Growth and carbon sequestration of poplar plantations on the Tibetan Plateau

Tree radial growth has long-term adaptation and rapid responses to climate, manifested as age-dependent low-frequency and climate-sensitive high-frequency signals. Although the former is usually removed in climate-growth analyses, its overall change still profoundly affects forest biomass and carbon sequestration. The itera-tive growth model (IGM) reveals the underlying links among organism lifespan, growth rate, and respiration, providing a set of theoretical indicators to evaluate or predict growth. Here, IGM was extended to the tree-ring scale (IGMR) to study the low-frequency growth signals of poplar plantations in the Yarlung Tsangpo River, Tibetan Plateau. As predicted by the IGMR, the low-frequency growth signals all follow a unimodal pattern over the diameter at breast height (DBH) gradient while constraining the high-frequency signals. The unimodal growth curves' length (maximum DBH), height (maximum growth rate of tree DBH), and resulting tree lifespan could be used to assess and predict tree growth. The results showed that the maximum DBH, growth rate and inverse of the longevity of the trees were greater at lower elevations. The indicators of Populus x beijingensis (PB) were better than those of P. alba (PA). Overall, poplars adapted to the plateau climate by reducing growth rates and increasing longevity. Temperature was the key factor affecting these trade-offs, with the best temperature at 14.69 celcius. Combined with stand density, PB plantations (11695.58 +/- 1704.98 g/m2) had greater potential maximum biomass than PA plantations (9032.50 +/- 2031.21 g/m2). This study highlights that the response of low-frequency growth signals to environments is holistic, and the resulting indicators have important value for evaluating and predicting tree growth and forest carbon sequestration. Moreover, the results have important practical significance for reasonable plantations and proper assessment of the ecological contribution of plan-tation forests on the Tibetan Plateau.

Automated summary

Tree radial growth shows long-term adaptation and rapid responses to climate, with age-dependent low-frequency signals and climate-sensitive high-frequency signals. The iterative growth model (IGM) provides theoretical indicators to evaluate or predict growth by linking organism lifespan, growth rate, and respiration. This study extended the IGM to the tree-ring scale (IGMR) and found that low-frequency growth signals in poplar plantations on the Tibetan Plateau follow a unimodal pattern over the diameter at breast height (DBH) gradient while constraining high-frequency signals. The resulting indicators of growth, such as maximum DBH, growth rate, and tree lifespan, are important for assessing and predicting tree growth and forest carbon sequestration.