Tree Heaven Trail

Eastern hemlock often grows in nearly pure (single-species) stands. It is very tolerant of shade, which allows it to grow underneath other trees, including itself. At the same time, hemlocks leave very little light for plants beneath them due to the structure and arrangement of their branches and needles. Barely 1% of full sunlight reaches the ground beneath a deep, closed-canopy hemlock forest on sunny days. Consequently, hemlock stands are dark and the ground vegetation is scarce.

After agricultural abandonment, this stand consisted of old-field white pines that grew in a former pasture around 1890. Most of the overstory old-field pine was removed from this stand in a salvage cutting made after the 1938 hurricane, releasing the hardwoods and hemlocks. The few large pines that remain here are now what we call emergents: their crowns reach up above the main canopy. The trees in this stand are of different species and sizes, but they are all approximately the same age — the pine is only 20-30 years older than the other species.

A crown thinning in 1984 opened up space and light from the canopy, allowing pines to regenerate in some of the openings following a mast (a year when the mature pines produced an exceptionally large amount of seeds) in 1993. However, this cohort of younger pines are now being suppressed by shade as the canopy closes.

To age the pine regeneration, you can count the whorls of growth. Each whorl (i.e. one level of branches arranged concentrically around the main stem) represents one year of growth.  If you count the whorls, you will see that the pines here are all the same age, but they are growing at different rates depending on which microsites they occupy and how densely they are spaced.

Mountain laurel is an evergreen shrub (and the State Flower of Connecticut) that we often find in the understory of our forest. It responds vigorously to canopy openings and can quickly occupy recently released growing space. Its fast growth is aided by its ability to produce clones through layering (the process by which individual branches take root and expand the plant’s reach across the ground). It can also re-sprout after fire and is well adapted to the nutrient poor soil often associated with abused and degraded pastures.

All of these characteristics make mountain laurel a strong competitor with tree seedlings that might otherwise establish on the forest floor. Mountain laurel is often associated with hardwood forests that arose after pastures were abandoned, because sheep would graze on all the other species and leave the poisonous laurel behind.

Just as mountain laurel is affected by both site conditions and land use history, so too is the regeneration of evergreen tree species (the most common here are hemlock and white pine). Hemlock seedlings tend to dominate on shady, wetter sites, where soil water is more freely available to their shallow roots. On drier or less shady sites, white pines are more common. White pines have deep taproots, making them less susceptible to drought, and they are less shade-tolerant than hemlock. In addition to sprouting up as old-field pine in abandoned fields, pines are also often found underneath hardwood canopies on drier sites.

From this stop, we can see all three of our most common evergreen tree and shrub species, growing from the understory to the canopy. This spot was previously an old, abandoned pasture (indicated by the prevalence of mountain laurel), and is positioned on a hill between a moister downslope (which favors hemlock) and a drier upslope (more conducive to pine).

Forest dynamics are tied both to disturbances and to different growth patterns among species. Large disturbances, such as hurricanes or timber harvesting, remove many trees and provide opportunities for new trees to grow in their place. Often, a number of different species will establish soon after a disturbance, but some grow quickly while others grow more slowly and steadily. The fast-growing trees soon reach the canopy, but over time the slower trees will pass them by. In general, species that require more light grow faster, while those that are more shade-tolerant will grow more slowly, essentially waiting for their turn to dominate the canopy. At any given time, the composition of different layers of the canopy will depend on not only the species present, but also the amount of time since the last major disturbance.

The stand that we see today originated after the destruction of old field pine in the 1938 hurricane. In the 1950s and 1960s, foresters were puzzled that this same stand was seemingly dominated by red maples, with only a few red oaks. This was a widespread phenomenon for stands released beneath cut-over or hurricane-destroyed old field pine. In the same stands during the 1970s, studies revealed that the slower-growing red oaks overtopped the faster-growing red maples and black birches after only about 30 years. The oaks grew steadily in height while the other species slowed down, likely because they shut down height growth when water is unavailable, and unlike the oaks, cannot resume it later. Pioneer gray birches had already dropped out, but a few natural hybrids of gray and paper birch still persist today. There were probably some chestnuts initially in the emergent layer, but they succumbed to the chestnut blight around 1925.

At this stop, you’ll see different colors of paint marking the trees. “Trees of the past,” those that have already been passed by (such as black birch and red maple), are painted orange. “Trees of the present,” which make up the current canopy (the oaks), are blue, and “trees of the future” (such as beech and sugar maple) are painted green. Notice that both the trees of the past and future are found underneath the oaks that now dominate the canopy. The light-demanding gray and paper birch does not last long after the oaks have surpassed it.

In contrast, red maple and black birch can survive under the oaks even though their crowns show significant signs of dieback. The more shade tolerant hemlock, sugar maple and beech represent the future. When the oaks are removed or lost to disturbance, those usually longer-lived trees will be ready to replace them.

Trees need growing space. This “space” includes light, nutrients, water, and literal physical space. Physical space is important because tree growth is closely related to crown size. At this stop, not all the oaks are growing well. Although the oaks have already outgrown the red maples and birches, their crowns cannot fully expand because they are competing amongst themselves. The smaller, more suppressed oaks have responded by extending their crowns downward along the stem, but this is less effective at capturing light than having a larger crown in the canopy.

If the goal is to accelerate stand development here, trees with crowns adjacent to the larger, well-formed oaks should be removed. This silvicultural treatment is called a crown thinning, and it focuses only on the trees that make up the present canopy. In this case, some of the oaks marked in blue should be cut for the benefit of the neighboring oaks, marked in orange, whose crowns they are competing with. These orange-painted oaks represent crop trees that will be harvested in the future. Not all of the blue trees would be cut at once; instead, we would cut every other or every third, focusing most on the ones that are competing directly with the orange crop trees. In a crown thinning, any trees that are not competing with trees of the present canopy are ignored. Because this treatment focuses only on oaks, it will not alter the species composition of the stand. Rather, it encourages structural diversity by both encouraging the growth of the remaining canopy trees and also letting some light into the lower strata to release the more shade-tolerant species below.

There are research plots for several studies at this stop, which examine how variations in site affect species composition of tree seedlings in forest understories. The fifteen sites used for these studies are all located under closed canopies and include ridges, midslopes, and valleys across this forest. The ridges are shallow to bedrock with dry soils. The midslopes have moister and deeper soils than the ridges, and the valleys have the deepest, most fertile soils. The particular site before you is considered a midslope.

There are many tree species that are found in different forests or in different parts of the same forest. As part of a long-term study asking what accounts for this diversity, we planted seedlings in 1990. The purpose of this research is to investigate the performance of different oak species, and to discover which species grow best in the different topographical positions of a slope. The results will improve our understanding of why certain oak species are found where they are and will lead to better guidelines for forest management and planting guidelines. You could ask the same questions regarding other closely related species, such as the hickories, birches, and maples, that co-occur in this forest.

The young sapling stand before you is one of four gaps that were created in 1999 in a study designed to examine how forest vegetation responds to variation in microsite conditions. The long side of this rectangular gap lies in an east-west direction. This orientation, combined with the asymmetry of light in northern latitudes, result in different combinations of light, moisture, and other related differences in the microenvironment across the gap and the adjacent understory following forest harvesting.

This site illustrates how past land use can have a dramatic and long-lasting effect on a forest stand. This stand was once a farmer’s woodlot. Oak, maple, birch, and chestnut would have grown here alongside the hemlock. These hardwoods are much better for fuel and timber than hemlock, so the farmer would have preferred to cut and use them. Repeated cutting of the hardwoods allowed the remaining hemlocks to take over the site. Since then, the heavy shade cast by the hemlock has effectively prevented other, less shade-tolerant trees from returning to the stand.

The first part of the demonstration area is the control. No trees were cut here, so this section can be used as a basis for comparison. Like many stands in this area, the control is a single-aged stand that arose from old pasture abandonment around 1900 and has been crown thinned twice. As you walk through the demonstration area, you will see three types of shelterwood: uniform, irregular, and group. In the control and the three treatments, seedlings and remnant trees are being measured to see which shelterwood is best for oak regeneration and overstory growth. Evident throughout the demonstration area is the concept of “past, present, and future trees” and the concern for adequate spacing between oaks. The shelterwood treatments were created in the fall of 1998.

In a uniform shelterwood, the oak trees retained are distributed in a regular, equally spaced pattern. These reserve trees were originally selected because of their height, their long, clear, straight trunks, and their large crowns. The 50-foot spacing of these trees allows for even spread of seedlings and relatively uniform partial shade. Although it is not easy to judge by eye, the average light level beneath the crowns of the remaining trees is 50% of the amount of sunlight in the open. You can think of this treatment as simulating a very tidy tornado or hurricane.

Oak regeneration here is present, though scarce. Look for the small, pole-sized oaks, which represent seedlings that were released by the shelterwood harvest. You’ll also see abundant black birch and sprout-origin red maple (which you can identify by its multi-stemmed trunks). Following the harvest in 1998, the regenerating stand comprises canopy emergents of sprout origin red maple, a more uniform and thick canopy of black birch, occasional red oak and pine saplings in the midstory, and seedlings of hemlock and sugar maple in the understory. The large oaks have remained in the overstory and would normally have been removed by now in a Uniform Shelterwood. We have left them, making these an older legacy of the original stand that will remain as reserves and therefore would be now termed an irregular shelterwood. When a good crop of acorns is produced in a mast year and new seedlings are well established, the trees left after the first cutting could be removed. This second cut would make growing space available to the seedlings so they develop into the future crop of tall trees.

This type of treatment is a modification of the uniform shelterwood. The first treatment was in 1999 and the second occurred in 2011. Here, a second layer of trees has been left in addition to the tall oaks of the uniform shelterwood: shorter and more shade-tolerant sugar maples. The oaks and maples are the same age, but the oaks are the trees of the present canopy while the sugar maple is the tree of the future canopy.

This shelterwood is more diverse in structure, with two layers that provide extra habitat and additional shade. The shadier forest may provide better habitat for amphibians (newts and salamanders), rodents (mice and voles), and interior forest birds (ovenbirds, black and white warblers). The shade will also benefit the more shade-tolerant seedlings of black birch, red maple, sugar maple, and pine, but the oak and the fast-growing shade-intolerant seedlings such as paper birch and black cherry will not do as well. Provided the sugar maple trees are adequately spaced, they can be left when and if the tall oaks are removed and a new mixed stand will grow up around them. However, the maples must be left at a distance such that their crowns will not grow into each other and impede the growth of the newly regenerating cohort beneath. When some of the tall oaks are removed, a two-aged stand will be created, comprised of the older sugar maples and some oak, and the younger mixed cohort of mostly birch and maple that grow up around them.

Group shelterwoods are created when patches of trees are removed and the majority of the stand is initially left intact. In this demonstration, there are four gaps in the canopy, each 100 feet in diameter. After successful establishment occured in the center of the gaps, a second cut in 2012 removed more of the trees along the edge of the gap, expanding it. Another treatment further expanding the gap and taking out some of the subcanopy is intended in 2024. A third and final treatment in 2031 should finish regenerating the complete area. This type of treatment is also called a femelschlag in German, which means "expanding group shelterwood."

This sequence of establishment and subsequent expanding cuts will gradually increase the size of the gap, as well as increase the regeneration in the center of the gap. Over time, this is intended to create a forest with groups of trees with a wider range of ages. This could be beneficial to wildlife because certain wildlife species need different sized and aged trees during different parts of their lives.

Throughout this area, and within the shelterwood treatments, are trees that are relics of the former pastures and open fields. These trees, called wolf trees, grew in the open before old-field pine colonization and the establishment of second-growth hardwoods. Most of the wolf trees are sugar maples that are likely to be over 250 years old and are a signature of the forest composition prior to land clearance for agriculture. At Yale-Myers Forest, we do not cut wolf trees, regardless of the stand treatment. Wolf trees are important sites for roosting birds and dens for nocturnal animals. They are also important legacies of the history of human land use across the forest.

Here, you’ll notice that the sugar maple wolf tree that served as the focal point for this trail stop for many years has finally succumbed to its old age. It has left behind a small gap, allowing for light to reach the ground and promote the growth of a new cohort of trees. It is also now serving another important role as woody material on the forest floor, where it provides shelter for wildlife and will enrich the soil as it decomposes.

In this type of shelterwood, scattered groups of trees are left uncut in addition to the well-spaced oak and pine. These reserve groups are primarily composed of hemlock of various heights, but other single-tree reserves of other species are also represented. Whereas in previous stops we used the term “reserves” to indicate trees that might later be harvested, here the reserve groups are intended to remain in the stand.