Plant growth - stoichiometry and competition

In four different studies, this thesis addresses issues concerning plant nutrition and growth from a theoretical perspective. Terrestrial plants require nutrients, water, light and space for their existence. All of these resources may be limiting for growth, however in this thesis the main focus is on the nutrients. The first paper is part of an environmental impact assessment on introducing logdepole pine (pinus contorta) as a replacement for the domestic Scots pine (pinus sylvestris) in Swedish forests. The long-term development of carbon and nutrient pools was simulated in a mathematical model. Higher yield of P. contorta meant higher acidification of the soil. Lower decomposability of P. contorta litter had minor impact on the soil carbon pool after one rotation, but lead to significantly larger storage at steady state. The subject of paper II was plant nutrient ratios. Lab experiments have suggested that nutrients are required in similar proportions for a number of species. We wanted to know if the same proportions could be detected also in the field. We first made the assumption that nitrogen is either limiting growth or when available in larger amounts, only taken up moderately in excess of requirements for growth. Then we found that nutrient ratios determined from field data corresponded well to the optimum ratios determined in the lab. In paper III we addressed the competitive exclusion principle, which predicts that for plants occupying the same niche and competing for a single limiting nutrient, the stronger competitor will outcompete all the others. The competitive exclusion relies on the assumption that growth is proportional to biomass. However, growth is commonly assumed proportional to the concentration of the limiting nutrient. We showed that it is highly unlikely that potential nutrient uptake increases proportionally to plant biomass, but rather at a slower rate. When this scaling relation is included in a competition model, plants are allowed to coexist without niche separation. Finally, in paper IV, feedback of carbon was added to plant nitrogen and phosphorus relations in an ecosystem model. For nutrient acquirement, plant carbon can be invested in roots, be exchanged for nutrients in the symbiotic relation with myccorhiza or exudated, where the exudates stimulate nutrient availability in different ways. We suggest that the plant partly can direct this carbon investment to the nutrient most limiting growth. We also suggest that a smaller fraction of available carbon is invested as nutrient availabilities increase. The model then predicts 1) The plant nutrient ratio partly reflects availabilities and partly plant requirements. 2) When co-limited by N and P, plant growth will increase at increased availability of either of these, because a larger fraction of carbon can be directed for uptake of the more limiting nutrient.