Growth ranking of hybrid aspen genotypes and its linkage to leaf gas exchange

Abstract

Background: Afforestation of non-forestland is a new measure by the European Union to enhance climate mitigation and biodiversity. Hybrid aspen (Populus tremula L. × P. tremuloides Michx.) is among the suitable tree species for afforestation to produce woody biomass. However, the best performing genotypic material for intensive biomass production and its physiological adaptation capacity is still unclear. We compared 22 hybrid aspen genotypes growth and leaf physiological characteristics (stomatal conductance, net photosynthesis, intrinsic water-use efficiency) according to their geographical north- or southward transfer (European P. tremula parent from 51° to 60° N and North American P. tremuloides parent from 45° to 54° N) to hemiboreal Estonia (58° N) in a completely randomized design progeny trial. We tested whether the growth ranking of genotypes of different geographical origin has changed from young (3-year-old) to mid-rotation age (13-year-old). The gas exchange parameters were measured in excised shoots in 2021 summer, which was characterised with warmer (+ 4 °C) and drier (17% precipitation from normal) June and July than the long-term average.

Results: We found that the northward transfer of hybrid aspen genotypes resulted in a significant gain in growth (two-fold greater diameter at breast height) in comparison with the southward transfer. The early selection of genotypes was generally in good accordance with the middle-aged genotype ranking, while some of the northward transferred genotypes showed improved growth at the middle-age period in comparison with their ranking during the early phase. The genotypes of southward transfer demonstrated higher stomatal conductance, which resulted in higher net photosynthesis, and lower intrinsic water-use efficiency (iWUE) compared with northward transfer genotypes. However, higher photosynthesis did not translate into higher growth rate. The higher physiological activity of southern transferred genotypes was likely related to a better water supply of smaller and consequently more shaded trees under drought. Leaf nitrogen concentration did not have any significant relation with tree growth.

Conclusions: We conclude that the final selection of hybrid aspen genotypes for commercial use should be done in 10-15 years after planting. Physiological traits acquired during periods of droughty conditions may not fully capture the growth potential. Nonetheless, we advocate for a broader integration of physiological measurements alongside traditional traits (such as height and diameter) in genotype field testing to facilitate the selection of climate-adapted planting material for resilient forests.

Keywords: Genotype ranking; Nitrogen; Populus; Water-use efficiency.

Fig. 1

Temperature and precipitation of 2021 in comparison with the long-term climate normal values (Estonian Environment Agency)

Fig. 2

Diameter at breast height (DBH) (a), height (H) (b), stem mass (M) (c), current annual stem mass increment (CAIM) (d) yearly relative growth (RG) (e) and relative growth rate (RGR) (f) of hybrid aspen clones from four different origin groups at the age of 13 years. Different letters denote significant differences in mean values between the groups

Fig. 3

Ten-year (3- to 13-year-old) change of relative height ranking among the genotypes` geographical origins. The significance of the mean change of ranking inside the origin group (i.e. difference from zero): *** P < 0.001, ** P < 0.01, * P < 0.05, n.s. - not significant

Fig. 4

Mean net photosynthetic rate (P_n) (a), leaf stomatal conductance (g_day) (b), intrinsic water use efficiency (iWUE) (c), leaf nitrogen content (N) (d), leaf water potential (e) and shoot hydraulic conductance (K_shoot) (f) of hybrid aspen clones from four different origin groups measured between 22nd and 30th July 2022. Different letters denote significant differences in mean values between the groups

Fig. 5

The effect of daytime stomatal conductance (g_day) and the origin group on net photosynthetic rate (P_n) on hybrid aspen origins. Both, g_day and P_n values are on a logarithmic scale. Model R² and model fit line equation values for the origin groups were as follows, respectively: Finland − 0.49, P_n = 0.47 × g_day + 0.08; Germany − 0.75, P_n = 0.75 × g_day − 1.36; Latvia − 0.86, P_n = 0.71 × g_day − 1.15; Sweden − 0.61, P_n = 0.59 × g_day − 0.49, and P < 0.001 for all the origin groups. Different colors denote different origin groups

Fig. 6

The effect of daytime stomatal conductance (g_day) (a), net photosynthetic rate (P_n) (b), and leaf nitrogen content (N) (c) on intrinsic water use efficiency (iWUE) on hybrid aspen origins. Model R² and model fit line equation for the origin groups were as follows, respectively: (a) Finland − 0.58, iWUE = − 0.13 × g_day + 92.08; Germany − 0.34, iWUE = − 0.13 × g_day + 96.05; Latvia − 0.55, iWUE = − 0.15 × g_day + 100.06; Sweden − 0.50, iWUE = − 0.20 × g_day + 111.06, and P < 0.001 for all the origin groups; (b) Latvia − 0.21, iWUE = − 1.80 × P_n + 97.34, and P < 0.001; Finland, Germany, Sweden - nonsignificant, P > 0.05 (R² and fit line equation not shown) (c) Germany − 0.06, iWUE = 11.47 × N + 50.31, and P < 0.05; Finland, Latvia, Sweden - nonsignificant, P > 0.05 (R² and fit line equation not shown). Different colors denote different origin groups