Density - The density is the mass of a substance divided by its volume.  Denser materials weigh more for the same given volume.  For example, steel is denser that styrofoam.  Density is often defined in various units, such as pounds per cubic foot, kilograms per cubic meter and others.  Another way to define density is called the specific gravity.  This is the ratio of the density of the material in comparison to that of water.  A specific gravity of 0.1 indicates that the density of the material is 0.1 times the density of water.  Irrespective of the units, materials with higher densities weigh more for same given volume.  In regards to a stringer, for the same stringer size, a more dense material will weigh more.  The wood properties included in this section have units of density as specific gravity.  

The table shows the referenced data sorted in order of increasing density.  The lightest woods are first and as the list proceeds the wood gets more dense.  The general trend of this data is that most woods have a specific gravity of 0.3 to 0.6 which is a factor of 2.  Balsa is half again the lowest at about 0.16.  

Impact Bending - This is a standard test where a specific weight is dropped on a beam at varying heights until the beam fails or bends excessively (greater than 6 inches).  This test helps describe the ability of a specific wood to absorb severe shocks.  Larger values are greater height for failure, so larger is stronger. 

Modulus of Elasticity - This parameter helps define how "stiff" a material is.  Materials which are very stiff would have a large Modulus of Elasticity.  Steel have a much high Modulus of Elasticity than wood, so for the same dimensions, a part made from steel will be much stiffer than one made from wood.  In terms of whether stiff is desirable, this is not necessarily clear.  A very stiff material is often also very strong, so a stiff material may be better for strength.  For some structures, failures can be mitigated if the structure bends a little, so some flexure may be desirable.  In the composite structure of a surfboard, the flexure of the board can also create undesirable loads on the composite sheets covering the foam core, so a stiff stringer could reduce overall flexure of the board and thereby reduce deflections of the glassing thereby reducing the tendency of the glassing to fail.  OR to help make it more confusing, functionally one may WANT the board to flex slightly for a specific board handling behavior, in which case the actual flexure is some specific value, not to loose, not too stiff.  The desired stiffness of a wood material is therefore less easy to define as good or bad, but it is highly likely to vary from wood to wood.  For at least uniformity of stiffness, it would be good to understand how this parameter varies.  In general, higher values of the Modulus of Elasticity means a material which is stiffer.

The data table shows the variation in Modulus of Elasticity from the most compliant to the stiffest.  One sees a range of 0.5 to 2.0 (in units of millions of psi) or a range of 4 times.  Again, there is a trend for moisture content where aged 12% moisture wood is stiffer than green wood.  

The associated graph shows the Modulus of Elasticity versus wood density with the same groupings for green and aged wood.  The data scatter is more severe but a simple trend of denser wood is more stiff and aging increases stiffness is seen.