Primary and secondary stem growth in arctic shrubs: implications for community response to environmental change

1 Shrubs are among the tundra plants most responsive to environmental change. We measured primary and secondary stem growth in a retrospective analysis of ramets of three codominant shrubs (Betula nana, Salix pulchra, and Ledum palustre ssp. decumbens) exposed to long-term field treatment with greenhouses and N + P fertilizers at Toolik Lake, Alaska. 2 Ramets of Salix had the greatest primary stem growth under control conditions, because of their relatively high branching rate. Under fertilization, however, Betula produced much more primary stem growth than the other species, because axillary buds that would have grown as short shoots in control ramets were instead stimulated to produce long shoots (structural branches). There appeared to be a trade-off between allocation to length per stem segment and number of stem segments produced in a given year, for both Betula and Ledum. 3 Although secondary growth in stems is the largest component of above-ground net primary production in forests, it is often ignored in shrub-dominated ecosystems. We derived an expression for secondary growth in shrubs based on distributions of stem mass and length with age, and allowing for experimentally induced changes in secondary growth rate. 4 There was good agreement between measured ramet stem mass and calculated values for all three species, validating our mathematical analysis of secondary growth. 5 Fertilization greatly increased the relative rate of secondary growth only in Betula, consistent with observed accumulations of its stem mass in ecosystem-level quadrat harvests. Secondary growth of Betula was a major component of ecosystem NPP in fertilized plots and probably contributes significantly to ecosystem carbon storage. 6 The increase in its secondary growth enabled Betula to become dominant under fertilization, whereas the inability of older stems of Leduna to respond in this way prevented it from growing into the canopy.