A general hypothesis of forest invasions by woody plants based on whole-plant carbon economics

Although closed-canopy forests are characterized by low-light availability and slow population dynamics, many are under threat from non-native, invasive woody species that combine high colonization ability and fast growth potential with high low-light survival. This ‘superinvader’ phenotype contravenes expected trade-offs predicted by successional niche theory, posing a challenge to both invasion and forest succession theory. We propose a parsimonious conceptual model based on the whole-plant light compensation point (WPLCP) that, across a variety of plant strategies and growth forms, can explain greater competitive abilities of forest invaders in the context of both high-light growth rate and shade tolerance. The model requires only that non-native species experience relatively fewer carbon costs than native species, enabling resource-acquisitive species to establish in low-light conditions. We review evidence for lower carbon costs in invasive species resulting from (1) enemy release, (2) recent environmental changes that favour less stress-tolerant phenotypes and (3) phylogenetically constrained native floras. We also discuss implications of invader shade tolerance in the context of other life-history strategies that, combined with canopy disturbances, facilitate their rapid numerical dominance. Synthesis. An invasion framework driven by carbon dynamics suggests renewed focus on whole-plant carbon costs, including below-ground respiration and tissue turnover, which are rarely measured in functional studies of forest invaders.