Gap size through the lens of understory light environment and shade intolerant species regeneration: Historic and current tree spatial patterns in southwestern Oregon mixed conifer/hardwood forests

Background/Question/Methods

Across the west, changes in disturbance processes have shifted forests to be dominated by crowded, smaller trees of fire sensitive species resulting in more continuous tree cover, and a reduction in the number and size of gaps. Although much research has been conducted to measure tree spatial patterns, gaps have typically been reported as a proportion of stand area and spatial patterning often is reported as Moran’s I or Ripley’s K. These global statistics do little to provide ecological context for spatial patterning. Understanding of the importance of gap requires an ecological rationale which ultimately will allow quantification of gaps size, shape, and function.

Our research compares historic and contemporary gap sizes as a function of understory light availability and shade intolerant species regeneration. Within the diverse mixed-conifer hardwood forests of southwest Oregon, 4 three-hectare plots were stratified by biophysical setting. All live and dead stems greater than 10 cm diameter at breast height were mapped. Using growth rates, snag fall rates, and age of death equations, all trees were reconstructed to the date of last disturbance (1914) and 150 years prior to the sample year (1861). This provided precise estimates of species composition and diameter distributions of historical forests. Light maps were generated with the forest simulation software SORTIE, and parameterized with tree allometry and species specific light transmittance data. Species specific crossover point irradiance values were used to determine locations where light availability would allow shade intolerant species to grow faster than shade tolerant species, and thus locations of gaps were shade intolerant species may effectively regenerate.

 Results/Conclusions

We found that in 2011 there were fewer opportunities for shade intolerant species. Understory light availability was greatest in 1861, and there was a broader range of gap sizes in 1861 relative to 2011 with a greater proportion of plot area in gap. Historical species composition reflected a more open forest structure and contained a greater proportion of large fire resistant trees than in 2011.

Differences in life history among tree species increase the complexity of forest restoration in mixed conifer/hardwood forests relative to forests dominated by an individual (or few) conifer species.  Greater understanding of forest spatial patterning creates opportunities for implementation of treatments designed to provide diverse habitats that will sustain a range of species and thus helps promote healthy resilient forests.