Per-Ola Hedwall

Fertilization in northern forests - biological, economic and environmental constraints and possibilities

Forests of northern ecosystems respond slowly to management activities and the possibilities to increase the growth in a short-term perspective and meet swift increases in societys demand for biomass are small. An exception among the silvicultural measures is fertilization which can be applied in combination with present management systems and, almost instantly, enhances forest productivity. There may, however, be both economic and environmental constraints to large-scale applications of fertilizers in forest. Here we review the literature concerning biomass production of forests under different fertilization regimens, environmental constraints and possibilities in northern forests on mineral soils. Further on we discuss the implications of both extensive and more intensive fertilization in relation to the developing bioeconomy, which encompasses the production and conversion of renewable biological resources into food, health and industrial products and energy. Fertilization in Sweden and Finland is currently practiced by extensive fertilization regimens where nitrogen fertilizers are applied once, or up to three times, during a rotation period, mainly in mature forest. This type of fertilization gives, in most cases, a small and transient effect on the environment as well as a high rate of return to the forest owner with low-economic risk. The increase in biomass production, however, is relatively small and consequently the impact on the processing industry and the bioeconomy is limited. More intensive fertilization regimens implying intensive fertilization starting in young forests may, on the other hand, considerably increase the biomass supply and value for the industry. The economic and environmental risks of this type of fertilization may, however, be larger and more research is needed on the effects on the stand level, and especially on the landscape level, including late rotation management of the forest.

Trait variations of ground flora species disentangle the effects of global change and altered land‐use in Swedish forests during 20 years

Northern forest ecosystems are exposed to a range of anthropogenic processes including global warming, atmospheric deposition, and changing land-use. The vegetation of northern forests is composed of species with several functional traits related to these processes, whose effects may be difficult to disentangle. Here, we combined analyses of spatio-temporal dynamics and functional traits of ground flora species, including morphological characteristics, responses to macro- and microclimate, soil conditions, and disturbance. Based on data from the Swedish National Forest Inventory, we compared changes in occurrence of a large number of ground flora species during a 20-year period (1994-2013) in boreal and temperate Sweden respectively. Our results show that a majority of the common ground flora species have changed their overall frequency. Comparisons of functional traits between increasing and declining species, and of trends in mean trait values of sample plots, indicate that current floristic changes are caused by combined effects of climate warming, nitrogen deposition and changing land-use. Changes and their relations with plant traits were generally larger in temperate southern Sweden. Nutrient-demanding species with mesotrophic morphology were favored by ongoing eutrophication due to nitrogen deposition in the temperate zone, while dwarf shrubs with low demands on nitrogen decreased in frequency. An increase of species with less northern and less eastern distribution limits was also restricted to temperate Sweden, and indicates effects of a moister and milder macroclimate. A trend toward dense plantation forests is mirrored by a decrease of light-demanding species in both vegetation zones, and a decrease of grassland species in the temperate zone. Although denser tree canopies may buffer effects of a warmer climate and of nitrogen deposition to some extent, traits related to these processes were weakly correlated in the group of species with changing frequency. Hence, our results indicate specific effects of these often confounded anthropogenic processes.

Decreased variation of forest understory vegetation is an effect of fertilisation in young stands of Picea abies

Abstract The substitution of fossil fuels with biofuels to mitigate climate change has caused increased interest in enhancing forest biomass production through fertilisation. We investigated the effects of different fertilisation frequencies on the diversity of understory vegetation in young stands of Picea abies on five sites distributed in regions in the middle and south of Sweden. The treatments included fertilisation conducted annually, every second year or every third year, as well as an unfertilised control. A lower number of vascular plant species was observed on fertilised plots than on control plots, whereas the number of bryophyte species remained unchanged. Fertilised plots also showed a lower variance in species composition and a lower Shannons diversity index than unfertilised plots. Fertilised plots were more similar to each other than unfertilised plots were to each other over the geographical range. The two most intensive fertilisation treatments had similar effects on the vegetation, whereas the effects of fertilisation conducted every third year were not as substantial. However, the treatment in which fertilisation occurred every third year implies a lower stem-wood production, and there is little knowledge of the long-term differences between the treatments. We conclude that fertilisation of young stands will lead to long-term changes in understory vegetation at the stand scale, whereas the effects at the landscape level are still largely unknown.

The Nutritional Balancing Act of a Large Herbivore: An Experiment with Captive Moose (Alces alces L).

The nutrient balancing hypothesis proposes that, when sufficient food is available, the primary goal of animal diet selection is to obtain a nutritionally balanced diet. This hypothesis can be tested using the Geometric Framework for nutrition (GF). The GF enables researchers to study patterns of nutrient intake (e.g. macronutrients; protein, carbohydrates, fat), interactions between the different nutrients, and how an animal resolves the potential conflict between over-eating one or more nutrients and under-eating others during periods of dietary imbalance. Using the moose (Alces alces L.), a model species in the development of herbivore foraging theory, we conducted a feeding experiment guided by the GF, combining continuous observations of six captive moose with analysis of the macronutritional composition of foods. We identified the moose’s self-selected macronutrient target by allowing them to compose a diet by mixing two nutritionally complementary pellet types plus limited access to Salix browse. Such periods of free choice were intermixed with periods when they were restricted to one of the two pellet types plus Salix browse. Our observations of food intake by moose given free choice lend support to the nutrient balancing hypothesis, as the moose combined the foods in specific proportions that provided a particular ratio and amount of macronutrients. When restricted to either of two diets comprising a single pellet type, the moose i) maintained a relatively stable intake of non-protein energy while allowing protein intakes to vary with food composition, and ii) increased their intake of the food item that most closely resembled the self-selected macronutrient intake from the free choice periods, namely Salix browse. We place our results in the context of the nutritional strategy of the moose, ruminant physiology and the categorization of food quality.

Interactions with successional stage and nutrient status determines the life-form-specific effects of increased soil temperature on boreal forest floor vegetation

The boreal forest is one of the largest terrestrial biomes and plays a key role for the global carbon balance and climate. The forest floor vegetation has a strong influence on the carbon and nitrogen cycles of the forests and is sensitive to changes in temperature conditions and nutrient availability. Additionally, the effects of climate warming on forest floor vegetation have been suggested to be moderated by the tree layer. Data on the effects of soil warming on forest floor vegetation from the boreal forest are, however, very scarce. We studied the effects on the forest floor vegetation in a long-term (18 years) soil warming and fertilization experiment in a Norway spruce stand in northern Sweden. During the first 9 years, warming favored early successional species such as grasses and forbs at the expense of dwarf shrubs and bryophytes in unfertilized stands, while the effects were smaller after fertilization. Hence, warming led to significant changes in species composition and an increase in species richness in the open canopy nutrient limited forest. After another 9 years of warming and increasing tree canopy closure, most of the initial effects had ceased, indicating an interaction between forest succession and warming. The only remaining effect of warming was on the abundance of bryophytes, which contrary to the initial phase was strongly favored by warming. We propose that the suggested moderating effects of the tree layer are specific to plant life-form and conclude that the successional phase of the forest may have a considerable impact on the effects of climate change on forest floor vegetation and its feedback effects on the carbon and nitrogen cycles, and thus on the climate.

Phosphorus and nitrogen co-limitation of forest ground vegetation under elevated anthropogenic nitrogen deposition

Plant growth in northern forest ecosystems is considered to be primarily nitrogen limited. Nitrogen deposition is predicted to change this towards co-limitation/limitation by other nutrients (e.g., phosphorus), although evidence of such stoichiometric effects is scarce. We utilized two forest fertilization experiments in southern Sweden to analyze single and combined effects of nitrogen and phosphorus on the productivity, composition, and diversity of the ground vegetation. Our results indicate that the productivity of forest ground vegetation in southern Sweden is co-limited by nitrogen and phosphorus. Additionally, the combined effect of nitrogen and phosphorus on the productivity was larger than when applied solely. No effects on species richness of any of these two nutrients were observed when applied separately, while applied in combination, they increased species richness and changed species composition, mainly by promoting more mesotrophic species. All these effects, however, occurred only for the vascular plants and not for bryophytes. The tree layer in a forest has a profound impact on the productivity and diversity of the ground vegetation by competing for both light and nutrients. This was confirmed in our study where a combination of nitrogen and high tree basal area reduced cover of the ground vegetation compared to all the other treatments where basal area was lower after stand thinning. During the past decades, nitrogen deposition may have further increased this competition from the trees for phosphorus and gradually reduced ground vegetation diversity. Phosphorus limitation induced by nitrogen deposition may, thus, contribute to ongoing changes in forest ground vegetation.

Fertilization in Boreal and Temperate Forests and the Potential for Biomass Production

Forest fertilization is one of the most efficient methods in forest management to increase the short-term production of biomass. In this context, this chapter provides a brief background of the physiological response of trees to fertilization and increased nutrient availability. Furthermore, we shall describe different fertilization regimes and demonstrate the potentials of fertilization in enhancing biomass production, which will be performed by presenting relevant literature and some unpublished results. This chapter will also elaborate on some ideas for developing fertilization in operational forestry.