DOI: 10.5061/dryad.v9s4mw6zq
title: New tree‐level temperature response curves document sensitivity of tree growth to high temperatures across a US‐wide climatic gradient

description: Temperature is a key climate indicator, whose distribution is expected to
 shift right in a warming world. However, the high temperature
 tolerance of trees is less widely understood than their drought tolerance,
 especially when it comes to sub-lethal impacts of temperature on tree
 growth. I use a large data set of annual tree ring widths,
 combined with a flexible degree-day model, to estimate the relationship
 between temperature and tree radial growth. I find that tree
 radial growth responds non-linearly to temperature across many ecoregions
 of the US: across temperate and/or dry ecoregions, spring-summer
 temperature increases are beneficial or mostly neutral for tree growth up
 to around 25-30°C in humid climates and 10-15°C in dry climates, beyond
 which temperature increases suppress growth. Thirty additional degree-days
 above the optimal temperature breakpoint lead to an average decrease in
 tree ring width of around 1-5%, depending on ecoregions, seasons, and
 inclusion or exclusion of temperature-mediated drought impacts. High
 temperatures have legacy effects across a 5-year horizon in dry
 ecoregions, but none in the temperate-humid South-East or among
 temperature-sensitive trees. I find limited evidence that trees
 acclimatize to high temperatures within their lifetime: local variation in
 exposure to high temperatures, which stems from local variation in the
 timing of tree birth, does not significantly impact the response to high
 temperatures, although temperature-sensitive trees acquire some heightened
 sensitivity from early exposure. I also find some evidence that
 trees adapt to high temperatures in the long-run: across humid ecoregions
 of the US, high temperatures are 40% less harmful to tree growth, where
 their average incidence is one standard deviation above
 average. Overall, these results highlight the strength of a new
 methodology which, applied to representative tree ring data, could
 contribute to predicting forest carbon uptake potential and composition
 under global change.
description: The data used in this paper have been compiled from
 freely and publicly available sources. Data processing is described in
 detail in the associated paper. Processing of tree ring data relies in
 part on Zhao et al. 2018 "The International Tree‐Ring Data Bank
 (ITRDB) revisited: Data availability and global ecological
 representativity". Tree ring width data and associated metadata come
 from the International Tree-Ring Data Bank (ITRDB; https://www.ncei.noaa.gov/access/paleo-search, accessed February 2019). I use a subset of these data, which covers the contiguous United States, and years 1903 to 2016. Temperature and precipitation data are based on the "Parameter-Elevation Regressions on Independent Slopes Model" (PRISM), which covers the contiguous US at a 4 km resolution. These data come from a version of PRISM, which is adapted to panel analyses (http://www.columbia.edu/~ws2162/links.html). Drought data come from the Standardised Precipitation-Evapotranspiration Index (SPEI) database (https://spei.csic.es/database.html). Ecoregion information come from the Köppen-Geiger climate classification (http://koeppen-geiger.vu-wien.ac.at/present.htm).