Growth characteristics of root-shoot relations of three birch seedlings raised under different water regimes

Three birch species (Betula ermanii, B. maximowicziana, B. platyphylla var. japonica) widespread in northern Japan were raised under different water regimes (ca. 23, 35, and 60%) to study root-shoot increment and gas exchange traits in relation to their habitat preferences in natural conditions. Total biomass of all birches was larger for medium, wet and dry treatment. Maximum root length of B. platyphylla raised at dry or wet condition was higher than other species. Root growth rate of three birches peaked around mid July to early September while shoot growth rate was found maximum between late June and mid August. Root growth of three birches was suppressed under dry and wet treatment, especially for B. ermanii. Allocation percentage of biomass to roots of three birches ranged from 30 to 40% but it mainly allocated to elongate the root length under dry and wet condition. Fine roots of B. ermanii and B. maximowicziana under wet condition were distributed mainly on soil surface. In the dry treatment, B. platyphylla allocated photosynthates to elongate the root length and fine root production (<2 mm) and had highest plasticity of roots to different water conditions among the three birches. Specific leaf area (SLA) of three birches decreased with decreasing soil moisture content. Plasticity in SLA of B. platyphylla was found largest. Net photosynthetic rate (Pn) and water use efficiency (WUE) of B. platyphylla showed highest among all the treatments. Leaf nitrogen content of B. platyphylla was also relatively higher under different water conditions as compared to the other two birch species, suggesting B. platyphylla may enable to invade to various growing conditions with different water regimes.