Compartmentalization, Resource Allocation, and Wood Quality

Curr Forestry Rep (2015) 1:8–15 DOI 10.1007/s40725-014-0002-4 PHYSIOLOGICAL PROCESSES (M DAY, SECTION EDITOR) Compartmentalization, Resource Allocation, and Wood Quality Kevin T. Smith Published online: 1 February 2015 # Springer International Publishing AG (outside the USA) 2015 Abstract The concept of a trade-off of tree resources between growth and defense is readily grasped. The most detailed development of the concept is for the growth-differentiation balance hypothesis that predicts that resources for normal growth and primary metabolism are diverted to support plant defense and secondary or stress metabolism. This hypothesis has been applied to the biosynthetic cost of stress metabolites that protect wood and foliage from herbivory. This review suggests that the trade-offs of primary to stress metabolism is an ongoing theme throughout the evolution of land plants. This review extends the concept of the growth/defense trade-off to processes of apoptosis in the constitutive development of tracheary elements and heartwood as well as to the induced boundary-setting processes of compartmentalization of decay in living trees. For wood utilization, the confusion of heartwood with wound-initiated discoloration continues to obscure the sources of value loss, particularly for high-value hardwood lumber. Examples are drawn from the anatomical effects of tree injury from fire, storms, and vascular wilt disease. Keywords Wood discoloration . Growth-differentiation hypothesis . Heartwood formation . Vascular wilt disease . Mineral stain . Fire damage . Secondary metabolism must grow fast enough to compete for light, water, and essential mineral elements while providing defense against pests, pathogens, and episodically or chronically adverse environments. While annual plants may require active defense for a single growing season lasting 1 year or less, the challenge of temperate zone trees includes the frequent potential for great size and long life with periods of annual growth and apparent dormancy. Although the concept of a growth/defense trade-off in the allocation of resources is readily grasped, the validity and breadth of applicability of the concept are still being assessed [1]. The most frequently cited example of a growth/defense trade-off involves chemical biosynthesis to protect foliage and wood from herbivory. Various competing schemes for the theoretical and mechanistic basis of the trade-off continue to be tested [2, 3]. One of the dominant schemes is the growthdifferentiation balance (GDB) hypothesis extended by Herms and Mattson [4] to describe the trade-off in within-tree allocation between tree growth and protection from herbivory. Compartmentalization of decay in trees represents a set of resource trade-offs that includes both the programmed loss of wood function and the diversion of tree energy resources through biosynthesis. This review presents the compartmentalization process in the wood of living trees: (1) as a critical conceptual example of resource allocation trade-offs and (2) as a major factor for tree health and wood quality. Introduction Plants face an existential dilemma in the allocation of internal resources to individual survival and reproductive effort. Plants This article is part of the Topical Collection on Physiological Processes K. T. Smith (*) United States Department of Agriculture Forest Service, Northern Research Station, 271 Mast Road, Durham, NH 03824, USA e-mail: Metabolism and Specialization Growth results from primary metabolism: the photosynthetic capture of solar energy and atmospheric carbon which is fixed into sugar, respired to yield metabolic energy, and converted into other organic compounds. These compounds include structural and non-structural carbohydrates, amino acids for protein, and nucleotides for energy transfer compounds (e.g., ATP) and the genetic code (DNA and RNA). Under Curr Forestry Rep (2015) 1:8–15 conditions of attack, infection, or abiotic stress, the tree shifts the allocation of energy and biosynthetic feedstocks to defensive processes. This diversion of photosynthetic carbon compounds and associated energy costs is traditionally termed “secondary” metabolism. This review uses the term “stress metabolism” for those pathways to reduce minimization of their central role for organism survival. The structure and function of present-day forest flora are the legacy of lasting trade-offs in resource allocation. Natural selection acts on reproductive performance and not directly on individual traits [5]. Allocation trade-offs extend the ecological range or amplitude, but incur an investment cost in stress metabolism. For potentially long-lived forest trees, survival depends at least as much on effective responses to periodic or episodic disturbance as on optimal primary production. A brief sketch of a few key developments in the long history of trees shows a common theme of success from the diversion of resources from primary to stress metabolism. Stress metabolites such as phenolic acids and complex compounds such as lignin are often assumed to be limited to seed plants (embryophytes) or even more restricted to vascular plants (tracheophytes), the latter containing a vascular system to conduct water and photoassimilates. Although there is some controversy as to the role of convergent evolution or direct descent from a common ancestor [6, 7], diversion of carbohydrates to simple phenolic compounds and more complex lignin or lignin-like polymers likely occurred in red and green algae prior to land invasion [8, 9]. Lignin, composed of crosslinked phenolic propane molecules, likely protected algal cell walls and cytoplasm from UV-induced oxidation, mechanical buffeting, and intermittent desiccation of intertidal or coastal zones [10]. Development of these stress metabolites improved the fitness of algae in the prevailing marine or freshwater environment. Following land invasion, cellular specialization in the development of the stele of prototracheophytes enabled increased height growth and greater competitive exploitation of available sunlight. This specialization included the secondary thickening of cell walls which allowed the tracheary elements (vessel segments and tracheids) to function at negative pressures without collapsing. The tracheary elements also lacked living contents at maturity and functioned with open lumens, greatly improving hydraulic efficiency and flow rates [11]. The open cell lumens result from apoptosis or programmed cell death. Although there are differences in the apoptosis program among cell types, the mechanism for cell death was in place early in the processes of plant evolution and cell differentiation [11]. Tree Development The hallmark of broadleaved and coniferous trees is the vascular cambium (VC), the bifacial meristematic layer 9 that divides to form secondary xylem (which matures into sapwood, SW) and phloem (or inner bark) [12, 13]. The development of the V (...truncated)