Across all taxa, amphibians exhibit some of the strongest phenological shifts in response to climate change. As climates warm, amphibians and other animals are expected to breed earlier in response to temperature cues. However, if species use fixed cues such as daylight, their breeding timing might remain fixed, potentially creating disconnects between their life history and environmental conditions. Wood frogs (Rana sylvatica) are a cold-adapted species that reproduce in early spring, immediately after breeding ponds are free of ice. We used long-term surveys of wood frog oviposition timing in 64 breeding ponds over 20 years to show that, despite experiencing a warming of 0.29 °C per decade in annual temperature, wood frog breeding phenology has shifted later by 2.8 days since 2000 (1.4 days per decade; 4.8 days per °C). This counterintuitive pattern is likely the result of changes in the timing of snowpack accumulation and melting. Finally, we used relationships between climate and oviposition between 2000 and 2018 to hindcast oviposition dates from climate records to model longer-term trends since 1980. Our study indicates that species can respond to fine-grained seasonal climate heterogeneity within years that is not apparent or counterintuitive when related to annual trends across years.

Climate data: We extracted daily meteorological data, including maximum and minimum temperature, precipitation, and snowpack (quantified as snow water equivalent) from the DayMet database (v.3 (Thornton et al. 2016)). The Daymet algorithm uses daily meteorological observations from ground stations to interpolate estimates at 1km square cells across North America between 1980 and 2018. We extracted estimates for the cell centered nearest the geographical center of our site (tile: 11754, N 41.9198 W -72.1604).

Oviposition data: These data include records of annual egg-mass surveys collected between 2000 and 2019 for 64 wood frog breeding ponds at Yale Myers Forest, a 3,213 ha research forest in northeastern Connecticut, USA. During surveys, the entire pond was closely searched by observers. If eggs were found, the observers independently counted and then averaged the estimates (or, in cases of surveys by a single observer, egg masses were counted twice and averaged).

Microhabitat data: We include microhabitat data for wood frog breeding ponds at our site including: latitude, elevation, aspect, and a suite of canopy metrics estimated from repeated hemispherical photos. Briefly, five hemispherical photographs were take along the shore at each cardinal point and at the center of each pond during leaf-off and leaf-on seasons. We used Gap Light Analyzer (Frazer 1999) to estimate average leaf-on and leaf-off global site factor (GSF; the ratio of above-canopy radiation to under-canopy radiation (Anderson 1964)) and a weighted GSF value integrated over the duration of wood frog larval and embryonic life cycle (Halverson et al. 2003). Additionally, we included the within-pond variance in GSF values, which captures important components of the canopy shape and structure above and surrounding the ponds. Point values for aspect and elevation were estimated from the USGS National Elevation Dataset (U.S Geological Survey 2002). In addition, we included site-wide daily average temperature, precipitation, and SWE between day-of-year 0 and 120 (Jan 1 ~ April 29) from the DayMet dataset. This range spans the earliest and latest observed oviposition dates (day-of-year 74 and day-of-year 113) with a margin.

Code: R code to replicate the analysis is included with further instructions inline in the scripts.