Home / Blogs / Unseasonable Conditions and the Responses Found in Our Forests
March 8, 2023
As I write this article, the temperature outside my window here in northern Maryland is 60 degrees (and the sun isn’t even shining). Just a week ago (late February) it was 70+ degrees across much of the State. It’s hard to believe that we’re hitting these temperatures in February and March! Walking and driving around, I see red and silver maple trees in flower, and they have been for the past two weeks. I know that throughout the Chesapeake Bay Watershed, we aren’t unfamiliar with wacky weather and inconsistent temperatures. However, I can’t help but dwell on the mild conditions over the last few winters, and particularly this “winter” we just experienced. In truth, I find these unseasonably warm days both enjoyable, and worrying.
I’m sure I’m not alone when I say that I’ve greatly enjoyed sitting on my back porch on evenings like this, basking in the sun and dreaming of the spring season to come. Almost as soon as I find my mind drifting into that space, something snaps me back to the present, acting as a silent alarm. This alarm rings to the tune of “Where did winter go? Was it ever here?”. I can’t help but think about the life cycles of our plants, their seeds, insects, and animals which all respond to the seasonal conditions around them (length of day, temperature, soil moisture, etc.).
Let’s start off by discussing everyone’s (unofficial) favorite season, fall. Who doesn’t enjoy the change of season marked most notably by our deciduous hardwood trees, with leaves of green turning to shades of orange, yellow, and vibrant red! We also get terrific seasonal fruits such as apples and the largest fruit indigenous to the United States, the paw paw (Asimina triloba). You can even find witch hazel (Hamamelis virginiana) flowering in the fall. It’s during this season that our trees are preparing themselves for winter in response to longer nights and dropping temperatures. When we see the colors change in the leaves of our deciduous trees, we are actually watching the breakdown of chlorophyll, which are produced throughout the growing season and give the leaves their green color. What is left behind are the other pigments found within each leaf, namely carotenoids and anthocyanin.
Images left to right: Redbud (Cercis canadensis) left and two yellow poplar (Liriodendron tulipifera) leaves changing color, both photos taken in October of 2022 (Photo by Zach Carnegie).
Carotenoids are produced throughout the year, and can be noticed when our trees exhibit signs of chlorosis (yellowing of the leaves, sometimes observed to be pale-green). This can be caused by a myriad of stressors such as drought, iron deficiency, even high concentrations of lead in the soil, which lead to an underproduction of chlorophyll. However, we observe yellowing of leaves in the fall as part of a natural cycle. Beginning sometime when temperatures begin to drop, particularly at night, deciduous trees produce a corky membrane which effectively seals off the leaves from the branches and twigs they are attached to. Without the flow of water and nutrients to sustain and replace the chlorophyll cells, they break down. Species which contain higher levels of carotene turn a vibrant yellow. In others, the breakdown of the sugars contained in the last of the chlorophyll cells reacts to sunlight, turning red! Eventually these leaves fall from the canopy and carpet the floor of our forests. Now, where I live in Maryland, we haven’t had the greatest display of colors in our leaves over the past several autumns. Our yellows have been spotty and turned brown rather fast, and our reds have not been very vibrant, and if you keep a close eye on the conditions, and weather patterns of the season, you’ll know why.
There are several factors that contribute to a strong vibrant color change. Mother nature’s peak fall foliage recipe follows a few conditions: sunny days, cool temperatures (especially at night) but generally above freezing, and relatively dry conditions. When we have warmer fall weather the chlorophyll doesn’t break down as rapidly, and higher rates of rainfall dilutes the sugar concentrations within the leaves, reducing anthocyanin production. Cloudy days block sunlight which would also help break down chlorophyll. If patterns like this persist, our fall foliage winds up being drab. In recent years, I’ve had to travel to parts of the Bay watershed that are at higher elevations in order to get a good taste of the vibrant fall color.
While I do wish for more vibrant fall foliage, I have a greater concern for the milder temperatures we’ve experienced over the course of winter. During this time of year, our trees stand dormant, meaning they’ve drastically reduced metabolic activity, and no growth occurs, especially in the above ground trunk and branches. Within the layers of sapwood, the sugars that were produced during the growing season are stored. This cache helps keep the living wood alive over the winter since there are no leaves to produce more “food”. The stored energy will also be used to produce the flush of new growth in the Spring, starting with flowers, buds and leaves.
What concerns me with these later winter months is the drastic temperature swings that are becoming more common and less predictable. For weeks we may have little to no freezing temperatures, many days at close to 50 degrees or warmer. Recently, I was at several reforestation sites in Washington County, MD where some of the young trees had already broken buds and leafed out. This was in late February mind you! While this early leaf out may give them a head start on food production, there is a high probability that a cold snap sets in before winter is truly over. When temperatures drop below freezing, it can kill off those newly grown leaves, wasting precious resources and forcing the tree to invest more stored energy to the growth of a new set. This might not sound like much of a setback, but if you consider that the other conditions may not be ideal, such as drought, these young trees may experience extra stress on top of wasting energy regrowing leaves. I also find myself thinking about the cycle of our native trees/shrubs and the relationships they have with the insects which rely on them.
Many of our insect and other wildlife species respond to changes in the seasons, ultimately driving their life cycles. Thanks to evolution, it’s no coincidence that these cycles closely match up with those of our native plants. Some insect species rely on a small set of host plants to carry out their larval stage, such as the spicebush swallow-tail and the spicebush shrub (Lindera benzoin) or sassafras tree (Sassafras albidum). When insects emerge, it’s critical that their host plant(s) be ready to receive them. If the seasonal conditions have been erratic enough, they may emerge to find less food available to them during those early stages of their life cycle. If the insects emerge too early, or a cold snap sets in later than usual, it can affect their survival as well. However, some of the arachnids found within our forests and grasslands are taking full advantage of these unseasonable conditions.
Black-legged or Deer tick (Ixodes scapularis) (Photo from the CDC website, tick gallery).
I’m talking about ticks. Whether it was a day in the field or a weekend hike, there wasn’t a month this winter that I didn’t find ticks crawling on me or my dogs. It wasn’t just one or two, but many. This wasn’t much of a surprise, given the temperatures here in northern MD generally stayed above freezing, and the ground barely froze solid. It’s said that ticks become active (searching for a host) when the temperature is about 40 degrees and warmer. This means that during a mild winter, they may never be forced into dormancy, free to crawl around and find their way onto humans and critters alike that happen to come along, increasing the rates of dispersal throughout the winter. Researchers are spreading warnings that this could lead to increased rates of tick-borne illnesses, the most notorious of which is Lyme disease. Unfortunately, this is likely to become a greater issue as climate change continues to alter our seasonal patterns.
Whether you missed cold temperatures, or you’re anxiously awaiting spring to set in fully, I’d encourage you to observe the native plants around you. Take notice of when their buds begin to swell, they first begin to flower, or they fully leaf out. Observe when leaves change colors in the fall, and their relative vibrancy. Personally, I’ve started to keep a journal, with observations from one season to the next, and I hope to add general weather patterns for reference! It’ll enable me to keep a close record of the climatic conditions in my area, and the subsequent responses by the species found in our forests. Many naturalists of the past centuries kept similar records, some of which have been referenced in studying the impacts of climate change. If nothing else, it provides us a convenient reminder of how the natural world around us is responding to the seasonal conditions, however unseasonable they may be.
Forests for the Bay