Abstract:

Objective: Fertilization is a critical intensive management practice to increase the productivity of fast-growing tree species. Nutrient addition would increase soil nutrient resource availability, and thus benefit the tree growth, which may subsequently result in plasticity in tree growth as well. Although a large body of literature exists on the response of tree growth to nitrogen (N) fertilization, the influence of N supply on the plasticity responses of phenological characteristics and tree growth is still not well understood for fast-growing tree species. This study aims to clarify the phenological characteristics and growth plasticity of fast-growing tree species in response to fertilization, providing a theoretical basis for the development of efficient nutrient management strategies for fast-growing and high-yield forests. Method: A three-year fertilization experiment was conducted in Populus tomentosa plantations, which species is the main afforestation tree species for constructing fast-growing and high-yields plantations in northern China. The experiment included three N application rate treatments (115, 230, 345 kg?hm^?2a^?1) and a control treatment (0 kg?hm^?2a^?1), with a randomized complete block design with three replicates. The leaf area index (LAI), tree diameter at breast height (DBH), and biomass were measured consecutively during the experimental period. The relationships of the biomass allometric growth among different organs were also analyzed. Result: 1) The results showed that all experimental treatments exhibited approximately the same phenological characteristics of dynamics of LAI and stem growth rate during the experimental period: the growing duration of P. tomentosa at this site lasted about four and a half months, and the growth often began in mid-April and ceased in early September. The monthly DBH growth rate showed two peaks, the first peak occurred from late April to mid-May, and the second, a minor peak, was detected from late June to mid-July. However, both the DBH growth rate and LAI were higher in the fertilization treatments, especially at the late stage (August) of the growing season when the fertilized stands had 71.6% and 85.1% higher (P<0.05) DBH growth rate in 2011 and 2012, respectively. 2) The analysis results of the biomass allocation showed that: the slope of allometric function for wood biomass and DBH under CK and fertilization treatments was 2.4, indicating that the growth relationship between stem biomass and DBH was allometric. The increase in stem biomass allocation in the fertilization treatments was induced by accelerating ontogenetic development and was not due to the ‘true’ plasticity response of growth to N addition. In contrast, the slopes of the allometric function for leaf and root biomass under CK and fertilization treatments were 1.2 and 1.0, respectively, suggesting that the growth relationship between leaf and root biomass allocation was isometrical. Compared with CK, the value of the allometric constants of the allometric function of different fertilization treatments was significantly higher by 0.4-0.6 (P<0.05), suggesting more leaf biomass accumulated per unit root biomass under treatments than that in CK, indicating that fertilization increased the biomass allocation from roots to leaves and induced a ‘true’ shift toward more leaf biomass per unit root biomass. Conclusion: Consequently, under fertilization conditions, the growth plasticity responses of P. tomentosa not only include a ‘true’ plasticity, but is also strongly controlled by the ontogenetic development. The findings of this study can help us to better understand the response and adaptation strategies of fast-growing tree species to the variation of environmental nutrient availability. Furthermore, the study can also provide theoretical references for refining high-efficient nutrient management techniques according to the plasticity response strategies of plants.

Key words: phenotype plasticity, fertilization, phenology characteristics, biomass allocation, ontogenetic development