https://orcid.org/0000-0002-3862-9563
All major lineages of living vascular plants have tried their hands at being trees; this repeated evolution of the woody, single-trunked growth form has been driven over the eons by competition for light and a complementary life history strategy emphasizing longevity (King, 1991; Piovesan and Biondi, 2021). Forests of long-lived tree species have dynamically shifting patterns of competitive and facilitative interactions between neighbours, where the growth of individual trees is released or suppressed by sometimes sudden changes in light availability as trees die and others rush to replace them (Lutz, 1928; Franklin and Van Pelt, 2004). In this issue of Annals of Botany, Decombeix et al. (2024) describe a fossil tree trunk that preserves a record of relatively quick growth followed by abrupt and lasting suppression. Although we can imagine that the biotic and abiotic challenges of trees in the Triassic were much the same as those faced by trees today, this is the first time a tree fossil convincingly capturing an ancient ecological growth-suppression event has been found (Decombeix et al., 2024). Competition among trees is the general rule, with neighbour-induced growth suppression typically a product of one ‘dominant’ tree outgrowing and overshadowing another; however, lost facilitative interactions – where a large tree has been subsidizing the growth of a smaller one – also induce suppression when the donor tree dies (Chin et al., 2023). Shade-tolerant trees can establish beneath an overstorey, waiting in a suppressed state until their spot in the upper canopy opens (Franklin and DeBell, 1988; Sinton et al., 2000), while some of the longest-lived trees establish in the open but have the phenotypic plasticity to acclimate to long periods of light limitation (Piovesan and Biondi, 2021). In addition to competition from other trees, a strategy of longevity requires the ability to resist once-in-a-century (or far more!) climatic events, fires, lightning, windthrow, pests, falling neighbours, disease and the many other vagaries of the forest. Such is the life of trees.
What would evidence of among-tree interactions look like in a fossil tree trunk? After removing interannual climatic variation, and assuming a generally favourable region, we can picture the growth history of four trees within the same species and form simplified hypotheses (Fig. 1). Tree A has a favourable, low-competition site and lots of luck – it grows consistently fast (Fig. 1A); Tree B has a limiting site, perhaps due to poor local soil or the presence of strong competitors – it grows slowly over its whole life (Fig. 1B). The other two trees experience an abrupt ecological event resulting in persistent changes to growth-limiting factors. For Tree C this event is negative, and limitations increase, resulting in growth suppression (Fig. 1C), while for imaginary Tree D, conditions improve and growth is released (Fig. 1D). Trees A and B have consistently low or high levels of local environmental limitation (Fig. 1), making them useful to palaeodendrochronologists and morphologists – and thus what we see in the literature. Focusing on the potential to uncover ecological interactions, we might hunt for the fossils of Trees C and D, where a novel biotic or abiotic environmental change altered the tree’s growth trajectory (Fig. 1).
Simple hypotheses for reflections of ecological conditions in fossil tree rings. Trees A and B are null scenarios where ecological conditions are so constant that it is hard to detect among-tree interactions. Following an abrupt event leading to lasting changes to growth limitation (suppression or release) we would expect to find Trees C and D. While Tree C has abiotic explanations, Tree D provides strong evidence of ancient growth release due to neighbour loss/gap formation.
In Antarctica’s central Rennick Glacier area in the Middle to Upper Triassic Helliwell Formation, Decombeix et al. (2024) discovered the equivalent of our imaginary Tree C (Fig. 1C), an individual conifer where suppression was abruptly induced at minimum-year 30, and then a very slow growth rate was maintained for the following ~90 years until tree death. A truly enviable find. After making a strong case for a mild regional climate, the authors propose that this tree exhibited suppressed growth due to either soil limitation or shading by a dominant neighbour (Decombeix et al., 2024). Thinking about our expectations for Tree C, we can contemplate some scenarios for the tree of Decombeix. Was it soil? There might be a very few events that could rapidly decrease local soil quality, but poor soil seems most plausible for Tree B, where a consistently limiting environment leads to a lifetime of suppressed growth. The sudden loss of a water source can be excluded because tracheid dimensions did not decrease, and pest or disease damage seems unlikely to last 90 years with no sign of recovery or worsening (Decombeix et al., 2024). Physical damage to trees includes scarring from falling neighbours, rock strikes, burns, wind snapping and lightning. Rock strikes, damage from neighbours and most survivable burns would have impacted one side of the cambium, making such damage unlikely to be the case here because post-suppression radial growth was consistent around the trunk (Decombeix et al., 2024). Lightning and wind can potentially lead to the loss of most of the upper crown of a tree, so we cannot rule out severe topping but might expect some signs of recovery after nine decades. Lost facilitative interactions seem unlikely; it is hard to believe that the death of a neighbour would lead to 90 years of dramatic and unrelenting growth suppression. With this information, it seems that new competition is the most reasonable explanation, where a rapidly growing neighbour shaded Decombeix’s tree, and this suppression of growth continued in a similar manner for the rest of the small tree’s life.
However, with a few grains of salt, we can perhaps deduce a bit more about the ecological context of Decombeix’s Triassic tree through its morphology and the environments similar conifers grow in today. The fossil trunk segment was 2 cm long, with about three nodes each bearing original branches roughly 3 mm in diameter and 1.5 cm apart (Decombeix et al., 2024). These thin branches do not appear buttressed (see images in Decombeix et al., 2024), suggesting that they are very short if plagiotropic, and more likely geotropic (pendant) giving the tree a narrow, ‘weeping’ aspect. Such trees exist in polar regions today, where the form promotes light interception at a low solar angle. Can light dynamics of high latitudes support vertically stratified canopies? Modern shade-tolerant trees typically have widely spaced plagiotropic branches leading to flat crowns (Niinemets, 2010). Would weeping dwarf conifers be found in an understorey? If the fossil tree was semi-exposed, then a dramatic topping event may be more likely than shading, and further, explains how such densely packed branches are maintained for 120 years without self-thinning.
To be fully convinced that we have fossil evidence of among-tree interactions, we really need to find Tree D, a specimen exhibiting clear and prolonged growth release (Fig. 1D). These fossil trees must be there; the competitive processes that drove their evolution promise it (King, 1991). Discoveries such as this will require a focus on fossil trees as individuals, some of which may be awaiting notice by ecologists in existing palaeobotanical collections. Centring on the ecological struggles of individual trees may allow us to detect facilitation as well as competition (Chin et al., 2023), and is a promising new approach for glimpsing the deep past of forest ecology. This will necessitate the development of clear a priori hypotheses of what we expect to find (i.e. more nuanced than in Fig. 1) with support from the detailed literature on wood and growth release (e.g. Renninger et al., 2006), and a combination of dendrology with morphoecology. No scenario explaining the growth suppression preserved in Decombeix’s tree is completely satisfying. That is some of the fun of it. What we do know, is that in a lush polar forest in the depths of time, something really bad happened to an individual conifer tree. The little tree survived, as trees do, and continued growing for nearly a century, only to be transported to Germany and immortalized in the Annals of Botany over 200 million years later, long after its species faded. The most improbable scenario of all.
