Plastic Tubes or Metal Cages? Rethinking How We Protect Young Trees

Planting trees has many benefits. Whether trees are planted along streams, on agricultural lands, in community public places, or in someone's yard, they can play a critical role in improving air and water quality, restoring or enhancing wildlife habitat, reducing urban heat, and enhancing any landscape's ecological services. These benefits are part of the reason people feel satisfaction when planting a tree. However, planting a tree is just the first step. All young trees, especially those under 5 feet tall, need protection to survive long enough to mature. After all, the benefits just mentioned typically come from mature trees.

One of the best ways to ensure newly planted trees survive to maturity is to use tree shelters (Photo 1). Tree shelters can protect young trees from wildlife browsing. Specifically, deer exert significant pressure on newly planted trees in Pennsylvania. Shelters keep vital woody and leaf material out of a deer's reach by creating a barrier around the tree.

Tree shelters come in two main types: tubes and cages. The tree tubes are typically made of plastic, and their walls limit the amount of wind that can get through. Some of these plastic tubes can be vented, but the small holes still limit wind from entering. On the other hand, tree cages are typically made of metal fencing and provide an open-air version of a tree shelter.  Each of these shelter types has pros and cons (Table 1).

Table 1.  Pros and Cons of plastic tree tubes and metal tree cages

Tree Tubes

Pros

• Protection against herbicide drift
• Can decrease drought stress and aid in tree growth if anchored into the ground.
• The tube can act like a greenhouse
• Cheaper ($5-$7 per tube)
• Lighter weight and easier to handle

Cons

• Could act like a wind tunnel if not anchored into ground
• Can reduce wind stress and temporarily weakens tree trunks

Tree Cages

Pros

• Natural growing conditions and allows for wind hardiness
• Thought to reduce over wintering rodent stress
• Protection from larger wildlife (bears)

Cons

• No protection against herbicide drift
• More expensive ($15-$20 per cage)
• Heavier

One of the most striking differences between tree tubes and cages for individuals investing in new tree plantings is the price differential. Tree tubes can range from $5 to $7, depending on how many are purchased at once. The tree cage price depends on the gauge of metal fence used to make the cage, but it's about 3 to 4 times that of a tree tube, typically between $15 and $20. In addition to price concerns, the weight of a specific gauge of metal fence used in the tree cage creation should also be considered. Metal tree cages are heavier than tree tubes, and this can be a physical limitation for some individuals.

Since tree tubes provide a wind barrier around the young tree, they can be seen as offering both the benefits and drawbacks that a wind barrier would naturally provide. First, for benefits, these tree tubes protect the tree from herbicides used to kill weeds around its base. This is typically called spot treating and is a common new tree-planting maintenance strategy, especially in riparian buffers. Further, because the wind is severely reduced within the tree tube, it has often been thought to provide increased temperature and humidity for the young tree, akin to a greenhouse effect.

The increased temperature comes from the tube absorbing solar radiation during the day and slowly releasing it as heat at night. The elevated temperatures inside the tree tubes are sometimes seen as a Catch-22. A higher tree tube temperature in the spring can buffer the tree and its buds against thermal extremes if spring warms early, but then freezing temperatures occur at night. However, the downside to this thermal difference can be during the summer. Already warm air temperatures can lead to an even greater temperature extreme inside the tube after it is warmed by solar radiation. This extreme heat can cause tree stress, especially for native tree species in Pennsylvania. The amount of stress would depend on the species and the temperature difference between inside and outside the tree tube. For instance, species like American beech, eastern hemlock, sugar maple, eastern white pine, yellow birch, and red spruce have been shown to have negative impacts with elevated temperatures. However, species like black oak, northern red oak, pignut hickory, sweetgum, and white oak have been shown to be more resilient or sometimes favored by increasing temperatures (Butler-Leopold et al., 2018)

Increased humidity levels can be caused by the tree leaves inside the tree tube. During the day, the tree's leaves have holes on their surfaces, called stomata, that open to the air. These stomata allow carbon dioxide into the leaves for photosynthesis, but they also allow some moisture to escape from the tree. This moisture tends to stay in the tube longer because less wind blows over the stomata. Overall, the amount of water that leaves a tree through open stomata during the day depends on the humidity difference between inside the plant and the air directly around the stomata. This difference is called a moisture gradient. Overall, the general thought behind tree tubes is that the moisture locked into the tube itself from a tree's stomata moisture loss will reduce the moisture gradient and lead to less water loss by the tree. This lower water loss is thought to reduce drought stress during drier parts of the growing season, since the tree uses less water from the ground and can photosynthesize longer before the soil reaches a low water content called the wilting point. This theory is similar to studies showing that forests in humid regions, compared to those in arid regions, can be more efficient at using water to fix carbon through photosynthesis (Sun et al., 2024; Winbourne et al., 2020).

It should be noted that the opposite humidity effect of a tree tube can occur if the tree tube itself is not installed correctly. Tree tubes should be pushed into the ground, typically 2 to 4 inches, to prevent wind from entering the bottom of the tree tube and blowing it up past the tree leaves, which is known as the wind tunnel effect. This wind tunnel effect can push a large volume of air past the stomata on tree leaves, causing the leaves to lose more moisture than they would in open-air conditions. Leaves losing additional moisture can lead to even worse drought conditions in drier parts of the growing season, thereby causing additional stress on the young tree.  Another reason tree tubes should be secured into the soil is to prevent rodents from entering the bottom of the tube, making a nest inside, and chewing the tree, potentially killing it.

All the previously mentioned temperature and humidity trends with tree tubes are based on scientific theories from individuals who have spent significant time planting trees. However, since these tree tubes are used on a large scale throughout Pennsylvania, members of the Penn State Extension Water Resources Team, as well as Dr. Tyler Groh's Laboratory, put together a scientific study to directly assess the temperature and humidity differences between trees grown in open-air systems like the metal tree cages and those grown in the tree tubes. This research was initially funded by a Science to Practice grant from the Penn State College of Agricultural Sciences.

For this study, temperature and humidity sensors (OMEGA, OM-92) were hung both inside and outside of tree tubes on one of Penn State’s Research and Extension Riparian Buffers (Photo 2). The sensor on the outside of the tree tube was placed in an upside-down, reflective cup. This cup allowed sunlight, and therefore heat, to be reflected away from the sensor so that it can measure the open-air temperature directly. The sensor inside the tree tube was allowed to hang from the branches within the tube to measure the temperature and humidity that the leaves experience.

The temperature and humidity sensors for this study recorded measurements every 30 minutes. Due to the high measurement frequency and the many sampling points, the readings were separated into nighttime and daytime measurements to assess daily averages for each period. Further, day and night measurements were averaged each day to yield a single daily value representing the impact of tree tubes. To directly compare the conditions inside and out of the tree tube, both the temperature and humidity data outside of the tube were subtracted from the same measurement from inside of the tube and then graphed (Photos 3 and 4). Any temperature or humidity difference below zero represents that the temperature or humidity inside the tube was less than outside. The opposite was also true: positive values indicated that the inside measurements were greater than the outside measurements.  Precipitation data taken from the National Oceanic and Atmospheric Association (NOAA) data collected at the State College, PA airport, was also used to inform discussions of humidity data.

Photo 3 shows the precipitation and humidity difference data for the first two years of this study. A pattern that emerged when assessing data from November 1st, 2022, through May 20th, 2023, was that nighttime average humidity differences were positive, up to 6.7%, while daytime average humidity differences were mostly negative, down to -13%. This meant that at night, the tree tubes stayed more humid, while during the daytime they were less humid. However, during the drought from May 6th, 2023, through June 12th, 2023, when only 0.4 inches of rain were recorded, daytime humidity differences became positive, reaching a maximum of 12%. This indicates that when trees planted in tree tubes are exposed to localized drought, the tubes can create a microclimate of more humid air around the tree, limiting moisture loss from the leaves while the stomata remain open for photosynthesis, thereby reducing tree stress. This chunk of data supports the theory that tree tubes can create a more humid environment for young trees. However, it should be noted that this more humid environment was measured only when air humidity was low during a localized drought.

This trend continued into year two of the data. Most of the time, when it was more humid inside the tree tubes, it was at night. The nighttime humidity differences reached a maximum of 28% on April 3^rd, 2024. The opposite was true for daytime averages, with most of the daytime humidity differences being negative, indicating the air being drier inside of the tree tubes. While this does indicate that the trees would be losing more water during photosynthesis than if they were growing outside of tubes, it should be noted that there wasn't a prolonged localized drought during the 2024 growing season, thus soil water content was not as much of a limiting resource as during the 2023 drought.

For both years of data, the air temperature, calculated as the average of the temperature sensors placed outside the tree tubes, displayed the normal annual temperature swing in Pennsylvania. Temperatures reached a minimum of -2℉ on December 23^rd, 2022, and a maximum of 91℉ on June 2nd, 2023. From November 1^st, 2022, through May 21st, 2023, the daytime average differences indicated that it was warmer inside the tube than outside, while the nighttime average differences were right around zero. This changed right at the time of the localized drought of 2023 and one theory for the lower daytime differences in temperature was that the elevated humidity inside the tubes during this dry time provided evaporative cooling, thus limiting higher temperatures in the tube.

The second year of data, with more consistent rainfall and less extreme air temperatures throughout the year, continued on the initial trend. Most of the daytime temperatures inside the tube were greater than outside the tubes, with a maximum difference of 30℉ on October 20^th 2024. These elevated daytime tree tube temperatures may cause tree stress for native Pennsylvania trees that are not acclimated to such high temperatures. Because of this, the researchers in this experiment wanted to test the relative stress levels of each tree species on the site using a chlorophyll meter (Opti-Sciences CCM-200 Plus).

The species measured during the 2023 growing season were swamp white oak, hackberry, basswood, and white oak. These species were growing in both tree tubes and tree cages at the Penn State Research and Extension Riparian Buffer. The Chlorophyll Content Index (CCI) was measured on 5 randomly selected leaves for each of the 4 species. CCI estimates the amount of chlorophyll in a plant’s leaf, with a greater number meaning more chlorophyll and a higher potential for greater photosynthetic rates. The CCI values in this study were compared for each species between those grown in tubes and those in cages (Photo 5). Ultimately, the greater the CCI, the less stressed the tree.

The CCI results were species-specific. Swamp white oak had significantly greater chlorophyll content, and therefore less stress, in the open-air tree cage, while basswood had significantly greater chlorophyll content and less stress in tree tubes. The hackberry trees seemed to shift from preferring tree tubes earlier in the growing season (June) to open-air cages later in the growing season (September). White oak had greater chlorophyll content in tree tubes in on June 21^st, 2023 and September 28^th, 2023, but had greater chlorophyll content in cages for both August 2023 sampling dates.

So, what do all these results mean for tree tubes and their supposed greenhouse effect around young trees? Overall, the tubes themselves do not provide as much of a greenhouse effect for humidity as originally thought. The only time the young trees experienced a more humid growing environment than the air outside the tube was during a drought. These results should not be minimized, though, as this dry period is when trees, especially those just planted with smaller root systems, need additional humidity around their stoma to reduce drought stress. Also, the ability of tree tubes to buffer temperature extremes during early growing seasons, when a late frost could damage buds, seems limited. The near-zero difference in tube temperatures at night showed that tubes did not retain heat for very long and therefore would do little when temperatures in spring could fall below freezing for extended periods of time. Further, tree tubes seem to elevate temperatures by as much as 30℉ during the daytime. These greater temperatures could prove to stress some tree species more than others. A greater variety of trees species is needed to assess stress from shelter type to determine which species should be in cages or tubes. For now, the take-home message is that tubes can help mitigate some of the tree’s drought stress in very dry conditions, but they also provide much warmer temperatures during the day.

Acknowledgements:

The research work described here was made possible by the Science to Practice grant opportunity through The Pennsylvania State College of Agricultural Sciences. Further, the co-investigators of this work were Emily Rojik, Jennifer Fetter, David Jackson, Danielle Rhea, Kristen Koch, and Sarah Xenophon.

Citations:

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