Nine years of liana removal does not alter a tropical rainforest’s fine root biomass or morphology

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Audrey Massmann and Stephen Porder, Ecology & Evolutionary Biology, Brown University, Providence, RI, Audrey Massmann and Stephen Porder, Institute at Brown for Environment and Society, Brown University, Providence, RI, Stefan A. Schnitzer, Department of Biological Sciences, Marquette University, Milwaukee, WI, Stefan A. Schnitzer, Smithsonian Tropical Research Institute, Balboa, Panama

Background/Question/Methods:
Lianas contribute ~40% to the leaf area of the top of tropical rainforests canopies, but have little aboveground woody biomass and greatly reduce the woody growth of their tree hosts. Both greenhouse studies and field observations suggest that belowground liana-tree competition could be more important than aboveground competition in explaining reduced tree growth; however, the effect of lianas on tree fine roots had never been tested. We hypothesized that lianas reduce tree fine roots, and that trees should compensate following liana removal. To test this, we compared fine roots in plots with both lianas and trees and in plots that had been maintained as liana-free for nine years.
The experiment consists of 16 plots of 0.64ha each in Gigante, Panama, half of which have had all lianas regularly removed by machete since March 2011 when the forest was ~60 years old. To test how liana removal affects fine roots, we collected nine soil samples per plot (144 samples total) in January 2020. Each sample consisted of two cores of 2cm diameter and 10cm depth, stored at 4oC until roots (≤2mm) were isolated. Roots were washed, photographed for length and mean diameter, dried to constant mass at 65oC, and then weighed.
Results/Conclusions:
Standing fine root biomass did not differ between the treatments. Control plots, where both lianas and trees were present, had 289±12 g/m2 of fine root biomass in the upper 10 cm of soil, while liana removal plots had 295±10 g/m2 (n=8, p=0.4). Mean root diameter also did not differ, at 488±15 μm in control plots and 498±12 μm in liana removal plots (n=8, p=0.4). The only hint of a treatment effect was in standing fine root length, which was 3.02±0.08 km/m2 in the control plots, and 2.87±0.11 km/m2 in the liana removal plots (n=8, p=0.1).
The striking similarity between the two treatment types suggests that trees compensated for the loss of liana fine root mass, resulting in remarkably similar amounts of fine root mass in the soils of the liana removal and control treatments. Liana roots have previously been shown to have a greater specific root length than tree roots. This could have resulted in the somewhat reduced fine root length observed with liana removal, though the difference is marginal. Our findings show that tropical rainforest fine root biomass, fine root length, and mean fine root diameter are largely unaffected by removal of a major plant lifeform.