## 3 Results  
Figure 1 provides strong support for the basic assumption that a general relationship exists between LAI\({}_{\text{max}}\) and site growth potential (MAI\({}_{\text{max}}\)). The relationship applies over the full range of values in western North America [13].  
Figure 2 contrasts modeled changes in GPP and LAI\({}_{\text{max}}\) for Douglas-fr between the two periods. As expected, the spatial patterns are similar, because soil fertility, which exerts the major control on the partitioning of carbon above or below ground in the model, remained the same for both periods. An increase in drought or VPD between the two periods could result in reduced photosynthesis, causing a decrease in LAI when monthly litterfall exceeds new foliage production. A reduction in the occurrence of frost or an increase in solar irradiance associated with a reduction in cloud cover would have the opposite effect. The smallest percentage change in LAI\({}_{\text{max}}\) occurred in the Marine West Coast Forest Zone. However, more than a 20% increase in LAI\({}_{\text{max}}\) was simulated from the cooler to warmer period on the east side of the Sierra Mountains in California and Nevada. The model predicts \(>\)20% decreases in LAI\({}_{\text{max}}\) since 2000 for parts of Alberta and the northern Rocky Mountains. Figure 3 provides  
Figure 1: Simulated maximum leaf area index (LAI\({}_{\text{max}}\)) for the period 2000–2009 is closely related to the maximum mean annual increment (MAI\({}_{\text{max}}\)) estimated from height and age measurements acquired from 2000 to 2006 on \(>\)3356 FIA (Forest Inventory and Analysis) survey plots in Oregon and Washington [15].  
insights into the underlying causes for simulated changes in GPP and LAI\({}_{\text{max}}\) displayed in Figure 2. The most consistent change across the study area is a general decrease in spring frost (Figure 3a). The coastal region and also larger parts of the interior of the Pacific Northwest of the U.S. and Southwestern Canada experienced a particularly strong decline in spring frost across much of the natural range of Douglas-fir.  
Figure 2: Modeled mean LAI\({}_{\text{max}}\) for Douglas-fir (maximum 12.0 m\({}^{2}\) m\({}^{-2}\)) from (**a**) 1950–1975 and (**b**) 2000–2009; and the mean gross primary production (GPP) for (**c**) 1950–1975 and (**d**) 2000–2009; along with the percentage change of (**e**) LAI and (**f**) GPP for the two time periods.  
A reduction in frost permits trees to start growth earlier, but at the same time, warmer temperatures melt snow faster and increase transpiration, which, on shallow soils, leads to soil water deficits. Since 2000, summer evaporative demand (Figure 3b) has increased most notably in the Southwest USA [51]. Although summer soil water deficits were also predicted to increase across parts of the Southwest, the  
Figure 3: Modeled average changes from 2000 to 2009 in **(a)** spring frost frequency, **(b)** summer evaporative demand, **(c)** summer soil water stress and **(d)** fall soil water stress compared to the cooler and wetter period (1950–1975). The units represent the proportional change in environmental constraints on GPP in reference to the first period. The range of Douglas-fir is from [28].  
largest deficits were predicted for the forests situated along the cordillera through western Alberta into Montana, a condition that continued in some places into the fall (Figure 3d).