The vascular cambium
is part of a plant's primary growth
produces secondary phloem and xylem tissue
decreases in ring size each year

The vascular cambium produces secondary phloem and xylem tissue.
Vascular cambium, a plant tissue located between the xylem and the phloem in the stem of a vascular plant. It is also the source of both secondary xylem growth and the secondary phloem growth.  Vascular cambium is usually found on dicots and gymnosperms not on monocots which usually lack secondary growth. It does not transport water, dissolved food or minerals by plants. Vascular cambia are cylinders of unspecialized meristem cells which divide  to make new cells which specialize to form secondary vascular tissues.