When does Acacia koa know it’s time to grow up?

Sometimes the best strategy for a young plant is not the same as the best strategy for an old plant. Some heteroblastic plants can switch how they grow but what triggers this?

In Hawaii, you can find the tree Acacia koa that grows without leaves. Instead it uses phyllodes. Phyllodes develop from petioles, leaf stalks. In A. koa the phyllodes develop into vertical sickle shapes, but the tree doesn’t start that way. As a sapling, it grows pairs of compound leaves that attach to a long petiole. Some time after six months, something in the plant suddenly switches, and the plant starts making phyllodes. This is heteroblasty.

Acacia koa
Image: Young koa tree in Hawai’i, showing two types of “leaves”; photographed by Eric Guinther (Marshman) at Nā Pōhaku o Hauwahina planting project. Marshman / Wikipedia

A new study by Kyle Rose and colleagues investigates heteroblasty in A. koa. Dr Rose said; “We have a long on-going series of collaborations with many institutions and researchers in Hawaii looking a wide range of questions related to koa, which is a very valuable hardwood worldwide.”

“Although other acacias have been studied with regards to heteroblasty, we hypothesized that the longer transition window would help us detect mechanisms that might be missed during a faster transition in another species. We did find a new mechanism, which doesn’t mean it was missed in other species.”

“Further, increasing our understanding of the interaction between koa development and its growth form can help managers in Hawaii develop systems to promote growth that can be both ecologically and economically important for Hawaiians.”

The team investigated the roles of light and water availability in triggering the transition to the mature leaf form in contrasting wet/dry ecotypes. It’s the combination of light and water that makes the advantages of the juvenile and mature forms different, Dr Rose explained: “As koa’s leaves transition from bipinnately compound and horizontally-oriented true leaves to vertically oriented phyllodes, the ability of the plant to capture light in gaps in the forest canopy is diminished. However, phyllodes increase drought tolerance in koa because they have greater stomatal control in low-water conditions. It’s a trade-off between fast growth early during development (when the germinate in gaps in the forest, hypothetically) and resilience to stress post-establishment.”

Dr Rose thought that the fact that Koa has such a wide climactic range was a clue to water stress being one of the triggers for change. “The hypothesis would be that the driving selector in resource-rich environments would be light availability (because of more competition for light in denser forests), but this would differ from resource-poor environments with sporadic rainfall (where koa is also the dominant species) where an earlier transition to phyllodes would be advantageous. It just made sense.”

A factor the team had to deal with was the speed of Koa’s growth. You can have a tall tree after just a few years. This rapid climb from the plants caused a few problems. Dr Rose said: “After six months, they outgrew the greenhouse space and consequently, we did not record as many transitions in the lower light conditions as we needed to be able to detect any interactions if present.” So if you’re appealing to your department for more space, Koa would be the plant to research.

Identifying what makes Koa grow so well, or not, will be useful to conservationists said Rose: “Obviously, for species with wide climatic ranges where the local ecosystem relies on their continued presence for healthy functioning, identifying vulnerable populations in the face of climate change is very important. Increased understanding about the role of development in plant establishment across this range could help us target restoration activities where they are most needed.”

“For this reason, this paper is relevant to people that work in restoration, whether in tropical or temperate regions, who are interested in how manipulating the overstory environment might affect plant growth of planted seedlings. Additionally, anyone that is interested in variation in developmental trajectories and how they might affect survival and performance of plants would be interested in this.”

And of course, anyone interested in heteroblasty, in general, should find plenty to occupy them in the paper.

Further reading

Rose, K. M. E., Mickelbart, M. V., & Jacobs, D. F. (2019). Plasticity of phenotype and heteroblasty in contrasting populations of Acacia koa. Annals of Botany. https://doi.org/10.1093/aob/mcz083