Winter Embolism and Recovery in the Conifer Shrub Pinus mugo L.

Stefan Mayr,Peter Schmid,Sabine Rosner

doi:10.3390/f10110941

Stefan Mayr, Peter Schmid + Show 1 more

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https://doi.org/10.3390/f10110941

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Abstract

Research Highlights: Pronounced winter embolism and recovery were observed in the Alpine conifer shrub Pinus mugo L. Data indicated that the hydraulic courses and underlying mechanism were similar to timberline trees. Background and Objectives: At high elevation, plants above the snow cover are exposed to frost drought and temperature stress during winter. Previous studies demonstrated winter stress to induce low water potentials (Ψ) and significant xylem embolism (loss of conductivity, or LC) in evergreen conifer trees, and recovery from embolism in late winter. Here, we analyzed xylem hydraulics and related structural and cellular changes in a conifer shrub species. Materials and Methods: The uppermost branches of Pinus mugo shrubs growing at the Alpine timberline were harvested over one year, and the Ψ, water content, LC, proportion of aspirated pits, and carbohydrate contents were analyzed. Results: Minimum Ψ (−1.82 ± 0.04 MPa) and maximum LC (39.9% ± 14.5%) values were observed in mid and late winter, followed by a recovery phase. The proportion of aspirated pits was also highest in winter (64.7% ± 6.9% in earlywood, 27.0% ± 1.4% in latewood), and decreased in parallel with hydraulic recovery in late winter and spring. Glucose and fructose contents gradually decreased over the year, while starch contents (also microscopically visible as starch grains in needle and stem tissues) increased from May to July. Conclusions: The formation and recovery of embolism in Pinus mugo were similar to those of timberline trees, as were the underlying mechanisms, with pit aspiration enabling the isolation of embolized tracheids, and changes in carbohydrate contents indicating adjustments of osmotic driving forces for water re-distribution. The effects of future changes in snow cover regimes may have pronounced and complex effects on shrub-like growth forms, because a reduced snow cover may shorten the duration of frost drought, but expose the plants to increased temperature stress and impair recovery processes.

Highlights

At high elevation, plants, unless protected by the snow cover, are exposed to high stress intensities during winter
Materials and Methods: The uppermost branches of Pinus mugo shrubs growing at the Alpine timberline were harvested over one year, and the Ψ, water content, loss of conductivity (LC), proportion of aspirated pits, and carbohydrate contents were analyzed
The formation and recovery of embolism in Pinus mugo were similar to those of timberline trees, as were the underlying mechanisms, with pit aspiration enabling the isolation of embolized tracheids, and changes in carbohydrate contents indicating adjustments of osmotic driving forces for water re-distribution

Summary

Introduction

Plants, unless protected by the snow cover, are exposed to high stress intensities during winter. They have to cope with temperature stress (i.e., low minimum temperatures and high temperature fluctuations) as well as winter drought. Evergreen trees were demonstrated to experience frost drought, because water is lost via (cuticular) transpiration over the crown, while the frozen soil blocks water uptake for months [1]. Low water potentials (Ψ) develop, which, in combination with frequent freeze–thaw events, can lead to the formation of xylem embolism. Embolism blocks the water transport and the water supply of distal tissues.

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