Genetic diversity, spatial patterns, and growth of root sprouts in a temperate tree at the northern distribution limit

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In many plant species, population persistence at the limit of their distribution relies on clonal growth, but little is known about the consequences for genetic diversity, spatial patterns, and growth of the offspring. The rare European tree Sorbus torminalis (Rosaceae) was chosen to explore the patterns and limitations of clonal growth. Offspring were mapped in 7 small populations at the northern limit of the species' distribution, their size was recorded in 2 consecutive years, and the degree of shading (diffuse site factor, DSF) was measured at the forest floor. A subsample of offspring and adults was genotyped with microsatellites to determine the frequency of vegetative reproduction, and another group of offspring was protected against browsing. Clonal origin was observed in 98% of the offspring, and thus they were classified as root sprouts. The fine-scale distribution of the root sprouts was mainly clumped, and variation of DSF explained some of this pattern. Mixed linear modeling of root sprout size, with DSF, sprout density, and distance to the nearest adult tree as explanatory variables, suggested inter-offspring competition, but there were significant interactions among the explanatory variables, In addition, browsing by deer reduced growth of the root sprouts. Root sprout mortality was 17% after 1 y, and highest for young, small, and isolated offspring growing in high-light microhabitats. Root sprout growth and mortality were not directly affected by distance to adult trees. We conclude that clonal growth is common in the northern populations of the study species, and that root sprout distribution and growth are controlled by complex interactions between light, browsing, distance to the parent tree, and sprout density.