About

N. Michele Holbrook is a professor whose research focuses on the physics and physiology of vascular transport in plants. Her work aims to understand how constraints on the movement of water and solutes between soil and leaves influence ecological and evolutionary processes. Her research addresses questions related to cavitation, stomatal mechanics, solute transport, cell wall mechanics, and wood anatomy. She is actively involved in studying the physiological mechanisms underlying plant vascular systems and their ecological implications.

Research topics

• Geology
• Meteorology
• Geography
• Atmospheric sciences
• Agronomy
• Environmental science
• Composite material
• Botany
• Ecology
• Materials science
• Remote sensing
• Biology
• Soil science
• Chemistry

Selected publications

• Mapping temperature thresholds and safety margins of temperate plant species under global change

  2026-03-13

  articleOpen accessSenior author

  Global climate change imposes growing challenges to vegetation thermoregulation through rising temperatures and increasing drought frequency. Understanding plant thermal limits (e.g., Tcrit and T50) and their associated temperature safety margins is essential to evaluate canopy resistance to thermal stress. Despite intensified heatwave events in temperate regions, research on plant temperature thresholds has predominantly focused on tropical ecosystems, and methodological inconsistencies have limited cross-study comparability. In this study, we address these knowledge gaps by: (1) quantifying thermal thresholds (Tcrit, T50) for temperate plant species through field sampling, (2) compiling published datasets standardized under a homogenized methodology, (3) analyzing the global drivers of T50 and the inter- and intraspecific variability linked to temperature, phenology, genetics, and methodological factors, and (4) mapping temperature safety margins by integrating field data, upscaling models, and satellite-derived land surface temperatures. Finally, we project future temperature safety margins for temperate vegetation under anticipated climate scenarios. Our findings provide a comprehensive framework to assess and predict the thermal resilience of temperate plant species under ongoing and future climatic stress.

  PublisherDOI

• Apparent surface conductance sensitivity to vapour pressure deficit in the absence of plants

  Nature Water · 2023-10-16 · 16 citations

  article
  PublisherDOI

• On the path from xylem hydraulic failure to leaf cell death

  HAL (Le Centre pour la Communication Scientifique Directe) · 2022-06-04

  preprint

  International audience

  Publisher

• Phloem turgor is maintained during severe drought in <i>Ricinus communis</i>

  Plant Cell & Environment · 2022-07-20 · 11 citations

  articleSenior author

  The phloem is a key player in whole plant functioning-transporting carbon from sites of production to sites of demand-and is likely influenced by drought due to its dependence on water for generating pressure-driven bulk flow transport. Yet, phloem functioning during drought remains largely unknown due to a lack of experimental studies. Here, we use a phloem-bleeding species, Ricinus communis, to investigate phloem loss-of-function in the context of leaf physiological processes, the mechanisms of phloem turgor maintenance during drought, and the role of turgor in phloem loss-of-function. We found that the solute concentration in the phloem sap doubled over the drought, which allowed phloem turgor to be maintained past the point at which leaves have reached permanent stomatal closure. We also found that phloem turgor did not decline before bleeding ceased, which suggests that phloem bleeding ceassation (interpreted as the cessation of transport) occurred when the phloem still had turgor. In sum, our findings highlight the robustness of phloem functioning, with important implications for forecasting whole-plant carbon dynamics and drought-induced tree mortality.

  PublisherDOI

• Sieve tube structural variation in Austrobaileya scandens and its significance for lianescence

  2021-10-28

  preprintOpen accessSenior author

  Lianas combine large leaf areas with slender stems, features that require an efficient vascular system. The only extant member of the Austrobaileyaceae is an endemic twining liana of the tropical Australian forests with well-known xylem hydraulics, but the vascular phloem continuum aboveground remains understudied. Microscopy analysis across leaf veins and stems of A. scandens revealed a low foliar xylem to phloem ratio, with isodiametric vascular elements along the midrib, but tapered across vein orders. Small sieve plate pore radii increased from 0.08 µm in minor veins to 0.12 µm in the petiole, but only to 0.20 µm at the stem base, tens of meters away. In searcher branches, phloem conduits contained a pectin-rich wall and simple plates, whereas in twinning stems, conduits connected through highly-angled-densely populated sieve plates. Twisted and elongated stems of A. scandens display a high hydraulic resistance of phloem conduits, which decreases from leaves to stems, efficiently delivering photoassimilate from sources under Münch predictions. Sink strength of a continuously growing canopy might be stronger than in self-supporting understory plants, favoring resource allocation to aerial organs in angiosperms that colonized the vertical niche.

  PublisherOA PDFDOI

• Evaluating the benefits of chlorophyll fluorescence for in-season crop productivity forecasting

  Remote Sensing of Environment · 2021 · 36 citations

  • Environmental science
  • Remote sensing
  • Atmospheric sciences
  PublisherOA PDFDOI

• Wood day capacitance is related to water content, wood density, and anatomy across 30 temperate tree species

  Plant Cell & Environment · 2020 · 51 citations

  Senior authorCorresponding
  • Materials science
  • Botany
  • Biology

  Abstract Water released from wood during transpiration (capacitance) can meaningfully affect daily water use and drought response. To provide context for better understanding of capacitance mechanisms, we investigated links between capacitance and wood anatomy. On twigs of 30 temperate angiosperm tree species, we measured day capacitance (between predawn and midday), water content, wood density, and anatomical traits, that is, vessel dimensions, tissue fractions, and vessel–tissue contact fractions (fraction of vessel circumference in contact with other tissues). Across all species, wood density (WD) and predawn lumen volumetric water content (VWC L‐pd ) together were the strongest predictors of day capacitance ( r 2 adj = .44). Vessel–tissue contact fractions explained an additional ~10% of the variation in day capacitance. Regression models were not improved by including tissue lumen fractions. Among diffuse‐porous species, VWC L‐pd and vessel–ray contact fraction together were the best predictors of day capacitance, whereas among semi/ring‐porous species, VWC L‐pd , WD and vessel–fibre contact fraction were the best predictors. At predawn, wood was less than fully saturated for all species (lumen relative water content = 0.52 ± 0.17). Our findings imply that day capacitance depends on the amount of stored water, tissue connectivity and the bulk wood properties arising from WD (e.g., elasticity), rather than the fraction of any particular tissue.

  PublisherDOI

• Wood diurnal capacitance, water storage and their anatomical drivers across 30 temperate angiosperm tree species.

  2020-03-09

  articleOpen accessSenior author

  &amp;lt;p&amp;gt;Water released from storage into the transpiration stream (termed: capacitance) can play an important role in tree every day hydraulic functioning as well as in tree drought response. However, anatomical underpinnings of capacitance and water storage remain unclear, impeding better understanding of capacitance mechanisms. Across 30 temperate angiosperm tree species, we measured &amp;lt;em&amp;gt;in natura&amp;lt;/em&amp;gt; twig wood diurnal capacitance and water content, wood density and anatomical properties: vessel dimensions, tissue fractions and vessel-tissue contact fractions (proportion of vessel circumference in contact with other tissues). We found that wood density and predawn lumen volumetric water content (proportion of wood volume that is occupied by water in lumen) together were the strongest predictors of capacitance (&amp;lt;em&amp;gt;r&amp;lt;sub&amp;gt;adj&amp;lt;/sub&amp;gt;&amp;lt;/em&amp;gt;&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;=0.44***). Vessel-tissue contact fractions&amp;amp;#8212;vessel-ray, vessel-axial parenchyma and vessel-fibre&amp;amp;#8212;each explained an additional &amp;amp;#8764;10% of variation in capacitance. Parenchyma fraction did not correlate with capacitance challenging the common assumption that parenchyma acts as the main source of capacitance water. Anatomical structure, water content and capacitance relationships differed significantly between diffuse-porous and ring-porous species. Predawn relative water content (water in a fresh sample relative to saturated sample) was on average 0.65&amp;amp;#177;0.13 implying that parts of wood were devoid of water.&amp;lt;/p&amp;gt;

  PublisherDOI

• Idioblasts and peltate hairs as distribution networks for water absorbed by the leaves of Capparis odoratissima

  2020-09-22

  preprintOpen accessSenior author

  Capparis odoratissima is a tree species native to semi-arid environments of the northern coast of South America where low soil water availability coexists with frequent nighttime fog. A previous study showed that water applied to leaf surfaces enhanced leaf hydration, photosynthesis, and growth, but the mechanisms of foliar water uptake are unknown. Here we combine detailed anatomical evaluations with water and dye uptake experiments in the laboratory, and use immunolocalization of pectin and arabinogalactan protein epitopes to characterize water uptake pathways in leaves. Abaxially, the leaves of C. odoratissima are covered with peltate hairs, while the adaxial surfaces are glabrous. Both surfaces are able to absorb condensed water, but the lower surface has higher rates of water uptake. Numerous idioblasts connect the adaxial leaf surface and the abaxial peltate hairs, both of which contain hygroscopic substances such as arabinogalactan proteins and pectins. The highly specialized anatomy of the leaves of C odoratissima fulfills the dual function of minimizing water loss when stomata are closed, while maintaining the ability to absorb liquid water. Cell-wall related hygroscopic compounds in the peltate hairs and idioblasts create a network of microchannels that maintain leaf hydration and promote water uptake.

  PublisherOA PDFDOI

• Combined influence of soil moisture and atmospheric evaporative demand is important for accurately predicting US maize yields

  Nature Food · 2020 · 242 citations

  • Environmental science
  • Atmospheric sciences
  • Soil science
  PublisherDOI

Recent grants

• Collaborative Research: Physiology of Long Distance Assimilate Transport

  NSF · $337k · 2015–2020

• Collaborative Research: Testing the Munch Hypothesis: Hydraulics of Phloem Transport in Vines and Trees

  NSF · $373k · 2010–2014

Frequent coauthors

• Peter Huybers

  Harvard University

  82 shared

• Nathaniel D. Mueller

  University of Nebraska–Lincoln

  73 shared

• Ethan E. Butler

  Minnesota Department of Natural Resources

  70 shared

• A. N. Rhines

  Netflix (United States)

  67 shared

• D. K. Ray

  University of Minnesota

  65 shared

• Stefan Siebert

  University of Göttingen

  65 shared

• Maciej A. Zwieniecki

  University of California, Davis

  50 shared

• C. Kevin Boyce

  Stanford University

  27 shared

Labs

• Holbrook GroupPI