3. In multi-aged forests, the canopy of older trees shades young trees. The growth of these young trees is often suppressed because they receive little sunlight. If a portion of the canopy is removed (through selective logging or natural process like winter storms), the growth of the young trees may accelerate. The pattern of suppressed and accelerated growth is often seen in the widths of the annual tree rings.

Selective logging was conducted in the Cowichan Valley in 1993. Tree cores were collected from 15 young trees in 1999. For each tree, ring widths were measured for year 1990 (before logging) and year 1996 (after logging). The data are located in the file Tree.sav.

As a forestry researcher, you are investigating whether there is a significant increase in tree growth after the canopy is removed. Before canopy removal, the yearly rings were approximately the same width so there was little difference in ring width each year. Therefore, we will use 0 as the known value for comparing the difference before and after logging.

1. Which test would you use to answer this question? Explain your choice.
    
2. Conduct the analysis by hand (omit p-values from your analysis). What do you infer?
    

To learn more about tree ring research, visit the UVic Tree Ring Lab and the Arizona Laboratory of Tree Ring Research.