The other day I was in the middle of the stream and took a rare moment to glance up. Towering above were several large hemlocks playing gatekeeper to the hot sun trying to peak through. In addition to being one of the few trees I can identify, hemlocks are one of the most common species of riparian vegetation in trout streams. But, invasive insects are threatening the health of hemlocks, which could have devastating effects on brook trout populations.
I know nearly nothing about trees and certainly couldn’t do this topic justice. So, I asked my colleague Erynn Maynard, a PhD student in the Ecology Program at Penn State studying invasive plants, to catch me up to speed on the plight of the hemlocks.
Do you have a favorite hemlock grove? If you spend time outdoors in the northeastern United States and in the southern Appalachian Mountains, you likely can think of a place where this species casts their deep purple shade. Within this region, hemlocks flourish in riparian areas near clear, flowing water, and even thrive in rocky outcroppings and stream-edges. Their roots appear to cling and hug to rocks and squeeze between crevices. They are frequently, although not always, accompanied by the graceful sweeping branches and large leaves of the evergreen shrub, rhododendron (Rhododendron maximum).
While the eastern hemlock (Tsuga canadensis) has tiny needles (1/3 to 2/3 inch in length), they are arranged in a single plane along their many branches to provide dense, evergreen shade (a must-have for trout!). Hemlock seedlings can only grow in a dark, moist litter bed, and so they require the deep shade of a hollow and/or other trees to first pave the way and grow large enough to cast shade on the soil below. Once a hemlock seedling germinates, slowly, over many decades, the hemlock will grow and continue to reach for the canopy. Once it inches above its fellow competitors, all other tree species fail to regenerate and only the occasional beech tree will survive in the shady understory. This is why old hemlock stands are what ecologists call a ‘climax stage,’ meaning that the trees species that make up a forest doesn’t change much once hemlock take over. Well, that is until the hemlock woolly adelgid was introduced.
The hemlock woolly adelgid (Adelges tsugae) is an insect that resembles an aphid and sucks sap from its host trees near the base of the needles. Its native range in East Asia, the woolly adelgid is usually not capable of killing or even severely impacting the health of its host tree. However, in the United States, trees do not have defenses against the pest, and hemlocks typically succumb to adelgid infestation within five to seven years. If you do have a favorite hemlock grove in the southern parts of the hemlock range, you may have noticed it thin out and lose the purple cast shade over the recent years. Sadly, in many areas, the grove may be gone completely.
Why does this matter for brook trout? This thinning of the canopy over coldwater streams allows more sunlight to hit the water causing increased stream temperature in the summer, which brook trout are very sensitive to. In the winter, because air is cold and the ground is warm (comparatively), streams are actually a few degrees warmer when under a dense hemlock canopy and this warmth could facilitate development of brook trout eggs. In some cases, rhododendron shrubs, which have large evergreen leaves, and already existed in the understory can thicken and provide a similar shade and buffering effect. However, in many cases, this shade may also prevent trees from re-establishing at the site, which has many other impacts to the terrestrial and aquatic ecosystems.
Rhododendron is also not an equal replacement for hemlock because they are smaller and are not able to regulate water levels in the same way as the larger, denser hemlock. You can think of each hemlock tree as a storage container for water. When it rains, hemlocks soak up water preventing it from immediately flooding streams. When it’s dry, hemlocks slowly release this water back into the atmosphere. So, loss of each hemlock means an increased risk of flooding and less water available during droughts.
When trees are able to re-colonize after hemlock departs, they tend to be deciduous trees, meaning that they lose their leaves seasonally and are bare in the winter. Unlike the evergreen hemlock, deciduous trees are not actively storing and using as much water during the winter. This results in higher stream flows in the winter when deciduous trees are dormant, but also lower stream flows during the spring and summer as deciduous trees are using a lot of water to grow and maintain their big floppy leaves. In fact, streams with hemlock have a much lower chance of going dry in the summer than streams lined with deciduous trees in the same watershed. This water regulation service provided by hemlock helps keep stream flows consistent for fish, but also invertebrates. An increased chance of seasonally drying is one of many factors that impacts the abundance and diversity of insects and other invertebrate organisms in the stream. Hemlock streams have more of these delicious creepy-crawlies than hardwood streams do, which may be why some studies have found up to three times as many brook trout in hemlock streams as compared to deciduous streams.
What is being done about this? Well, individual trees can be chemically treated with insecticide (on the trunk or in the soil where it is absorbed into the entire tree) to prevent adelgids from eating them. This is obviously time and cost prohibitive in large and remote areas, and is usually only done in parks and landscaping settings. However, this is not a permanent solution, but merely a hold-over to keep large trees alive until predators are established for the adelgid. While the hemlock woolly adelgid was introduced accidentally on shipped plant material, its insect predators were left behind and nothing in the United States finds the adelgid quite as delicious as its predators from East Asia. Currently, there is a mad scramble to find something from the adelgid’s native range that will eat only the adelgid and that can be reared and released here in the United States. But, this type of biological control agent comes with risks as well. It’s difficult to predict what an introduced insect predator will and won’t eat in the wild in a new range. This means that many people are very opposed to intentionally introducing a new non-native species, however, no tree species native to the U.S. will replace the niche of the eastern hemlock.