Influence of hydrological variability on inherent water use efficiency of pine forests with different disturbance histories in semi-arid Pacific Northwest climate

As temperature rapidly rises, it is predicted that drought-induced forest mortality will increases. With the increased morality over forests, the strength of carbon sink and evapotranspiration is expected to decline. Pine forests in the semi-arid Pacific Northwest climate often face drought stress, and hydrological variation can generate changes in carbon and water balances. We conducted eddy covariance measurements at three ponderosa pine forests (i.e., a mature ponderosa pine forest and two young ponderosa pine forests) and one Douglas fir forest over 25-site years. Inherent water use efficiency (WUEi) was used to assess differing shift of carbon and water balance and drought tolerance over the pine forests. Seasonal variation of daily WUEi indicated that the young pine forests responded to the seasonal drought sooner and to more severe degree than the mature pine forest. Annual WUEi at the mature and young ponderosa pine forests were mostly determined by WUEi during the drought season. At Douglas fir forest with no drought stress throughout the year, seasonal variation of daily WUEi was less than the mature and young pine forests. We found a decreasing trend of annual WUEi in the mature pine forest (slope of the trend line of -1.89 g C per kg H2O hPa), which is a contrasting trend found in temperate and boreal forests of the Northern Hemisphere over the past two decades. The two young pine forests and the Douglas fir forest showed an increasing trend of annual WUEi with the slope of 1.30 and 0.83 g C per kg H2O hPa, respectively. The observed decrease at the mature pine forest was primarily associated with the trend of increasing evapotranspiration over the measurement period, whereas the observed increase at the other forests was due to the trend of increasing gross primary production. We will conduct further examination to assess the driving mechanisms of different trends of WUEi and identify the relationship between WUEi and drought tolerance over the pine forests.