The ratio of hardwood to conifer trees in temperate forests is predicted to change under global warming, which could lead to major changes in ecosystem function.

Results/Conclusions

We report historic landscape-scale evidence of this phenomenon: a significant increase over 70 years in the abundance and extent of hardwood trees along a 300 km conifer-hardwood ecotone. These changes were detected through comparison of two independent historical vegetation data sets to contemporary data. Change is found along an elevational ecotone measuring 300 km on the western flank of the Sierra Nevada mountains, California, USA. Comparison within 200 m elevation bands of historic and contemporary vegetation plots shows an increase in hardwood abundance at all elevations measured but decreasing conifer recruitment at lower elevations, and increasing recruitment at higher elevations. Comparison of historic to contemporary vegetation maps shows an increase in hardwood extent of 1452 km², with 1115 km² replacing conifer dominated lands on the upslope side of the hardwoods’ range. These dynamics occurred in a region where the May-October nighttime lows have increased by over 0.85 C, but with little change in precipitation, effectively extending summer drought conditions.

This change in vegetation physiognomy is emperical evidence of projected future climate change effects, captured over recent historic time, on an area of sufficient size to represent a lanscape-level trend. We expect mountain ranges in other arid and semi-arid regions to exhibit similar dynamics.