Forest decline caused by high soil water conditions in a permafrost region

Abstract

Abstract. In the permafrost region near Yakutsk, eastern Siberia, Russia, annual precipitation (June–May) in 2005–2006 and 2006–2007 exceeded the 26-year (1982–2008) mean of 222±68 mm by 185 mm and 128 mm, respectively, whereas in 2007–2008 the excedent was only 48 mm, well within the range of variability. Yellowing and browning of larch (Larix cajanderi Mayr.) trees occurred in an undisturbed forest near Yakutsk in the 2007 summer growing season. Soil water content at a depth of 0.20 m was measured along a roughly 400 m long line transect running through areas of yellowing and browning larch trees (YBL) and of normal larch trees (NL). In the two years of supranormal precipitation, soil water content was very high compared to values recorded for the same area in previous studies. For both wet years, the mean degree of saturation (s) was significantly greater in YBL than NL areas, whereas the converse was the case for the relative gas diffusivity (DP/D0). This implies that rather than mitigating water stress suffered during normal precipitation years, elevated soil water conditions adversely affected the growth of larch trees. Eastern Siberia's taiga forest extends widely into the permafrost region. Was such supranormal annual precipitation to extend for more than two years, as might be expected under impending global climate changes, forest decline would be expanded and a danger of accelerating greenhouse gas emissions could result.

Satellite-Derived Spatiotemporal Variations in Evapotranspiration over Northeast China during 1982–2010

“…However, we found a strong spatial correlation between PET trend changes and ET trend changes in wet regions of Northeast China, which demonstrates that PET may be the dominant factor determining the trend changes in ET (Figure 11d). Moreover, previous substantial studies suggest that Tair is a key controlling factor influencing ET in high latitude region [75], because it can be recognized as a surrogate for atmospheric demand [76]. Our results illustrate that the trend of Tair is positively related to ET trends in these areas (Figure 11c), but the correlation is weaker than PET.…”

Too wet for oaks? Inter-tree competition and recent persistent wetness predispose oaks to rainfall-induced dieback in Atlantic rainy forest

“…For instance, in a previous study of Q. robur mortality in northeastern France, a severe reduction in radial growth in the years 1995-1996 coincided with an excess of water in the studied forest stand, due to an especially rainy spring in 1995 (Marçais and Bréda, 2006). Likewise, exceptionally rainy conditions in a boreal Siberian forest of Larix cajanderi greatly increased the water saturation of the soils, and critically reduced their relative gas diffusivity in declining stands in comparison to nondeclining ones (Iwasaki et al, 2010). Even in a semiarid ecosystem, a trend of increasing rainfall during past decades has driven woodland dieback, causing growth decline and massive mortality of the droughttolerant species Prosopis caldenia due to sustained groundwater level rise in the sedimentary plains of the Argentinean Pampas (Bogino and Jobbágy, 2011).…”

A review of global terrestrial evapotranspiration: Observation, modeling, climatology, and climatic variability

“…However, this is not always the case. In high latitudes and cold areas, temperature as a major control of λE is generally positively related to λE [ Iwasaki et al , 2010; Nemani et al , 2003]. Therefore, the T s ‐VI method is most suitable for a growing season (range of VI is large enough) in middle‐latitude areas where soil moisture rather than T a or available energy is the key control of λE.…”