Intra-Annual Growth and Its Response to Climatic Factors of Two Salix Species under Warm Temperate Environment

Monitoring cambial activity and intra-annual growth dynamics is an effective method for identifying tree growth response to climate change. However, there have been few pieces of research on intra-annual wood formation of diffuse-porous species under monsoonal warm temperate environment. Here, we monitored weekly the cambial activity and xylem differentiation of Salix babylonica and Salix matsudana by collecting microcores during the 2018 growing season. Two willow species exhibited similar cambial activity and xylem differentiation processes, of which the onset and cessation of xylem growth was from middle/late March to early/middle November. The onset and cessation of cambial activity were slightly earlier for Salix matsudana (19 March and 12 October) than for Salix babylonica (26 March and 17 October), which peaked on 2 June and 31 May, respectively. Salix babylonica showed wider xylem increment and higher growth rate than that of Salix matsudana, of which the intra-annual xylem width was 8525 +/- 1201 mu m and 7603 +/- 826 mu m, respectively, fitted by Gompertz function. Moreover, the maximum growth rate of Salix babylonica and Salix matsudana was 79.75 mu m day(-1) and 66 mu m day(-1), respectively, occurring on 4 June (DOY155) and 26 May (DOY146). Both temperature and water availability were important factors influenced the xylem growth for two species, which Salix matsudana had a stronger response to temperature but not to water availability than Salix babylonica. These results suggested that Salix babylonica seem to grow better under moist warm temperate environment due to it being a more conservative response to the climate than Salix babylonica. These observed species-specific differences at the intra-annual scale may help researchers more accurately anticipate the species suitability in temperate forests.