Many terrestrial ecosystems evolved with growing season fires caused by lightning. Fire top-kills vegetation and causes a nutrient pulse in regenerating tissues that generally attracts herbivores. Because the nutrient pulse often co-occurs with the heightened nutritional stress of lactation, ungulates often shift diet selection to nearly exclusively target the ephemeral resource pulse creating a positive feedback between herbivores and plants (i.e., continuous herbivory extends the nutrient pulse by perpetuating leaf regeneration). Ultimately, the positive feedback leads to a fire-herbivory trap on plant succession and accelerates nutrient cycling during fall through leaf consumption and increased leaf litter input quality. However, we have a 36-year data set from the southeastern United States demonstrating that climate change has caused lightning-fire phenology to shift almost a month earlier. Also, we have a 25-year data set demonstrating that the reproductive phenology of the largest herbivore (Odocoileus virginianus) shifted a few days in the opposite direction. The disparate shifts created a consumer-resource mismatch because the resource pulse now occurs before peak lactation. To determine the potential consequences of the consumer-resource mismatch, we conducted a two by two field experiment crossing deer access with top-kill phenology on an aggressively resprouting tree species (Acer rubrum). Fifty trees were top-killed during March (i.e., early phenology), 30 trees during June (i.e., historical phenology), and resprouting vegetation on half of the trees in each phenology were protected from deer herbivory. We measured nutritional quality of the resprouting trees during peak lactation, the date of leaf senescence, and sprout height after senescence. Also, we collected senesced leaves and conducted a common-garden leaf decomposition experiment to measure the effects of the mismatch on leaf litter decomposition rate.
Results/Conclusions
Nutritional quality estimates are not available yet. However, we found a significant herbivore by phenology interaction. Early phenology sprout heights were similar with and without herbivore access but herbivory decreased sprout height by 17% following historical phenology. The historical phenology delayed leaf senescence a month later than the early phenology, but herbivory delayed senescence on the historical phenology an additional 2 months until frost killed immature leaves. The common-garden leaf decomposition experiment revealed historical phenology and herbivore access collectively accelerated leaf litter decomposition rate by 24% compared to early phenology excluding herbivory. Shifting lightning phenology may indirectly slow nutrient turnover because the consumer-resource phenological mismatch allows plants to escape the fire-herbivory trap.