Browsing by Author "Johnson, Daniel M"

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    Co-occurring woody species have diverse hydraulic strategies and mortality rates during an extreme drought.
    (Plant Cell Environ, 2018-03) Johnson, Daniel M; Domec, Jean-Christophe; Carter Berry, Z; Schwantes, Amanda M; McCulloh, Katherine A; Woodruff, David R; Wayne Polley, H; Wortemann, Remí; Swenson, Jennifer J; Scott Mackay, D; McDowell, Nate G; Jackson, Robert B
    From 2011 to 2013, Texas experienced its worst drought in recorded history. This event provided a unique natural experiment to assess species-specific responses to extreme drought and mortality of four co-occurring woody species: Quercus fusiformis, Diospyros texana, Prosopis glandulosa, and Juniperus ashei. We examined hypothesized mechanisms that could promote these species' diverse mortality patterns using postdrought measurements on surviving trees coupled to retrospective process modelling. The species exhibited a wide range of gas exchange responses, hydraulic strategies, and mortality rates. Multiple proposed indices of mortality mechanisms were inconsistent with the observed mortality patterns across species, including measures of the degree of iso/anisohydry, photosynthesis, carbohydrate depletion, and hydraulic safety margins. Large losses of spring and summer whole-tree conductance (driven by belowground losses of conductance) and shallower rooting depths were associated with species that exhibited greater mortality. Based on this retrospective analysis, we suggest that species more vulnerable to drought were more likely to have succumbed to hydraulic failure belowground.
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    Conversion of natural forests to managed forest plantations decreases tree resistance to prolonged droughts
    (Forest Ecology and Management, 2015-01-01) Domec, Jean-Christophe; King, John S; Ward, Eric; Christopher Oishi, A; Palmroth, Sari; Radecki, Andrew; Bell, Dave M; Miao, Guofang; Gavazzi, Michael; Johnson, Daniel M; McNulty, Steve G; Sun, Ge; Noormets, Asko
    © 2015 Published by Elsevier B.V.Throughout the southern US, past forest management practices have replaced large areas of native forests with loblolly pine plantations and have resulted in changes in forest response to extreme weather conditions. However, uncertainty remains about the response of planted versus natural species to drought across the geographical range of these forests. Taking advantage of a cluster of unmanaged stands (85-130year-old hardwoods) and managed plantations (17-20year-old loblolly pine) in coastal and Piedmont areas of North Carolina, tree water use, cavitation resistance, whole-tree hydraulic (Ktree) and stomatal (Gs) conductances were measured in four sites covering representative forests growing in the region. We also used a hydraulic model to predict the resilience of those sites to extreme soil drying. Our objectives were to determine: (1) if Ktree and stomatal regulation in response to atmospheric and soil droughts differ between species and sites; (2) how ecosystem type, through tree water use, resistance to cavitation and rooting profiles, affects the water uptake limit that can be reached under drought; and (3) the influence of stand species composition on critical transpiration that sets a functional water uptake limit under drought conditions. The results show that across sites, water stress affected the coordination between Ktree and Gs. As soil water content dropped below 20% relative extractable water, Ktree declined faster and thus explained the decrease in Gs and in its sensitivity to vapor pressure deficit. Compared to branches, the capability of roots to resist high xylem tension has a great impact on tree-level water use and ultimately had important implications for pine plantations resistance to future summer droughts. Model simulations revealed that the decline in Ktree due to xylem cavitation aggravated the effects of soil drying on tree transpiration. The critical transpiration rate (Ecrit), which corresponds to the maximum rate at which transpiration begins to level off to prevent irreversible hydraulic failure, was higher in managed forest plantations than in their unmanaged counterparts. However, even with this higher Ecrit, the pine plantations operated very close to their critical leaf water potentials (i.e. to their permissible water potentials without total hydraulic failure), suggesting that intensively managed plantations are more drought-sensitive and can withstand less severe drought than natural forests.
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    Leaf hydraulic parameters are more plastic in species that experience a wider range of leaf water potentials
    (Functional Ecology, 2018-01-01) Johnson, Daniel M; Berry, Z Carter; Baker, Kathryn V; Smith, Duncan D; McCulloh, Katherine A; Domec, Jean‐Christophe
    © 2018 British Ecological Society. Many plant species experience large differences in soil moisture availability within a season, potentially leading to a wide range of leaf water potentials (Ψ LEAF ). In order to decrease the risk of leaf dehydration, among species, there is a continuum ranging from strict control (isohydry) to little control (anisohydry) of minimum Ψ LEAF . In central Texas USA, species are exposed to a range of soil moisture from wet springs to hot, dry summers. There are diverging water management strategies among the four dominant woody species in this system; two of these species are more isohydric (Prosopis glandulosa, Quercus fusiformis) while two others are more anisohydric (Diospyros texana, Juniperus asheii). To maintain leaf turgor and photosynthesis during periods of limited soil moisture, anisohydric species may adjust leaf hydraulic parameters more than isohydric species. To test this hypothesis, we quantified iso/anisohydry from 3 years of Ψ LEAF predawn and midday measurements, and we measured the changes in turgor loss points (Ψ TLP ), osmotic potential at full hydration (Ψ π100 ), and resistance to leaf hydraulic dysfunction (leaf P 50 ) throughout the spring and summer of 2016. Diospyros and Juniperus experienced more negative Ψ LEAF and adjusted Ψ TLP and Ψ π100 in response to both drying soils during the summer also in response to rainfall events during September. In contrast, the more isohydric species (Quercus and Prosopis) did not appear to adjust Ψ TLP or Ψ π100 in response to soil moisture. The more anisohydric species also adjusted leaf P 50 during periods of reduced soil moisture. Our results suggest that species that experience wider ranges of Ψ LEAF have a greater ability to alter leaf hydraulic properties. This provides insight on how species with different strategies for water potential regulation may modify properties to mitigate drought effects in the future. A plain language summary is available for this article.
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    Role of aquaporin activity in regulating deep and shallow root hydraulic conductance during extreme drought
    (Trees, 2014-05-30) Johnson, Daniel M; Sherrard, Mark E; Domec, Jean-Christophe; Jackson, Robert B
    Key message Deep root hydraulic conductance is upregulated during severe drought and is associated with upregulation in aquaporin activity. In 2011, Texas experienced the worst single-year drought in its recorded history and, based on tree-ring data, likely its worst in the past millennium. In the Edwards Plateau of Texas, rainfall was 58 % lower and the mean daily maximum temperatures were >5 °C higher than long-term means in June through September, resulting in extensive tree mortality. To better understand the balance of deep and shallow root functioning for water supply, we measured root hydraulic conductance (KR) in deep (~20 m) and shallow (5-10 cm) roots of Quercus fusiformis at four time points in the field in 2011. Deep roots of Q. fusiformis obtained water from a perennial underground (18-20 m) stream that was present even during the drought. As the drought progressed, deep root KR increased 2.6-fold from early season values and shallow root KR decreased by 50 % between April and September. Inhibitor studies revealed that aquaporin contribution to KR increased in deep roots and decreased in shallow roots as the drought progressed. Deep root aquaporin activity was upregulated during peak drought, likely driven by increased summer evaporative demand and the need to compensate for declining shallow root KR. A whole-tree hydraulic transport model predicted that trees with greater proportions of deep roots would have as much as five times greater transpiration during drought periods and could sustain transpiration during droughts without experiencing total hydraulic failure. Our results suggest that trees shift their dependence on deep roots versus shallow roots during drought periods, and that upregulation of aquaporin activity accounts for at least part of this increase. © 2014 Springer-Verlag Berlin Heidelberg.