Response of Pinus cembra to modified soil temperatures

Several studies have been published which indicate a tight control of shoot and root growth by root-zone temperature and cold soils are considered as a limiting factor for tree growth at the treeline. By now studies dealing with soil temperature effects on tree growth have mostly been conducted in laboratories using potted seedlings, whereas in situ studies are quite rare and manipulative experiments on soil temperature-growth relationships within the alpine treeline ecotone are still missing. In the proposed study the impact of elevated and reduced soil temperature on growth of Pinus cembra within the treeline ecotone (c. 2000 m a.s.l.) will be evaluated using naturally grown ca. 50 year old trees. Soil temperature will be reduced and elevated by shading of the root zone with insulating-plates and heat-trapping by Perspex- panels, respectively. The influence of altered soil temperature on radial growth will be determined by automatic circumference dendrometers, while fine root dynamics and turnover will be quantified by minirhizoscope technique and root coring. Measurements of photosynthesis and soil respiration as well as analysis of needle, soil and root nutrient content and ectomycorrhizal abundance and composition will provide insight on above and below ground metabolic activity. Soil temperature and soil moisture as well as macroclimate will be monitored continuously during the study period of 3 years. Aim of the proposed study is to improve our understanding of the relationship between soil temperature and tree growth within the treeline ecotone using in situ experimental designs, where only soil temperature is altered while trees grow under natural environmental conditions. Analysis of above and below ground growth, as well as monitoring of metabolic activity and nutrient status will enable a detailed interpretation of soil temperature-tree growth relationships.

Temperature is the paramount factor controlling tree growth at high altitudes and it is suggested that aside of aboveground temperature also soil temperature has a direct effect on tree growth. We have evaluated the impact of experimentally modified soil temperatures and nutrient availability on radial tree growth, fine root growth, mycorrhization, photosynthesis and water use of Pinus cembra at the treeline in the Central Eastern Alps (c. 2150 m a.s.l., Tyrol, Austria). Soil temperature in the rooting zone of naturally grown c. 25 year old trees (n=6 trees per treatment) was altered by shading and heat-trapping using non-transparent and glasshouse foils mounted c. 20 cm above soil surface. Additional trees were selected for a nitrogen fertilisation treatment and as controls. The study was conducted during the vegetation period of three growing seasons (2012 to 2014). Mean soil temperatures at 10 cm depth were reduced by c. 3C at the cooled vs. warmed plots, while soil moisture was not influenced due to soil water transport along the slope. Results revealed that changed soil temperatures did not significantly affect radial tree growth, while nitrogen fertilization and a reduction of interspecific root competition by the experimental shading of the soil, led to significantly raised radial stem growth. On the other hand we found an elevated fine root biomass and a higher fine root turnover in colder compared to warmer soils, which may account for lower nutrient availability in cold soil areas. Sap flow density was significantly raised in trees growing on warmer soils while photosynthesis and transpiration were scarcely affected. Also the degree of mycorrhization and the number of mycorrhized root tips, as well as needle nutrient content and specific leaf area were not altered by soil temperature manipulation. The results indicate that tree growth was not limited by soil temperature at the selected study area, while interspecific competition for nutrients among trees and low stature vegetation (dwarf shrubs, grasses) had significant impact on tree development. Therefore, we suggest that root competition with alpine grassland and dwarf-shrub communities will hamper temperature driven advance of alpine treeline in the course of climate warming.