LAUSR

I here use Dante Alighieri's quote to set the scene for the scope of this essay: that above‐ and belowground subsystems of any terrestrial ecosystem studied so far are intimately linked and influence each other, mostly through plants as a conduit. Accordingly, the end of the 1990s and the early 2000s saw a surge of research and syntheses on the ecological linkages between above‐ and belowground subsystems (Van der Putten et al., 2001; Wardle et al., 2004b), how they influence each other—mainly through changes in plant traits (Bardgett and Wardle, 2003; van Dam et al., 2003) —and what are the ecological and ecosystem consequences of changes to them (De Deyn and Van der Putten, 2005). Through all of this work, we were able to conclude that aboveground–belowground interactions seem to be widespread—if not omnipresent—in the plant kingdom. However, based on the experimental evidence so far, another major conclusion on this topic is that the magnitude and net effect of these interactions is highly context dependent. Context dependency on aboveground–belowground interactions can arise from numerous factors. For instance, aboveground herbivory seems to change the activity of soil microbial communities, but this effect can vary over time. Over short periods (days or weeks), foliar herbivory can cause plants to release carbon‐rich molecules into the soil. This observation led to predictions that aboveground herbivory should stimulate the activity of soil microbial communities, which in turn may increase the size of the soil inorganic nitrogen pool available to plants (Hamilton and Frank, 2001). Over longer periods of time, aboveground herbivores may stimulate plant compensatory growth to some extent, which can itself promote further herbivory and increase the return of plant litter, in the form of debris or herbivore feces, to the soil. In turn, such additional litter is generally more labile and more assimilable by the plants than normal plant litter, which can feed back to spur plant productivity (Bardgett and Wardle, 2003). On the contrary, aboveground herbivory can have negative effects on soil communities, if for example foliar herbivores stimulate the production of chemical defences in roots (Bezemer and van Dam, 2005) or over time promote the dominance of unpalatable plant species with poor litter quality (Wardle et al., 2002). Therefore, whether the net effects of the aboveground subsystem on belowground communities via feedback loops on plant productivity are positive or negative clearly depends on context. Effects of belowground subsystems on aboveground communities can similarly be positive or negative. Root herbivory can cause a reduction in plant growth or yield (Blossey and Hunt‐Joshi, 2003), while beneficial soil microbes and fungi can directly stimulate plant growth and nutrient acquisition (Dellagi et al., 2020). Accordingly, reviews on below‐to‐aboveground processes generally show that the effects of soil microbial and animal communities on aboveground plant community productivity, composition,