Temperature extremes from a leaf perspective: Micro- vs macro-climate predictors of dwarf-shrub thermal tolerance limits

Despite their broad climatic and geographic ranges and dominant ecosystem roles across boreal, alpine and arctic vegetation zones; dwarf shrubs can be sensitive to climatic changes, in particular shifting thermal regimes. But questions remain as to the macro-or micro-climatic conditions we should focus upon when considering these changing plant-climate relationships. As a case study highlighting how microclimate insights may contribute at scales from the field through to land-surface models, in the DURIN project we explore how thermal microclimate measures at plant- and leaf-levels can be used to better inform models regarding the thermal tolerance limits of leaves. At high-latitudes, increasing summer heat extremes and aseasonal freezing events associated with changing snowpack dynamics, will expose dwarf-shrubs to potentially stressful conditions for photosynthesis and carbon gain. We explore thermal damage by quantifying temperature at which there is a 50% decline in the maximum quantum yield of photosystem II (FV/FM), for a range of dwarf shrub species growing across habitats and bioclimatic zones. We then compare the extent of photosystem damage to various estimates of temperature extremes as derived from plant-level climate sourced from TOMST loggers, FLIR imagery, and leaf-level thermocouples deployed in dwarf-shrub and non-dwarf-shrub plots. The time-series of fine-scale characterization of dwarf-shrub microclimates in-situ will be correlated with downscaled microclimate estimates from NicheMapR, along with nearby weather station data to highlight the discrepancies between macro- and microclimates. These comparisons allow us to additionally ask fundamental questions about the ways in which we should assess thermal tolerance taking into greater consideration methods for quantifying heat stress.  Classical assays of photosynthetic thermal tolerance limits have focused on singular and short exposure times to temperature stress, but increasingly the field is moving towards providing more biologically meaningful insights into thermal tolerance exposures, enabling us to better define and experimentally impose thermal stress events. An emerging discussion surrounds the use of the thermal death time framework, where cumulative thermal stress is applied, e.g. a range of exposure times at varying temperatures. Our work will help develop protocols for the thermal death time framework which requires a more nuanced understanding of thermal stress events at plant and leaf-level, integrating our micro-and macro-climate insights.