Divergent phenology response to nitrogen addition between a Mediterranean and a boreal forest

Anthropogenic nitrogen (N) deposition and fertilization in the past decades shifted the ecosystem stoichiometry with potentially profound impacts on vegetation activity and ecosystem functioning. Current N-addition experiments mostly focus on leaf-level or individual plants at the plot scale, whereas studies investigating the responses of vegetation dynamics to N-addition at the landscape level are lacking. It is especially unclear how ecosystems with different water availability (water-limited versus water-surplus) respond to elevated N input. We compared vegetation dynamics and ecosystem functioning in two unique ecosystem-scale N addition experiments – one Mediterranean tree-grass ecosystem and one boreal evergreen forest. At each site, one pair of landscape-scale N addition was set up by adding N onto the footprint area of one eddy covariance (EC) tower, with another EC tower without N addition used as a control. We hypothesize that their different water availability can exert different responses in vegetation phenology and gross primary productivity (GPP).  Since the start of the experiments, we found that the fertilized treatments in both experimental sites have had higher amplitudes of GPP and more rapid green-up and leaf senescence compared to the control. However, phenological transition dates (PTDs) defining the start and end of the growing seasons (SOS, EOS), derived from GPP at the two sites show different patterns. During the green-up period, SOS was similar between the fertilized and control treatments at the Mediterranean site since the vegetation green-up here was mainly controlled by the timing of precipitation. In the boreal forest, however, the fertilized treatment was greening slightly later than the control. In the leaf senescence period, the fertilized treatment senesced earlier in the Mediterranean ecosystem compared to the control. In contrast, the fertilized treatment delayed in EOS compared to the control in boreal forests.  We propose that increased leaf area index and canopy greenness in the fertilized (based on the observed increasing evapotranspiration) compared to the control treatment in both ecosystems, along with different vegetation composition, probably contributes to the divergent response of EOS at the two sites with different water availability. This study underscores the necessity to take different water availability into account when evaluating the effects of N addition on vegetation dynamics.