Positive mixture effects in pine-oak forests during drought are context-dependent
Canyon Vista Campground area of the Coconino National Forest consists of ponderosa pine, mixed conifer, aspen and oak,
The increasing severity and frequency of droughts is expected to play a pivotal role in shaping future forest ecosystems worldwide, raising questions about how to manage forests under these conditions. Trees growing together in mixtures of different species are thought to be less susceptible to drought stress, but evidence for such positive effects remains limited and may vary based on region and forest type. To address this knowledge gap, a scientist from the Rocky Mountain Research Station engaged in collaborative research with European scientists examining how the composition of “neighborhoods” of different types of trees growing together affected the growth of pine and oak species experiencing recurring drought stress.
The scientists conducted their research in naturally-occurring, unmanaged, mixed forest stands in two different forest ecosystems: semi-arid Arizona, U.S., and sub-humid Bavaria, Germany. They found that the mixture of tree species impacted tree growth in response to stress, but this response varied between the two ecosystems. In Arizona, they focused on Gambel oak (Quercus gambelii) and ponderosa pine (Pinus ponderosa) and found that greater species mixtures buffered trees from drought stress, especially for Gambel oak. However, in Bavaria where they focused on pedunculate oak (Quercus robur) and Scots pine (Pinus sylvestris), the effects were much more variable and did not appear to have a strong positive effect.
These results indicate that the mixture of tree species in a particular tree’s “neighborhood” may be an important feature of forest management, especially in the face of worsening drought. Overall, these results contribute to a larger body of work suggesting that silvicultural practices on dry sites would be wise to favor mixed stands of different species with traits that complement each other, regardless of the particular ecosystem.