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• 1. [Article] Silvicultural Treatment Impacts on Understory Trees and 20-Year Understory Vegetation Dynamics in Mature Douglas-Fir Forests

  Policy regarding the management of public forests has undergone a drastic shift over the past couple of decades due to the loss old-growth forests in the Pacific Northwest (PNW) of the United States. For ...

  Citation × Citation Citation Abstract Copy Title: Silvicultural Treatment Impacts on Understory Trees and 20-Year Understory Vegetation Dynamics in Mature Douglas-Fir Forests Author: Priebe, Jim E. Policy regarding the management of public forests has undergone a drastic shift over the past couple of decades due to the loss old-growth forests in the Pacific Northwest (PNW) of the United States. For much of the 20th century, forest management on public lands emphasized timber production through the use of even-aged management practices. There has been increasing recognition, however, that traditional even-aged management approaches are unable to support species that rely on the complex, heterogeneous structures provided by old-growth forests. In response, public forest managers have redirected their focus to developing more ecologically sustainable forests capable of meeting a broad array of objectives including an increasing emphasis on the development of late-seral and old-growth characteristics. Thinning has been identified as a promising method for promoting late-seral characteristics in managed stands. Recent long-term studies have shown that thinning stands does indeed accelerate the development of at least some late-seral structure characteristics, particularly when varying levels of thinning intensity and non-uniform retention patterns are incorporated into silvicultural prescriptions. Likewise, thinning has also shown some ability to increase the abundance of late-seral associated plant species in the understory. The impacts of thinning on vegetation dynamics are complicated by external factors such as natural disturbance events and the influence of pre-treatment vegetation on post-treatment communities. Within the context of managing for late-seral attributes, thinning is used to imitate natural disturbance processes. However, this does not preclude the occurrence of natural disturbances, which may either disrupt or compound treatment effects. Initial site conditions create another potential complication for the development of late-seral attributes by limiting the potential for change in understory communities. While some studies have shown that thinning improves late-seral plant abundance, others have found that the legacy of pre-treatment vegetation has a stronger impact on post-treatment communities. This study focused on the impacts of ice storm disturbance and pre-treatment vegetation on the understory of mature Douglas-fir forests using the ongoing Mature Forest Study (MFS), a long-term silvicultural experiment evaluating the effects of thinning and understory vegetation management treatments, as a framework. The first study examined the impact of an ice storm (glaze disturbance) on planted understory trees. Specifically, I looked at the effect of understory tree species, tree size, and overstory neighborhood environment on the type (bending, crown loss), source (ice loading, falling debris), and severity of damage experienced by planted understory trees at one of the MFS sites. Tree species, size, and overstory environment all affected the amount of understory glaze damage. Frequency and severity of damage both varied among underplanted tree species. In general, smaller trees were more prone to being bent, while larger trees were more susceptible to crown loss. The Douglas-fir component of the overstory provided enough additional sheltering that the increased risk to understory trees from falling debris was balanced by a corresponding decrease in the odds of damage by ice loading. This was not the case for the hardwood component; increasing risk of damage to understory trees from falling debris with increasing hardwood basal area drove an overall increase in the risk of understory damage as hardwood basal area increased. This study suggests that species, tree size, and overstory environment all need to be considered by managers hoping to reduce glaze damage risk to younger cohorts in multi-aged stands. The second study investigated the impacts of thinning intensity and herbicide application on the long-term (20-year post-treatment) development of understory vegetation communities on both of the MFS sites. Trends were examined with a focus on the ability of herbicide application, in concert with thinning treatments, to reduce the legacy of common pre-treatment species and promote the abundance of late-seral associates. Results indicated that both thinning intensity and herbicide application affected 20-year changes in understory plant community composition. Herbicide application was associated with a decrease in the abundance of common pre-treatment species, suggesting that it did reduce the legacy effect. However, this was not associated with any change in the abundance of late-seral species. While light thinning showed some ability to mitigate decline in late-seral species relative to higher intensity thinnings, there was no evidence of treatment interaction with herbicide application. These results suggest that while managers may be able to reduce the influence of initial site conditions on post-treatment vegetation communities, the use of herbicides offers little control over the successional trajectory of the understory. Light thinning appears to be the most effective means of increasing late-seral species abundance, although the use of herbicides to meet other management objectives is not contraindicated by the results of this study. Overall, these results suggest that the best options available to managers to both reduce glaze disturbance impacts to understory trees and hasten the development of late-seral plant communities are heavy thinning with unmanaged leave patches to provide late-seral refugia, or light thinning with gaps to provide growing space for better tree regeneration. Title: Silvicultural Treatment Impacts on Understory Trees and 20-Year Understory Vegetation Dynamics in Mature Douglas-Fir Forests Author: Priebe, Jim E. Policy regarding the management of public forests has undergone a drastic shift over the past couple of decades due to the loss old-growth forests in the Pacific Northwest (PNW) of the United States. For much of the 20th century, forest management on public lands emphasized timber production through the use of even-aged management practices. There has been increasing recognition, however, that traditional even-aged management approaches are unable to support species that rely on the complex, heterogeneous structures provided by old-growth forests. In response, public forest managers have redirected their focus to developing more ecologically sustainable forests capable of meeting a broad array of objectives including an increasing emphasis on the development of late-seral and old-growth characteristics. Thinning has been identified as a promising method for promoting late-seral characteristics in managed stands. Recent long-term studies have shown that thinning stands does indeed accelerate the development of at least some late-seral structure characteristics, particularly when varying levels of thinning intensity and non-uniform retention patterns are incorporated into silvicultural prescriptions. Likewise, thinning has also shown some ability to increase the abundance of late-seral associated plant species in the understory. The impacts of thinning on vegetation dynamics are complicated by external factors such as natural disturbance events and the influence of pre-treatment vegetation on post-treatment communities. Within the context of managing for late-seral attributes, thinning is used to imitate natural disturbance processes. However, this does not preclude the occurrence of natural disturbances, which may either disrupt or compound treatment effects. Initial site conditions create another potential complication for the development of late-seral attributes by limiting the potential for change in understory communities. While some studies have shown that thinning improves late-seral plant abundance, others have found that the legacy of pre-treatment vegetation has a stronger impact on post-treatment communities. This study focused on the impacts of ice storm disturbance and pre-treatment vegetation on the understory of mature Douglas-fir forests using the ongoing Mature Forest Study (MFS), a long-term silvicultural experiment evaluating the effects of thinning and understory vegetation management treatments, as a framework. The first study examined the impact of an ice storm (glaze disturbance) on planted understory trees. Specifically, I looked at the effect of understory tree species, tree size, and overstory neighborhood environment on the type (bending, crown loss), source (ice loading, falling debris), and severity of damage experienced by planted understory trees at one of the MFS sites. Tree species, size, and overstory environment all affected the amount of understory glaze damage. Frequency and severity of damage both varied among underplanted tree species. In general, smaller trees were more prone to being bent, while larger trees were more susceptible to crown loss. The Douglas-fir component of the overstory provided enough additional sheltering that the increased risk to understory trees from falling debris was balanced by a corresponding decrease in the odds of damage by ice loading. This was not the case for the hardwood component; increasing risk of damage to understory trees from falling debris with increasing hardwood basal area drove an overall increase in the risk of understory damage as hardwood basal area increased. This study suggests that species, tree size, and overstory environment all need to be considered by managers hoping to reduce glaze damage risk to younger cohorts in multi-aged stands. The second study investigated the impacts of thinning intensity and herbicide application on the long-term (20-year post-treatment) development of understory vegetation communities on both of the MFS sites. Trends were examined with a focus on the ability of herbicide application, in concert with thinning treatments, to reduce the legacy of common pre-treatment species and promote the abundance of late-seral associates. Results indicated that both thinning intensity and herbicide application affected 20-year changes in understory plant community composition. Herbicide application was associated with a decrease in the abundance of common pre-treatment species, suggesting that it did reduce the legacy effect. However, this was not associated with any change in the abundance of late-seral species. While light thinning showed some ability to mitigate decline in late-seral species relative to higher intensity thinnings, there was no evidence of treatment interaction with herbicide application. These results suggest that while managers may be able to reduce the influence of initial site conditions on post-treatment vegetation communities, the use of herbicides offers little control over the successional trajectory of the understory. Light thinning appears to be the most effective means of increasing late-seral species abundance, although the use of herbicides to meet other management objectives is not contraindicated by the results of this study. Overall, these results suggest that the best options available to managers to both reduce glaze disturbance impacts to understory trees and hasten the development of late-seral plant communities are heavy thinning with unmanaged leave patches to provide late-seral refugia, or light thinning with gaps to provide growing space for better tree regeneration. Close

• 2. [Article] Establishment and growth of natural and planted conifers ten years after overstory thinning and vegetation control in 50-year-old Douglas-fir stands

  Forest managers of public lands in western Oregon and Washington have become increasingly interested in creating additional conifer cohorts in young, even-aged, second-growth Douglas-fir stands. The purpose ...

  Citation × Citation Citation Abstract Copy Title: Establishment and growth of natural and planted conifers ten years after overstory thinning and vegetation control in 50-year-old Douglas-fir stands Author: Nabel, Mark R. Forest managers of public lands in western Oregon and Washington have become increasingly interested in creating additional conifer cohorts in young, even-aged, second-growth Douglas-fir stands. The purpose of our research was to assess the establishment, survival, and growth of naturally-regenerated and underplanted conifers 10-13 years after overstory thinning and understory vegetation control in 50-year-old Douglas-fir stands. Two sites within the Oregon Coast Range were used in our study. One site was relatively dry (~130 cm of annual precipitation) and contained a mostly pure Douglas-fir overstory. The second site was moister (~175 cm of annual precipitation) and contained a Douglas-fir/western hemlock mixed-species overstory. At each site, stands were thinned to basal areas ranging from approximately 18 to 32 m2/ha using either a uniform or gappy thinning pattern. In the gappy treatment, 20% of the total area was comprised of 0.06- and 0.10-ha gaps. Understory vegetation was controlled across a portion of each thinning plot using a broadcast application of herbicides prior to thinning. After thinning, the following species were underplanted: Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), and grand fir (Abies grandis, dry site only). Establishment of natural regeneration and survival of underplanted conifers was greater for all species under lower overstory densities at the drier site, but on the moister site, where western hemlock regenerated prolifically in the understory, no differences were detected. At both sites, underplanted and dominant naturally-regenerated conifers growing under lower overstory retention levels were generally taller and had larger diameters after ten years. Thinning pattern had no effect on the establishment, survival, or growth of understory conifers at the overall stand level, but the tallest individuals were located within gaps. Controlling competing vegetation increased the rate of establishment for naturally-regenerated Douglas-fir and the rate of survival for planted Douglas-fir and western hemlock. Vegetation control generally resulted in larger underplanted seedlings of all species after ten years, but the size of dominant naturally-regenerated conifers was not affected. Shade-tolerant species generally outperformed Douglas-fir in understories except where western redcedar suffered heavy browsing damage. Natural regeneration of western hemlock may contribute to the development of an understory conifer layer when a seed source is present in the overstory, but in mostly pure Douglas-fir stands, underplanting would likely be required to supplement sporadic and slow-growing natural regeneration. In either case, future thinnings of rapidly closing overstory canopies would be required to maintain the long-term development of an understory conifer layer. Title: Establishment and growth of natural and planted conifers ten years after overstory thinning and vegetation control in 50-year-old Douglas-fir stands Author: Nabel, Mark R. Forest managers of public lands in western Oregon and Washington have become increasingly interested in creating additional conifer cohorts in young, even-aged, second-growth Douglas-fir stands. The purpose of our research was to assess the establishment, survival, and growth of naturally-regenerated and underplanted conifers 10-13 years after overstory thinning and understory vegetation control in 50-year-old Douglas-fir stands. Two sites within the Oregon Coast Range were used in our study. One site was relatively dry (~130 cm of annual precipitation) and contained a mostly pure Douglas-fir overstory. The second site was moister (~175 cm of annual precipitation) and contained a Douglas-fir/western hemlock mixed-species overstory. At each site, stands were thinned to basal areas ranging from approximately 18 to 32 m2/ha using either a uniform or gappy thinning pattern. In the gappy treatment, 20% of the total area was comprised of 0.06- and 0.10-ha gaps. Understory vegetation was controlled across a portion of each thinning plot using a broadcast application of herbicides prior to thinning. After thinning, the following species were underplanted: Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), and grand fir (Abies grandis, dry site only). Establishment of natural regeneration and survival of underplanted conifers was greater for all species under lower overstory densities at the drier site, but on the moister site, where western hemlock regenerated prolifically in the understory, no differences were detected. At both sites, underplanted and dominant naturally-regenerated conifers growing under lower overstory retention levels were generally taller and had larger diameters after ten years. Thinning pattern had no effect on the establishment, survival, or growth of understory conifers at the overall stand level, but the tallest individuals were located within gaps. Controlling competing vegetation increased the rate of establishment for naturally-regenerated Douglas-fir and the rate of survival for planted Douglas-fir and western hemlock. Vegetation control generally resulted in larger underplanted seedlings of all species after ten years, but the size of dominant naturally-regenerated conifers was not affected. Shade-tolerant species generally outperformed Douglas-fir in understories except where western redcedar suffered heavy browsing damage. Natural regeneration of western hemlock may contribute to the development of an understory conifer layer when a seed source is present in the overstory, but in mostly pure Douglas-fir stands, underplanting would likely be required to supplement sporadic and slow-growing natural regeneration. In either case, future thinnings of rapidly closing overstory canopies would be required to maintain the long-term development of an understory conifer layer. Close