The team at GMO posted asset class return predictions for the next 7 years here. The link is only a PDF there was no commentary but the info is still worth thinking about.
Number one on the list for expected returns is timber. The PDF does not specify timberland but many times in the past he has talked specifically about timberland. This of course will get investors thinking about timber REITs and the timber ETFs WOOD from iShares and CUT from Guggenheim.
Any of those may be fine holdings but expecting a precise correlation to actual timberland is to expect too much. WOOD allocates about 60% to industrials and CUT allocates 74% to materials; those numbers come from the funds' webpages, but there are a lot of overlapping names as you might expect. Over the last four years (since WOOD's inception) the chart of both looks like the S&P 500 but have lagged that index.
Plum Creek Timber (NYSE:PCL) is the most widely known and followed timber REIT. We used to own PCL for clients and sold it because I felt it was over-owned such that the correlation to the market would increase. After a short period in early 2008 when it looked very little like the market it has since tracked very closely to the Utility Sector SPDR (NYSEARCA:XLU). If you want to own the name because you like the fundamentals and the yield then go for it (I am not implying anything wrong with the fundies) but it has lost much of the low correlation effect it once had.
GMO thinks emerging market stocks will do well in the next seven years, thinks US large cap will have zero return and that TIPS will have negative returns. There are plus or minus numbers tied to each asset class mentioned (there are others besides the ones I mentioned).
Accessing actual attributes associated with timberland even with PCL is difficult because the share prices are subject to the various emotions that all stocks are subject to. This is similar to farmland stocks. Adecoagro (NYSE:AGRO) and Cresud (NASDAQ:CRESY) have lagged badly behind the S&P 500 for the last couple of years and although they haven't looked much like the stock market, investors haven't really benefited from theme.
There is no reason to think that access to these spaces can't evolve but for now the stocks don't offer as much diversification as the concepts.