  The time dependence of the recently introduced minimum spanning tree
description of correlations between stocks, called the ``asset tree'' have been
studied to reflect the economic taxonomy. The nodes of the tree are identified
with stocks and the distance between them is a unique function of the
corresponding element of the correlation matrix. By using the concept of a
central vertex, chosen as the most strongly connected node of the tree, an
important characteristic is defined by the mean occupation layer (MOL). During
crashes the strong global correlation in the market manifests itself by a low
value of MOL. The tree seems to have a scale free structure where the scaling
exponent of the degree distribution is different for `business as usual' and
`crash' periods. The basic structure of the tree topology is very robust with
respect to time. We also point out that the diversification aspect of portfolio
optimization results in the fact that the assets of the classic Markowitz
portfolio are always located on the outer leaves of the tree. Technical aspects
like the window size dependence of the investigated quantities are also
discussed.
