Influence of concurrence of extreme drought and heat events on carbon and energy fluxes in dominant ecosystems in the Pacific Northwest region

Abstract. The impacts of drought intensities and vapor pressure deficits (VPD) beyond historic norms in Pacific Northwest (PNW), USA, are critical in understanding the potential future function and resilience of ecosystems in the region. While ecosystems in this region are adapted to seasonal droughts, June 2015 temperatures were the highest recorded in the region and strongly coupled with relatively low soil moisture. June is usually the best month for growth in the PNW. Here, we examined the impact of the June 2015 climate extremes on carbon and energy fluxes at sagebrush in the high desert, young and mature ponderosa pine in the semi-arid Great Basin, and Douglas-fir in the mesic Coast Range ecoregion compared to an average climate year (2014). We assessed if the ecosystems recover from extreme climate stress within the growing season. The monthly anomalies in temperature and VPD were 3 standard deviations, and precipitation was 1 standard deviation, outside the 30-year mean at all sites. In sagebrush, the carry-over effect of precipitation (i.e., intensive precipitation prior to the drought and heat) mitigated the immediate impact of extreme climate stress, leading to 25–40 % increase in net ecosystem production (NEP) and gross primary production (GPP), with little change in ecosystem respiration (RE) and 65 % increase in latent heat flux, compared to the June 2014. The drought and heat lowered NEP by 35–65 % and GPP by 15–33 % in ponderosa pine and Douglas-fir. A greater increase in latent heat flux was observed in Douglas-fir (110 %) than in ponderosa pine (< 10 %) driven by increased evaporation. The decline in NEP was significantly correlated with VPD (R² of 0.4–0.7 and p < 0.001), but not with soil moisture (0–100 cm). NEP recovered in October (the beginning of the rainy season) in ponderosa pine following the lowest NEP in August (the driest and hottest month). Douglas-fir showed partial recovery by October, resulting in the largest seasonal reduction (May–October) in carbon fluxes (64–128 g C m⁻² season⁻¹). The seasonal changes in Douglas-fir were corresponding with 30–40 % decline and were greater than in mature ponderosa pine (5–20 % decline), when compared to the long-term seasonal means (2002–2015), respectively. Our results suggest that the responses of carbon and energy fluxes to climate extremes differ depending on site- and species-specific characteristics. Douglas-fir is likely to experience more constraints on carbon fluxes than ponderosa pine if the hot and dry season intensifies in the PNW. Given the likelihood of future drought and heat extremes, identifying these anomalous ecological responses to anomalous climate (e.g., the combination of VPD, heat, and dry soil) is critical to improve predictions of physiological thresholds and tolerance of different tree species.