During 1995-98 Michelle Crowell studied the ecological processes which influence the performance of Scots pine seedlings, which we planted at Athnamulloch in Glen Affric in 1991-92, for her doctorate at Cambridge University.
Here is Michelle's summary of the results of her research.
Restorationists in Scotland are busy in all kinds of places. Trees for Life volunteers find themselves working in plantations of exotic conifers, in overgrazed native woodlands, as well as in open areas where trees have been absent for decades or centuries.
At treeless sites, the main types of restoration work have been deer exclusion from project sites and low-impact planting of Scots pine and other native tree species. The long term goal of restoration of such barren areas is the development of Caledonian Forest, a structurally diverse woodland that includes native ground flora, animals, and soil organisms. When we carry out this work we are making several assumptions about the return of the Caledonian Forest. Two of the more important assumptions are:
- that the basic conditions required for tree seedling growth into mature trees persist at treeless sites, and are facilitated by low-impact planting.
- that successful establishment of mature trees will encourage the natural development of the other elements of Caledonian Forest, including colonisation by native plants, animals, and micro-organisms.
These types of assumptions are necessary and common to many restoration projects. However, we do need to think carefully about the basic conditions required in order for planted seedlings to grow on a treeless site. We also need to assess our assumption that the establishment of trees will lead to a diverse Caledonian Forest.
My research at Cambridge University was motivated by the need to examine these two assumptions. I was interested to devise a case study of a treeless restoration site that not only examined the ecological processes which influence the performance of planted Scots pine seedling, but also established a baseline for monitoring ecological change over the long term. Between 1995 and 1998, I carried out an integrated set of component studies in an area of the Athnamulloch exclosure (west of Loch Affric) planted in 1992. The research design owed much to our historical understanding of deforestation in Scotland and our ecological knowledge of Scots pine. I used these two sources to isolate five ecological processes that were likely to affect seedling performance at a treeless, heather-dominated site. These processes were:
- soil drainage
- competition from ground vegetation (Common heather, feathermosses, Sphagnum moss)
- wind exposure
- soil nitrogen mineralisation
- the formation of ectomycorrhizas (symbiotic relationships between the seedling roots and specialised soil fungi.)
I carried out two detailed field surveys in the study area in 1996 and 1997 and 'located' the seedlings, vegetation, and soil data on a computer-generated three-dimensional terrain model of the site. This Geographic Information System (GIS) provided the basic set of data for the thesis, around which the five component studies were built.
One of the interesting results from the thesis pertained to the spatial distribution of seedling size and health. The largest, healthiest seedlings were consistently observed at well-drained, heather-dominated locations, whereas poorly drained, moss-dominated locations were associated with the smallest, most chlorotic (yellowed) seedlings. This was an unsurprising result, but it has at least two important implications. First, this confirms the importance of choosing good planting locations. Given that our restoration goals require that seedlings grow into mature trees, we should avoid planting seedlings in wet, moss-dominated places. Second, this result emphasises the discrepancy in growing conditions between a treeless heathland and a native woodland. When choosing a planting location and technique, we need to take into account the important changes that have taken place at the restoration site since trees were last present. In particular, we should try to find locations that resemble regenerating areas in native woodland. This might include good soil development or a disturbed soil surface.
Differences between Athnamulloch and a native woodland were also evident from the results of other component studies. I monitored the supply of mineral nitrogen during the growing season, using field incubations in experimental plots. The rates of mineral nitrogen supply in the plots were much lower than the rates observed in several comparable studies in temperate or boreal forests. This level of low availability of mineral nitrogen could limit seedling growth.
The pilot study on ectomycorrhization analysed the changes in the number and types of fungi colonising the roots of Scots pine seedlings at different stages of the restoration process, ie. after nursery treatment, after one year in the field, and after five years in the field. There was a significantly lower number of ectomycor-rhizal roots on planted seedlings than on nursery seedlings. This could be related to the changes to the soil and ground flora that have occurred in the absence of Scots pine at Athnamulloch.
The processes of nitrogen supply and ectomycorrhization could be taken into account when we plan where and how to plant the seedlings. For example, localised site preparation could be considered as a way of stimulating mineralisation and improving the rooting environment. Nursery practices could also be adapted to facilitate the retention of beneficial fungi in the field.
The three-year period of thesis research was too short to investigate the second assumption, that successful tree establishment will foster the development of Caledonian Forest. Instead, I established a baseline for studying this assumption over the long term. All of the seedlings examined in the thesis were tagged with an identification number. This identification number links the seedling to the (GIS) at the core of my research. This should allow us to monitor changes at Athnamulloch over the long term, with a bit of field work every few years to record the condition of the trees and the ground flora. If anything, the most exciting results are yet to come! My thesis provides a 'snapshot' of seedling performance four to five years after planting. We may find that the patterns of both tree performance and ground flora change drastically in the years to come.