Effects of Overstory Retention and Understory Vegetation on Small Tree Height Growth in the Inland Northwest

With the increasing use of partial retention silvicultural systems in the Inland Northwest, managers must be able to anticipate rates of growth and the degree of vegetative competition control that is necessary to recruit younger cohorts into the canopy and sustain desirable long-term growth rates. This study was designed to collect long-term understory tree growth data under a broad range of conditions, with the objective of separately evaluating the effects of site quality, overstory retention, and understory competition levels on tree height growth. After four years, mean periodic annual height growth increment (PAI) for 1,708 ponderosa pine (Pinus ponderosa) and 1,286 Douglas-fir (Pseudotsuga menziesii var. glauca) were analyzed from 21 research sites across a gradient of site productivity and stand conditions. At each research site, seven 0.2 ha plots were established under homogeneous overstory and understory conditions, with subsequent herbicide treatment of randomly selected plots to create a range of understory competition levels. Research sites were categorized into three site quality classes (Low, Medium, High) based on plant associations, with sites ranging from warm dry Douglas-fir/Pinegrass to more moist Grand fir/Twinflower. Reducing competing understory vegetation significantly improved PAI for ponderosa pine trees in Low and Medium sites (p<0.001), and for Douglas-fir in Low sites (p<0.06). PAI in untreated control plots was 42% - 59% lower for PIPO and 23% - 34% lower for Douglas-fir, relative to plots with multiple herbicide treatments. Initial size and overstory density were the most influential variables explaining growth in the linear regression analyses. Tree level analysis for initial tree size displayed more predictive value for Douglas-fir (R2 = 48%) than for pine (R2 = 29%). Plot level overstory variables explained more variation for pine (R2 =44%) than for Douglas-fir (R2 =25%). Multiple regression analyses produced models using the two prominent variables, with additional significant variables (i.e. sapling competition index, vegetation volumes, others) to explain 57%, 59%, and 60% of the variation in PAI for pine in the Low, Medium, and High sites, respectively. Models for Douglas-fir explained 50%, 82%, and 60% of the variation for PAI in the same classes. PAI rates in seedling sized conifers showed more sensitivity to increased basal area retention than larger trees, suggesting that understory tree size ought to be considered when planning silvicultural treatments. Reducing overstory to levels to between 5-10 m2-ha-1 can achieve partial retention objectives if PAI losses of 7%-26% for pine and 26% - 28% for Douglas-fir are acceptable by the landowner (compared to more full removal harvest). Results suggest that together understory vegetation and overstory density can significantly affect the rate of height growth in these species.

Name thesis:degree_name

Master of Science (MS)

Grantor dc:publisher

University of Montana

Year

2009

Author dc:creator

• McHugh, Ryan Partrick

• conifer height growth
• Douglas-fir
• herbicide treatment
• ponderosa pine
• competing vegetation
• overstory retention

Repository record dc:identifier

https://scholarworks.umt.edu/etd/363

OAI identifier oai:identifier

oai:scholarworks.umt.edu:etd-1382