Climates in northeastern North America are predicted to warm by 3-5 ˚C in the coming century. Understanding the effects of warming on tree seedling performance is thus critical for predicting population recruitment in the future hardwood forests in the northeast. Red maple (Acer rubrum) is a common canopy tree with a wide distribution and ecological amplitude, and is becoming a dominant late-successional species in some northeastern forests. Despite the ubiquity of this species, it is poorly understood how key performance traits, particularly survival, growth, biomass allocation and damage probability, respond to warming during the early seedling stage, a critical filter for stand recruitment. Warming can increase metabolic rates and accelerate phenology in tree seedlings, but may also increase biotic and abiotic stresses such as natural enemy loads and/or tissue damage by fungal pathogen infection. We conducted a field pot experiment at the Harvard Forest in Petersham, MA, using wild-collected A. rubrum seeds from local genotypes. We used electric warming mats to examine how 5˚C soil warming during the growing season affected germination rate, survival, leaf phenology, growth, above- and belowground biomass allocation and herbivory/fungal damage in first-year A. rubrum seedlings.
In first-year A. rubrum seedlings, germination rates and overall seedling survival did not differ between control and warming treatments. Soil warming increased the probability of leaf damage, particularly by fungal infection in leaf tissues, but were not associated with growth reductions or damage to root or meristematic tissue by the end of the first growing season. Most importantly, soil warming significantly accelerated first leaf expansion, delayed final autumn leaf senescence, and drastically increased height, leaf number, leaf area, and total biomass production. Soil warming significantly increased biomass allocation to root structures, but there was no evidence of any allocation tradeoff, as shoot biomass production was also much greater with soil warming. Our results suggest A. rubrum seedlings capitalize on soil warming using highly plastic spring and autumn phenology, resulting in a longer period of carbon uptake and leading to higher overall biomass production. Overall, we show a strongly positive effect of soil warming, particularly on biomass production in first-year A. rubrum seedlings, which suggests this species may be able to exploit windows of opportunity for high recruitment pulses under warming climate conditions in Northeastern forests.