring Precipitation Research Articles

Weakened Subtropical Westerlies and Their Deflection by the Tibetan Plateau Contribute to Drying Southeastern China in Early Spring

AbstractAn obvious long‐term drying trend in recent early springs (February–March–April) is observed over southeastern China (SEC). Here, we attribute this drying to the weakened subtropical westerlies and deflected by the Tibetan Plateau (TP). Climatologically, the low‐level southwesterlies at the southeastern margin of the TP, a branch of the upstream subtropical westerly jet deflected by the TP terrain, bring water vapor to SEC and the southerlies move upward over SEC mainly through isentropic gliding mechanism, inducing persistent precipitation in early spring. However, the subtropical westerlies weakened significantly in recent decades due potentially to the decreased Eurasian snow cover. Consequently, an easterly trend appears along the southern margin of the TP with anomalous northeasterlies over SEC. These northeasterlies suppress both moisture supply and upward motions over SEC, and reduce regional early spring precipitation. Our results highlight the interaction between the TP terrain and the weakened subtropical westerlies that leads to the drying SEC.

An Unexpected Decline in Spring Atmospheric Humidity in the Interior Southwestern United States and Implications for Forest Fires

Abstract On seasonal time scales, vapor pressure deficit (VPD) is a known predictor of burned area in the southwestern United States (“the Southwest”). VPD increases with atmospheric warming due to the exponential relationship between temperature and saturation vapor pressure. Another control on VPD is specific humidity, such that increases in specific humidity can counteract temperature-driven increases in VPD. Unexpectedly, despite the increased capacity of a warmer atmosphere to hold water vapor, near-surface specific humidity decreased from 1970 to 2019 in much of the Southwest, particularly in spring, summer, and fall. Here, we identify declining near-surface humidity from 1970 to 2019 in the southwestern United States with both reanalysis and in situ station data. Focusing on the interior Southwest in the months preceding the summer forest fire season, we explain the decline in terms of changes in atmospheric circulation and moisture fluxes between the surface and the atmosphere. We find that an early spring decline in precipitation in the interior region induced a decline in soil moisture and evapotranspiration, drying the lower troposphere in summer. This prior season precipitation decline is in turn related to a trend toward a Northern Hemisphere stationary wave pattern. Finally, using fixed humidity scenarios and the observed exponential relationship between VPD and burned forest area, we estimate that with no increase in temperature at all, the humidity decline alone would still lead to nearly one-quarter of the observed VPD-induced increase in burned area over 1984–2019. Significance Statement Burned forest area has increased significantly in the southwestern United States in recent decades, driven in part by an increase in atmospheric aridity [vapor pressure deficit (VPD)]. Increases in VPD can be caused by a combination of increasing temperature and decreasing specific humidity. As the atmosphere warms with climate change, its capacity to hold moisture increases. Despite this, there is a decrease in near-surface air humidity in the interior southwestern United States over 1970–2019, which during the summer is likely caused by a decline in early spring precipitation leading to limited soil moisture and evaporation in spring and summer. We estimate that this declining humidity alone, without an increase in temperature, would cause about one-quarter of the VPD-induced increase in burned forest area in this region over 1984–2019.

Weakened Subtropical Westerlies Reduced Early Spring Precipitation in the Southeast Tibetan Plateau

Abstract The Southeast Tibetan Plateau (SETP) is a major region where many low-latitude glaciers are located, with spring precipitation being a major input of the glacier mass balance. This study shows that early spring precipitation has decreased significantly since 1999, which is attributed to declined moisture contribution from the far-field sources (west of 70°E) induced by the weakened subtropical westerlies. The possible physical mechanism underlying this change has also been revealed. It is found that snow-cover extent (SCE) in March reduced in midlatitude Eurasia after 1999; meanwhile, strong solar radiation during this month may have exacerbated snow melting through snow albedo–radiation interactions. These two processes led to warming and caused a strong anticyclone over midlatitude Eurasia that weakened the subtropical westerlies near 30°N. This decadal change in the subtropical westerlies led to a decrease in moisture transport upstream. As a result, the windward slopes of large terrain along the latitudinal belt near 30°N received less precipitation, and the decrease in SETP precipitation was part of this change. A further analysis shows that the positive correlation between the westerlies and precipitation has weakened since 1999. Significance Statement The purpose of this study is to reveal the decreased early spring precipitation and explore its possible physical mechanism in the Southeast Tibetan Plateau (SETP), which is crucial to understand the shrinkage of the local glacier. Our results indicate that the reduction of snow cover in midlatitude Eurasia since 1999 and the strong solar radiation in March contributed to the weakening subtropical westerlies, which further resulted in the decreasing precipitation in the SETP and other windward slopes of large terrain along the latitudinal 30°N belt in Eurasia.

Site-contingent responses to drought of core and relict Tetraclinis articulata populations from Morocco and Spain

The growth responses to climate variability are still unknown in locally threatened conifers from dry regions, but this information is necessary for improving the conservation of relict populations under increasing aridification. We characterized the radial growth patterns and responses to climate of Tetraclinis articulata, a Cupressaceae tree endemic to the western Mediterranean Basin, in a relict population located in southwestern Spain (Doñana) and two populations from the northern Morocco where the species core habitat is found (Tétouan, Ifarten). We assessed climate-growth relationships by using tree-ring width, climate data, drought and North Atlantic Oscillation (NAO) indices. Climate-growth analyses were refined using the climwin R package to select the most informative statistical models. The main climatic constraints of growth were inferred by using the process-based Vaganov-Shashkin (VS) model explicitly considering non-linear climate-growth relationships. Tetraclinis articulata growth was favored by wet conditions from the prior autumn to the spring of the growth year. In Doñana, warmer May conditions led to growth decline but this negative effect could be reversed by wet-warm conditions in the prior late autumn. Growth in the two Moroccan sites was constrained by 6- to 18-month long droughts peaking in summer, which account for cumulative water deficit since the previous autumn. Winter and early spring precipitation were the main climate drivers of growth in the Moroccan T. articulata populations, and their year-to-year variability was linked to the NAO. The VS model simulations showed that T. articulata growth is enhanced by wet soil conditions in late winter and early spring, probably recharging shallow soil water pools. The VS model also confirmed that warmer spring-summer conditions could amplify drought stress and threaten the long-term persistence of the relict Doñana population.

The capability of CMIP6 models on seasonal precipitation extremes over Central Asia

The accurate extreme precipitation simulations of the GCMs during the historical period are more likely to offer reliable forecasts in the future. In this study, we evaluated the abilities of 22 GCMs of CMIP6 in simulating extreme precipitation over Central Asia in the early spring, summer, and late winter, when extreme precipitation can be very threatening. Based on seven extreme precipitation indices, robust skill score techniques such as Taylor diagrams and DISO (Distance between Indices of Simulation and Observation) are employed to rank the overall model performance. The results show that all models have difficulties in simulating the overall spatial patterns of all extreme precipitation in early spring and late winter. Differently, in summer, most of them can fairly well capture the spatial pattern of PRCPTOT and the CDD. Most models show consistent bias of extreme precipitation indices in early spring and late winter, with overall overestimated CDD and CWD and underestimated SDII, Rx5day, R95PTOT, and R10mm. However, there is high inter-model variability in the regional bias of summer, and the areas of inconsistency that emerge always contain most of Xinjiang or its surrounding mountain regions, possibly due to a lack of topographic correction in these areas with very complex topography. The model ensemble simulations generally outperform the individual models in terms of CC (Correlation coefficient) and DISO, while the improvement is limited regarding RMS (Centered mean square error) and RSD (Relative standard deviations). This study may serve as a reference to further investigate the sources of persistent systematic biases in current GCMs over Central Asia, which is expected to assist policymakers in adopting strategies reflecting the state of scientific understanding of the likelihood.

Tree-Ring Patterns and Growth Response of Abies jaliscana to Climate along Elevational Gradients in the Mountains of Western Jalisco, Mexico

Jalisco fir (Abies jaliscana) is an endemic tree with restricted distribution, and is considered a Pliocene relict inhabiting the cloud montane forests in the humid-temperate upper zone of western Jalisco Mountains. Despite its ecological importance, to date, there are no studies for this taxon aimed to understand its vulnerability to regional climate variability. We used dendrochronological techniques to investigate tree-ring patterns and the effects of climate variability on the radial growth of Jalisco fir through its elevational range in both southern and northern aspects. We developed tree-ring series that ranged in length from 79 to 152 years. High-elevation trees (2400 m.a.s.l.) showed higher annual radial growth compared to those present in low-elevation sites. Over the last 150 years, Abies jaliscana trees have had higher annual radial growth rates compared to those exhibited by other Abies species. Both winter and early spring precipitation are the main factors determining the growth rates at all the elevation sites for both aspects. In the cold season, mean maximum temperature negatively affected ring-width growth at most of the elevation zones except for two study sites. We conclude that the radial growth of A. jaliscana appears to be smaller during lower rainfall periods in the dry seasons (winter and early spring), and A. jaliscana annual radial growth responds differently to regional climate variability along the elevational gradient and mountain aspects.

Topographical and Thermal Forcing in Favorable Circulation Pattern to Early Spring Precipitation over the Southeastern Tibetan Plateau

During the boreal spring (March–May), the precipitation that occurs from March over the southeastern Tibetan Plateau (TP) can account for 20–40% of the total annual amount. The origin of this phenomenon has not been clearly understood from a climatological perspective. In this study, the role of topographical and thermal forcing on the precipitation over the southeastern TP in early spring (March) was investigated through sensitivity numerical simulations based on general circulation model. The simulated results show the favorable circulation and static stability to early spring precipitation over the southeastern TP when the model is simultaneously forced by realistic topography, zonal symmetric radiative equilibrium temperature, and diabatic heating over the TP and its surrounding areas. The quasi-stationary wave pattern over the Eurasian continent forced by realistic and TP topographical forcing leads to prolonged low pressure and intensified zonal winds over the southeastern TP due to quasi-steady wave activities. Thermal forcing experiments reveals that sensible heating over the southeastern TP not only strengthens the cyclonic circulation, ascending motion and statically unstable over the southeastern TP through thermal adaptation and the Sverdrup balance, but also triggers an anticyclone at upper tropospheric level extending from north of the Bay of Bengal to the eastern TP, which further favors precipitation over the southeastern TP. This work will provide useful background information for spring climate prediction over the TP.

Climate warming and extended droughts drive establishment and growth dynamics in temperate grassland plants

Current climate warming and extended droughts have major impacts on plant performance, with consequences for biodiversity and ecosystem functioning. However, unlike in trees, little is known about species-specific responses in grassland plants and the role of their different life histories in mitigating climate change impacts. We studied the climate-related plant establishment and growth dynamics over the last thirty years in four Central European grassland species with contrasting rooting depth, seed mass and xylem anatomy, and hence ability to cope with drought-induced restriction on establishment and growth. To assess the annual and seasonal dependency on fluctuating temperature and precipitation, we reconstructed establishment and growth chronologies from 174 individuals spanning 1–29 years, and more than 2,500 annual growth increments. We identified contrasting climatic controls of establishment and growth among focal species, which were likely related to their different traits. The establishment of deep-rooted, heavy-seeded species profited from increased winter and early spring precipitation, while the establishment of shallow-rooted, light-seeded species was weakly influenced by climate fluctuations. However, their growth was more adversely affected by high summer temperatures and drought than the growth of deep-rooted species. Our results show that the change towards a warmer and drier climate over the past thirty years profoundly reduced the growth and establishment of studied grassland plants. Most individuals established in the wet period of the 2000s, but the establishment reduced considerably during the dry and warm period of the 2010s. Our results provide novel insights into species-specific climate dependency of plant establishment, growth and population dynamics, suggesting the high vulnerability of Central European grasslands to climate warming.

Solar activity modulates the El Niño‐Southern Oscillation‐induced precipitation anomalies over southern China in early spring

AbstractThe present study identifies an evident solar cycle modulation upon the spatial pattern of the El Niño‐Southern Oscillation (ENSO) impact on the southern China precipitation in early spring (February–March). During high/low solar activity (HS/LS) years, the significant positive correlation between the wintertime Niño 3.4 index and the precipitation in the following early spring mainly locates at the southeastern China/Yangtze River Valley (SEC/YRV). Our analysis also suggests that the precipitation anomalies show asymmetries between ENSO warm and cold events under both solar phases. In HS years, the ENSO warm events‐related rainfall anomalies over the southern China are weak, but during the ENSO cold events significant dry anomalies appear at the SEC region. In LS years, significant wet anomalies over the YRV region are found during the ENSO warm events, whilst the rainfall anomalies are insignificant for the ENSO cold events. These modulations on precipitation are mediated by the changes in the ENSO‐associated anomalous western North Pacific anticyclone/cyclone, which may be attributed to the solar modulations on the ENSO‐associated sea surface temperature anomalies. Our results highlight that the level of solar activity should be taken into account when ENSO is used as a predictor for the precipitation over East Asia. [Correction added on 18 June 2021, after first online publication: The text for the abstract and graphical abstract were interchanged in the original‐published article. This updated version rectifies the error.]

A 20 y Analysis of Weather and Management Effects on a Small White Lady's-slipper (Cypripedium candidum) Population in Manitoba

The small white lady's-slipper, Cypripedium candidum, is a rare perennial orchid with a limited distribution in Canada, occurring as isolated populations in remnant tallgrass prairie in southern Manitoba and Ontario. The species is listed as endangered in both provinces and as threatened federally. Despite its status, information on how environmental conditions and land management affect population size and persistence of this species is limited. We used 20 y of monitoring data collected for a subset of the largest population in Canada to evaluate the response of small white lady's-slipper to land management and weather. Long-term monitoring suggests the population is in decline and may not persist under the current climate and management regime. Temperature appears to regulate vegetative growth and flowering proximately. Warm temperatures early in the spring, when shoots are emerging, appear favored, but high temperatures during anthesis appear detrimental, reducing both vegetative growth and flowering. In contrast, precipitation appears to have a lag effect on growth and flowering. However, snow depth was identified as a positive influence on vegetative growth, suggesting precipitation in early spring, when shoots are emerging, is also important for above-ground growth. Some grazing appears to benefit the species presumably by reducing competition and shading, but frequent grazing may increase the risk of direct damage to individuals from cattle consumption and trampling and does not provide sufficient time for individuals to recover following grazing events. Our findings add to the knowledge of orchid conservation and management, highlighting the importance of long-term monitoring in detecting population trends in species with erratic life cycles and fluctuating populations, such as the small white lady's-slipper.

A 20 y Analysis of Weather and Management Effects on a Small White Lady's-slipper (Cypripedium candidum) Population in Manitoba

The small white lady's-slipper, Cypripedium candidum, is a rare perennial orchid with a limited distribution in Canada, occurring as isolated populations in remnant tallgrass prairie in southern Manitoba and Ontario. The species is listed as endangered in both provinces and as threatened federally. Despite its status, information on how environmental conditions and land management affect population size and persistence of this species is limited. We used 20 y of monitoring data collected for a subset of the largest population in Canada to evaluate the response of small white lady's-slipper to land management and weather. Long-term monitoring suggests the population is in decline and may not persist under the current climate and management regime. Temperature appears to regulate vegetative growth and flowering proximately. Warm temperatures early in the spring, when shoots are emerging, appear favored, but high temperatures during anthesis appear detrimental, reducing both vegetative growth and flowering. In contrast, precipitation appears to have a lag effect on growth and flowering. However, snow depth was identified as a positive influence on vegetative growth, suggesting precipitation in early spring, when shoots are emerging, is also important for above-ground growth. Some grazing appears to benefit the species presumably by reducing competition and shading, but frequent grazing may increase the risk of direct damage to individuals from cattle consumption and trampling and does not provide sufficient time for individuals to recover following grazing events. Our findings add to the knowledge of orchid conservation and management, highlighting the importance of long-term monitoring in detecting population trends in species with erratic life cycles and fluctuating populations, such as the small white lady's-slipper.

Radial growth responses of two dominant conifers to climate in the Altai Mountains, Central Asia

The boreal forests of Central Asia play a vital role in biodiversity protection and regional economic development. It is important to study potential changes in the growth dynamics of boreal species in a context of global change. In this study, we developed a network of 34 tree-ring chronologies for two tree species, Siberian larch (Larix sibirica Ledeb.) and Siberian pine (Pinus sibirica Du Tour). The network extended across a large latitudinal gradient (45°N to 55°N). Principal component analysis (PCA) was used to detect spatial patterns in tree radial growth during a common period 1943–2004. Results indicated an obvious clustering pattern with chronologies being divided into a northeastern (NR) and a southwestern (SR) region. Bootstrapped correlation analyses of regional climate versus aggregated chronologies showed that tree radial growth in both regions was positively associated with summer temperature (June and July). Tree radial growth in the northeastern region was however positively associated with early spring precipitation and spring Palmer Drought Severity Index (PDSI) whereas, in the southwestern region, it was characterized by negative correlations with early summer precipitation and summer PDSI. The warm pool El Niño-Southern Oscillation (WP ENSO) and North Atlantic Oscillation (NAO) regulated tree radial growth through their influence on regional precipitation and temperature. Results suggest that tree radial growth in the region may decline with future projected climate change. This study provides a more comprehensive understanding to tree growth-climate associations across Central Asia.

Middle East Climate Response to the Saharan Vegetation Collapse during the Mid-Holocene

AbstractUnderstanding climate change in the Middle East (ME) is crucial because people’s living environment depends on rain-fed crop systems. It remains unclear whether the ME climate would be affected by the Saharan vegetation collapse at the end of the mid-Holocene (MH). Proxy data suggest a transition from humid to more arid ME conditions during the period of 6.5–5 kyr BP. Using a set of idealized sensitivity experiments with an Earth system model (EC-Earth), we infer that the shift of Saharan vegetation plays a role in this wet-to-dry transition over the ME. The experimental results show that the Saharan greening can significantly increase the late winter and early spring precipitation over the ME. The reason is that the vegetation decreases the surface albedo, which induces a warming in North Africa and generation of an anomalous low-level cyclonic flow, which transports moisture from tropical North Africa and the Red Sea to the ME. The moisture also flows from the Mediterranean Sea region to the ME through the enhanced mid- to upper-level westerlies. The enhanced moisture carried by westerly and southwesterly flows is lifted upon reaching Mesopotamia and the Zagros Mountains, substantially increasing the precipitation there. When the Sahara greening is removed, a drier condition happens in the ME. The crop model simulation further shows a substantial decrease in wheat yield in Mesopotamia with the reduction of Saharan vegetation, which is consistent with paleoclimatic reconstructions. These results imply that future changes in Saharan land cover may have climatic and agricultural impacts in the Middle East.

Flowering season of vernal herbs is shortened at elevated temperatures with reduced precipitation in early spring

Vernal herbs are exposed to the risk of climate change under spring frost and canopy closure. Although vernal herbs contribute to the biodiversity of the understorey layer in temperate forests, few studies assessed the effect of climate change on the phenology of the herbs. To examine phenological shifts in flowering seasons of vernal herb species caused by climate change, a greenhouse experiment was conducted using four species (Adonis amurensis, Hepatica nobilis var. japonica, Viola phalacrocarpa, and Pulsatilla cernua) under two temperature conditions (ambient or elevated temperature) and two precipitation conditions (convective or reduced precipitation). Experimental warming advanced overall aspects of the flowering timing including the first and last day of flowering. The growth of flowering stalk was also promoted by elevated temperature. Effects of decreased precipitation varied among species, which advanced the last day of the flowering of the later flowering species. Consequently, a decrease in overall flowering period length was observed. These results indicate that overall, climate change results in a shortening of the flowering season of vernal herb species, specifically at a higher temperature and under conditions of less precipitation.

Warming and precipitation addition interact to affect plant spring phenology in alpine meadows on the central Qinghai-Tibetan Plateau

Temperature and precipitation are primary regulators of plant phenology. However, our knowledge of how these factors might interact to affect plant phenology is incomplete. The Qinghai-Tibetan Plateau, a cold and high region, has experienced no consistent changes in spring phenology, despite a significant warming trend. We conducted a manipulative experiment of warming and precipitation addition in an alpine meadow on the Qinghai-Tibetan Plateau in 2015 (cold and wet), 2016 (warm and dry) and 2017 (mild and very wet). We found that warming increased annual variability of plant spring phenology. Warming delayed green up of all monitored species in 2016, advanced green up of early flowering species in 2015, and did not alter green up in 2017. For example, green up of the shallow rooted Kobresia pygmaea advanced 8 (± 2) days in 2015 and was delayed by 23 (± 3) days in a dry year (2016) under warming compared with control. Early spring precipitation addition can offset the delaying effects of warming in a dry year on the Qinghai-Tibetan Plateau. Under warming plus precipitation addition, community average green up advanced compared to control plots in 2015 and 2016, and community average flowering advanced for all three years. In 2016, flowering of K. pygmaea (an early flowering species) advanced under warming plus precipitation addition compared to control while flowering of other species did not change. Our results highlight that annual variation of soil moisture condition plays a critical role in determining the magnitude and direction of spring phenology response to warming. We provide insights in how plant spring phenology might change in a warmer future in the presence or absence of precipitation increase.

Contrasting Response to Drought and Climate of Planted and Natural Pinus pinaster Aiton Forests in Southern Spain

Extreme drought events and increasing aridity are leading to forest decline and tree mortality, particularly in populations near the limits of the species distribution. Therefore, a better understanding of the growth response to drought and climate change could show the vulnerability of forests and enable predictions of future dieback. In this study, we used a dendrochronological approach to assess the response to drought in natural and planted forests of the maritime pine (Pinus pinaster Aiton) located in its southernmost distribution (south of Spain). In addition, we investigated how environmental variables (climatic and site conditions) and structural factors drive radial growth along the biogeographic and ecological gradients. Our results showed contrasting growth responses to drought of natural and planted stands, but these differences were not significant after repeated drought periods. Additionally, we found differences in the climate–growth relationships when comparing more inland sites (wet previous winter and late spring precipitation) and sites located closer to the coast (early spring precipitation). Response functions emphasized the negative effect of defoliation and drought, expressed as the June standard precipitation-evapotranspiration index calculated for the 12-month temporal scale and the mean temperature in the current February, on growth. The strong relationship between climatic variables and growth enabled acceptable results to be obtained in a modeling approach. The study and characterization of this tree species’ response to drought will help to improve the adaptive management of forests under climate change.

Climatic determinants of introduced Sitka spruce in Hiiumaa Island, Estonia

Tree-ring records of Sitka spruce growing in Hiiumaa (Estonia) were investigated to illustrate their growth variability and its climatic determinants. A chronology comprising ring-width series of eight big individuals from the Suuremõisa forest park was correlated with local climatic records. The growth variability of this species introduced to Hiiumaa was statistically explained profoundly by winter temperature and early-spring precipitation. Comparisons were also made with local tree-ring data of Norway spruce. Interestingly, both the native and non-native species responded positively to precipitation in June and negatively to precipitation in April. Previous studies have shown that the winter temperature response, demonstrated here for Sitka spruce, is found as Norway spruce growth responses in eastern Estonia, whereas in western Estonia the growth of the latter species is more clearly connected, similar to our findings, to early-summer precipitation. These findings indicate that while the both spruce species remain sensitive to spring/summer moisture regime, the growth of Sitka spruce may actually be less tolerate to winter temperatures, as evident here in western Estonia where the winters may likely be milder than in eastern Estonia. Common to Sitka spruce results from Hiiumaa, tree-ring data representing conspecific native populations from north-west North American sites indicated positive responses to mid-winter temperatures. Based on these results, low winter temperatures and early-summer droughts may both threaten the survival of the remnant individuals of this species in Hiiumaa.

Climate-growth relationships in a Larix decidua Mill. network in the French Alps

A better understanding of the respective role of key climatic variables on tree growth is crucial for an accurate assessment of how ongoing global changes may affect both dynamics and distribution of forest tree species in the future. The aim of this study was (i) to explore growth patterns of European larch (Larix decidua Mill.) through a network of tree-ring chronologies developed for the French Alps and (ii) to identify the main climatic drivers explaining radial growth. Climate-growth relationships were coupled with a hierarchical analysis. This relationship revealed significant variability expressed spatially by the existence of five clusters, initially discriminated by an elevational contrast related to (i) a negative correlation between summer temperatures and larch growth at lower elevations and (ii) a stronger response of low-elevation larch stands to winter precipitation. In the high-elevation clusters, tree growth depends on previous autumn and current summer temperatures and water supply in July. The differentiation, that portrays a strong geographical coherence, is mainly related to the latitudinal gradient; (i) the northwestern stands are mostly sensitive to high temperatures in summer; (ii) the growth of the southernmost clusters is equally driven by temperatures during autumn, winter, and summers; (iii) the populations of a cluster located in a transitional zone of the inner French Alps, subject to both Mediterranean and Continental influences, exhibit negative correlations to late winter and early spring precipitation. This significant spatial heterogeneity of climate–tree ring relationships in L. decidua clearly underlines the high plasticity of the species to adapt its growth to local climate conditions.

Fully coupled heat and water dynamics modelling of a reclamation cover for oil sands shale overburden

Numerical models of soil-water dynamics have been widely used in the design of soil cover systems for mine site reclamation; however, in most cases these models only consider water dynamics without consideration of heat dynamics. For cover systems in northern climates, such as those associated with oil sands mines in northern Alberta, Canada, freezing conditions exist for approximately 6 months of the year and snow melt comprises approximately 25% of annual precipitation. This study attempts to assess whether a fully coupled water and heat flow model (CM) provides additional insights into cover performance as compared with a water flow model (FM). The CM and FM are developed for a monitored reclamation cover constructed over oil sands shale overburden. The validated results indicated that the key limitation of the CM was its inability to simulate frozen ground infiltration. This limitation results in an overestimate of snow melt runoff and does not replicate the development of perched conditions on the shale overburden surface as a result of snow melt infiltration. The FM is also unable to simulate the observed infiltration of snow melt deep into the cover when snow melt is represented simply as surface precipitation in early spring following ground thaw. Both models are improved if snow melt infiltration is represented in the model by the preferential filling of macropores across the full depth of the cover and oxidized shale prior to ground thaw. This methodology is incorporated with the CM to produce a coupled water and heat model with enhanced infiltration (CM-EI), and with the FM to produce a flow model with enhanced infiltration (FM-EI). The CM-EI provided an improved simulation of soil temperature dynamics under frozen and unfrozen conditions, as well as soil water dynamics under unfrozen conditions, including the improved representation of the annual water balance components over each water year. Given the small difference in annual water balance components between the CM-EI and FM-EI modelling approaches (∼5 mm/year), it is concluded that a FM-EI provides the best tool with which to assess the performance of these reclamation covers.

Vegetation phenology on the Qinghai-Tibetan Plateau and its response to climate change (1982–2013)

Using NDVI3g vegetation index, we defined 18 phenology metrics to investigate phenological change on the Tibetan Plateau (TP). Considering the heterogeneity of vegetation phenology, we divided TP into 8 vegetation clusters according to a 1:1000000 vegetation cluster map. For regions where phenology is highly sensitive to climate, we investigated the impact of climate variables, such as temperature, precipitation, and solar radiation on phenology using the partial least squares regression (PLS) method. Results indicated (1) that turning points of the starting date of the growing season (SOS) metrics were in 1997–2000, before which SOS metrics advanced 2–3d/10a. The ending date of the growing season (EOS) and the length of growing season metrics (LOS) turning points were 2005 and 2004–2007, respectively. Before the turning points, the EOS metrics had a delayed tendency of 1–2d/10a, and the LOS metrics also had a prolonging tendency of 1–2d/10a. After the turning points, the significant levels of SOS and EOS metrics’ tendency only reached 0.1, and LOS’s tendency was insignificant at the 0.1 level. (2) Alpine meadows and alpine shrub meadows changed most intensely on TP. Advanced SOS and delayed EOS were the main reasons of the alpine meadow LOS extension. Advance SOS mainly contributed to the alpine shrub meadow LOS extension. (3) We used meteorological variables, such as temperature, precipitation and solar radiation, to analyze the drastic change of the phenology of alpine meadows and alpine shrub meadows through the PLS method. Temperature was found to be the dominant meteorological variable impacting phenology. In those regions, the previous year autumn and early winter temperature had a positive effect on the SOS phenology. The high temperature in this period would postpone previous year EOS, indirectly delaying SOS in the current year. On the other hand, warming autumn and early winter may slow the fulfilment of chilling requirements and lead to later SOS, which would have a delayed effect on SOS. Except summer, the minimum temperature had a similar effect on vegetation phenology, as average and maximum temperature. Furthermore, the effect of precipitation on phenology fluctuated widely across different months. The previous year autumn and winter precipitation had a negative effect on the SOS phenology, and early spring precipitation had a positive effect. The main factor limiting vegetation development in August was precipitation, and during this month precipitation had a positive impact on the EOS phenology. The influence of solar radiation was mainly during summer and early fall. This study will contribute toward vegetation phenology model improvement.