21 January, 2020 (Brugnera, GCB, Liana)

di Porcia e Brugnera, M, Meunier, F, Longo, M, et al. Modeling the impact of liana infestation on the demography and carbon cycle of tropical forests. Global Change Biology, 2019; 25: 3767– 3780. https://doi.org/10.1111/gcb.14769

• Question:

  1. How much does integrating lianas into the ED2 influence the estimates of forest carbon cycle?
  • The authors hypothesized that the impact of lianas on carbon uptake by forest would be larger in the secondary forests than in the old-growth forests since lianas showed high density in young forests.

• Context:

  • Lianas (woody vines) have been proposed as a possible cause of tropical forest structural changes, which limits our understanding of how tropical forests respond to perturbations and quantifying their contribution to the global carbon cycle.
  • Despite the demand for inclusion of lianas in models, no single dynamic global vegetation model so far includes lianas.
  • ED2 is a suitable model for examining the influence of lianas on carbon cycle because the density of lianas changes with the succession stage of the forest, which demography models take into consideration.

• Answer

  1. Liana negatively affected the above ground biomass of early- to mid- successional trees and favored shade-tolerance trees.
     • These were because lianas were treated as a pioneer and light-demanding species in ED2.
  2. The effect of introducing lianas was correlated with liana density.

• Implication:

  • Liana could influence the carbon cycle and the structure of forests, although this paper included some debatable assumptions and had limited observations on lianas.

• Unanswered:

  • How does drought stress affect the simulation results with lianas?
    • ED2 used in this study could not consider the water stress effect (and below-ground competition between different growth forms).
  • More data on lianas are needed. For example, lianas display a variety diversity of growth forms, climbing mechanisms, dispersal types, leaf photosynthetic properties, and allocation patterns.

• Comment:

  • The authors did a great job for incorporating lianas into the ED2 with careful considerations of allometry, light competition, and demography.
  • The lianas brought a large change in above ground biomass because the authors assumed that lianas less allocate biomass to their stems. However, this assumption may not be true; Smith‐Martin et al. (2019) explained that mature lianas and trees had a similar proportion of biomass investment to stems and comply with the same allometric scaling law. This contradictory result may introduce wholly different results to the simulation with lianas.
  • I like the opening of the discussion; it say the value of the modeling experiment: to enable to investigate the effect of a treatment on variables which are usually not or cannot measured.
  • Also, this study indirectly showed that abundant measurements are indispensable for formulating and parameterizing the model; the authors had difficulties in determining the characteristics of lianas such as climbing mechanism and biomass allocation.