The high mortality of seeds and seedlings of species in old-growth forests is influenced by environmental and neighborhood conditions, which may vary greatly from the conditions for germination and establishment experienced by current adults. The individuals that survive to join the forest canopy are those whose seeds land in suitable abiotic conditions and are able to withstand pressure from competitors, pathogens, and herbivores. Abiotic conditions are likely to change as global climate warms, which could influence the species that tend to survive as well as the composition of the surrounding biotic neighborhood. How great a filter this creates for future forest composition depends on the sensitivity of regeneration to variation in environmental conditions. We quantified seed production and seedling dynamics at 40 census sites across gradients in temperature, topography, light availability and snowmelt date within the Wind River Forest Dynamics Plot in southwestern Washington, an old-growth conifer forest dominated by Douglas fir and western hemlock. We compared rates of germination, establishment, growth, and survival from 2018 and 2019 to variation in light, moisture, and temperature to ask whether several common species varied in their regeneration niche.
Results/Conclusions
Germination rates varied among census sites, but were low overall, for example with just 0.4% of hemlock seeds and 0.5% of Douglas-fir seeds successfully germinating. Abiotic conditions also varied among census sites, and this variation was predictive of differences across sites in germination success for some species.Snow melted over a two week period, varying among sites along a 30 m elevation gradient. Maximum soil surface summer temperatures ranged from 29.4-47.8 °C, with an average of 35.9 °C. . Some of this environmental variation was related to germination success, while other abiotic factors seem to be less important. For example, western hemlock seeds were significantly more likely to germinate in sites with later snowmelt dates, at lower elevation, and under lower light availability. Because summers in the Pacific Northwest are very dry and precipitation occurs during the cold winter months, spring snowmelt could provide the additional moisture seeds need to germinate, this moisture could become more important as the climate of the Pacific Northwest warms and dries. Understanding the sensitivity of each species’ regeneration niche to changing environmental conditions will be crucial to predicting how the old-growth forests of the Pacific Northwest are likely to change.