The dry Douglas-fir forests of the Cariboo-Chilcotin in British Columbia are difficult to reforest following severe disturbances. The recent warming trends due to climate change have exacerbated this by exposing spring planted seedlings to more extreme summer temperatures and drought.
Under the Forest Carbon Initiative program, we have begun to investigate the potential for fall planting in this region. In the fall of 2019 we planted 155 000 seedlings. Within the planted area, we established plots on different microsites where we monitored the health and vigour of seedlings and collected microsite specific data. At each plot we closely monitored the condition of seedlings along with soil temperature and moisture, air temperature, and vapour pressure deficit (VPD) for 1 year.
After analysing the data we were able to determine what environmental factors proved most significant to the survival of the seedlings. These factors introduced some unique management considerations relative to spring planting that silviculturists must take into account when planting seedlings in the fall.
Initial establishment success of fall planted seedlings is directly related to environmental conditions. Due to this, best management practices should direct the use of on-site field observations to ensure fall planting occurs when seedlings have access to adequate soil moisture, moderate air and soil temperatures, and low VPDs.
During the fall, plants and other vegetation are senescing and/or hardening off for the winter. This makes the supple and sweet foliage of newly planted seedlings particularly appetizing and more susceptible to herbivory.
Throughout the coldest months of the winter snow cover plays a substantial role in seedling survival by providing insulation from the most extreme temperatures. For this reason, seedlings should not be planted beneath anything that will intercept snowfall such as live canopies.
When developing planting prescriptions, silviculturists need to take into consideration such factors as cold air drainage. While large surface features such as logs can provide seedlings with shade during the hot summer months, thereby lowering surface temperatures and increasing access to soil moisture, they can also obstruct cold air drainage. This leads to the pooling of cold air and greater occurrences of frost, which can cause seedling damage and morality. Therefore, the orientation of large surface features relative to slope must be considered when assessing suitable microsites.