Mycorrhiza and fine root dynamics in Sitka spruce
The surface horizons of north temperate forests are intensively colonised by the fine roots of trees and their mycorrhizas. Boreal and north temperate forest ecosystems are often nitrogen stressed and N fertilisation is an option for forest management. This research was designed to provide information about fine root dynamics in Sitka spruce plantations and to identify the role of nitrogen in these dynamics. The objectives were to gain information on the effect of N fertilisation on (i) numbers, biomass and seasonal variation in fine roots and mycorrhizas; (ii) to quantify the dynamic processes by estimating production, mortality and dead fine root disappearance from these data and, (iii) to identify effects of fertilisation on mycorrhizal frequency and type. In a replicated field experiment, up to 300 kg ha-1N as (NH4)2SO4 was applied to the forest floor in a 35 year old plantation of Sitka spruce (Picea sitchensis). The fine root system in the organic horizons was monitored over two years by making monthly assessments. This was carried out through a regime of soil coring followed by wet sieving and the manual extraction of root material. Fluctuations in numbers and biomass of fine roots and mycorrhizas exhibited a normal seasonal pattern. Maxima were observed in autumn and early spring; minima in summer and winter. This pattern was most pronounced in the finest of root diameter classes including the mycorrhizas. Major periods of root activity preceded formation of maximal numbers and biomass. These were marked by both growth and mortality. The fine root litter production in the forest floor of this stand may be between 0.25 and 0.5 times the above ground litter input. Calculated turnover times showed the finest material to be turning over very much more rapidly than the larger material. Indeed the mycorrhizas on average did not seem to live longer than around 4 months. Turnover times calculated from comparative data in the literature provide similar brief longevities for finest roots. Seasonal fluctuations in the concentration of nitrogen in the living fine root system were also recorded. High concentrations occurred in the winter, low in late summer and autumn. Nitrogen requirement for fine root production more or less equalled nitrogen throughput in dead roots at around 5.5 kg ha -1 but the data suggests that further nitrogen is found via retranslocation from dying to growing root tissue. Seasonal patterns in Cenococcum geophilum mycorrhizas showed peaks in number during winter and summer. This pattern indicates the probability that the proportions of mycorrhizal type in the population of mycorrhizas on the fine root system undergo considerable variation through the year. Under N fertilisation, fine root dynamics were considerably altered. Numbers and biomass in the most heavily fertilised plots did not decline during the summer following fertilisation and the winter decline occurred one month later. Fertilisation increased fine root biomass. In the second year following application of fertiliser, biomass remained high but there were fewer root tips in the fertilised plots and seasonal fluctuations were less marked. These patterns reflect changes in the production, longevity and death of the fine roots and mycorrhizas as a result of fertilisation. Although the periodicity of activity was similar, annual production and mortality were markedly reduced. Mycorrhiza and fine root lifespans were increased. In the months following fertilisation the fine roots remained an effective sink for carbon by virtue of their increased capacity to supply N and were not shed. At intermediate fertilisation levels response was delayed because of immobilisation of applied nitrogen in microbial populations. In the second year, there was a readjustment of the functional balance between root and shoot, which in the light of increased N availability led to reduced root growth. Finally, the increased net assimilation resulting from fertilisation permitted an overall increase in growth and by spring 1981 this was reflected in increased fine root biomass, particularly in the larger more structural size class. At the same time a reduced density of root tips indicated less intensive exploitation of the substrate. In a pot experiment investigating the response to different levels of available nitrogen by mycorrhizal and uninfected Sitka spruce seedlings this interpretation was given support by data on assimilate allocation between root and shoot, efficiency of nitrogen absorption and changes in root system morphology. The mycorrhizal effect and mycorrhizal intensity was greatest at low levels of nitrogen availability. Root:shoot ratios, rooting density and root tip frequency were all greatest in mycorrhizal seedlings at low levels of N availability. Fertilisation decreased the intensity of substrate exploitation but because of increased efficiency of N absorption by root tips, shoot growth was enhanced.