The Root of the Red Maple Paradox: Two distinct regeneration mechanisms determine leaf morphology, physiology, and competitive success in Acer rubrum.

Red maple is one of the most abundant and wide-ranging tree species in eastern North America and has rapidly increased in abundance over the past century following forest disturbances. This species is both a site- and light-generalist and has the unique ability to regenerate in forms that represent multiple stages of forest succession. However, red maple has modest physiological traits compared to its competitors, including low maximum photosynthetic rate, low photosynthetic nitrogen-use efficiency, and low foliar nutrient content. Red maples unremarkable physiology contradicts its competitive success. To untangle this paradox, we examine red maples two distinct regeneration mechanisms: seedlings and vegetative sprouts. Red maple can regenerate from seed, but can also sprout clonally from a stump following forest disturbance. We compare the morphology, physiology, and plasticity of these two regeneration mechanisms over 24 years of forest succession using a chronosequence of regenerating forest stands. We found that sprout-origin maples grow on average 6.5x taller and 5.5x faster than seed-origin maples. Sprout-origin trees display greater leaf spectral reflectance in the near-infrared range and greater stomatal density than seed-origin trees, demonstrating vegetative sprouts low water-use efficiency. Sprout-origin trees have more robust light-capture traits, including thicker palisade mesophyll than seed-origin trees. Sprouts also have a high plasticity between upper and lower leaves in many morphological and physiological traits, while seed-origin trees exhibited much less plasticity. The two distinct regeneration mechanisms of seed-origin and sprout-origin give rise to red maple trees with dramatically different leaf traits, allowing red maples to regenerate and thrive in a range of ecological conditions. Seed-origin maples are slow-growing, late successional, and shade-tolerant. Sprout-origin maples are fast-growing, early successional, and shade intolerant. This unique bimodal regeneration strategy may explain the red maple paradox and help predict forest composition and structure following disturbance.