Climate warming, acorn masting and the dynamics of rodent populations: Comparing long-term studies

Pulsed resources, including mast production by forest trees, often have knock-on effects on consumer populations and their prey, predators, parasites and mutualists. Response by small rodents to fluctuating acorn production in temperate forests is a widespread example. Long-term research in Maine, USA, recently suggested combined effects of a warming climate and forest maturation on acorn production by red oak trees, leading to directional increases in average population density and body mass of white-footed mice. To foster reproducibility in long-term ecological research, we analysed data from our long-term study in southeastern New York, USA, which used similar field methods. Such a comparison allowed us to assess impacts of climate warming and forest growth on the same pulsed resource and responses by the same consumer species over time and at different latitudes. Despite a clear directional increase in mean minimum temperature and considerable growth in the average size and total basal area of trees during our 33-year study, neither acorn production by red oak trees nor abundance of white-footed mice showed directional increases. Similarly, average body mass of the mice did not change through time. Abundance of mice in mid-summer increased with increasing red oak acorn production the prior autumn. Mouse abundance also was higher in warmer years, although the effect of acorn abundance on mice was stronger. We found no evidence that temperature modified the acorn-driven population responses by mice. Long-term studies are notoriously hard to maintain and even harder to replicate between sites. The direct comparison of similar studies between Maine and New York provides an opportunity to assess the generality of mechanistic models linking climate change, mast seeding and consumer responses.