Effects of excess nitrogen on frost hardiness and freezing injury of above‐ground tissue in young oaks (Quercus petraea and Q. robur)

Abstract

The effects of excess nitrogen (N) on the frost hardiness and freezing injury of bark and buds were tested in 2‐yr‐old sessile oaks (Quercus petraea) and pedunculate oaks (Q. robur) that had been grown outdoors in sand culture with normal or luxurious N supply during the growing season. Some trees from both N treatments were subjected to drought stress in summer, whereas others were adequately watered. Between January and March, whole plants were exposed to artificial freezing treatments at −25 °C of different durations (10 d, 21 d, or two periods of 4 d interrupted by an 8 d frost‐free period, referred to as a freeze–thaw cycle). The frost hardiness and freezing injury of the bark were determined by two different versions of the electrolyte leakage method (calculation of an index of injury, I−25, from the relative conductivity after a fixed time; and measurement of the electrolyte leakage rate). In addition, the vitality of bark and buds was assessed visually with the 2,3,5‐triphenyltetrazolium chloride (TTC) method. In comparison with the control, the oaks that had been luxuriously supplied with N exhibited distinctly higher N concentrations and lower C:N ratios in their leaves and bark, higher relative shoot increments, delayed leaf discoloration in autumn, and earlier budbreak in spring. Oaks kept outdoors during the winter were frost‐hardy from December until the end of February without showing differences between species or N treatments. In comparison with the pedunculate oak, the sessile oak was more susceptible to freezing injury induced by prolonged artificial freezing or by the freeze–thaw cycle. Increased freezing injury in trees with high N supply was detected only by the TTC test of the bark of sessile oaks subjected to the freeze–thaw cycle. In all oaks, except for the pedunculate oaks grown with normal N, the freeze–thaw cycle resulted in significantly lower frost hardiness, as indicated by increased I−25 values. This treatment also led to increased freezing damage of bark and buds, as revealed by the TTC test. In drought‐stressed oaks, the electrolyte leakage rate was significantly elevated after the freeze–thaw cycle. It is concluded that, in the sessile and pedunculate oaks, the water supply before frost stress and the course of the temperature in winter have a greater effect on frost hardiness and freezing damage than excess N.

Assessment of the impacts of climate change and weather extremes on boreal forests in northern Europe, focusing on Norway spruce

“…Spring frost damage will affect not only the vitality of larger trees but also, and perhaps more importantly, the establishment phase of seedlings and planted trees. Nitrogen deposition, once again, poses an additional stress factor as it reduces frost tolerance in plants (Skre 1988, Thomas & Ahlers 1999, Jönsson & Welander 2002, Jönsson et al 2004c); current deposition is greatest in the southwestern and southern parts of Sweden (Sverdrup et al 2002).…”

Effects of defoliation on the frost hardiness and the concentrations of soluble sugars and cyclitols in the bark tissue of pedunculate oak (Quercus robur L.)

“…Frost hardiness of the bark tissue was determined by artificial freezing according to Kolb et al (1985) [23], modified according to Thomas and Ahlers (1999) [44]. The glass vials with one bark sample each were frozen in a cryostat (Fryka FT 10-44; National Lab., Mölln, Germany) from +5°C to -30°C with a cooling rate of 5°C·h -1 (extreme minimum air temperatures below -25°C had occasionally occurred in Northern Germany during the past decade).…”

“…A respective number of control samples remained unfrozen in a refrigerator at +5°C during that time. Electrolyte leakage from the samples was measured with a conductivity sensor (sensor LTA 1 and conductometer LF 2000/C; WTW, Weinheim, Germany) after incubation in 6 mL of 3% (v/v) propanol in distilled water for 24 h. The relative conductivity (RC; %) of the medium was determined after killing the tissue by autoclaving as described by Thomas and Ahlers (1999) [44]. From the RC values of frozen and control samples, an index of injury by freezing at -30°C (I -30 ) was calculated according to Flint et al (1967) [13].…”

Effects of fertilisation on growth and nutrition of Norway spruce on a harsh mountain site

“…Though, the luxurious N is often viewed rather as a secondary factor than the decisive one. For instance Thomas and Ahlers (2007) admitted that N excess might play a role in the increased frost sensitivity of oaks, but these authors considered the water supply prior to frost stress onset and the course of winter temperature much more important than N excess.…”