What You Might Notice in a Warmer than Average Spring

Written by Jeff Osborne

With numerous days with temperatures in the 80s this March and April, you may wonder, “Have temperatures been warmer than average?” You also may wonder how much earlier can plants break bud, how insects and migratory birds can be affected by a warmer spring, and whether there are winners and losers when trees flush leaves early. Some of these questions are easier than others to answer as we consider a few of them.

Have temperatures been warmer than average this spring? This is a question you can easily find an answer for without taking your own thermometer readings. The National Oceanic Atmospheric Administration (NOAA) website has maps which show the ranking of average temperatures down to the county level over various periods as short as one month. Temperatures in the database go back 129 years. These maps show that small parts of Pennsylvania experienced record high average temperatures in January and February, but March temperatures were near average, with less than half of the state experiencing a 0-3°F increase and just a few areas experiencing a 3-6°F increase. April seems like it will be above average when comparing average and observed high temperatures, but NOAA maps for April will not be published until after this article is in print.

Image 1. Map showing deviation from average
observed temperatures for March 2023
https://www.ncei.noaa.gov/

So, after you establish this early spring was warmer than average, what implications can that have on the timing of natural events? In the forest, spring ephemeral plants emerge and capture most of the sunlight they use to create carbohydrates and grow in just a few weeks before trees and shrubs cast much shade. Most of these flowers are pollinated by bees and flies that time their emergence, based on temperatures, to hopefully occur while there are ample blooms to provide pollen for food. Also, in spring, neotropical birds move north to breed and eat emerging insects and ripening fruits and seeds. In general, these events occur earlier in warmer springs.

One study toward the beginning of the millennium noted how warmer temperatures affected bud break on lilac and birds’ spring migration. It found that for each 1.8°F increase in temperature, the arrival of the many bird species recorded occurred an average of one day earlier, and lilac would break bud three days earlier with the same temperature increase.¹ An article on pollinators and plants syncing up mentions many studies on various topics, including a study of bee emergence and the flowers they pollinate. This study examined collection records of bees and associated plants from the 1880s and determined bees were active and flowers were in bloom in the Northeastern United States about 10 days earlier in 2010 vs. the 1880s, but in some cases, there could be specialist bees active in a time with scarce preferred pollen sources for four to six days.² A study done by researchers to compare observations in spring wildflower emergence in Concord, MA examined first flowering dates of 14 herbaceous species and the first leaf dates of 15 overstory tree species taken during three periods from 1852 to 2018, compared them to temperature data, and determined that trees were more apt to flush leaves earlier in warmer springs than wildflowers, and were putting on leaves nearly 13 days earlier. They also measured spring and summer carbon gain in common wildflowers in a garden at Trillium Trail Nature near Pittsburgh, PA. They then estimated the wildflowers in the 1850s were able to gain 12-26% more carbon through photosynthesis than today. There have been several studies in Japan’s deciduous forest recording reduced seed production in some spring wildflowers during very warm springs, but not others.³

Generalist species, or species that depend more on overstory trees, may be winners in warmer springs. Some of the losers may be shrubs and trees that flower early like Amelanchier (serviceberry), and many orchard trees. They may be influenced to flower earlier in April by warmer temperatures in e, only to have flowers or fruits damaged by May frosts. Plants specializing in growing in a forest during the early spring—and species that utilize them during their lifecycle—may have more hardship in years with a warm spring. There seems to be so much more to be learned in this area as many of the spring wildflower studies state that there are few other similar studies measuring canopy closure and wildflower growth and reproduction.

Image 2. A gray squirrel with a mouthful of
oak flowers. Taken by author on April 17^th
in University Park, PA

What would happen to trees if there were no freezing period in the Northeast? Although we can’t know all the effects, we can look toward apple trees as a clue. Apple trees were imported into tropical regions over the years, notably into a few areas in Indonesia like Batu in the 1930s. What happened to these trees? Without tending, they had no cold temperatures or droughts to trigger leaf die-off and fall, and thus were essentially evergreen. The end of the limbs grew small tufts of leaves and trees were not likely to flower or branch from lateral buds. The apples were cultivated more heavily in the 1960s due to pest problems with citrus fruits. Under cultivation, the trees were completely defoliated by hand after the fruit was harvested. Defoliation triggered flower and vegetative buds to break, and, in this manner, orchard owners could realize two apple harvests per year.

Are there management opportunities to consider if warmer springs continue? If you have spring wildflower populations, you could take your own notes on their vigor and seed production to determine if their population seems stable. If they seem in decline, removing low shade to make small nearby canopy gaps should provide them a boost of sunlight. If you have moist, fertile soils and invasive shrubs, you can remove those shrubs to provide a possible space for wildflowers to take root. Most spring wildflowers are less likely to inhabit dry or nutrient-poor soils.

Natural systems are dynamic, resilient, and fascinating, but they do have their limits, which, in some areas, are yet to be known. So, enjoy unseasonably warm weather as much as you can, and if you are concerned with successive warm springs and the effects on the forest or specific plants around you, learn more about those effects and whether you can make a positive impact. Creating a diversity in plant communities is a good first step.

¹ Marra, P.P. et al. The influence of climate on the timing and rate of spring bird migration. Oecologia 142, 307–315 (2005). https://doi.org/10.1007/s00442-004-1725-x

² Willmer, P. Ecology: Pollinator–Plant Synchrony Tested by Climate Change. Current Biology 22, R131-R132 (2012). https://doi.org/10.1016/j.cub.2012.01.009.

³ Heberling J.M. et al. Phenological mismatch with trees reduces wildflower carbon budgets. Ecology Letters 22, 616-623 (2019). https://compassknox.com/wp-content/uploads/2019/03/Heberling_et_al-2019-Ecology_Letters.pdf