Nima Ehsani

Water Scientist

Spring plant phenology and false springs in the conterminous US during the 21st century


  1. Spring onset will occur earlier throughout the conterminous US by the year 2100.
  2. Projections of earlier spring onset indicate that recent historical trends may continue, but not everywhere.
  3. The averaging of many models canceled out the effects of the simulated internal variability and emphasized the role of greenhouse-forced climate change.
  4. Future changes in false spring risk depend on the relative change in the timing of spring onset and the last spring freeze. While both relate to changes in temperature, differences arise because spring phenology is a cumulative process throughout the spring whereas a single cold night can cause a hard freeze.
  5. Plants face an evolutionary tradeoff between the benefit of earlier leaf emergence, and hence, a longer growing season, and the risk of tissue damage from a false spring. Given changing plant phenology, maintaining current plant-animal interactions will require evolutionary or behavioral adaptation by dependent animals.

Allstadt, Andrew J., Stephen J. Vavrus, Patricia J. Heglund, Anna M. Pidgeon, Wayne E. Thogmartin, and Volker C. Radeloff. “Spring plant phenology and false springs in the conterminous US during the 21st century.” Environmental Research Letters 10, no. 10 (2015): 104008.

Spring plant phenology and false springs in the conterminous US during the 21st century

Figure 1. BCCA model average day of year for spring onset defined by (left) leaf out and (right) first bloom date. (a), (b) Average leaf out and first bloom dates during the climatological base period 1950–2005. Numbers denote the Omernik ecoregions: (1) North American Desert, (2) Mediterranean California, (3) Southern Semi-Arid Highlands (4) Temperate Sierras, (5) Tropical Wet Forests, (6) Northern Forests, (7) Northwestern Forested Mountains, (8) Marine West Coast Forests, (9) Eastern Temperate Forests, and (10) Great Plains. Remaining subplots indicate the mean projected changes in each spring onset definition relative to the historical period during the (c), (d) middle 21st century (2041–2070) and (e), (f) late 21st century (2071–2100) for the RCP8.5 high emission scenario. Changes at all locations were statistically significant among the climate models, so no changes were masked.


The onset of spring plant growth has shifted earlier in the year over the past several decades due to rising global temperatures. Earlier spring onset may cause phenological mismatches between the availability of plant resources and dependent animals, and potentially lead to more false springs, when subsequent freezing temperatures damage new plant growth. We used the extended spring indices to project changes in spring onset, defined by leaf out and by first bloom, and predicted false springs until 2100 in the conterminous United States (US) using statistically-downscaled climate projections from the Coupled Model Intercomparison Project 5 ensemble. Averaged over our study region, the median shift in spring onset was 23 days earlier in the Representative Concentration Pathway 8.5 scenario with particularly large shifts in the Western US and the Great Plains. Spatial variation in phenology was due to the influence of short-term temperature changes around the time of spring onset versus season-long accumulation of warm temperatures. False spring risk increased in the Great Plains and portions of the Midwest, but remained constant or decreased elsewhere. We conclude that global climate change may have complex and spatially variable effects on spring onset and false springs, making local predictions of change difficult.

Source: Spring plant phenology and false springs in the conterminous US during the 21st century – IOPscience

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