Evaluating the ecological impacts of EAB and climate change on black ash forests (Chippewa NF)

To create and maintain a structurally and compositionally complex uneven-aged stands dominated by non-ash canopy tree species, including current climate-adapted (e.g., northern white cedar, red maple, yellow birch, and American elm) and future climate-adapted (swamp white oak, hackberry) species.  Black ash will remain a canopy tree species in the near-term, but will be preferentially removed in group selection harvest entries over the next four cutting cycles and replaced via planting and ultimately from natural seeding of other species.

The stands in this large black ash wetland complex in 2010 were similar to many WFn55 and WFn64 communities in the region with no history of active management. Maximum black ash canopy tree ages ranged from 150 to 274 years; however, stands were strongly uneven-aged with distinct recruitment periods corresponding to past gap-scale disturbance events, including drought in the 1930s and recent mortality of canopy American elms in the 1980s. Black ash represented 80-95% of pre-treatment basal area, with minor components of American elm, basswood, and trembling aspen. Other species found locally at low levels of abundance were balsam fir, bur oak, and balsam poplar. Black ash was found across all size classes, whereas most other canopy tree species were < 12 inches DBH. Relative to other black ash-dominated ecosystems in the region, these areas were quite healthy in regards to the degree of black ash crown dieback and decline documented; however, Dutch elm disease was prevalent throughout the stands. Given the considerable acreage of black ash forest wetlands across the Chippewa National Forest (CNF), the objectives for these areas were to reduce the stand and ownership-level vulnerability of this resource to EAB. This experiment reflects the genesis of several years of trials and discussions led by Gary Swanson, silviculturist on the CNF, aimed at proactively managing ash stands to increase their resilience to EAB by diversifying tree species.

Figure 1: Staff from USFS Northern Research Station and Chippewa National Forest in black ash swamps prior to treatment

This experiment includes four different treatments (no-harvest control, simulated EAB infestation, clearcutting, and group selection); however, the following description is focused solely on the group selection prescription given the initial effectiveness of this approach.  Results from other treatments are provided in the following sections to serve as points of contrast regarding options for managing black ash systems.

Create 0.1 acre group selection openings throughout 20-25%% of the stand removing all black ash but reserving individuals of other canopy tree species should these be present. Harvest during frozen ground conditions. Plant each harvest gap with 11 potential black ash replacement species at approximately 150 TPA per species favoring best microsites over rigid spacing.  Replacement species will include northern white cedar, tamarack, black spruce, swamp white oak, hackberry, red maple, yellow birch, American elm (Valley Forge variety), balsam poplar, cottonwood, trembling aspen, and Manchurian ash and there will be no site preparation or plan for protection. Note that the native range of hackberry and swamp white oak is currently located to the south of the treatment areas.  After three growing seasons, release planted stock with a target density of free-to-grow non-ash species of 1000 TPA (approximately 60% survival).  Treatment sequence will be repeated in ten years in unharvested portions of stand. 

Fall 2010: Timber was appraised and treatments were laid out by Doug Kastendick from the USFS Northern Research Station and the stumpage was sold to CO Johnson Logging, Inc.

Summer 2011:  Permanent plots were installed for pre-treatment measurement of regeneration, understory plant species, and overstory trees.  In addition, permanent monitoring wells were installed in each treatment unit for measuring water table responses to treatments and black ash mortality.

Fall 2011: Half of the seedlings were planted in the September-October prior to harvest to evaluate the potential for underplanting ash replacement species prior to EAB or overstory treatments.  All seedlings were containerized stock, with the exception of swamp white oak, Manchurian ash, and cottonwood.  Note American elm was large stock (0.5 gallon pots).  Rationale for the fall planting was to take advantage of “drier” planting conditions in fall, which would allow for evaluation of suitable planting microsites.

Winter 2012: The stand was harvested by Clarence Johnson of Black Duck, MN with a cut-to-length processor and forwarder. A lack of snow cover allowed for solid, frozen ground conditions and logging operations began in early January and ended in March.  This lack of snow cover also had a negative impact on seedling survival due to lack of protection from exposure, browse, and machine traffic.  Plots simulating EAB mortality were also girdled during this period (and again in Winter 2013)

Spring 2012: The remainder of the seedlings was planted in May-June.

Summer 2012-2014: Regeneration, understory vegetation, and hydrologic data were collected for three seasons post-treatment by field crews led by Mitch Slater from the USFS Northern Research Station with additional long-term measurements ongoing.  Upcoming treatments will include released treatments around planted seedlings in Summer 2015. Gallery (photos 1-6)

Table 1. Three-year average survival rates (%) for planted seedlings.

Overall survival of planted non-ash seedlings was low, with average three-year survival rates across treatments of 32 and 29% survival for fall and spring planting, respectively. Survival rates were significantly lower in clearcutting treatments (22.1%) compared with group selection treatments (34.5%). Regardless of treatment, swamp white oak, American elm, hackberry, and Manchurian ash had the highest survival rates (Table 1).  The native species with the highest survival rate in the group selection treatment was American elm (87%), followed by balsam poplar (45%) and red maple (33.5%).  Swamp white oak (76.5%), hackberry (51.6%), and Manchurian ash (78.3%) also had high survival rates in this treatment. (Gallery photos 7-8).  In general, for species with high survival, fall planting generally had higher survival than spring planting (Note balsam poplar, swamp white oak, and Manchurian ash were planted the fall after harvest).                                                             

Table 2. Natural regeneration (trees ac^-1) following treatments.

Natural regeneration following the group selection treatments was dominated by American elm and black ash stump sprouts.  A small amount of burr oak and basswood were also recruited in group openings.

Effect of Treatment on Ground Layer and Hydrology

Clearcutting had a dramatic influence on the structure of the ground layer resulting in a shift from herbaceous communities composed of a mixture of facultative and obligate wetlands species, to one now dominated by wetland graminoids, including lake sedge.  Group selection harvests also increased the abundance of lake sedge and other graminoids in group openings; however, these changes were less pronounced and facultative species, including rough bedstraw, sweet cicily, and wood nettle still made up a considerable portion of the understory. (photos 8-9)

Water table responses to the group selection treatments indicated that these initial entries did not increase the level of flooding in these areas relative to pre-treatment and control conditions.  In contrast, clearcutting and EAB-simulation (girdling) treatments resulted in an average increase in flooding duration by 24 days (Appendix). These responses should be considered in the context of flood tolerance of potential replacement species being encouraged in ash-dominated stands.

Figure 2: Group selection harvest immediately post treatment (2012)

Figure 3: Group selection harvest two years post-treatment (2013)

Figure 4: Girdle (EAB infestation) treatment two years post-treatment (2013)

Figure 5: Clearcutting treatment immediately post-treatment (spring 2012)

Figure 6: Fall planting (2011) and planted northern white cedar seedling in summer 2013

Figure 7: Clearcutting treatment with monitoring well in August 2013

Future treatments in these areas will include release of surviving, planted seedlings in 2015.  Depending on funding and interests, the second set of group selection harvests and plantings will occur in 2022.

The cost for laying out treatments and planting was $135/acre.  On a per acre basis the CNF received $140/acre on the group selection treatments. The nursery stock costs were roughly $1562 for 4224 trees across the group selection treatments. 

Based on the initial responses to this prescription, group selection appears to be a viable option for establishing non-ash species in black ash wetlands (WFn55 and WFn64) communities.  The high survival rates of seedlings in the girdle and control sites also suggests that other regeneration methods, such as shelterwood methods, that rely on overstory ash to buffer hydrologic responses and minimize understory competition may also be viable options in these communities.  While we relied primarily on planted seedlings, additional work using direct seeding should be tried given the logistical challenges with planting in these systems.  Finally, the high success of more southern species, namely swamp white oak and hackberry, argues for thinking broadly regarding suitable species for addressing the challenges posed to these forest types by EAB and future climate change.