Tropical fine-root trait strategies for phosphorus acquisition and their response to hurricane disturbance in a wet tropical forest of Puerto Rico

Daniela Yaffar, Kristine Grace Cabugao, Richard Norby, Camille E. Defrenne and Joanne Childs, Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, Daniela Yaffar and Richard Norby, Ecology and Evolutionary Biology, University of Tennessee Knoxville, Knoxville, TN, Kristine Grace Cabugao, Climate and Ecosystem Sciences, Lawrence Berkley National Laboratory, Berkley, CA

Background/Question/Methods
Tropical trees commonly present adaptive functional root traits to live in low available phosphorus (P) ecosystems. Some of these root traits include mycorrhizal colonization, root phosphatase activity (phosphomonoesterase; PME), and root branching. However, due to the high cost of investment in any one of these root traits, species may rely on one more than the others, creating a gradient of strategies for nutrient acquisition. These strategies are part of the root economics spectrum (RES) framework. However, root morphological traits have largely dominated the RES space, and few studies that have evaluated species-specific root traits in addition to functional or physiological traits. Further, no study that has evaluated root traits related to P acquisition has measured their response to the disturbance in the aftermath of two consecutive hurricane events. We ran two types of studies, one at species-specific level and another one at the forest level. At the species-specific level we aimed to evaluate root trait strategies related to soil P acquisition from five common species in Puerto Rico and test the responsiveness of these root traits to hurricane disturbances. At the forest level we aimed to identify which root morphological traits and soil variables influence root phosphatase activity.
Results/Conclusions
In our species-specific studies, we found that species with greater mycorrhizal colonization had a smaller root branching ratio driven mostly by pioneer species (Cecropia schreberiana and Spathodea campanulata) as opposed to non-pioneers (Dacryodes excelsa, Prestoea montana, and Calophyllum calaba). Further, mycorrhizal colonization was positively related to PME. The species with greater root mycorrhizal colonization had 17 times higher root PME than the species with less mycorrhizal colonization, which suggests a combined contribution of root PME by the root and their mycorrhizal symbiont partners. Thus, species will either invest in greater root branching to increase soil nutrient exploration, or in their mycorrhizal fungal partner to increase nutrient uptake. We also found that root PME decreased in conservative species after hurricanes, suggesting a cost-benefit trade-off for producing these enzymes as well as species dependency on nutrient acquisitive enzymes. At the forest level study, we showed that root PME, which other studies have shown to be closely related to P uptake, was well predicted by a combination of soil available P (resin P) and specific root length. These findings can improve estimates of biochemical mineralization in ecosystem models that contain fine-root morphological traits and soil P distributions.