Secondary succession, the postdisturbance transition of herbaceous to woody-dominated ecosystems, occurs faster at lower latitudes with important ramifications for ecosystem processes. This pattern could be driven by the direct effect of temperature on tree growth; however, an alternative mechanism is tree-herb competition, which may be more intense in more fertile northern soils. We manipulated soil fertility and herbaceous species composition in identical experiments at six sites spanning the Eastern United States (30-43° N) and monitored the growth and survival of four early successional trees. Tree seedling mass 2 years after sowing was strongly associated with site differences in mean growing season temperature, regardless of species or soil treatment. The effect of temperature was twofold: seedlings grew faster in response to warmer site temperatures, but also due to the reduction of competitive interference from the herbaceous community, which was inhibited in warmer sites. Our results suggest that increasing temperatures will promote a faster transition of fields to forests in temperate ecosystems.
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