Late Spring Precipitation Research Articles

JAMES BUTTLE REVIEW: A resilience framework for physical hydrology

AbstractHydrological systems across the globe are increasingly subjected to pressures from a warming climate and anthropogenic disturbance. Responses to stress can be nonlinear and variable; therefore, it is imperative to improve understanding of resilience and tipping points of these systems. Previous applications of resilience concepts to physical hydrology have been disconnected, and an increasing range of definitions and approaches has often led to improper applications. Here, upon synthesizing relevant literature, we define physical hydrological resilience as the ability of a watershed to maintain hydrological function while exposed to a stressor or perturbation, thus remaining within the same regime. From this definition a new framework for physical hydrological resilience is forwarded which includes four key traits that must be part of a robust application of resilience for physical hydrology. Any evaluation of resilience should (1) identify and justify a baseline regime; (2) evaluate hydrological function, one or more of collection, storage and release; (3); assess physical hydrological systems for both resistance and latitude and (4) evaluate important perturbations and processes and how they interact to manifest into resistance, latitude, and tipping points. The framework is applied to the example of the Elbow River in Alberta, Canada, using baseline (1979–2015) and future (2050–2080) conditions. Two key hydrological processes, snow accumulation and streamflow, are found to have low resistance to winter duration and late spring precipitation, respectively. The collection function is resilient to a warmer, wetter climate due to a large latitude in relation to air temperatures, while the release function is not resilient, shifting from a streamflow‐ towards an evapotranspiration‐dominated regime in the future. The successful application of the concepts of resilience to this complex catchment demonstrates how the framework could be applied across a diversity of catchments. Results of resilience studies can improve environmental monitoring and evaluation programs and complement social‐ecological resilience frameworks and water management strategies.

Major ions and potentially toxic elements in atmospheric precipitation during the COVID-19 lockdown in Moscow megacity

For the first time, the change in the physicochemical properties, ions composition, concentrations, and ratio of soluble and insoluble potentially toxic elements (PTEs), as well as sources contribution to the content of PTEs using principal component analysis with multiple linear regression (PCA-MLR) were determined in the Moscow precipitation before, during and after the lockdown (January–July 2020). The impact of the lockdown on the precipitation composition was ambivalent. The decrease in the precipitation pollution with PTEs (by 10–99% for soluble PTEs and 9–61% for insoluble ones) was caused by the purification of the atmosphere from aerosols during their long-term washing out by precipitation and a decrease in anthropogenic emissions. Air advection to Moscow from the suburbs, where wood, coal, household and agricultural wastes were burned, on the contrary, contributed to the growth of precipitation pollution with insoluble P, Pb, Cd, soluble P, Ag, Pb, Sb, As, Cd, as well as [Cl−] and [K+]. After the lockdown, the restoration of the level of precipitation pollution by PTEs occurred gradually due to the time lag between the increase in atmospheric pollution and the washing out of aerosols by precipitation, as well as dilution by exceptional rainfall amount. Faster restoration rates of insoluble PTEs compared to soluble ones are associated with the rapid increase in the activity of the urban source (road and construction dust, industrial and traffic emissions). The lifting lockdown restrictions reduced the contribution of industrial sources to the content of soluble PTEs forms (from 38–66% to 6%) due to an increase in the contribution of road dust and non-exhaust emissions, soil particles resuspension, waste and fuel combustion, and vehicle emissions. A decrease in the contribution of vehicle emissions, road dust resuspension, and construction dust to the content of insoluble PTEs from winter to summer due to the lockdown influence and a large amount of precipitation in late spring and early summer was confirmed. The results highlighted the need for further studies of the chemical composition and properties of precipitation in the city in similar periods for the correct separation of the influence of social, economic, emission, meteorological, and physicochemical factors on the content and the ratio of PTEs forms.

2021 February Texas Ice Storm Induced Spring GPP Reduction Compensated by the Higher Precipitation

AbstractClimate extremes are more frequent and affect the carbon cycle. On 10–20 February, an exceptional winter storm in Texas brought severe ecological and economic effects. However, its impacts on vegetation activities and the carbon cycle are unclear. This study evaluated the early 2021 ice storm effects on spring phenology and the carbon cycle. The results showed that the ice storm led to a significant later vegetation spring phenology and a reduction of 42.32 Tg C in gross primary production (GPP) and a reduction of 34. 47 Tg C net ecosystem production (NEP) in spring. In the 2021 spring, Texas became a carbon source from the carbon sink. More precipitation in late spring and summer precipitation recharged the rootzone soil moisture, which enhanced summer vegetation GPP and NEP and compensated for the effects of the ice storm on spring carbon uptake. The summer GPP enhancement offset all the spring GPP reduction, and thus GPP was increased by 4.97% during 2021. 91.47% of the spring NEP reduction was compensated by summer NEP enhancement due to higher root zone soil moisture. Due to the NEP enhancement during autumn, NEP did not decrease as well and increased by 10.88% during 2021. This observation‐based research highlights the seasonal compensation effects on the interactions between extreme climate events and the biosphere.

Synergistic impacts of westerlies and monsoon on interdecadal variations of late spring precipitation over the southeastern extension of the Tibetan Plateau

AbstractBased on multiple long‐term observational and reanalysis datasets, this study investigated the characteristics and physical mechanisms of the interdecadal variations in late spring (i.e., May) precipitation (LSP) over the southeastern extension of the Tibetan Plateau (SETP) since 1900. It was revealed that by and large, LSP over the SETP experienced interdecadal decrease during the period preceding 1927, 1962–1988, and 2004 onwards, but saw an increase during the periods of 1928–1961 and 1989–2003. The atmospheric circulations responsible for interdecadal variations in LSP over the SETP were also analysed. These analyses identified significant synergistic impacts of decreased mid‐latitude upstream westerlies and increased low‐latitude monsoonal southerlies over the Central North Bay of Bengal (CNBOB) on interdecadal variations in precipitation, suggesting striking interactions between extratropical eastward cold air and tropical northward warm/humid air. Further observational and modelling evidence suggested that Atlantic Multidecadal Oscillation (AMO) was likely to be a salient oceanic driver for the interdecadal synergy between upstream westerlies and CNBOB monsoonal southerlies. The elevated sea surface temperature anomalies associated with the warm phase of the AMO could spark favourable local atmospheric anomalies, forcing an upper‐tropospheric, planetary‐scale teleconnection emanating from the east of the North Atlantic sector, which may serve as an effective bridge linking the remote AMO signal and the synergy between westerlies and monsoonal southerlies around the SETP on interdecadal timescales. Our findings provided new insights into the understanding of the synergistic roles of westerlies and monsoons in the modulation of interdecadal LSP over the SETP, prior to the peak Asian summer monsoon season.

Toward role of westerly‐monsoon interplay in linking interannual variations of late spring precipitation over the southeastern Tibetan Plateau

AbstractA recent study reported a notable westerly‐monsoon interplay (WMI) to the northeast of the Tibetan Plateau in boreal summer. However, this study introduces another striking and unique WMI over the southeastern TP (SETP) in late spring (May), emphasizing the critical roles of the upstream mid‐latitude westerly and the downstream Bay of Bengal (BOB) summer monsoon (BOBSM) in linking interannual variations of precipitation over SETP. In this study, the research SETP domain (22.5°–27.5°N, 87.5°–97.5°E) as well as a westerly index (WI) over the pivotal upstream westerly domain (22.5°–30°N, 65°–80°E) and a monsoon index (MI) over the pivotal downstream BOBSM domain (12.5°–22.5°N, 87.5°–100°E) are defined to investigate connections between the westerly and BOBSM and their combined effects on precipitation over SETP. The results indicate that the mid‐latitude westerly and BOBSM show opposite effects on precipitation over SETP. The weaker westerly and the stronger BOBSM correspond to enhanced SETP precipitation, and vice versa. The enhanced BOBSM acts as a predominant role on the strengthening of precipitation, while the decelerated westerly plays a secondary dynamical amplification role. To quantify their synergistic roles in linking SETP precipitation, a WMI index is defined. Statistical analysis suggests that this newly defined WMI index has a higher positive correlation with the precipitation over SETP. The correlation has a prominent increase compared to individual correlations between WI/MI and SETP precipitation, and dynamic conditions in high WMI index years are more favorable for the formation of SETP precipitation. The possible mechanism is that the BOBSM southerly strengthens and the southern branch of the upstream mid‐latitude westerly weakens in high WMI index years, resulting in an anomalous cyclone over the northwestern BOB and an anomalous anticyclone over the western TP. The anomalous cyclone resembles a Rossby wave response to the increased precipitation over SETP, and the precipitation in turn contributes to maintaining the BOB cyclonic anomaly and the above anticyclonic anomaly via the latent heating release. This positive feedback effect is conducive to enhanced SETP precipitation.

ОСОБЕННОСТИ ФЛУКТУАЦИЙ ЭФЕМЕРОВО-ЭФЕМЕРОИДОВОЙ СИНУЗИИ В ПОЛУПУСТЫННЫХ ФИТОЦЕНОЗАХ КУРА-АРАЗСКОЙ НИЗМЕННОСТИ В СВЯЗИ С КЛИМАТИЧЕСКИМИ ИЗМЕНЕНИЯМИ

The dynamics of semi-desert herbaceous phytocenoses was studied under the conditions of the Kura-Araz lowland on permanent model sites (10×10 m). Based on seven-year (2014–2020) stationary observations, regularities of seasonal and annual fluctuations of cereal-forb and legume-forb phytocenoses were revealed in connection with weather conditions. The analysis of data on the long- term dynamics of their species composition, structure, stock of the raw above-ground part is carried out. The α-diversity is estimated by generally accepted indices, the results of phenological observations are given. It was found that in the seasonal dynamics of plants, the beginning of the growing season depends on the air temperature in March, and the end of the cycle depends on the air temperature in May-early June. The optimal dates for the mass development of grasses are the beginning and the middle of May. In the case of an increased norm of atmospheric precipitation and a late onset of summer heat, this pattern may be violated, and the dates are shifted to the end of May – beginning of June. Over the years of observations in the cereal-forb phytocenosis of 19 species, 9, and in the legume-forb phytocenosis of 17 species, 5 turned out to be constant. The total projective cover ranged from 50 (dry years) to 100 % (wet years). The assessment of α-diversity showed that the species diversity of phytocenoses increased in years with a high amount of atmospheric precipitation in May. A linear dependence of phytomass on the total amount of atmospheric precipitation for March-June was revealed. It was found that the best accumulation of herbal phytomass is influenced by the amount of precipitation in late spring. The most resistant to warming plant species have been identified. These are mainly cereals with an average feed value: Avena eriantha, Bromus japonicus, Bromus rubens, Eremopyrum orientale, Lolium rigidum and weed herbs – Capsella bursa- pastoris, Carduus pycnocephalus, Convolvulus arvensis, Erodium cicutarium, Lepidium draba, Xanthium strumarium, the last species is an invasive plant. Among the representatives of legumes, only one species, Medicago minima, turned out to be relatively resistant to high air temperatures. It was found that of the total herbage, only 35.5 % of species can withstand a warming of 1.5–2 °C.

Long-term trends in the body condition of parents and offspring of Tengmalm\u2019s owls under fluctuating food conditions and climate change

Physical condition is important for the ability to resist various parasites and diseases as well as in escaping predators thus contributing to reproductive success, over-winter survival and possible declines in wildlife populations. However, in-depth research on trends in body condition is rare because decades-long datasets are not available for a majority of species. We analysed the long-term dataset of offspring covering 34 years, male parents (40 years) and female parents (42 years) to find out whether the decline of Tengmalm’s owl population in western Finland is attributable to either decreased adult and/or juvenile body condition in interaction with changing weather conditions and density estimates of main foods. We found that body condition of parent owl males and females declined throughout the 40-year study period whereas the body condition of owlets at the fledging stage very slightly increased. The body condition of parent owls increased with augmenting depth of snow cover in late winter (January to March), and that of offspring improved with increasing precipitation in late spring (May to June). We conclude that the decreasing trend of body condition of parent owl males and females is important factor probably inducing reduced adult survival and reduced reproduction success thus contributing to the long-term decline of the Tengmalm’s owl study population. The very slightly increasing trend of body condition of offspring is obviously not able to compensate the overall decline of Tengmalm’s owl population, because the number of offspring in turn simultaneously decreased considerably in the long-term. The ongoing climate change appeared to work in opposite ways in this case because declining depth of snow cover will make the situation worse but increased precipitation will improve. We suggest that the main reasons for long-term decline of body condition of parent owls are interactive or additive effects of reduced food resources and increased overall predation risk due to habitat degradation (loss and fragmentation of mature and old-growth forests due to clear-felling) subsequently leading to decline of Tengmalm’s owl study population.

Factors Influencing the Population Fluctuations of Euproctis chrysorrhoea (Lepidoptera: Erebidae) in Maine.

The browntail moth (Euproctis chrysorrhoea (L.)) is a forest pest that was accidentally introduced in the late 1800’s and spread throughout New England in the early part of the 20th Century. At its peak range expansion in 1915 it encompassed an area of 150,000 km2 after which populations declined. By the 1960s, its distribution had receded to relic populations on outer Cape Cod, MA, and islands in Casco Bay, ME. In 1989 browntail moth resurged in Maine, with periodic, moderate outbreaks before a dramatic increase of the population occurred in 2016. We examined the pattern of annual defoliation by browntail moth since its resurgence in the 1990s as well as variation in populations throughout infested areas in Maine during three years of the recent outbreak, 2016–2018, relative to differences in weather, parasitism and habitat characteristics. Levels of defoliation over 24 yr were predicted by the preceding spring precipitation (−, negative effect) and the year’s previous late summer and early fall temperatures (+, positive effect) when first to third instar larvae feed and then construct winter hibernacula. Late summer temperatures predicted the abundance of hibernacula across outbreak areas (+). Early spring temperatures (+) and early and late spring precipitation (−) predicted early summer larval and pupal nest abundance. Warmer fall temperatures result in more mature populations coming out of winter hibernacula in the spring, whereas spring precipitation drives epizootic outbreaks of Entomophaga aulicae (Reichardt in Bail) Humber (Entomophthorales: Entomophthoraceae). with parasitoids playing a lesser role. Climate trends indicate continued increases in fall temperatures since browntail moth resurgence.

When do rock glacier fronts fail? Insights from two case studies in South Tyrol (Italian Alps)

AbstractThe fronts of two rock glaciers located in South Tyrol (Italian Alps) failed on 13 August 2014, initiating debris flows in their downslope channels. A multimethod approach including climate, meteorological, and ground temperature data analysis, aerial image correlation, as well as geotechnical testing and modeling, led to the reconstruction of the two events. An integrated investigation of static predisposing factors, slowly changing preparatory factors, and potential triggering events shed light on the most likely reasons for such failures. Our results suggest that the occurrence of front destabilization at the two rock glaciers can only partly be explained by the occurrence of heavy rainfall events. Indeed, antecedent hydrological and thermal ground conditions were characterized by a saturated active layer favored by a snow‐rich winter and extensive precipitation in late spring and summer. Also, the rising trend of air temperature during spring and summer months since 1950s might explain the concurrent marked displacement of the two rock glaciers. Indeed, geotechnical investigations have provided strong indications that one of the investigated rock glacier fronts was at a marginally stable state prior to 2014. As rainfall events more intense than the one that occurred in August 2014 were previously recorded in the same area without resulting failures at the studied rock glaciers, we propose that both predisposing and preparatory destabilizing factors have played a key role in the 2014 rock glacier front failures.

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.

Hydroclimatology of the Missouri River Basin

Abstract Despite the importance of the Missouri River for navigation, recreation, habitat, hydroelectric power, and agriculture, relatively little is known about the basic hydroclimatology of the Missouri River basin (MRB). This is of particular concern given the droughts and floods that have occurred over the past several decades and the potential future exacerbation of these extremes by climate change. Here, observed and modeled hydroclimatic data and estimated natural flow records in the MRB are used to 1) assess the major source regions of MRB flow, 2) describe the climatic controls on streamflow in the upper and lower basins , and 3) investigate trends over the instrumental period. Analyses indicate that 72% of MRB runoff is generated by the headwaters in the upper basin and by the lowest portion of the basin near the mouth. Spring precipitation and temperature and winter precipitation impacted by changes in zonal versus meridional flow from the Pacific Ocean play key roles in surface water supply variability in the upper basin. Lower basin flow is significantly correlated with precipitation in late spring and early summer, indicative of Atlantic-influenced circulation variability affecting the flow of moisture from the Gulf of Mexico. Although increases in precipitation in the lower basin are currently overriding the effects of warming temperatures on total MRB flow, the upper basin’s long-term trend toward decreasing flows, reduction in snow versus rain fraction, and warming spring temperatures suggest that the upper basin may less often provide important flow supplements to the lower basin in the future.

Interannual variability of the Arctic freshwater cycle in the second half of the twentieth century in a regionally coupled climate model

We use a regionally coupled ocean-sea ice-atmosphere-hydrological discharge model to investigate the influence of changes in the atmospheric large-scale circulation on the interannual variability of the Arctic freshwater (FW) components. This model includes all sinks and sources of FW and allows for the analysis of a closed FW cycle in the Arctic. We show that few atmospheric winter modes explain large parts of the interannual variability of the Arctic FW cycle. A strong Icelandic low causing anomalous strong westerlies over the North Atlantic leads to warmer and wetter conditions over Eurasia. The ocean circulation is then characterized by a strong transpolar drift leading to increased export of FW in liquid and solid form into the North Atlantic. In contrast to this, a weaker than usual Icelandic low and a strong Siberian high is associated with a strong Beaufort Gyre and thus an accumulation of FW within the Arctic Ocean. Not only specific winter conditions but also increased precipitation in late spring and summer, caused by enhanced cyclone activity over land, lead to increased Eurasian runoff, which is responsible for most of the variability in Arctic river runoff.

Mid Holocene vegetation reconstruction from Vanevan peat (south-eastern shore of Lake Sevan, Armenia)

A sediment core has been retrieved from Vanevan peat (south-eastern shore of Lake Sevan, Armenia), which is today disconnected from Lake Sevan thanks to an artificial shallowing of the lake. Based on 5 radiocarbon dates, Vanevan record covers the Mid Holocene (from ca. 7800 to ca. 5100 cal. BP). The Late Holocene is today absent in the peat stratigraphy due to modern peat exploitation by surface mining. This study focuses on a multi-proxy approach including pollen, charcoals, and pollen-inferred climate reconstruction.An open-land, steppic vegetation is recorded up to ca. 7700 cal. BP, followed by a more forested landscape during the Mid Holocene (up to ca. 5700 cal. BP), and ending again with an open-land vegetation (to the end of record, 5100 cal. BP). This vegetation dynamics responds to general climate changes documented in the Near East. Whether human activities are documented since ca. 7500 cal. BP (Late Neolithic) in Vanevan, they remain marginal and probably did not affect the area. Early Holocene dry climate, which caused the steppic environment to be widespread through the Near East, is strongly related to low late spring precipitation (PMay–Jun = 180 mm). Mid Holocene forested landscape and increasing lake-level seem related to late spring precipitation (+28%), which is the main change in estimated climate parameters. This has to be linked with reinforcement of the Westerlies and less active Siberian High, which are inversely involved in the following, dry phase starting at ca. 5700 cal. BP.

Exploring Growth Variability and Crown Vitality of Sessile Oak (Quercus Petraea) in the Czech Republic

Unraveling climatic effects on growth of oak - Europe’s most ecologically and economically important forest species - has been the subject of many recent studies; however, more insight based on field data is necessary to better understand the relationship between climate and tree growth and to adapt forest management strategies to future climate change. In this report, we explore the influence of temperature, precipitation and drought variability on the productivity and vitality of oak stands in the Czech Highlands. We collected 180 cores from mature oaks (Quercus petraea) at four forest stands in the Czech Drahany Highlands. Standard dendromethods were used for sample preparation, ring width measurements, cross-dating, chronology development, and the assessment of growth-climate response patterns. Crown vitality was also evaluated, using the modified ICP Forests methodology. Late spring precipitation totals between May and June as well as the mean July temperature for the year of ring formation were found to be the most important factors for oak growth, whereas crown condition was significantly affected by spring and summer drought. This study is rep-resentative for similar bio-ecological habitats across Central Europe and can serve as a dendroclima-tological blueprint for earlier periods for which detailed meteorological information is missing

Relationships between climate variability and radial growth of Nothofagus pumilio near altitudinal treeline in the Andes of northern Patagonia, Chile

Global warming is expected to enhance radial tree growth at alpine treeline sites worldwide. We developed a well-replicated tree-ring chronology from Nothofagus pumilio near treeline in a high precipitation climate on Choshuenco Volcano (40°S) in Chile to examine: (a) variation in tree radial growth in relation to interannual climatic variability; and (b) relationships of radial growth to variability in El Niño Southern Oscillation (ENSO) and the Antarctic Oscillation (AAO) at interannual and decadal time scales. A tree-ring chronology based on 99 tree-ring series from 80 N. pumilio trees near treeline showed a high series intercorrelation (0.48) indicating a strong common environmental signal. Radial growth is negatively correlated with precipitation in late spring (November–December). Temperature and tree growth are positively correlated during late spring and early summer (November–January). Interannual variability in both seasonal climate and in tree growth is strongly teleconnected to ENSO and AAO variability. Radial growth of N. pumilio in this humid high-elevation forest does not show a positive trend over the past half century as predicted from global treeline theory and broadscale warming in the Patagonian-Andean region. Instead, tree growth increased sharply from the 1960s to a peak in the early 1980s but subsequently declined for c. 30years to its lowest level in >100years. The shift to higher radial growth after c. 1976 coincides with a shift towards warmer sea surface temperatures in the tropical Pacific which in turn are associated with warmer growing season temperatures. The decline in tree growth since the mid-1990s is coincident with the increasingly positive phase of the AAO and high spring precipitation periods associated with El Niño conditions. The recent shift towards reduced growth of N. pumilio at this humid high-elevation site coincident with rising AAO mirrors the reduced tree growth beginning in the 1960s for trees growing in relatively xeric, lower elevation sites throughout the Patagonian-Andean region. The current study indicates that N. pumilio growth response in humid high-elevation environments to recent broad-scale warming has been non-linear, and that AAO and ENSO are key climatic forcings of tree growth variability.

Interacting influences of climate factors and land cover types on the distribution of caddisflies (Trichoptera) in streams of a central European low mountain range

Abstract Effects of various climatic factors, altitude and land cover types on the distribution of eight caddisfly species were modelled from occurrence data by the use of the maximum entropy method. The predicted habitat use and the most contributing environmental factors differed considerably among species. Those differences of responses may be a basis for changes of assemblage structures with climate changes. Precipitation in late spring to early summer, minimum temperatures in the second half of the year, and temperature seasonality, or other bioclimatic factors that give information about the temporal stability of temperatures (‘diurnal range’, ‘isothermality’), were key climate factors influencing the distribution of most of the studied species. Therefore, climate change may affect Trichoptera assemblages not only by the height of mean temperatures but also by temperature variability. Land cover types contributed to the development of distribution models for several caddisfly species. Forests promoted most of the species, whereas cultivated and managed areas were less preferred. Few species concentrated on individual land cover types, e.g. Wormalida occipitalis on coniferous forest. Therefore, different adaptations of caddisfly species to land cover types, in addition to climatic factors, seam to occur, and are discussed to facilitate niche partitioning. Environmental variables that were observed to affect the distribution of caddisflies, i.e. temperature, precipitation, and vegetation, may interact with each other. Climate changes may influence caddisflies not only via temperature changes, but by related changes of precipitation regimes, and by a long‐term change of vegetation types, e.g. tree species predominant in forests.

Influence of land use and climate on recent forest expansion: a case study in the Eurosiberian–Mediterranean limit of north‐west Spain

Summary In Mediterranean mountainous areas, forests have expanded in recent decades because traditional management practices have been abandoned or reduced. However, understanding the ecological mechanisms behind landscape change is a complex undertaking because the influence of land use may be reinforced or constrained by abiotic factors such as climate. In this work, we evaluated their combined effects on recent forest expansion across climatic, topographic and management gradients. We used orthorectified aerial photographs from the second half of the twentieth century (1956, 1974, 1983, 1990 and 2004) to monitor changes in forest distribution in a set of 20 head‐water basins in the Cantabrian Mountains of north‐west Spain, at the Eurosiberian–Mediterranean limit. In particular, we evaluated the role of land‐use history (comparing natural vs. anthropic basins) and microclimate (comparing shaded vs. sunny aspects) of forest gain/loss rates and spatial distribution shifts. Finally, we applied Species Distribution Modelling techniques (MaxEnt and BIOMOD) in the stated scenarios of land‐use history and microclimate, to assess habitat suitability for forest expansion on the basis of topography, soil properties and mesoclimatic variables. Forest cover increased from 10.72% in 1956 to 27.67% in 2004 in the area. The rate of expansion was significantly higher in natural basins and, particularly, on shaded slopes. In all cases, the mean elevation of new forest patches increased during the study period, which was particularly evident on natural sunny slopes. The performance of the models and the magnitude of the effects varied across land‐use histories and microclimatic conditions. Soil properties and temperature and precipitation in late spring and early summer were the main drivers of forest expansion in modelling exercises, although expansion rates and upward altitudinal shifts were primarily controlled by land‐use history and the biogeographic origin of the forests. Synthesis. The combination of monitoring and modelling techniques used in this work contributed to the understanding of forest expansion in cultural systems, indicating that ecological succession is not a homogeneous process, but varies spatially due to human and abiotic constraints since historical times.

Cork oak physiological responses to manipulated water availability in a Mediterranean woodland

This study details the physiological responses of cork oak (Quercus suber L.) to manipulated water inputs. Treatments named as dry, ambient and wet, which received 80, 100 and 120% of the annual precipitation, respectively, were applied to a Mediterranean woodland in southern Portugal. Tree ecophysiology and growth were monitored from 2003 to 2005.The impacts of the water manipulation were primarily observed in tree transpiration, especially during summer drought. Rainfall exclusion reduced the annual stand canopy transpiration by 10% over the 2-year study period, while irrigation increased it by 11%. The accumulated tree transpiration matched precipitation in spring 2004 and 2005 at the stand level, suggesting that cork oak trees rely on precipitation water sources during the peak of the growing season. However, during the summer droughts, groundwater was the main water source for trees.Despite the significant differences in soil water content and tree transpiration, no treatment effects could be detected in leaf water potential and leaf gas exchange, except for a single event after spring irrigations in the very dry year 2005. These irrigations were intentionally delayed to reduce dry spell duration during the peak of tree growing season. They resulted in an acute positive physiological response of trees from the wet treatment one week after the last irrigation event leading to a 32% raise of stem diameter increment the following months. Our results suggest that in a semi-arid environment precipitation changes in spring (amount and timing) have a stronger impact on cork oak physiology and growth than an overall change in the total annual precipitation.The extreme drought of 2005 had a negative impact on tree growth. The annual increment of tree trunk diameter in the ambient and dry treatments was reduced, while it increased for trees from the wet treatment. Water shortage also significantly reduced leaf area. The latter dropped by 10.4% in response to the extreme drought of 2005 in trees from the ambient treatment. The reduction was less pronounced in trees of the wet treatment (−7.6%), and more pronounced in trees of the dry treatment (−14.7%).Cork oak showed high resiliency to inter-annual precipitation variability. The annual accumulated tree transpiration, the minimum midday leaf water potential and the absolute amount of groundwater used by trees appeared unaffected by the extreme drought of 2005. Our study shows that cork oak rapidly and completely recovered from the extreme dry year of 2005 or from rainfall exclusion. Our results support the eco-hydrological equilibrium theory by which plant acquire complementary protective mechanisms to buffer the large variability in water availability experienced in semi-arid ecosystems. In optimizing their structural biomass increase in response to increasing drought stress, cork oak trees succeeded in restricting water losses to maintain the minimum leaf water potential above the critical threshold of xylem embolism, though with narrower hydraulic safety margins in 2005.Our findings highlight cork oak's sensitivity to the amount and timing of late spring precipitation. This could be critical as future climate scenarios predict a reduction of spring precipitation as well as enhanced severity of droughts in the Iberian Peninsula by the end of the 21st century. In inducing water stress before the onset of summer droughts, the predicted spring precipitation decline could drive the species closer to the threshold of catastrophic xylem embolism at the peak of the drought period.

Modeling study of the effect of anthropogenic aerosols on late spring drought in South China

In this study, the mechanisms underlying the decadal variability of late spring precipitation in South China are investigated by using the latest Community Earth System Model version 1 (CESM1). We aim to unravel the effects of different climate forcing agents such as aerosols and greenhouse gases (GHGs) on the decadal variation of precipitation, based on transient experiments from pre-industry (for year 1850) to present-day (for year 2000). Our results reveal that: (1) CESM1 can reproduce the climatological features of atmospheric circulation and precipitation for the late spring in South China; (2) only simulations including the forcing of anthropogenic aerosols can reproduce the observed decreasing trend of late spring precipitation from 1950–2000 in South China; (3) aerosols affect the decadal change of precipitation mainly by altering the large-scale atmospheric circulation, and to a less extent by increasing the lower-tropospheric stability to inhibit the convective precipitation; and (4) in comparison, other climate forcing agents such as GHGs have much smaller effects on the decadal change of spring precipitation in South China.

Effects of hydrology on recruitment of Pilularia minuta Durieu (Marsileaceae), an endangered plant of Mediterranean temporary pools

Hydrological requirements for germination and development of P. minuta were assessed through a 5-year field survey in Garâa Sejenane, Tunisia, and an experiment under controlled conditions. The spatial distribution of P. minuta was recorded in the field annually, while the experiment tested the effects of water levels and flooding dates on germination (the emergence of new individuals) and development. Water level was found to be the major factor affecting the germination and the development of P. minuta with flooding date as a secondary limiting factor. For germination to occur, the sediment must be completely waterlogged. Water depths of 5–10cm are optimal for plant development. In addition, the plant needs a minimal flooding period of seven weeks to develop. Late-spring precipitation appears to be more critical than earlier rains.