Do lianas spend fewer resources on roots for support than other plants?

If a plant can lean on something else, does it really need to invest so much in its roots for support? Could resources usually used for roots be redirected to stems and leaves? In a new paper, Tomasz Wyka and colleagues examine the growth of lianas and find that they do grow differently to self-supporting plants.

The paper uses temperate lianas for the study. Far from being tropical vines for swinging from tree to tree, ‘Liana’ has a definite meaning for botanists. Tomasz Wyka explained: “Lianas are a broad category of climbing plants (i.e. plants that use other plants as support for vertical growth) that have woody stems (i.e. stems that are hard, last for years and gain in girth every year). Thus not all climbers are lianas, but all lianas are climbers. As with most terms, this definition is a bit fuzzy at edges, but it works in most cases.”

Lianas have been overlooked when it comes to resource allocation. It’s a gap that matters, as Dr Wyka explained: “To fully understand evolution one must account for costs and benefits of the particular forms, including the way plants partition biomass between photosynthetically productive parts (leaves) and accessory parts (stems and roots). Whereas relevant data on many self-supporting species have accumulated, there is very little information available on lianas, especially on larger individuals. We have summarized the previous state of science in our earlier Annals of Botany paper. Our current study fills the data gap and yields an unexpected insight into the structure of the liana body. Moreover, our approach of statistically removing plant size effect from comparisons of biomass partitioning proved useful and worthy of wider application.”

Picking lianas to work with isn’t just a case of finding ten common lianas, as this might bias the results. Dr Wyka said: “We aimed to avoid the situation where multiple related liana species (e.g., honeysuckles or clematises) would be treated as replicate examples of the liana habit, because in fact in their evolutionary history there might have been only one event in which lianescence had evolved. We instead used lianas from remotely related families thus making sure that we are actually studying several evolutionary events that led to the emergence of lianas from self-supporting plants.

To get a mix of lianas and a suitable comparison, Wyka and colleagues used a three-step process. Dr Wyka said: “First we listed lianas that are known to be hardy in botanical gardens in Poland. From those we chose botanical families that included both lianas and hardy self-supporting species, but excluded families with only lianas, such as grape family. Finally, the choice of liana and shrub species within each family was dictated by the availability of seeds, ease of germination and tolerance under cultivation. Ultimately our target was to use a set of species that would be both morphologically diverse and taxonomically balanced.”

After finding plants to compare, the team then grew them over three seasons. The results were clear, but the reasons for the results less so. Dr Wyka said: “It appears that lianas using short adhesive roots for climbing (’root-climbers’) also feature a particularly low investment in the whole root system, allowing more biomass to be used for aboveground parts. One explanation for this pattern is that the stems of these lianas adhere so tightly to tree bark, that they do not need strong ground anchorage of their own. Another explanation is that they tend to occur in fertile sites without having to explore large soil volumes for nutrients or water. As usual, more research is needed as to the causes of this pattern.”

Now the team have a baseline, they can consider changing some factors to see how that affects liana growth. Dr Wyka said: “Since our study was conducted under growth-promoting conditions, the obvious next step would be to investigate responses of lianas to reduced resources, especially water and nutrients. Will their partitioning patterns be altered to a larger extent than in self-supporting species, and will they be able to retain their growth advantages?”

The research may have started in Europe, but it has global significance. Dr Wyka concluded: “Although our lianas are representative for the temperate world, the true diversity of lianas is in the tropics where lianas are a major component of woodland vegetation (e.g., in some localities they constitute up to 25% of all flowering plant species!). It would be interesting to learn whether our findings are upheld for the tropical lianas and what is the basis of any differences. So far, we do not have a comprehensive understanding of differences between tropical and temperate lianas.”