Radiative heaters, which are in widespread use, do not simulate
convective (diffuse) warming, but exert a directional heat with vertical gradients,
unlike that of a warmer climate, even if mean temperatures may match. The same
applies to soil warming, which, for physical reasons, induces water diffusion away
from the heat source. In addition, step increases of temperature in soils represent a
major disturbance which may take years to lead to a new steady state, with initial
responses in essence documenting the disturbance of the rather delicate balance between
plant roots, fungi, microbes and the soil fauna associated with it. Given these
intrinsic constraints, it is far safer to build upon short-distance natural topographic
or narrow elevational gradients which easily can be found to offer, e.g., 2K warmer
condition under otherwise similar overall test conditions (soils, flora, precipitation).
An alternative is the use of soil monoliths at least in grassland. These can be transplanted
or transferred in the field to controlled environments, although the ‘step
change’ problem cannot be overcome. There are several psychological barriers to
the use of these elegant tools that nature offers to the experimentalist, who often
prefers to interfere with some technological glamour rather than capitalise on these
free-of-charge test conditions