Observed Changes

First Leaf Date

Tracking the timing of leaf emergence each season over time for different plant species provides valuable insight into our changing climate. Plant lifecycles respond to seasonal differences in weather, particularly temperature. Leaf emergence timing is a critical component of the food web and climate change has the potential to mismatch lifecycle timing for some species that are unable to adapt quickly enough.

For example, many insects, including pollinators, depend on the young leaves of specific plants for food. If leaves on these plants emerge earlier, insect species may not arrive in time to take advantage of their food supply. Many migrating birds depend on healthy insect populations to feed their young upon arrival to their breeding grounds. Citizen scientists are working with researchers to track timing of seasonal events in various plant and animal species to get a better understanding of how specific species may respond to changes in seasonal timing.

Researchers are already documenting changes in timing. For example, research by Courter et al. demonstrated that Ruby-throated hummingbirds are arriving at their spring breeding grounds 11-15 days earlier (depending on latitude) when comparing the data from 1880-1969 to data from 2001-2010. These birds rely on nectar for most of their nutritional needs. Scientists are uncertain just how much the changes in the timing of nectar blooms will affect their populations.

Source: EPA Climate Change Indicators

According to the EPA Climate Change Indicators, in Northwest Arkansas, the first leaf date is arriving on average approximately 8 days earlier than it did in the 1950’s. The map below shows trends in the timing of first leaf dates in lilacs and native honeysuckles across the contiguous 48 states. Blue circles represent later leaf dates, and red circles represent earlier. Earlier first leaf dates have been documented in much of the northern and western United States with trends in warming temperatures. Much of the Southeast region is experiencing later arrival of spring. The delay in leaf out date in this region may be due to a lack of chilling days needed by plants to respond to spring temperatures. Chilling day requirements refer to the number of cold hours a plant must experience before budding, generally the number of hours between 32 and 45 degrees Fahrenheit. Each species has different requirements.  

 Changes in Plant Hardiness Zone

Source: Arbor Day Foundation

Plant hardiness zones help farmers and gardeners decide which plants will likely survive in their area. The zones are based on average lowest temperatures recorded each winter. Plants are more sensitive to an extreme low temperature event than an extreme high temperature event. As temperatures warm in a location over time, new plants establish where they were previously unable to survive. These shifts in plant distributions affect the composition of the greater ecosystem. For example, as new plants move into or out of an area, wildlife species that depend on those plants often follow. From 1990-2015, Northwest Arkansas changed from plant hardiness zone 6 to zone 7.

Changes in Growing Season

Changes in growing season 1895-2015     Source: Climate Central

Changes in the length of the growing season directly impacts which types of crops can be planted and when they should be planted. Agricultural practices must adapt to shifts in seasons to be successful. These changes also impact the types of plants that can be successful in our ecosystems. The map below show that nationally the growing season is about 15 days longer than at the beginning of the twentieth century. The western region has seen the greatest change. With warmer temperatures, the growing season in Northwest Arkansas has increased by approximately 10-15 days in the last century.                                                                     

Allergies and Ragweed Season                                    

Change in Ragweed Pollen Season 1995–2015

Allergic illnesses, including hay fever, affect about 30% of the population in the United States. According to the EPA Climate Change Indicators, Northwest Arkansas has experienced an increase of approximately 11 days in the ragweed pollen season from 1995 and 2015. A study by Dr. Lewis Ziska and several colleagues found the longer pollen season is associated with the first fall frost appearing later in the season and longer frost-free periods. As temperatures continue to warm, Northwest Arkansas is likely to experience longer ragweed seasons.

Additional Information

Leaf Out Dates Highlight a Changing Climate

Hummingbirds at Home: The Effects of Climate Change on Feeding Behavior

Spring Arrival Maps 2017

Changes in Phenology: Southeast

Warming Temperatures Drive Up Pollen Production and Allergies