Effect of warming and degradation on phenophases of Kobresia pygmaea and Potentilla multifida on the Tibetan Plateau

Abstract Many studies have focused on the responses of phenophases to warming in natural, non-degraded grasslands. However, degraded grasslands widely distribute in the world due to climate change and overgrazing. So far, a comprehensive understanding of how plant phenology responds to warming, degradation and their interactions remains elusive. Here, the relative effects of warming and degradation on the phenological sequences of two dominant plant species were tested using a two factorial experiment of warming and moderate degradation (i.e., four treatments: NWND, WND, NWD and WD) during three successive years. We found that WND significantly delayed the onsets of early phenophases (i.e., leaf-out, booting-set, flowering and fruiting-set) and shortened the total activity period (TAP) only for the early spring flowering Kobresia pygmaea compared with NWND. NWD generally did not significantly affect the timing or duration of all phenophases for the two plant species depending on year relative to NWND. Interaction between warming and moderate degradation significantly delayed leaf-out and shortened total activity period of the two plant species. There were negative correlations between plant species richness and onset of vegetation after fruiting-set for K. pygmaea and the duration of fruiting-set for Potentilla multifida (a dominant mid-summer flowering species in degraded plots), but there were positive correlations between plant species richness and leafing duration for P. multifida. Positive correlations were found between first leaf-out and NO3−-N in soils for both species but negative correlations were found between first leaf-out and NH4−-N in soils for P. multifida. There were positive and negative correlations between TAP of P. multifida and NH4−-N and NO3−-N in soils, respectively. Therefore, our results suggest that moderate degradation could weaken the negative effects of warming on reproductive phenophases of K. pygmaea, and that we should take into account grassland degradation and its associated changes in plant species richness and soil inorganic nitrogen when we predict the response of plant phenology to future warming.