Abstract
Purpose Roots are the mean absorbing organ of plants and an important part of material circulation in ecosystems. Understanding long-term root system development and its response to soil water availability can help to elucidate carbon cycling and plant resource uptake strategies in forest plantations under climate change. Methods We created a soil water content (SWC) gradient in a poplar forest using irrigation. During the six-year rotation, we determined biomass allocation by dissecting 45 sample trees, and measured spatial-temporal distribution and morphology of fine roots through analysis of 3984 root samples. Observations occurred as stands developed through canopy closure and experienced increasing inter-tree competition. Results Biomass allocation initially responded positively to low SWC while rooting depth was unaffected. As stands developed, biomass allocation differences gradually disappeared, and low SWC caused deeper root distribution. Morphological responses to depth emerged after the first year. In shallow soil, specific root length and fine-root diameter were lower while root tissue density was greater than those below 50 cm depth. Conclusion Soil water availability can affect biomass allocation and determine the soil layer with high exploration intensity, but both of these effects depend on stand development. During development, horizontal root system expansion takes precedence over vertical expansion. Besides, our finding implies that functional divergence of fine roots may occur gradually with stand development, with finer diameter roots in shallow soil perhaps functioning mainly for absorbing soil resources while low-density, high-diameter roots in deep soil functioning primarily for expanding the root system and exploring for limiting soil moisture resource.