Impacts of restoration treatments on alien plant invasion in Pinus ponderosa forests, Montana, USA

Summary

In fire-adapted ponderosa pine Pinus ponderosa forests of western North America, the response of alien plants to restoration treatments is poorly known, particularly for those species capable of altering environmental conditions (known as transformers). Understanding their response is a pre-requisite to developing appropriate restoration treatments to minimize alien plant invasion. To assess the effects of restoration treatments on non-native plants, this study evaluated four treatments at different scales in second-growth ponderosa pine forests.

Study sites: The study was undertaken at the 11,000 ha University of Montana Lubrecht Experimental Forest, in Montana, USA.

Experimental design: Three blocks of 36 ha each (about 3 km apart) were established in second-growth stands comprised primarily of 80-90 year-old ponderosa pine Pinus ponderosa and Douglas-fir Pseudotsuga menziesii, with some western larch Larix occidentalis and lodgepole pine Pinus contorta. There were patches of regeneration and occasional trees up to 200 years old. Despite only modest grazing pressure, study sites were fenced to isolate treatment effects on plant invasion.

Each block was divided into four square 9 ha treatment units. One replicate of each treatment was then randomly assigned within each block, with the exception of two burn treatment units, positioned to allow fire containment. Ten, 20 × 50 m (1,000 m²) plots were established within each treatment unit. Each plot was subdivided into 10, 10 × 10 m subplots. Each subplot had two 1 × 1 m quadrats located in opposite corners (20 quadrats/plot), 12 of these randomly selected to sample understorey vegetation.

Treatments: Restoration treatments (prescribed burning in the spring; thinning; and thinning followed by spring burning, and an untreated control) were aimed at moving towards historical forest density and structure:

Thinning – felling was undertaken to reduce the density of small- and medium-sized trees, and leave an open fire-resistant stand with a target basal area (BA) of 11 m²/ha (resulting in about half of the basal area being removed). Large-diameter ponderosa pines were favoured as unfelled trees, although some were retained in all size classes, if available. Logging slash (non-saleable tree tops and branches) was left on site and driven over by the harvesting equipment to compress it. Thinning was conducted during the winter of 2001 on a snowpack (thus reducing damage to the underlying soil).

Burning - prescribed burns were implemented in May and June 2002, in each of six burn treatment units (3 × burn-only and 3 × thin-burn). Burning was conducted using a strip-head fire technique during calm weather (winds 2 to 13 km/h).

Vegetation sampling: All plant species in each 1,000 m² plot (and associated quadrats) were identified prior to treatment in the summers of 2000 (thin-only and thin-burn) and 2001 (burn-only and control) and after treatment in 2002, 2003 and 2004. Pre- and post treatment covers was visually estimated in quadrats for each species.
Alien transformer species (i.e. those that can alter environmental conditions and therefore are a management priority) were identified, these were: Bromus tectorum, Carduus nutans, Centaurea biebersteinii, Cirsium arvense, Cirsium vulgare, Cynoglossum officinale, Potentilla recta and Verbascum thapsus. In addition to the 1000 m² plot and 1 m² quadrat scales, cover of transformer species (and also exposed soil) was estimated in each of the 10 subplots (100 m²) per plot in 2003 and 2005.

Basal area was calculated pre-treatment and in 2003 for each overstorey tree > 10 cm diameter at breast height (d.b.h. = 1.37 m), and summed for each plot. Saplings (trees > 1.37 m in height but < 10 cm d.b.h.) were censused in five randomly selected 100 m² subplots per plot. The change in overstorey basal area and sapling density (stems/ha) was calculated. In 2003, live canopy cover was sampledwith a densitometer at the subplot corners in each plot.

Post-treatment cover was estimated in quadrats for: duff (i.e. partially decomposed and fully humified organic matter), litter, rock, woody stems, large  woody debris and logging slash. In 2002, maximum crown scorch height was measured for every tree > 10 cm d.b.h. Slope and aspect were measured for each plot.

Treatment effects: Over the course of the study, 178 native species and 25 aliens were recorded. There were no differences in richness or cover of either alien or transformer species prior to treatment, but numerous differences after, indicating differential responses to restoration treatments.

Alien richness and cover: Alien richness differed among treatments at the 1000 m² plot scale in all post-treatment years. In 2002 the thin-only and thin-burn treatments both had significantly higher alien richness than the burn-only and control. In 2003, alien richness in the thin-only treatment remained higher than the control, while the thin-burn had higher alien richness than all other treatments. This trend continued in 2004.
At the quadrat scale, no differences in alien richness were apparent among treatments in 2002 but there were marginally significant differences in 2003 and 2004. The thin-and-burn had significantly higher alien richness and cover than the other treatments.

Transformer species richness and cover: Following treatment, transformer species richness at the plot scale differed among treatments in each sampling year. The thin-only and thin-burn had significantly higher richness in 2002 than the burn-only and control. In 2003, transformer richness in the thin-only remained higher than the burn-only and control, while the thin-burn had higher transformer species richness than all other treatments. By 2004, both the burn-only and thin-only had higher transformer richness than the control, while the thin-burn maintained higher transformer richness than all other treatments.

Transformer species richness at the quadrat scale also differed among treatments. In 2002 and 2003, the thin-burn had significantly higher richness than the control. In 2004, the thin-burn had significantly higher richness than all other treatments. Transformer species cover at the quadrat scale was also significantly higher in the thin-burn than the control in 2002, and significantly higher than all other treatments in 2003 and 2004.
At the subplot level transformer cover differed among treatments in 2003 and 2005, the thin-only and burn-only having significantly higher cover than the control. The thin-burn had significantly higher transformer cover than other treatments in 2003, and higher cover than the thin-only and control in 2005.

Transformer cover & environmental variables: In both 2003 and 2005, transformer cover varied significantly among the three blocks, and was positively correlated with overstorey tree basal area removal and increasing crown scorch height. It was estimated that a 60% reduction in overstorey basal area would elicit nearly a 1% increase in transformer cover. With each 10 m of crown scorch height in 2003 and 7 m of scorch height in 2005, transformer cover would be expected to increase by 1%. In 2005, transformer species cover was positively correlated with the cover of duff and litter, and slash.

Indicator species: The thin-burn had four significant transformer indicators (Carduus nutans, Cirsium arvense, Cirsium vulgare and Verbascum thapsus) in both 2003 and 2005. The only other significant indicator, Cynoglossum officinale, was an indicator of the thin-only in both 2003 and 2005. There were no transformer species indicators for control or burn-only treatments.

Conclusions: The results show that alien species, including transformers, responded to all of the restoration treatments, especially the most intense, combined thin-burn treatment. The significant response of alien species to the thin-burn presents a dilemma because of its benefits, including killing fire-vulnerable Douglas-fir seedlings and saplings, reducing fuel build-ups, recycling nutrients bound in woody debris and increasing the sprouting of important wildlife forage species. Thus managers must weigh the benefits of restoration treatments against unwanted effects relative to their objectives. Although there was a clear response of alien species to the thin-burning in this study it was infact modest at about a 2% increase in cover. Conducting thinning treatments over a winter snowpack will help reduce soil disturbance and is a viable option over most of the range of ponderosa pine.


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