Through-growth by Pseudotsuga menziesii: A mechanism for change in forest composition without canopy gaps

The currently prevailing view is that saplings require gaps or larger disturbances in order to grow into the canopy. This study documents an exception. In California's Pseudotsuga-mixed hardwood forests, crowns of Pseudotsuga menziesii (Douglas fir) are within those of angiosperm trees (Arbutus menziesii and Quercus species). In the forests we examined, every Pseudotsuga was younger and all but one were growing more rapidly in girth than the Arbutus or Quercus whose crown it had penetrated. Furthermore, as saplings, the Pseudotsuga had grown at rates between those of suppressed saplings and canopy dominants. The recruitment of emergent Pseudotsuga substantially alters these canopies because of the large size Pseudotsuga attains. Given the density of Pseudotsuga growing in canopy crowns, such recruitment is likely. As a mechanism of recruitment, this through-growth differs from gap recruitment in that the turnover of canopy trees is determined by an understory species' growth rate rather than the overstory species' longevity, and community attributes may change rapidly by replacement of canopy dominants with a dissimilar species. Pseudotsuga could grow through the canopy because of its greater potential height (> 60m vs. 20-40m for the angiosperms), narrower crown and its branches suffering less mechanical damage than those of the angiosperms. In general, resource levels in the understory, canopy height, and interspecific differences in maximum height and crown architecture all influence the likelihood of through-growth. Therefore, for vegetation types whose dominants differ substantially in growth form, through-growth may be a mechanism for rapid ecosystem change.