What Makes Some Tree Species Prolific Stump Sprouters?

At some point, you have probably walked through a forest path and seen a cut stump on the ground. From this cut stump, amazingly, you could see fresh shoots with leaves growing up from its base. But why are some species better at it than others? Why do some trees fail to do it at all? Stump sprouting gives an extra chance for survival after extreme disturbance. To try to understand why some species may sprout in certain conditions, we first need information on how trees can sprout from their stumps.

Stump sprouts come about from suppressed dormant buds at the root collar of a tree – the meeting place of the stem and roots – that become active in the case of injury or extreme environmental changes that induce stress. While there are dormant buds that exist all over the tree, sometimes referred to as epicormic buds, they typically have a short lifespan after emergence and give rise to branches, not new stems (trunks). Dormant buds at the root collar, however, have traces all the way to the pith and in some species may live just as long, if not longer, than the main stem. They formed when the tree first put out roots and shoots.

The dormant buds grow slowly along with the tree, staying near the surface of the living wood beneath the bark. They are also believed to be genetically more juvenile, which means that these buds are not necessarily a continuation of the tree's life but more of a revitalization, rebirth.

From statistical analysis done in 2022, it was found that for some temperate hardwood species, stump sprouting probability had a relation to site conditions (Nieves Et Al. 2022). There was a strong negative correlation among certain species when it came to probability of sprouting in relation to age and size, excluding only Northern red oak from the trend.

It was also found that drier moisture regime sites (sometimes used as a predictor of wildfire frequency) saw an increased probability of sprouting. Because there are trends between and among species that are consistent with site factors, we can make an inference that among angiosperms, stump sprout likelihood is determined by underlying genetic factors that arose from evolutionary adaptations to disturbance regimes.

It is believed that all angiosperms have the ability to stump sprout. Gymnosperms, on the other hand, rarely do. One commonality between most of the gymnosperms that have this ability and angiosperms is the number of cotyledons. Cotyledons describe the embryonic leaves within seed-bearing plants. While angiosperms typically have two, gymnosperms can have between two to twenty-four. Both major stump-sprouting conifers we know of have two cotyledons and often reproduce from root suckers and stump sprouts. Those are Sequoia sempervirens (coast redwood) and Taxus brevifolia (Pacific yew). Our outliers, however, specifically two pines, P. Rigida (pitch pine) and P. echinata (short-leaf pine), are known to sprout but have more than 2 cotyledons. But, like some of the angiosperms described earlier, pitch pine's ability to sprout decays with age, and the older the tree is when it sprouts, the less likely it is to have long-surviving stems. This may indicate that while it sprouts from its stump, the dormant buds it uses to do so are morphologically different from the earlier described dormant buds arranged around the root collar because the sprouts show age effects. Knowing that pitch pine is a fire-adapted species, it could have evolved this way as an adaptation to fire. While the shoots may not start a fresh tree, they allow it to get some seed out before it perishes after a damaging disturbance. So even if it may sprout from its stump, it does not do it in the way that angiosperms and the 2-cotyledon conifers do; it rarely results in a fresh, genetically strong stem.

 So, stump sprouting seems to be an adaptation to disturbance. Some of the angiosperms that we discussed earlier as having lower sprout probability with greater age and size are also species that grow in sites with historic fire regimes. It is reasonable to assume that oaks with thicker bark would have less necessity to sprout in their later years. And the fact that red oak does not follow the trend could be explained by its tendency to grow in drier soil while black oak resides in wetter soil. Even in its later years, red oak will be susceptible to fire, but black oa,k with its greater access to moisture, may resist the typical fire regime of the region. The prolific stump sprouting conifers seem to have morphologically similar dormant buds in the root collar to angiosperms, leading to juvenile offspring, whereas the other conifers have a weaker ability or none. But all the conifers that sprout live in areas with fire regimes and have adaptations related to fire protection.

So, what makes some tree species prolific stump sprouters? The most likely answer is evolutionary adaptation in dormant and epicormic buds to disturbance regimes that allow species to reemerge swiftly after major disturbance.

Resources

Sander, Ivan. "Quercus Rubra L." USDA Gov,  Accessed 13 February 2023.

Jennifer M. Nieves, Jeffrey S. Ward, Alejandro A. Royo, Marc E. McDill, Jesse K. Kreye, Kim C. Steiner, Stand and site characteristics affect the probability of stump sprouting in some eastern North American hardwoods, Forest Ecology and Management, Volume 511, 2022, Accessed 13 February 2023.

Little, Silas, and Peter Garrett. "Pinus Rigida Mill." USDA Gov,  Accessed 13 February 2023.

Sander, Ivan. "Quercus Velutina Lam." USDA Gov,  Accessed 13 February 2023.

Del Tredici, P. Sprouting in temperate trees: A morphological and ecological review. Bot. Rev 67, 121–140 (2001).

Butts, Dennis and John Theodore Buchholz. "Cotyledon numbers in conifers." Transactions of the Illinois State Academy of Science 33 (1940): 58-62.