Seven-year survival of perennial herbaceous transplants in temperate woodland restoration

Summary

Following European settlement, much North American hardwood forest east of the Missouri River was cleared. There has been widespread reforestation in the east but not in the more agricultural Midwest. Here, little is known about restoring the native perennial herbaceous understory of Midwestern deciduous woodlands. Many understory species have reproductive characters that make reestablishment by seed slow or difficult, therefore the efficacy of transplanting as a strategy for introducing 24 species to a degraded early-successional woodland in central Iowa ( USA) was investigated.

Study site: The study was conducted at the Betz Site, Camp Dodge (an Iowa Army National Guard base), Johnston, Iowa. The 3 ha study site (a highly degraded woodland) on a gently sloping, well-drained west-facing slope, was considered ideal to test establishment by transplanting of understorey species that occur in shade/semi-shade, as the variable woodland canopy (black walnut Juglans nigra, hackberry Celtis occidentalis, white mulberry Morus alba and red elm Ulmus rubra) allowed examination of the influence of various light levels on establishment and survival. The pre-existing understory lacked herbaceous species characteristic of undisturbed woodlands in the region and was mostly composed of generalist and weedy species. The isolation of the site ensured that results would no be confounded by dispersal of target plants into the area. Total annual precipitation is 848 mm, 73% of which occurs from April to September.

Transplant species: Few shade-tolerant species were available locally and as a local (Iowa) genetic source was desired, species were limited to those that could be purchased, grown readily from seed, or transplanted from local populations (if able to withstand harvest). None of the 24 species planted were considered rare. Excepting side-flowered aster Aster lateriflorus and tall windflower Anemone virginiana, none were species readily able to recolonise woodlands.

Plot preparation: Thirteen 4 × 5.5 m permanent plots were subjectively distributed in May 1998. Plot locations were chosen to represent a range of light levels. Prior to planting, the plots plus a 1-m buffer strip were sprayed with RoundUp (glyphosate). By early June, except for white avens Geum canadense, the sedges Carex sparganoides and C.davisii, gooseberry Ribes missouriensis and black raspberry Rubus occidentalis, which appeared unaffected by the herbicide, most other vegetation was dead. Gas-powered string trimmers were used to cut down standing litter and stems.

Planting: Generally, single species groups of four individuals were randomly assigned to a 0.25-m² planting unit within a plot. The number of units per species and individuals per unit were determined by the plants available. Most planting was undertaken during 8-22 June 1998, watering was unnecessary as it rained frequently during this 2-week period. As they became available, some plants were added later. Plots were weeded periodically in 1998 to eliminate weed competition but were only weeded once in May 1999 and not at all from 2000–2005.

Plant survival & flowering: Survival of the 24 species was monitored five times over a 7-year period (1999-2005). In 2003, only 11 plots (rather than all 13) were monitored (two having been accidentally mown). Flowering (an indicator of seed production and establishment), was monitored for 15 species in 1999 and 2000. Data from three planting units of oak sedge Carex pensylvanica recorded flowering for the first time in 2003, was also included. Not all species were monitored for flowering due to time constraints. Survival data were based on presence or absence of species in a 0.25-m² planting unit. Plants outside the planting units and plots were noted to document species spread.

Light levels: To measure daily light levels (solar irradiance and photosynthetically active radiation), from June 30 to July 17, and September 15–18 1999, photon flux density (PFD; 400–700 nm) was measured at four locations per plot using Campbell Scientific 21X data loggers and quantum sensors (attached to vertical dowel rods 50 cm above the ground).

Soil characteristics: In August 1999, four soil cores per plot were taken, pooled and analysed for phosphorous, potassium, nitrogen, percent organic matter and pH.

Survival: Overall survival of the 24 planted species averaged 91% in year 1, 85% in year 2, and after 7 years was 57%. Survival in years 2003-2005 did not include individuals spreading beyond the original planting units (see below) and is therefore a minimum abundance estimate of many species. Brown-eyed Susan Rudbeckia triloba (a biennial or short-lived perennial) was not included in survival analysis after the first year. Only two species (great blue lobelia Lobelia siphilitica and gray goldenrod Solidago nemoralis) did not survive to year 5 possibly because the habitat was too dry for the lobelia and too shaded for the golden rod. For all other species (surviving to year 7), planting groups of three and four survived equally well(62%) indicating that this difference in number of individuals per group did not influence results.

By year 7, 17 of the 24 species were spreading by self-seeding and/or rhizomes. Although not quantified, the expansions of oak sedge, dutchman's breeches Dicentra cucullaria, bottlebrush grass Elymus hystrix, Virginia waterleaf Hydrophyllum virginianum, woodland phlox Phlox divaricata, cut-leaved coneflower Rudbeckia laciniata, brown-eyed susan and elm-leaved golden rod Solidago ulmifolia were particularly notable (e.g. oak sedge, bottlebrush, waterleaf and coneflower had formed some patches of at least 3 m². Others had scattered individuals established outside their planting areas). Six species, tall windflower, Virginia wild rye Elymus virginicus, bishop's cap Mitella diphylla, white lettuce Prenanthes alba, Culver's root Veronicastrum virginicum and blunt-lobed woodsia Woodsia obtusa persisted vegetatively but did not spread.

Flowering: Average flowering was 72% for the 15 species monitored and was positively correlated with percent photosynthetically active radiation. There were few strong correlations between the other measured environmental variables (both for flowering or survival) and none was consistent from year to year. In addition, eight other angiosperm species were observed flowering in the plots.

Conclusions: These survival and flowering rates are considered acceptable indicators of establishment success, especially given drought conditions in the first few years and lack of weed control beyond the first year, and evidence that transplanted species were spreading from planting locations.

The survival data for the perennial herbaceous species introduced at the Betz Site indicates that it is feasible to increase diversity of such plants by transplanting, despite no weed control beyond 2 years and below normal rainfall in 2000 and 2002–2003. High flowering rates, stable survival between years 5 and 7, and the spread of several species also indicated success. The results are encouraging for species like Jack-in-the-pulpit Arisaema triphyllum, wild ginger Asarum canadense, oak sedge, dutchman's breeches, wild geranium Geranium maculatum, Virginia waterleaf and woodland phlox which are unlikely to re-establish by seed. This study also suggests that detailed knowledge of variation in light and other abiotic factors may not be needed to undertake woodland perennial restoration, at least at the small scale of a 3-ha site.


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