Smaller pores reduce the chance that embolisms are introduced into vessels. In addition, they often have more pits in xylem vessels, which ensure that vessels are refilled and hydraulic connectivity is restored following cavitation. Interestingly, the stem can store some water. This buffers transpiration from water uptake. It occurs mainly in live parenchyma cells with non-lignified walls.
Trees shrink in diameter at midday.
This can act as protection against warm winter day when leaves photosynthesize while ground still frozen. However, other critically important, widespread stresses, such as freezing tolerance, are also based on plant water relations. Basically, to understand winter water relations, you must understand how drought kills plants. Let's see why. It determines water vapor concentration gradient in leaf.