170 Moffat et al.: Experimental Tree Planting on U.K. Containment Landfill Sites trast, Dobson and Moffat (1993) reviewed the results of tree growth on 19 restored landfill sites and found considerable varia- tion in the performance of different species, both native and nonnative, coniferous and broadleaved. This study has also found that nonnatives performed well on some sites but not others; there appears to be no universal truth in this respect. Instead, simple analysis of the performance of individual species over a 10-year period suggests that soil and site characteristics such as climate are very important. Like Rawlinson et al. (2004), we found conifers to perform badly, but this is probably the result of the alkaline nature of the soil substrates at almost all the experimental sites. Table 6 presents the yield class for selected species at Year 10 compared with regional averages derived from data from forests and woodlands on undisturbed land. The results confirm the relatively poor performance of trees at Grunty Fen and the impressive growth at Shaw Tip. The conifers naturally demon- strate a larger yield class than the broadleaves, and growth was better than average at Shaw Tip and Pimbo. Of the broadleaves, oak showed consistent growth closest to average expectations, although with only one or two exceptions (e.g., ash at Grunty Fen and sycamore at Yanley), growth of broadleaved species was reasonable given the comparatively hostile conditions at most sites. Implications for Woodland Establishment on Landfill Sites Taken together, the 10 years of monitoring field experiments on modern landfill sites suggest that it is possible to establish wood- land composed of tree species that grow reasonably well com- pared with equivalents established on greenfield sites. From an engineering standpoint, the results reinforce the need for uncom- pacted, rootable soil or soil-forming material, preferably placed by loose tipping or “complete cultivation.” The widespread N deficiency detected during the monitoring program was predict- able given the relatively infertile materials used in restoration at most sites (Table 1). However, there was no opportunity to use organic waste materials such as sewage sludge or composts as an overall treatment to the sites. The inability of artificial mineral fertilizers to redress permanent nutrient deficiency in the field experiments underlines the need for alternatives such as organic Pimbo amendments, which are often a more effective approach to re- store land amendment (Bending et al. 1999; Moffat 2006). Proper attention to provision of a suitable thickness of soil or soil-forming material, prevention of compaction, and site fertil- ity can maximize the likelihood of good tree performance. How- ever, there was some evidence from the field experimentation program that regional rainfall, or lack of it, was also important. For some parts of the country, notably the drier south and east, meticulous weed control is increasingly necessary, but even if undertaken according to good forestry practice, there remains a risk to tree survival and performance on those sites where plant- available water may be limited, for example at Grunty Fen where low rainfall combined with a clayey substrate of low available water capacity. The potential influence of climate change has been appreciated in recent guidance on suitable soil provision for landfill and other restored brownfield sites (Moffat 1995), but further work is needed to refine this in the light of government- predicted climate change scenarios (Hulme et al. 2002). Considered species selection is an obvious way to maximize the likelihood of a successful woodland, and the field experi- mental program has demonstrated that some tree species tested are more robust and/or provide a low risk of failure. These have been discussed in this article. Other tree species, especially Ley- land cypress, Japanese larch, and Norway maple, performed very poorly during the experiment and should be chosen with care in future schemes on similar sites. The basic maxim is to match species to site conditions as much as possible and to be conser- vative in the expectations for the new woodland, e.g., not to expect tree species typical of natural woodland to thrive on una- mended soil-forming materials, but to choose so-called pioneer species like poplars and alders, which are comparatively tolerant of infertility and exposure. However, climate change makes it more difficult to be certain about choice of species because the climate in future years is likely to be significantly different than today, especially in the south and east of the British Isles (Broadmeadow et al. 2004). In these regions, notably drought-tolerant species should be chosen. Some species such as white poplar may perform well as a re- sponse to climate change (Cannell et al. 1989), but others such as cherry may fare less well than today. It may also be necessary to consider species not tested in these experiments such as walnut Table 6. Measured and regional average general yield classes (m3/ha/year) at Year 10 for all sites except Beech Farm.z Grunty Fen Shaw Tip Species Ash Beech Corsican pine English oak Hybrid larch Italian alder Leyland cypress Silver maple Sycamore Whitebeam White poplar Wild cherry General yield class is expressed in terms of the maximum annual increment of cumulative stem volume per hectare for a given species assuming a standard management regime. NA yield class curves not available (trees less than 10 years old) for these species. z ©2008 International Society of Arboriculture 6 4 NA NA 4 4 4 15 5 NA NA NA 4 4 6 4 8 16 8 12 14 7 11 12 6 NA 6 NA 8 6 8 NA 6 10 6 24 10 NA 15 7 Measured 2 Regional average 5 Measured 8 Regional average NA Measured 10 10 Regional average 5 7 Measured 6 12 6 NA 2 8 4 6 5 NA 6 10 Yanley Regional average 5 16 5
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