The hidden life of tropical roots: functional root traits and their response to climatic disturbances

Roots play a critical role in plant nutrition, and terrestrial carbon cycling. However, they are often understudied compared to their aboveground counterparts; especially in the tropics, where more carbon is cycled than in any other ecosystem. Some tropical forests, like in Puerto Rico, are more represented in scientific studies than others. However, this information is sparse, complicating the interpretation of root trait patterns. Trees in Puerto Rico have adapted mechanisms for withstanding hurricane disturbances, including in their roots. Additionally, as many tropical forests, some in Puerto Rico have low available phosphorus (P); thus, trees rely on root traits to enhance P acquisition. For example, higher root mycorrhizal colonization, branching, and exudation of phosphatase enzymes can each optimize P uptake. Yet, the high cost invested on these traits is a trade-off, which results in a diversity of traits among species. The aim of this dissertation was to synthesize root studies from Puerto Rico for the past 50 years, measure root response to warming and hurricanes disturbances, and test root trait strategies related to P acquisition from common species of the island. I found from previous studies that rooting depth in Puerto Rico is shallow ( < 20cm), root nutrient concentrations do not vary much across forests, and there is under-representation of forests outside the Luquillo Experimental Forest. I measured a negative effect of warming on root production and biomass after 7 months of experimental warming. I captured an overall increase of root biomass after 10 months of the hurricanes (Irma and María) possibly due to the understory plant composition change. Yet, the increase in root biomass was less in previously warmed plots than in control plots, suggesting a legacy effect of the warming treatment on the recovery of roots. When testing root traits related to P acquisition, I found that species with high mycorrhizal colonization also had high root phosphatase activity, but low branching ratio. This suggests a combined contribution of phosphatase enzymes from the plant and their fungal partners to obtain soil P, and that plants will either invest in more branched roots, or more mycorrhizal colonization and more phosphatase exudation.