Effects of climate extremes on the terrestrial carbon cycle: concepts, processes and potential future impacts

Abstract

Extreme droughts, heat waves, frosts, precipitation, wind storms and other climate extremes may impact the structure, composition and functioning of terrestrial ecosystems, and thus carbon cycling and its feedbacks to the climate system. Yet, the interconnected avenues through which climate extremes drive ecological and physiological processes and alter the carbon balance are poorly understood. Here, we review the literature on carbon cycle relevant responses of ecosystems to extreme climatic events. Given that impacts of climate extremes are considered disturbances, we assume the respective general disturbance-induced mechanisms and processes to also operate in an extreme context. The paucity of well-defined studies currently renders a quantitative meta-analysis impossible, but permits us to develop a deductive framework for identifying the main mechanisms (and coupling thereof) through which climate extremes may act on the carbon cycle. We find that ecosystem responses can exceed the duration of the climate impacts via lagged effects on the carbon cycle. The expected regional impacts of future climate extremes will depend on changes in the probability and severity of their occurrence, on the compound effects and timing of different climate extremes, and on the vulnerability of each land-cover type modulated by management. Although processes and sensitivities differ among biomes, based on expert opinion, we expect forests to exhibit the largest net effect of extremes due to their large carbon pools and fluxes, potentially large indirect and lagged impacts, and long recovery time to regain previous stocks. At the global scale, we presume that droughts have the strongest and most widespread effects on terrestrial carbon cycling. Comparing impacts of climate extremes identified via remote sensing vs. ground-based observational case studies reveals that many regions in the (sub-)tropics are understudied. Hence, regional investigations are needed to allow a global upscaling of the impacts of climate extremes on global carbon-climate feedbacks.

Ecosystem impacts of climate extremes crucially depend on the timing

“…On a different note, Wolf et al (3) find that high spring uptake, particularly in the eastern temperate forests, prevented the United States from shifting from a carbon sink to a carbon source. The spatially nonuniform signal demonstrates how the impacts of climate extremes differ between ecosystems and illustrates the challenge associated with finding general response patterns to climate extremes (2). What can be beneficial for one ecosystem might be devastating for another (9).…”

“…On the one hand, the authors see depletion of soil moisture through early vegetation activity in a warm spring potentially amplifying summer heating, a typical lagged direct effect of an extremely warm spring (2). On the other hand, spring and summer, and photosynthesis and respiration, compensate each other with respect to the net annual effect on the carbon cycle, leading to a near-neutral same-year carbon balance.…”

“…Whether there are any impacts at all, and how these impacts manifest themselves, critically depends on the timing, magnitude, extent, and type of the climate anomaly. Although many studies have been undertaken to investigate the impacts of climate extremes on ecosystem functioning, attempts to build an overarching framework have had little success so far and many open questions remain (2). A study published in PNAS (3) provides new insights into the question of how impacts of climate extremes occurring during different periods of the year can interact and counteract each other.…”

Drought stress impact on vegetable crop yields in the Elbe River lowland between 1961 and 2014

“…Longer intervals between rainfall events may increase the duration and severity of soil drought stress. In contrast, longer intervals between heavy rainfall events may reduce periods of anoxia and be favourable to plant growth in more hydric ecosystems (Frank et al, 2015). Plant responses to water stress differ significantly depending on (i) the intensity and duration of stress, (ii) the vegetable types, and (iii) its stage of development.…”

Detecting impacts of extreme events with ecological in situ monitoring networks

“…Consequently, much research focuses on understanding how extreme hydrometeorological events affect ecosystems and their functioning (overviews of the state of research and concepts are given in, for example, Smith, 2011;Reyer et al, 2013;Niu et al, 2014;Frank et al, 2015). For instance, ecosystem responses could be manifested in extreme anomalies of phenology (Ma et al, 2015), biogeochemical fluxes , or even in altered ecosystem structure due to induced mortality (Hartmann et al, 2015).…”