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Digging deeper? Biomass allocation patterns in trees and lianas in tropical seasonal forests.

trees and lianas in tropical seasonal forests

Lugli, L. F.

2020

trees and lianas in tropical seasonal forests

226(3)

639-640

How do plants grow? It seems like a simple question. Most of us believe that we know the answer, but our observations of the natura world indicate that it might not be so simple after all. A long- standing discussion in ecology aims to understand how plants allocate their biomass among roots, stems and leaves to acquire limiting resources aboveground and belowground, and how environmental factors and ontogeny can influence such patterns. wo theories are usually tested to explain plant allocation strategies: the allometric partitioning theory (APT) and the optimal partitioning theory (OPT). On the one hand, APT proposes a universal allocation pattern for all plants, following allometric rules that constrain biomass partitioning to plant organs (Enquist & Niklas, 2002; Niklas & Enquist, 2002). OPT, on the other hand, proposes that biomass allocation is flexible, allowing plants to optimize resource acquisition, depending on which resource is most limiting at a given point in time (Bloom et al., 1985; Chave et al., 2014). Due to the increasing dominance of lianas growing in tropical forests and their potential role in decreasing carbon accumulation in forests by intensifying tree mortality (van der Heijden et al., 2015), understanding how co-occurring trees and lianas share the same space and allocate carbon aboveground and belowground becomes crucial. In this issue of New Phytologist, Smith-Martin et al. (2020; pp. 714–726) dug deep into these questions by bringing together results from fieldwork, a common- garden experiment, and also by incorporating their results in an individual-based terrestrial biosphere model to test the effects of changing biomass allocation and the rules behind such patterns. While Smith-Martin et al. confirm some of the expectations regarding biomass allocation of lianas and trees in tropical dry forests, they also find surprising and important trends that contradict prevailing wisdom. Much of our understanding of biomass allocation patterns comes from ecosystems that are far less diverse than tropical forests, or from controlled experiments with young plants. Owing to the high species diversity, sometimes not so welcoming environmental conditions, and remote access, working in tropical forests is a major challenge that becomes even harder when harvesting whole mature trees and lianas in situ. Nonetheless, by using shovels and picks, Smith-Martin et al. shone a needed spotlight on ecological concepts that have been studied since the 19th century (Kny, 1894), proving that we may need to revisit some theories before generalizing them across biomes and life forms. It was hypothe- sized that because lianas use trees as support to grow, they would allocate less biomass to stems and potentially invest their resources in leaves to reach the canopy and also in roots to provide anchorage and to explore deeper soils in search for water and nutrients, especially during dry periods (Selaya et al., 2007; van der Heijden et al., 2013). Contrary to the general belief, Smith-Martin et al. found that mature lianas do not invest less in stem biomass and do not display deeper root systems when compared to deciduous and evergreen mature trees in a tropical dry forest. Their model simulations indicate coordination between belowground allocation and aboveground phenology, mainly related to water supply, where evergreen trees invest in deeper roots to sustain leaves for longer periods than deciduous trees and lianas, since the latter display shallower root systems. However, that lianas have root systems shallower than deciduous co-occurring trees could be attributed to the fact that lianas use other trees’ stems for support, not necessarily relying on roots for anchorage.

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The Liana Ecology Project is supported by Marquette University and funded in part by the National Science Foundation.