one of the key hypotheses of this research is that white birch that has evolved in wet areas will have larger vessels than those trees that evolved in dry areas. since the wet areas are less likely to experience drought they can therefore produce larger vessels while dry areas produce smaller vessels to ward off any embolism that may occur during a drought. the second hypothesis is that birch trees in dry areas will produce lower sapwood to leaf area ratios then those in wetter environments. trees in the wet areas can support higher leaf area per unit of sapwood area while trees in dry areas have made structural modifications to leaf area as a result of water stress. in total thirty sites were sampled across canada ranging from ontario to b.c.