Effect of shelterwood density on nocturnal near-ground temperature, frost injury risk and budburst date of Norway spruce

The near-ground temperature regime during summer periods in 1990-1992 was analysed in a high Scots pine (Pinus silvestris (L.)) shelterwood located in the interior of southern Sweden. Using a mobile climate measuring unit, the air temperatures 0.4 and 1.7 m above the ground were continuously recorded along a 400 m transect covering a clear-cut area and a shelterwood with stem density ranging from 10 to 420 trees ha(-1). Cuttings of two Norway spruce (Picea abies (L.) Karst.) clones were planted along the shelterwood transect. Bud development and shoot elongation of the cuttings were recorded several times during 1991, 1992 and 1997. The temperatures were measured close to each cutting. Soil surface moisture conditions influenced rates of nocturnal coolin. During clear and calm nights, the minimum 9 temperature was 3.2 degreesC higher in the densest part of the shelterwood compared to the clear-cut area when the ground surface was dry (soil water potential <-0.04 MPa), but only 1.7 degreesC higher at other times. The largest vertical temperature gradient between 1.7 and 0.4 m above the ground was found in the clear-cut area, while the gradient was near zero in the dense shelterwood. During the dry period, the minimum temperature difference between these points in the clear-cut area was larger, at 3.8 degreesC, than during other periods when it averaged 1.6 degreesC. Bud development assessments in 1991 and 1992 showed that budburst was delayed in the dense shelterwood, compared to the clear-cut area. A simple model for estimating the dates when the period in which the cuttings were most susceptible to frost began and ended were derived for each of the two clones, based on the accumulated daily maximum temperature above +10 degreesC. The model was applied to the growing seasons in 1990-1992 in an analysis of the frequency and severity of frosts during the frost-susceptible period. As a combined effect of higher minimum temperature and delayed flushing, on average two nights with frost occurred in the dense part of the shelterwood, compared to eight in the clear-cut area, during the frost-susceptible period for the early-flushing clone in 1990-1992. For the late-flushing clone, the number of nights with frost, experienced while the buds were in a potentially susceptible developmental stage, was lower compared to the early-flushing clone, in all shelterwood densities. Shelterwood regeneration systems are recommended on sites where early summer frosts are likely, since they can delay budburst date and reduce the risk of frost injury to seedlings, especially on dry soils. (C) 2002 Elsevier Science B.V. All rights reserved.