The real estate market will impact timber-related ETFs differently over time, however. For example, due to the fact that it sets aside 30% of its portfolio to real estate investment trusts, the iShares S&P Global Timber & Forestry Index Fund ( WOOD) should see some correlation with the action taking place in the housing market. The Guggenheim Timber ETF ( CUT), meanwhile, focused more heavily on companies dedicated to the commercial side of the timber industry. Ninety percent of the fund's portfolio is spread across the materials and consumer discretionary sectors. Steel, coal, and timber are a few of the industries bullish investors may want to keep an eye on. Investing in any these ETFs, however, is not for the faint of heart. The combination of a choppy investment atmosphere and these funds' concentrated subsector exposure will leave them vulnerable for wild day-to-day action. For now, I would encourage investors to keep these funds on the radar. Only the most aggressive should try their luck. However, even then, any position must be monitored closely. Written by Don Dion in Williamstown, Mass.