Collaborative Research: The risks of safety: xylem anatomy and tradeoffs between reproduction, growth, and drought survival in conifers

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Increasingly severe and frequent droughts threaten tree health in many regions. Key factors that affect whether a tree survives a drought include the anatomy of the cells (xylem) that conduct water and make up the wood, and how the tree invests resources toward wood growth versus reproduction versus defending itself from insects. These factors can all vary between individual trees. This project aims to improve scientific understanding of how investment of carbon resources affects survival in a severe drought by combining measurements of all these functions. The researchers will make use of forest plots in the Sierra Nevada mountains of California, many of which exhibited high tree death in the historic 2012-2016 drought. A small cylinder of wood will be removed from living and dead trees to measure overall growth, xylem cell anatomy, carbon in the wood, and production of defensive resins. Live twigs will be used to measure other traits related to water movement through the xylem. Cones produced by living pines will also be counted for several years to compare allocation to reproduction to amount of wood growth. These measurements will be combined to assess how growth relates to temperature and precipitation (climate) and how growth, xylem anatomy, and defense affect the chance that a tree will die in a drought, as well as whether these relationships differ between locations or species. These results will in turn be combined with the analysis of growth versus reproduction to identify which resource investment strategies work best under different drought scenarios, helping to identify conifer seed sources that will perform well under future climates. The project will provide research experience and training in the collection, processing, and/or analysis of dendrochronology and cell anatomy data for PhD students and undergraduates. The investigators will collaborate with the CalTeach program to develop learning modules that teach students about tree physiology and forest ecology while reinforcing other STEM skills.The two drivers of tree mortality during drought that have received the most theoretical attention are hydraulic failure and carbon depletion/starvation, but biotic agents are also an important cause of death. This project will A) determine the relationships between tree radial growth, xylem anatomy, and resin duct production and drought-related mortality risk in seven species of Sierra Nevada conifers, B) confirm whether thicker xylem walls consistently embody more carbon resources, C) test whether there are tradeoffs between cone production and carbon investment and embolism resistance in the stem in pines, and D) model the lifetime fitness consequences of different levels of investment in overall growth, tracheid walls, resin ducts, and reproduction under different levels of drought stress. Examining multiple traits through a carbon-allocation lens will also enable connection of carbon allocation in the stem to potential tradeoffs with seed production. Results will be an important step in modeling forest dynamics in drought-prone forests in a multigenerational eco-evolutionary manner. The findings of this study will help inform efforts to identify conifer seed sources that will perform well under future climates by providing evidence for the real-world consequences of traits such as hydraulic safety or resin production.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.