Growth rate rather than growing season length determines wood biomass in dry environments

A number of studies have suggested that growing season length determines carbon sequestration of forest ecosystems. Given the possibility that drought-induced growth decline will be caused by a prolonged growing season under a warming climate, we investigated the effect of growth rate and duration of xylem production on annual wood biomass in drought-prone environments. We analyzed the intra-annual dynamics of wood formation in Qilian junipers (Juniperus przewalskii) from the semi-arid north-eastern Tibetan Plateau, China (2009–2014) and in ponderosa pine (Pinus ponderosa) from the hyperarid Mojave Desert in Nevada, USA (2015–2016). Most variability in the number of xylem cells (Ncell) was related to growth rate (rm) rather than duration of cell production (Dcell). At the Tibetan sites, 69.9% and 54.7% of variability in Ncell was attributable to rm for the lower and upper treeline, respectively. Within the Mojave Desert site, 53.9% of the variability in Ncell was related to rm. The growth rate in the Tibetan Plateau forest is affected by minimum temperature and precipitation. Thus, rm is a primary control on wood biomass in conifer species of semi-arid forests. Under warmer and drier conditions, a longer growing season will not benefit xylem formation of conifers, and in turn warming-induced drought could limit carbon sequestration by reducing the rate of cell production.