Root dynamics response to warming and hurricane disturbance in a tropical forest of Puerto Rico

Background/Question/Methods: Increasing temperatures can affect plant root dynamics through multiple direct and indirect effects on carbon, water, and nutrient cycling processes. Temperature-induced changes to plant roots have numerous implications for ecosystem and global function, yet, we have little information regarding root responses to increased temperature in tropical forests. On top of this, almost exclusively in tropical forests, plant composition and structure are shaped by hurricane events, which are constantly changing forest dynamics and carbon balance. We aimed to answer the following questions:How did fine-root dynamics respond to the first warming experiment in the tropics, and how did warming affected root recovery from hurricane disturbance?

Results/Conclusions: We installed two minirhizotron tubes in three 12 m² warming plots and three control plots. The warming treatment was initiated in September 2016 with infrared heaters above the forest understory, and, a year later, the treatments were unexpectedly suspended by the two hurricanes: Irma and Maria. We took images of fine roots along the tubes with a minirhizotron camera every two weeks from February 2017 until the hurricane events. These events created a unique opportunity to study fine-root recovery and to explore warming legacy effect on fine-root recovery. Root observations were restarted three weeks after Maria. Only root biomass mortality was significantly affected by treatment prior the hurricane disturbances. After the hurricanes, we found that there was a shift in plant composition from woody species to herbaceous, through root morphological trait changes. Standing stock biomass, and biomass production increased more than double post hurricane, especially in the control plots. We found that soil temperature, soil phosphate (PO₄^3-), and ammonium (NH₄⁺) were correlated to root standing stock, root biomass production, and root surface area overall. Soil biogeochemistry constituted the material legacy responsible for root dynamics recovery. Our findings are the first of their kind, especially for the tropics, and show the effect of hurricanes in the belowground and highlight the potential for legacy effects of warming in the recovery of root dynamics.