Photosynthesis in black and red spruce and their hybrid derivatives: ecological isolation and hybrid adaptive inferiority

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Abstract
Photosynthetic responses of black and red spruce were used to define parameters of their fundamental niches. Grown at warm temperature, black spruce had highest rates of CO2 uptake at high light intensities, fitting it for a pioneering role, while red spruce had the lowest light compensation point, fitting it for a late successional role. Black and red spruce responded in different ways to low temperature acclimation. Low temperature, low light (12 °C, 100 ft-c (1 ft-c = 10.764lx)) acclimation stimulated CO2 uptake in black spruce under all conditions while it depressed CO2 uptake in red spruce relative to rates following high temperature (26 °C) acclimation. Niches defined by photosynthetic responses corresponded to observed habitats in the area of sympatry of black and red spruce and demonstrated the existence of ecological isolation, a premating restriction to hybridization.Hybrids, backcrosses, and recurrent backcrosses of black and red spruce had lower rates of CO2 uptake than one or the other species in all light and temperature environments tested. In most cases, the hybrid had lower rates than both species. Growth depends on photosynthesis and is one component of fitness. Therefore, negative heterosis in hybrids and hybrid derivatives represents another mechanism of reproductive isolation, the postulating barrier of hybrid adaptive inferiority. Selection against hybrids should be density dependent, explaining both their widespread occurrence in habitats opened by fire or logging and their paucity in closed stands.