3. RESULTS AND DISCUSSION
3.5. General acclimation mechanisms to limiting growing conditions
All studied species reacted to nutrient limitation or otherwise suboptimal growing conditions by actively modifying their absorptive root morphology, which was the first response both temporally (I) and in terms of treatment intensity (IV). At the FAHM site, morphological differences were largest in the first two study years (second and third year of treatment), being replaced by an increase in FRB by the third study year (fourth year of treatment). On the geothermal soil tem-perature gradient, root morphology responded to smaller temtem-perature increases, while FRB and aFRB decreased significantly at larger warming levels. At the same time, morphological changes proved to be persistent even after 9 years of warming in a 130-year-old forest (IV) and on the latitudinal gradient (II, III).
A general morphological reaction to environmental stress was forming longer and less-branched absorptive roots with higher SRL and SRA (Figure 6). This was observed for all the species included in this thesis and also for Scots pine, which has been analysed thoroughly in Publication III, but omitted from this thesis. Also, thinner absorptive roots were produced in the North by birch and pine and in relation to humidification by birch. A similar acclimation pattern of forming thinner roots with higher SRL in cold habitats and in infertile growing conditions has been demonstrated to occur globally across a large number of species (Freschet et al., 2017; Holdaway et al., 2011; Kramer-Walter et al., 2016), which permits us to to conclude that the observed responses reflect general accli-mation patterns to environmental stress and nutrient deficiency. Higher values of SRL and SRA have been shown to be indicative of malnutrition (Rosenvald, 2011) and are thought to reflect enhanced resource uptake at lower biomass invest-ments, high root cost efficiency and fast growth (Comas and Eissenstat, 2004;
Ostonen et al., 2011; Weemstra et al., 2016). Kramer-Walter et al. (2016) and Holdaway et al. (2011) reason that producing thinner roots with higher SRL may be useful when foraging for nutrients, which are spatially dispersed or display poor diffusivity, such as P, while forming thicker roots with lower SRL in productive environments may be beneficial as a thicker root cortex might support higher metabolic rates in fertile soils and have enhanced pathogen resistance, also necessary for growing in the warmer southern soils of our studied latitudinal gradient. Longer and less branched roots imply a shift from root tip formation to root elongation, increased soil exploration and a change in the EcM colonizers.
Figure 6. Main changes in absorptive root morphology (blue columns), absorptive root biomass (black columns) and the EcM fungal community (white columns) in A) the air humidification experiment, B) the soil warming experiments and C) northwards along the latitudinal gradient. Abbreviations: D – absorptive root diameter (mm), SRL – specific root
RTD stood out from the other morphological traits for its larger sensitivity to soil fertility, increasing towards soils with a higher C:N ratio both in the soil warming experiments (IV) and northwards on the latitudinal gradient (III), which is a shared response for a number of species (Kramer-Walter et al., 2016) and may reflect an alternatively advantageous strategy of forming longer-living roots in infertile and colder soils. On the other hand, experimental soil warming caused a reduction in tissue density, which indicates shorter lifespan in warmed soils.
Shorter spruce fine root lifespan has also been reported in relation to experimental soil warming in Flakaliden and southwards along a latitudinal gradient in Finland (Leppälammi-Kujansuu et al., 2014b, 2014a).
Both silver birch and Norway spruce responded to obstructed nutrient uptake, whether in northern forests with high soil C:N ratio or induced by air humi-dification, by increasing FRB/BA and aFRB/BA (Rosenvald et al., 2014 and III).
A proportional increase in root biomass, especially in the absorptive root fraction, has been shown to be a common pattern across a large number of species in colder conditions (Reich et al., 2014; Zadworny et al., 2016) and for Scots pine in our latitudinal gradient study (III).
The higher proportion of fine root biomass in colder climate seems to be largely genetically determined (Last et al., 1983; Reich et al., 2014; Zadworny et al., 2016). It has also been shown, that tree genotype influences the root-colo-nizing EcM community structure and absorptive root branching (Velmala et al., 2013). Although attempts have been made to evaluate the contribution of intraspecific heritability to variation in root morphology (Salmela et al., 2020;
Senior et al., 2019), the extent to which root morphological responses are under genetic control and how much they reflect phenotypic plasticity remains unclear.
Answering this question calls for the common-garden approach and using clone trees in climate change manipulation experiments. For example, the latter method has been employed at the FAHM-site.
The root associated microbial community – in the current work: EcM fungi and rhizosphere bacteria – showed high adaptability to changes in their growing environment. Regarding the EcM community, humidification led to an increase in the proportion of hydrophilic morphotypes and Tomentella spp. (I). In warmer soils, there was also a rise in the proportion of Tomentella spp. and fungi characterized by long-distance exploration type (species belonging to the /boletus lineage) (IV). These alterations in the EcM community could be related to increased root turnover concurring with both air humidification and higher soil temperatures. Both in the humidification experiment (Truu et al., 2017) and along the latitudinal gradient (III), multilateral relationships were observed between absorptive root morphology, functional types of colonising EcM fungi, and rhizosphere and soil bacterial community structure, which demonstrates that root-rhizobiome acclimation is characterized by strong inter-relatedness between plant traits and soil microbe communities.