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 (Pines ponderosa) from the hyperacid Mojave Desert in Nevada, USA (2015-2016). Most variability in the number of xylem cells (N-cell) was related to growth rate (r(m)) rather than duration of cell production (D-cell). At the Tibetan sites, 69.9% and 54.7% of variability in N-cell was attributable to r(m) for the lower and upper treeline, respectively. Within the Mojave Desert site, 53.9% of the variability in N-cell was related to r(m). The growth rate in the Tibetan Plateau forest is affected by minimum temperature and precipitation. Thus, r(m) 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.