Terminology is important in understanding what's going on. I'm going to do my best to explain what I know.
The strength of a material is a reference to how much stress a material can take before breaking. In the images below it's a reference to the failure region: the tensile strength (or compression strength) and/or fracture point. Like others have mentioned strength isn't really a good indicator of anything by its self for us bowyers.
resistance to compression/tension or as I say stiffness, has a couple of official terms: young's modulus or modulus of elasticity (MOE). In the images below its the initial slope. As bowyers we can think of it as how much the wood is going to resist bending. The higher the resistance/stiffness than the higher the poundage will be for that bend for a given amount of material. mikekeswick described it well. A stiffer wood will reach desired weight with less material than a less stiff wood. look at the second image. at a strain of 10 the first material is at a stress of 300 whereas the second is only about 150. This is the biggest factor in considering how strong our bows will be. The way we can effect this variable is by controlling the amount of material. We can have more material by either having the wood wider or thicker. However, there is a distinction between width and thickness. more of it in both cases will equal more poundage. However, more thickness will also increase the stress of the outer fibers whereas more width does not affect the stress the fibers undergo. So really the best way to meet your weight requirements is to ensure your bow is wide enough. Because of this distinction i think of manipulating the wood like this: the thickness determines how far it can bend and the width the poundage.
Now lets move on to the stresses.
In the images below we have strain and stress on the x and y axis. For the sake of applying it to us think of it as how far it can bend on the x axis and how much poundage it will give us on the y axis. a greater slope means that for less bend it gives us more poundage. This is good right? not necessarily because we also have to look at the elasticity of the material. a stiff material with low elasticity will give you high poundage for little bend but will break sooner. This describes a brittle material. High resistance to bending but easily breakable. There's also an important distinction between elasticity before breaking and the elasticity before set. These are indicated on the graph as the failure point described above and yield point. The yield point is the point the material will no longer return to its normal shape perfectly like it once did. It still can do work without breaking and still return a little bit, just not ideally like before. One it passes this point it becomes plastic. In our terminology we describe this as set. Set doesn't mean a broken bow, but it certainly doesn't mean an undamaged one either. Unfortunately, there is no real data out there on yield points (sometimes research is tested to failure and sometimes only to yield points but they generally treat them as the same so it's hard to distinguish what they actually mean). Therefore we mainly have to go off anecdotal experiences. Like your for example about plum. apparently plum is pretty elastic meaning it has a high yield point. If you want to get an idea for a woods elasticity using data you can take the failure point and divide it by the stiffness (MOE) and it will give you an idea. It's not perfect though because we have to use failure points instead of yield points.
Another important distinction needs to be made about compression vs tension. The same principles apply for both but they aren't the same values in wood. Generally wood is stiffer in tension then it is compression. Elasticity is a little less clear. I would say that wood more elastic in compression if you are referring to its elasticity before breaking, but if you are your referring to its elasticity before the yield point/set I believe it is weaker then its tension elasticity. A subject that I want to investigate further is whether wood will undergo set in tension or if it's only compression. It doesn't make sense that it would only be in compression yet that's what we see generally. So maybe its just that wood plastic region in tension is so small compared to compression.
