Andreas Brunner

Characterization of the structure, dynamics, and productivity of mixed-species stands: review and perspectives

The growth and yield of mixed-species stands has become an important topic of research since there are certain advantages of this type of forest as regards functions and services. However, the concepts and methods used to characterize mixed stands need to be understood, as well as harmonized and standardized. In this review we have compiled a set of measures, indices, and methods at stand level to characterize the structure, dynamics, and productivity of mixed stands, and we discuss the pros and cons of their application in growth and yield studies. Parameters for the characterization of mixed stand structure such as stand density, species composition, horizontal (intermingling) and vertical tree distribution pattern, tree size distribution, and age composition are described, detailing the potential as well as the constraints of these parameters for understanding resource capture, use, and efficiency in mixed stands. Furthermore, a set of stand-level parameters was evaluated to characterize the dynamics of mixed stands, e.g. height growth and space partitioning, self- and alien-thinning, and growth partitioning among trees. The deviations and changes in the behaviour of the analysed parameters in comparison with pure stand conditions due to inter-specific interactions are of particular interest. As regards stand productivity, we reviewed site productivity indices, the growth–density relationship in mixed stands as well as methods to compare productivity in mixed versus monospecific stands. Finally, we discuss the main problems associated with the methodology such as up-scaling from tree to stand level as well as the relevance of standardized measures and methods for improving forest growth and yield research in mixed stands. The main challenges are also outlined, especially the need for qualitatively sound data.

Site index prediction from site and climate variables for Norway spruce and Scots pine in Norway

Abstract Site index prediction models for Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) were developed using Norwegian National Forest Inventory data. A number of multiple linear regression models with different combinations of site and climate variables were developed in order to facilitate their application to a range of situations where the accessibility of various explanatory data differs. The best models used year of stand origin, temperature sum, vegetation type groups, soil depth, aspect, slope and latitude to predict site index. These models explained a large part of the total variation ( = 0.86 and 0.72 for spruce and pine, respectively) and had little residual variation (RMSE = 2.04 and 1.95 m for spruce and pine, respectively). Alternative models using only year of stand origin, temperature sum and vegetation type groups, or soil depth in addition, had slightly lower but still useful predictive power. All the developed models exhibited a strong non-linear effect of the year of stand origin on site indices, which varied when temperature sum was included. The increase in site indices along with increasing year of stand origin was significantly faster after about 1940 for both species. Similar time trends were observed for mean temperature and precipitation sums for the periods of stand growth, but only exhibited a faster increase after about 1960. Even though increased temperature and precipitation after 1990 seem to contribute to increased site indices, increased nitrogen availability and atmospheric CO2 levels may also be important factors.

Density–growth relationships in thinned and unthinned Norway spruce and Scots pine stands in Norway

Abstract Drawing on national forest inventory data representing a wide range of stand densities, site qualities, ages, and management practices in Norway, this study (1) analyzed the stand density–growth relationships in Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) dominated stands and (2) evaluated the influence of site index, stand age, and thinning on the density–growth relationships. We developed nonlinear growth models that described the periodic annual basal area or volume increment as a function of initial basal area, site index, and stand age. The results showed that for both species, basal area growth increases with stand density although degressive at higher densities. Volume growth increases nearly linearly with stand density. Nevertheless, as demonstrated by the similar shapes of the growth curves, the pattern of the density–growth relationships did not vary considerably by site index classes, by stand ages, and between thinned and unthinned stands. Thinned stands grew about 10% more than unthinned stands of the same density, site index, and age. Our models showed a much stronger decrease of volume growth with decreasing density than previously published results from thinning experiments for the two species in Fennoscandia. The study also demonstrated the suitability of national forest inventory data for analyzing the density–growth relationship.

Conversion of Norway Spruce: A Case Study in Denmark Based on Silvicultural Scenario Modelling

The aim of this study was to evaluate silvicultural alternatives for conversion of secondary plantations of Norway spruce [Picea abies (L.) Karst] in Denmark. To enable simulations that could possibly aid decision making for policy as well as for forestry practice, the German growth simulator SILVA was calibrated for Norway spruce and beech in Denmark. Calibration comprised changes to the main driving functions of SILVA, such as the height and the diameter growth potentials of individual trees. The calibrated model was evaluated against observed growth for a number of sample plots. Scenarios for different methods for conversion of Norway spruce on former heathland were calculated and analysed, including uniform shelterwood, irregular shelterwood, and target diameter harvest. The predictions of growth and regeneration did not show any limitations to a successful conversion, but resulted in different growth patterns and species compositions.

Stand responses to initial spacing in Norway spruce plantations in Norway

Abstract Using data from nine spacing experiments of Norway spruce (Picea abies (L.) Karst.) in Norway, covering wide ranges of site index and initial spacing, this study evaluated stand basal area and volume responses to initial spacing and examined whether these responses varied by stand ages or site quality. We developed nonlinear regression models that described the standing basal area or volume responses to initial spacing along with site index and stand ages. The results show that closer spacings produced higher standing basal area and volume than wider spacings. The response curves are highly nonlinear in younger stands and become nearly linear in mid-rotation stands, indicating stronger responses at younger ages and weaker responses as age increases. Furthermore, for young stands, spacing effects are stronger at closer than at wider spacings. The basal area and volume responses to initial spacing tend to be similar across site indices. However, the interaction of site index and stand age on spacing responses makes it difficult to isolate the effect of site index on spacing responses. Mortality is higher and begins earlier at closer spacings than wider. The mean diameter of the largest 100, 400, 600, and 800 trees ha−1 increased with spacing in three out of the nine experiments. Dominant height did not vary by initial spacing for any of the experiments. The findings suggest that the extra volume production in stands of closer initial spacing is restricted to early stand development.

Modeling individual tree height growth of Norway spruce and Scots pine from national forest inventory data in Norway

ABSTRACT We developed individual tree height growth models for Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) in Norway based on national forest inventory data. Potential height growth is based on existing dominant height growth models and reduced due to competition by functions developed in this study. Three spatially explicit and two spatially non-explicit competition indices were tested. Distance effects and diameter ratio effects were estimated from the data simultaneously with parameters of the potential modifier functions. Large height measurement errors in the national forest inventory data caused large residual variation of the models. However, the effects of competition on height growth were significant and plausible. The potential modifier functions show that height growth of dominant trees is largely unaffected by competition. Only at higher levels of competition, height growth is reduced as a consequence of competition. However, Scots pine also reduced height growth at very low levels of competition. Distance effects in the spatially explicit competition indices indicated that the closest neighbors are most important for height growth. However, for Scots pine also competitors at larger distance affected height growth. The five competition indices tested in this study explained similar proportions of the variation in relative height growth. Given that unbiased predictions can only be expected for the same plot size, we recommend a spatially explicit index, which describes the distance function with a negative exponential, for use in growth simulators.