Growth declines in natural ecosystems as assessed by Enhanced Vegetation Index (EVI) have been shown to be related to warmer temperatures and increased precipitation in the eastern US from 2000 to 2010 (Potter et al., 2012). However, many environmental factors affect plant growth including ozone which has detrimental effects (Gregg et al., 2003), and anthropogenic NOx emissions will add nitrogen(N) deposition which could increase or decrease forests growth depending whether the system is N-limited or N-saturated (Aber et al., 1989; Fenn et al., 2003; Thomas et al., 2010). NOx SIP Call started from 1998 reduced 57% nitrogen oxides emissions between 2000 and 2005 in the eastern US (Simon et al., 2015). Both nitrate and total nitrogen deposition declined throughout the US since the late 1990s, so for systems that are not N-saturated this could also account for growth reductions (Li et al., 2016). Here, we performed multiple regression analyses to determine the relative impacts of temperature, precipitation, ozone and NOx for explaining growth reductions in deciduous and evergreen forests as assessed via EVI throughout the US between 2000 and 2014.
Our results showed that 37% of deciduous and 34% of evergreen forests in the northeast (NE) and 49% deciduous and 53% evergreen forests in southeast (SE) showed a significant decline in EVI between 2000 and 2014. By contrast, only <1% deciduous and 2% evergreen forests in SE showed increased EVI with no changes for the remaining sites (P <0.05). The decline of EVI was significantly related to the decline in anthropogenic NOx emissions. For all deciduous and evergreen forests with no relationship to temperature, precipitation, ozone, or interactions between these factors except for SE evergreen forests which also showed a significant temperature by precipitation interaction (NE deciduous: P=0.085, R2=0.54; NE evergreen: P<0.05, R2 = 0.67; SE deciduous: P<0.05, R2 =0.86; SE evergreen: P<0.01, R2 = 0.94). Our results indicate N-limited status in the eastern US forests with substantial reductions in forest growth as a result of implementation of the NOx control measures. More work is needed to confirm these results with field measurements.