When speaking of sapwood heterogeneity, it is meant that there is phenotypic variation within an individual (Karban, 2015). As trees will respond at local levels to different conditions of the external and heterogeneous environment (Schlichting, 1986), pathogenic invasion may not necessarily be ‘straightforward’. Areas exposed to different levels of particular ‘resources’ (water, light, nutrients) may differ in their cellular structure and contents, and thereby create a texture that differs throughout the tree and in turn increases sapwood complexity (de Kroon et al., 2005). This means pathogens may have a harder time invading into different ‘segments’ of the tree that have responded differently to other parts of the tree, due to localised environmental differences that occur on such a micro level (Adler & Karban, 1994).
Adler, F. & Karban, R. (1994) Defended fortresses or moving targets? Another model of inducible defenses inspired by military metaphors. American Naturalist. 144 (5). p813-832.
de Kroon, H., Huber, H., Stuefer, J., & van Groenendael, J. (2005) A modular concept of phenotypic plasticity in plants. New Phytologist. 166 (1). p73-82.
Karban, R. (2015) Plant Sensing & Communication. USA: University of Chicago Press.
Schlichting, C. (1986) The evolution of phenotypic plasticity in plants. Annual Review of Ecology and Systematics. 17 (1986). p667-693.