Manuel Helbig

Spatial and seasonal variability of polygonal tundra water balance: Lena River Delta, northern Siberia (Russia)

The summer water balance of a typical Siberian polygonal tundra catchment is investigated in order to identify the spatial and temporal dynamics of its main hydrological processes. The results show that, besides precipitation and evapotranspiration, lateral flow considerably influences the site-specific hydrological conditions. The prominent microtopography of the polygonal tundra strongly controls lateral flow and storage behaviour of the investigated catchment. Intact rims of low-centred polygons build hydrological barriers, which release storage water later in summer than polygons with degraded rims and troughs above degraded ice wedges. The barrier function of rims is strongly controlled by soil thaw, which opens new subsurface flow paths and increases subsurface hydrological connectivity. Therefore, soil thaw dynamics determine the magnitude and timing of subsurface outflow and the redistribution of storage within the catchment. Hydraulic conductivities in the elevated polygonal rims sharply decrease with the transition from organic to mineral layers. This interface causes a rapid shallow subsurface drainage of rainwater towards the depressed polygon centres and troughs. The re-release of storage water from the centres through deeper and less conductive layers helps maintain a high water table in the surface drainage network of troughs throughout the summer.RésuméLe bilan d’eau estival d’un bassin d’alimentation typique de la toundra polygonale sibérienne a fait l’objet d’investigations afin d’identifier la dynamique spatio-temporelle des ses principaux processus hydrologiques. Les résultats montrent que, à côté de la précipitation et de l’évapotranspiration, un flux latéral influence considérablement les conditions hydrogéologiques spécifiques du site. La microtopographie marquée de la toundra polygonale contrôle fortement l’écoulement latéral et la modalité d’emmagasinement du bassin objet de l’investigation. Les bords intacts des polygones centrés bas constituent des barrières hydrologiques, qui libèrent l’eau accumulée en été plus tard que les polygones à bords dégradés et dépressions au dessus des biseaux glacées dégradées. La fonction barrière des bordures est fortement contrôlée par le dégel du sol, qui ouvre de nouveaux chenaux d’écoulement en subsurface et en accroît la connectivité hydrologique. C’est pourquoi, la dynamique du dégel détermine l’instant et le volume d’émission d’eau en subsurface et la redistribution de la réserve dans le bassin versant. Les conductivités hydrauliques des bordures polygonales élevées diminuent brusquement avec la transition d’horizons organiques à des horizons minéraux. Ceci cause un drainage superficiel rapide de l’eau de précipitation vers les centres déprimés des polygones et goulottes. Le relargage de l’eau en réserve depuis les centres à travers des couches plus profondes et moins conductrices aide à maintenir élevée la surface libre de l’aquifère sous le réseau de drainage superficiel des goulottes durant tout l’été.ResumenSe investiga el balance de agua de verano en una típica cuenca siberiana de polígonos de tundra para identificar la dinámica espacial y temporal de sus principales procesos hidrológicos. Los resultados muestran que, además de la precipitación y evapotranspiración, el flujo lateral influye considerablemente las condiciones hidrológicas del sitio específico. La destacada microtopografía de los polígonos de tundra controla fuertemente el comportamiento del flujo lateral y almacenamiento de la cuenca investigada. Los bordes intactos de los polígonos bajos centrados constituyen barreras hidrológicas, que liberan el agua de almacenamiento del verano después que los polígonos con bordes degradados y canales por encima de cuñas degradadas del hielo. La función de barrera de los bordes está controlada fuertemente por el deshielo del suelo, que abre nuevas trayectorias al flujo subsuperficial y aumenta la conectividad hidrológica subsuperficial. Además, la dinámica de deshielo del suelo determina la magnitud y el tiempo de la salida subsuperficial y la redistribución del almacenamiento dentro de la cuenca. Las conductividades hidráulicas en los bordes elevados del polígono disminuyen drásticamente con la transición de capas orgánicas a minerales. Esto causa un rápido drenaje subsuperficial somero del agua de lluvia hacia el centro de los polígonos deprimidos y canales. La reliberación del agua del almacenamiento desde el centro a través de capas menos conductivas y más profundas ayuda a mantener un nivel freático alto en la red de drenaje superficial de canales durante todo el verano.摘要为确定它的主要水文过程的时空动态,本文研究了一个典型的西伯利亚多边形冻原流域在夏季的水均衡。结果显示,除了降水和蒸发蒸腾,侧向流在相当程度上影响着特定场地的水文条件。多边形冻原主要的微地形特征强烈控制着研究流域的侧向流和储存行为。在中心偏低的多边形的完整边缘上建造水文屏障,以此可利用退化冰楔之上退化的边缘和凹槽在夏天释放储存的水。边缘上屏障的作用受到土壤解冻强烈的控制,解冻作用打开了地下水流的流径,提高了地下的水文连通性。因此,土壤解冻的动态决定了流出的地下水的规模和时间,以及流域内所储存水量的重新分配。升高的多边形边缘从有机层过渡到矿物层,渗透系数急剧减小。这造成了浅部雨水向凹陷的多边形中心和洼地迅速排水。所储存的水通过更深部的低渗透岩层从中心重新释放,这有助于夏季在地表凹槽排水系统中保持一个较高的水位。ResumoÉ investigado o balanço hídrico estival de uma típica bacia hidrográfica de tundra poligonal para identificar as dinâmicas espaciais e temporais dos seus principais processos hidrológicos. Os resultados mostram que, para além da precipitação e da evapotranspiração, o escoamento lateral influencia consideravelmente as condições hidrológicas em cada local. A característica microtopografia da tundra poligonal controla fortemente o escoamento lateral e o comportamento do armazenamento na bacia investigada. Os bordos intactos dos polígonos com centro deprimido constituem barreiras hidrológicas que libertam a água armazenada mais tarde no verão do que nos polígonos com bordos degradados e fendas situadas acima das cunhas de gelo degradadas. A função de barreira dos bordos é fortemente controlada pelo descongelamento do solo que abre novos percursos de escoamento subterrâneo e incrementa a conectividade hidrológica subsuperficial. Consequentemente, a dinâmica do descongelamento do solo determina a magnitude e a temporização do escoamento e a redistribuição do armazenamento dentro da bacia. A condutividade hidráulica nos bordos poligonais elevados diminui drasticamente com a transição entre as camadas orgânicas a as camadas minerais. Isto provoca uma rápida drenagem subsuperficial pouco profunda da água de chuva em direção aos centros deprimidos e às fendas. A re-libertação da água armazenada a partir dos centros através de camadas mais profundas e menos condutivas ajudam a manter ao longo do verão um nível de água elevado na rede de drenagem superficial formada pelas fendas.

Permafrost thaw and wildfire: Equally important drivers of boreal tree cover changes in the Taiga Plains, Canada

Boreal forests cover vast areas of the permafrost zones of North America, and changes in their composition and structure can lead to pronounced impacts on the regional and global climate. We partition the variation in regional boreal tree cover changes between 2000 and 2014 across the Taiga Plains, Canada, into its main causes: permafrost thaw, wildfire disturbance, and postfire regrowth. Moderate Resolution Imaging Spectroradiometer Percent Tree Cover (PTC) data are used in combination with maps of historic fires, and permafrost and drainage characteristics. We find that permafrost thaw is equally important as fire history to explain PTC changes. At the southern margin of the permafrost zone, PTC loss due to permafrost thaw outweighs PTC gain from postfire regrowth. These findings emphasize the importance of permafrost thaw in controlling regional boreal forest changes over the last decade, which may become more pronounced with rising air temperatures and accelerated permafrost thaw.

Direct and indirect climate change effects on carbon dioxide fluxes in a thawing boreal forest-wetland landscape.

Abstract In the sporadic permafrost zone of northwestern Canada, boreal forest carbon dioxide (CO2) fluxes will be altered directly by climate change through changing meteorological forcing and indirectly through changes in landscape functioning associated with thaw‐induced collapse‐scar bog (‘wetland’) expansion. However, their combined effect on landscape‐scale net ecosystem CO2 exchange (NEELAND), resulting from changing gross primary productivity (GPP) and ecosystem respiration (ER), remains unknown. Here, we quantify indirect land cover change impacts on NEELAND and direct climate change impacts on modeled temperature‐ and light‐limited NEELAND of a boreal forest–wetland landscape. Using nested eddy covariance flux towers, we find both GPP and ER to be larger at the landscape compared to the wetland level. However, annual NEELAND (−20 g C m−2) and wetland NEE (−24 g C m−2) were similar, suggesting negligible wetland expansion effects on NEELAND. In contrast, we find non‐negligible direct climate change impacts when modeling NEELAND using projected air temperature and incoming shortwave radiation. At the end of the 21st century, modeled GPP mainly increases in spring and fall due to reduced temperature limitation, but becomes more frequently light‐limited in fall. In a warmer climate, ER increases year‐round in the absence of moisture stress resulting in net CO2 uptake increases in the shoulder seasons and decreases during the summer. Annually, landscape net CO2 uptake is projected to decline by 25 ± 14 g C m−2 for a moderate and 103 ± 38 g C m−2 for a high warming scenario, potentially reversing recently observed positive net CO2 uptake trends across the boreal biome. Thus, even without moisture stress, net CO2 uptake of boreal forest–wetland landscapes may decline, and ultimately, these landscapes may turn into net CO2 sources under continued anthropogenic CO2 emissions. We conclude that NEELAND changes are more likely to be driven by direct climate change rather than by indirect land cover change impacts. &NA; Boreal forest–wetland landscapes in the lowlands of northwestern Canada store large organic carbon stocks and act as long‐term CO2 sinks to the atmosphere. Thaw‐induced wetland expansion has negligible effects on net ecosystem CO2 exchange of these landscapes as indicated by nested eddy covariance flux measurements. In contrast, boreal forest–wetland landscapes may no longer act as net CO2 sinks in an exceedingly warmer climate as indicated by combining climate projections with a simple CO2 flux model. These changes in net ecosystem CO2 exchange are five times smaller for a moderate warming scenario (RCP 4.5) compared to the scenario leading to the strongest warming (RCP 8.5). The fate of organic carbon in these landscapes depends therefore largely on the degree of warming during the 21st century. Figure. No caption available.