Root mechanical properties related to disturbed and stressed habitats in the Arctic

Various mechanical and architectural properties of roots were measured on plants characteristic of different levels of soil disturbance associated with frost-heave cycles on sorted polygons in Swedish Lapland: one of these measures, resilience, has not, apparently, been recorded previously in literature. Some roots were sampled from species which occurred on both disturbed and stable soils. Root length, root angle, numbers of roots and root diameter varied greatly between species but the degree of branching of roots was generally low. Among all species, and particularly forbs and graminoids which are the most common life forms on polygons, there was a clear trend of decreasing root diameter with increasing soil disturbance, while the species growing on both disturbed and stable ground showed no clear trend associated with soil disturbance. The small root diameter of species growing on the polygons could result from the different species and life forms found there together with a younger age class distribution of plants, younger plants having smaller roots. Only one third of the species growing on the most disturbed soils had rhizomes, whereas twice as many species possessed rhizomes in areas of lower disturbance. The tensile strength, breaking strain (a measure of how much a root is deformed by stretching), breaking stress (a measure of root strength corrected for the cross-sectional area) and the resilience (a measure of the elastic-recovery of stretched roots) of the roots varied highly significantly among the species. The tensile strength varied 6.5-fold, the breaking strain varied by up to almost 8-fold and the breaking stress 13-fold. The modulus of resilience varied least, by a factor of only two. There was no clear trend in the way mechanical properties of the roots of the three species occurring on both stable and frost-heaved soils varied between the two regimes. However, among all species, those from the frost-heaved soils tended to have the weakest roots, because of their small diameter, but the strongest roots per cross-sectional area (i.e. they had the greatest breaking stress). Among life-forms, graminoids toler̀Gated a significantly greater strain than forbs and dwarf shrubs and much higher stress before they broke. Dwarf shrubs and forbs had significantly higher resilience than graminoids and the highest tensile strengths. Forbs showed the lowest values for all of the measured variables except for resilience, which fell between the values for graminoids and dwarf shrubs. There were no significant differences in any of the mechanical measures of root resistance among species of different frost-heaving regimes, indicating that there are few architectural or mechanical properties which enable roots to survive in the most disturbed soil. Indeed, the absence of a complete plant canopy and an increase in abundance of young plants on the polygon area compared with the stable areas suggest that if there are any adaptations to strong disturbance, they are less than adequate and the hypothesis that arctic plants avoid, rather than adapt, to severe environment is supported.