Velocity of change in vegetation productivity over northern high latitudes.

Warming is projected to increase the productivity of northern ecosystems. However, knowledge on whether the northward displacement of vegetation productivity isolines matches that of temperature isolines is still limited. Here we compared changes in the spatial patterns of vegetation productivity and temperature using the velocity of change concept, which expresses these two variables in the same unit of displacement per time. We show that across northern regions (>50° N), the average velocity of change in growing-season normalized difference vegetation index (NDVIGS, an indicator of vegetation productivity; 2.8 ± 1.1 km yr-1) is lower than that of growing-season mean temperature (T GS; 5.4 ± 1.0 km yr-1). In fact, the NDVIGS velocity was less than half of the T GS velocity in more than half of the study area, indicating that the northward movement of productivity isolines is much slower than that of temperature isolines across the majority of northern regions (about 80% of the area showed faster changes in temperature than productivity isolines). We tentatively attribute this mismatch between the velocities of productivity and temperature to the effects of limited resource availability and vegetation acclimation mechanisms. Analyses of ecosystem model simulations further suggested that limited nitrogen availability is a crucial obstacle for vegetation to track the warming trend.