Daytime of direct radiation and growth of Norway spruce seedlings in spruce mountain forests

Peter Brang (Project leader)

Project aims

Methods

Work plan

Participants

Publications

Abstract: This project addresses a question often asked in forest management: At which daytime is direct radiation most effective in promoting Norway spruce seedling growth in subalpine Norway spruce forests?

Introduction

Norway spruce (Picea abies (L.) Karst.) stands are widespread in Switzerland in the upper montane and subalpine zone. Many of these stands protect against natural hazards and have reached an age where regeneration is urgent to permanently ensure their protective effect, and to create more natural stand structures. On steep north slopes of the subalpine zone, openings to favor regeneration have to be very small in order to prevent the release of snow avalanches. In these cood environments, Norway spruce regeneration is limited by soil temperature (Imbeck and Ott 1987). Seedling establishment has been successfully promoted by cutting elongated openings that enable the direct radiation of the sun to reach the soil surface. This leads to an increase in soil temperature near the surface, and promotes seedling root growth (Imbeck und Ott 1987, Brang 1996 und 1998), in contrast to diffuse radiation that does not have such a warming effect (Imbeck und Ott 1987).

Moreover, ground vegetation is well known to have a cooling effect on the soil (Imbeck und Ott 1987, Coates et al. 1991), although the magnitude of this effect remains unclear. However, the most urgent question is at which daytime direct radiation is most effective in promoting Norway spruce seedling growth. In practical forestry this has often been debated since the daytime of direct radiation can be easily influenced by the orientation of the elongated openings. Foresters have often assumed specific differences between different opening directions (e.g, Bischoff 1987), but scientific proof is lacking.

Project goals

The goals of this research project are:

1. To demonstrate the nature of the relationship between daytime of direct radiation and growth of Norway spruce seedlings, and the magnitude of any effects,
2. and to clarify the extent to which this relationship depends on ground vegetation.

Scientific methods

Starting from a priori-hypotheses I will investigate the relation between seedling growth, direct radiation and soil temperature using a field experiment. I will plant 3-year old Norway spruce seedlings in pots with a uniform substrate. The pots will be placed in a gap (> 1.0 ha) on a north-facing slope of the subalpine zone. Four groups of six pots will be placed below an artificial "window", which consists of two perpendicular walls (Fig. 1). The placement ensures that one group receives direct radiation from about 9 a.m. (morning), one from about 12 a.m. (noon), one from about 3 p.m. (afternoon), and one none at all, but only diffuse radiation (control treatment). This arrangement is repeated 15 times. Five of these "windows" are located on spots with dense ground vegetation, and 10 on spots without or with only scarce ground vegetation and where the vegetation is kept small. The walls are removed during winter and re-installed in spring.

The duration of direct radiation ensures that the received radiative energy is independent of daytime. This requires the edges of the walls to be formed, and the pots to be arranged, accordingly. Diffuse and direct radiation are assessed using fisheye photography. Soil temperatures are measured every 10 minutes during one growing season from the end of May until the end of September in a few pots. Thirty-two sensors are installed 4 cm below the soil surface, and connected to a data logger. Climate data are continuously recorded. During summer, the seedlings are protected from browsing ungulates with a fence.

Four of the six pots in each group are transferred to the valley during winter (beginning of November to mid May) to prevent browsing and black snow mould infection. In spring, they are re-arranged in the field, exactly in the same spots (location, orientation). Seedlings remaining in the field are protected from browsing, probably using a mechanical protection.

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Fig. 1. Experimental design. The pots with the seedlings are arranged in a way that they receive direct radiation either in the morning, around noon, in the afternoon, or none at all (only diffuse radiation).

Seedling growth is used as the variable reflecting seedling performance, mainly for three reasons: It is easy to influence using silvicultural treatments; it is related to seedling survival (Brang 1996); and it is easy to measure. On each seedling, the initial height and the basal diameter are recorded. After three growing seasons, the height growth and the biomass of the above-ground and below-ground parts are measured, and the number of shoots grown during these three seasons counted. The data are analyzed using analysis of covariance, with response variables reflecting seedling performance and soil temperature. The covariates for seedling performance are radiation variables, soil temperature variables and the initial seedling height and basal diameter. The relation between soil temperature and radiation is calculated and used to estimate soil temperature variables for pots without direct measurements.

Duration of the project and partners

The project will last four years. Much work has to be done in the first year: The study site will be selected, the location of the experimental installations chosen, the climate station installed, the walls constructed, the seedlings potted and put into the field, their initial status recorded, the field supervisors instructed, the fisheye photographs taken and the soil temperatures measured. In the second year, the seedlings are only loosely watched. In the third year, the seedling parameters are recorded. Data will be analyzed in the fourth year. Extension activities are planned for the whole project duration.

The project partners are the local forest service (supervision of the study site), the Forestry School in Maienfeld, Switzerland (supervision of the study site, extension) and the Federal Institute of Technology (FIT), Zurich, Chair of Silviculture (fisheye photography). The data are analyzed in a diploma thesis at FIT.

References