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Drought and freezing vulnerability of the isolated hybrid aspen Populus x smithii relative to its parental species, P. tremuloides and P. grandidentata - PubMed

  • ️Tue Jan 01 2019

. 2019 Jun 25;9(14):8062-8074.

doi: 10.1002/ece3.5364. eCollection 2019 Jul.

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Drought and freezing vulnerability of the isolated hybrid aspen Populus x smithii relative to its parental species, P. tremuloides and P. grandidentata

Nicholas J Deacon et al. Ecol Evol. 2019.

Abstract

Aim: We assessed the vulnerability of an isolated, relictual Pleistocene hybrid aspen population of conservation interest (Populus x. smithii) and the nearest populations of its parent species (Populus grandidentata and Populus tremuloides) to springtime post-bud break freezing and growing season drought stress. Response to these stressors in the three taxa was compared in terms of avoidance and tolerance.

Location: North American Midwest; USA.

Methods: Unique genets from the hybrid Niobrara River population and from the two parental populations were propagated in a common garden from rhizome cuttings. We tracked their phenology before and after bud break and measured their vulnerability to freezing (stem electrolyte leakage and leaf chlorophyll fluorescence) and to drought (stem hydraulic conductance, leaf osmotic potential, stomatal pore index, and gas exchange).

Results: Populus grandidentata was slower to leaf out, showed lower vulnerability to stem freezing and drought-induced cavitation, but exhibited a lower capacity to tolerate drought stress through leaf resistance traits compared to P. tremuloides. Hybrids were similar to P. grandidentata in their overwintering strategy, exhibiting later bud break, and in their higher resistance to stem freezing damage, but they were more similar to P. tremuloides in their higher vulnerability to drought-induced cavitation. The hybrids shared various leaf-level gas exchange traits with both parents. All aspens showed limited loss of leaf photosynthetic function following moderate freezing.

Main conclusions: The Niobrara River hybrid population is vulnerable to drought due to its combination of inherited drought avoidance and tolerance traits. As climate changes, P. x smithii will likely suffer from increased drought stress, while being unaffected by frost during warmer springs. The two parental species contrast in their survival mechanisms in response to climatic stress, with P. tremuloides tending toward freezing tolerance but drought avoidance and P. grandidentata tending toward freezing avoidance and drought tolerance.

Keywords: Niobrara National Scenic River; aspens; climate change; ecophysiology; electrolyte leakage; gas exchange; hydraulic conductivity; phenology.

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Figures

Figure 1
Figure 1

(a) Collections of Populus grandidentata (red triangles), Populus tremuloides (blue squares), and Niobrara Populus x smithii (purple circle) took place within and across the western boundary of the upper Midwestern ranges of Populus grandidentata (hatched) and P. tremuloides (light gray), and from isolated stands of P. tremuloides. Dark gray shading indicates areas where neither species is present. (b) Climatic (annual mean temperature and precipitation) envelopes of the ranges of P. grandidentata (red) and P. tremuloides (blue). A purple dot indicates the climatic conditions experienced by the hybrid P. x smithii population at Smith Falls. Details of climate envelopes in Appendix S3

Figure 2
Figure 2

Spring bud break phenology of aspens planted in a common garden in east‐central Minnesota, USA. (a) Populus tremuloides (blue) broke bud (reached stage 3; initial leaf emergence) roughly a week before Populus grandidentata (red) and their hybrid, Populus x smithii (purple). (b) Individuals of P. tremuloides varied significantly in their common garden phenology by state of origin. Trees from hot and dry western states (red, orange) showed advanced phenology compared to trees from cool and wet eastern states (green, blue, purple). Superscripts indicate significant (a) species or (b) state differences at the 0.05 level via Tukey test of an ANOVA model including date and either species or state

Figure 3
Figure 3

Branches of mature Populus grandidentata and Populus tremuloides collected from the area of the common garden in east‐central Minnesota on different dates and incubated in warm conditions in the greenhouse differed significantly in the days of warmth they required to break bud. Across a range of simulated winter lengths, P. tremuloides stems broke bud about four days earlier

Figure 4
Figure 4

Aspen stem tolerance of experimentally induced freezing depended on freezing temperature and species. Populus grandidentata (red) was less vulnerable to freezing at low temperatures (−15° to −10°C) than was Populus tremuloides (blue), with their hybrid intermediate (purple). Species did not differ and were relatively invulnerable to freezing at −5°C. Superscripts indicate significant species differences at the 0.05 level via Tukey test of an ANOVA model including freezing temperature and species

Figure 5
Figure 5

Aspen stem tolerance of experimentally induced freezing at −15°C depended on date of stem collection and species. Populus grandidentata (red) was less vulnerable to freezing early and late in the growing season than was P. tremuloides (blue), with their hybrid intermediate (purple). Species were most vulnerable to freezing in May, after they had leafed out but prior to acclimation to cool autumn temperatures. Superscripts indicate significant species differences at the 0.05 level via Tukey test of an ANOVA model including stem collection date and species

Figure 6
Figure 6

Populus grandidentata (red) showed lower loss of native stem hydraulic conductivity with increasing xylem tension (negative water potential) than did Populus tremuloides (blue). Hybrid Populus x smithii (purple) was intermediate. Shading around curves indicates 95% confidence intervals. Vertical lines show mean P50 (water potential at which 50% loss of conductivity occurred) for each species with values and confidence intervals provided above the plot

Figure 7
Figure 7

Species differed significantly in leaf traits related to water use and drought vulnerability. (a) Populus tremuloides (blue) leaves had a significantly more negative osmotic potential, allowing increased water access during drought (a drought avoidance strategy) than Populus grandidentata (red). (b) Compared to P. tremuloides, P. grandidentata had lower stomatal pore index (SPI), which allows leaves to minimize water loss. Hybrid Populus x smithii (purple) leaves showed lower negative osmotic potential similar to P. grandidentata but higher SPI similar to P. tremuloides. Letters indicate significant differences at the 0.05 level from a Tukey post hoc test

Figure 8
Figure 8

(a) Intrinsic water use efficiency (WUEi; light saturated A max/g s) in the common garden did not vary by species or in plants collected in more mesic, eastern sites. However, when only Populus tremuloides individuals from drier, hotter western sites (those with a high summer heat moisture index) were considered, plants from the driest and hottest sites had lower WUE in the common garden. The dashed line indicates the highest summer heat moisture index value recorded for a Populus grandidentata observation in the GBIF database. Populus tremuloides individuals collected from sites with a higher Index value than this are included in the regression. (b) Observed light saturated stomatal conductance (g s) and (c) photosynthetic rate (A max), in the common garden did not vary by species (red: P. grandidentata, purple: Populus x smithii, blue: P. tremuloides). There was no relationship between g s or A max and climate (summer heat moisture index) in the range of P. grandidentata and the eastern part of the range of P. tremuloides. Yet in the western part of the range of P. tremuloides, A max declined precipitously (and g s did to a lesser extent) contributing to a significantly lower WUEi of plants from warmer and drier climates

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