Riparian forest restoration: why each site needs an ecological prescription

Summary

In eastern North America, natural regeneration of forests in riparian areas is thwarted by competition from exotic plants and herbivory associated with large populations of mammals. Thus, proactive restoration (afforestation) tailored to maximise seedling survival and growth is sometimes required. The short-term (1-year) and long-term (5-year) effects of different treatments (tree shelters, herbicides, tree mats and mowing) intended to improve seedling survival and growth, were evaluated for five native deciduous tree species on the coastal plain of eastern Maryland, USA.

Study sites: The experiment was conducted at Chino Farms Inc. in Maryland, on two sites which had different soil and slope characteristics (see Table 1, attached): Site 1 (39°13'35''N 76°00'50''W); Site 2 (39°11'12''N 75°55'44''W).

Study species: In each plot seedlings of five tree species native to the region and recognised for their potential as marketable timber or for provision of 'ecosystem services' were tested:

i) Pin oak Quercus palustris
ii) Red oak Quercus rubra
iii) White oak Quercus alba
iv) Red maple Acer rubrum
v) Tulip poplar Liriodendron tulipifera

Experimental design and weed treatments: To examine the effects of weed competition on seedling survival and growth, four weed treatments were applied (Table 2, attached):

i) Mowing (weed-eater)
ii) Herbicide application (Roundup®)
iii) Tree mats (VisPore®)
iv) Control (no treatment)

Pre-planting preparation: Both sites were ploughed and disked 1 month before planting, making them initially weed free.

Planting of seedlings: Seedlings were planted on 26 and 29 April 1996, using a dibble bar on a 3 × 3 m spacing. One Right Start® fertiliser packet was placed in each hole before planting. All plants were 1-year old, bare-root seedlings.

Maintenance of plots: The vegetation between the rows of each plot and over 0.9 m from each seedling was mowed three times a year to prevent shading.

Mowing: The vegetation within 0.9 m of each seedling was mowed using a weed-eater. Mowing was conducted twice each growing season for the five years of the study (for specific dates see Table 3, attached).

Herbicide application: Herbicide was applied to an area of between 0.8 and 1.0 m² around each tree seedling. Herbicide was applied twice each growing season (for dates see Table 4, attached).

Tree mats: Tree mats (0.9 × 0.9 m) were placed around seedlings at time of planting.

Tree shelters: Each row within a plot contained two individuals of each species, planted in random order. One individual of each species was randomly selected to have a tree shelter (Tubex®), 1.2 m tall), placed over it.

Survival: Trees were classified either as alive or dead.

Growth: Height was defined as the highest vertical extent of either the stem or leaves. Thus, seedling growth was estimated by subtracting the average initial height at planting from the height at the end of the first and fifth growing seasons.

First-year and fifth-year survival: Seedling survival and growth is summarised in Table 5 (attached).

i) Survival at Site 2 was significantly higher compared to Site 1 (averaged across species and treatments), both for first-year and fifth-year survival.

ii) Pin oak, white oak, red oak and red maple displayed significantly higher survival than tulip poplar, both for first-year and fifth-year survival.

iii) Survival of sheltered seedlings was significantly greater compared to unsheltered seedlings for fifth-year survival.

iv) Survival significantly declined between the first and fifth year at both sites. The largest observed decline was for unsheltered trees.

v) Pin oak, white oak and red oak survival was significantly higher compared to tulip poplar when seedlings were unsheltered.

First-year and fifth-year growth:
i) Seedlings with shelters grew significantly faster (averaged across species, treatments, and sites) compared to unsheltered seedlings, at the end of the first and fifth year. Fifth-year growth for sheltered seedlings was 300% greater compared to unsheltered seedlings.

ii) Tulip poplar grew faster than all other species, as measured at the end of the first and fifth year.

iii) Only tulip polar and white oak increased in height after the first year. Fifth-year growth for pin oak was significantly greater than white oak and red maple while fifth-year growth for red oak was significantly greater than red maple.

iv) Tulip polar and red oak seedlings in herbicide plots grew significantly faster compared to seedlings in mat, mow or control plots at the end of the first and fifth year. Any increased growth due to tree mats or mowing did not differ significantly from controls for any of the species.

v) First year growth for tulip poplar was significantly greater at Site 2 compared to Site 1. White oak displayed significantly greater growth at Site 1 compared to Site 2.

vi) First year growth for unsheltered seedlings at Site 2 was significantly greater compared to Site 1. There was no significant difference in growth between the two sites for sheltered seedlings

vii) Sheltered seedlings grew significantly faster in herbicide plots compared to mow or control plots, at the end of the fifth year. Conversely, no significant fifth-year growth differences were observed between weed treatment plots for unsheltered seedlings.

viii) Sheltered tulip poplar grew significantly faster than sheltered pin oak however, unsheltered tulip poplar did not. Unsheltered tulip poplar and pin oak grew significantly faster than white oak and red maple, and unsheltered red oak and white oak grew significantly faster than red maple.

Site-specific prescriptions: Strong variations among the two test sites demonstrates that the choice of species selected, where and how they are planted and to what extent they are managed, must fit a site-specific prescription.

Methods for enhancing survival and growth:

Tree shelters - Tree shelters altered the relative growth relationships among species of seedlings i.e. how fast or slow a species grows relative to another species under the same conditions.

Weed abatement - Control of plant competition was not a significant factor affecting seedling survival in this study, although it has been at other sites in the region (Sweeney et al.. 2002) and elsewhere (e.g. Bendfeldt et al.. 2001). However, survival on control plots was higher in this study compared with other sites in the region. This suggests that control of plant competition is more crucial on marginal sites where soil fertility, moisture, and temperature are limiting. The different weed-abatement treatments did however exert a significant beneficial effect upon growth rates of seedlings. When weed-abatement treatments (particularly herbicide application) were used in conjunction with tree shelters seedling growth was even more enhanced.

Herbivory: Low fifth-year survival rates for unsheltered red maple and tulip polar (despite the latter's fast growth rate) indicated that herbivores feed selectively. Knowledge of selective herbivory could enhance the cost effectiveness and efficiency of restoration because the use of tree shelters could be restricted only to those species sought by herbivores.

Conclusions: In this study, herbivory was an important consideration when devising a site prescription because it effectively masked other factors, including site-to-site differences in soil moisture and fertility, and the effects of weed control measures. When seedlings were protected from herbivory, it became apparent that conditions for growth were superior at site 2 compared to site 1 for all seedlings and that seedlings grew significantly faster when freed from plant competition. Protecting seedlings from herbivory was more important than protecting seedlings against plant competition.


References
Bendfeldt E.S., Feldhake C.M. & Burger J.A. (2001) Establishing trees in an Appalachian silvopasture: response to shelters, grass control, mulch, and fertilisation. Agroforestry Systems, 53, 291-295.

Sweeney B.W., Czapka S.J. & Yerkes T. (2002) Riparian forest restoration: increasing success by reducing plant competition and herbivory. Restoration Ecology, 10, 392–400.


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