Born in 1930 in Calcutta, India and educated in George Watson's College, Edinburgh, he did National Service in the Royal Navy before reading Forestry at Edinburgh University.

Graduating BSc Forestry with 1st Class Honours in 1954 he joined the Forestry Commission as District Forest Officer in North West England Conservancy.

In 1961 he was appointed Lecturer in silviculture in the Department of Forestry and later became Senior Lecturer and director of Studies, and in 1987 became Head of the Department of Forestry and Natural Resources.

Research interests have focussed on climatic and soil interactions with tree growth but have included oak population studies, spruce provenance, mycorrhizal relations, effects of fertilization on organic soils and currently the N economy of mixed species stands

He is a member of several scientific societies holding various offices, was Editor of Forestry from 1978-86, is on the editorial board of three international journals and is currently Vice-President of the Institute of Chartered Foresters.

Introduction
At the end of the First World War the total forest area in Britain had reached its lowest extent and amounted to only 3 per cent of the land surface. Starting in 1919 a programme of afforestation began which, accelerating after 1945, has raised the forest cover to about 11 per cent, that is about 2 m ha of managed plantation forest. British foresters were fortunate to have at their disposal for this programme, a number of introduced species to supplement the only native conifer, Scots Pine (Pinus sylvestris L.). Most of the exotics had been introduced by David Douglas and other collectors in the years after 1830 from Northwest America. The most important of these introductions has been Sitka spruce (Picea sitchensis (Bong.) Carr.) which Douglas foresaw would be well adapted to the oceanic climate prevailing over much of Britain and to the moist organically rich soils of the uplands. Although initially planted on the more fertile soils at lower elevations, it was soon discovered that Sitka spruce was better able to withstand the exposure to wind and other climatic hazards at higher elevations than any other species. It also proved to be a highly productive species which was also versatile in utilization.

The emphasis on agricultural production in Britain denied foresters the opportunity of achieving much forest expansion on the better, lower-lying sites so that they had to extend progressively on to the less fertile uplands where the soils are generally impoverished and ill-drained. These sites became available to afforestation because of their low productivity for extensive grazing but many could not be planted effectively until the techniques of site amelioration through drainage, cultivation and appropriate fertilization had been worked out. The advent of tracked vehicles, particularly after 1945, enabled the treatment of increasingly difficult sites, until by about 1960 only the shallowest rankers or open pool bogs could not be cultivated or drained for planting. The other important factor in successful plantation establishment was the early discovery (about 1910) of the need for phosphatic fertilizer on all soil types in the uplands other than the freely drained brown earths.

Having succeeded in empirically improving these difficult soils for planting, species selection became less a question of silvicultural choice and more of an economic calculation. The alternatives on most sites were between adopting a low input system based on the inexacting lodgepole pine (Pinus contorta), which performed well with a single application of phosphorus and on the worst peat soils an application of potassium, or attempting to grow the more demanding Sitka spruce, which it was thought could provide an increased mean annual increment (pine 8-10 m³ ha^-1 yr^-1, spruce 14-16 m³ ha^-1 yr^-1) in comparable climates. The anticipation that nitrogen would become a limiting factor lead to the establishment in the 1960's of a number of trials of Sitka spruce on both oligotrophic heath soils (iron-pan podsols on indurated glacial material) and deep peat soil (> 45 cm, often raised bogs) and it is in these experiments that the alternative to repeated fertilizer N applications has become apparent and in which the studies described here have been based.

The effect of Calluna on spruce growth
A feature of the vegetation on upland Britain is the prevalence of plant communities dominated by the ericaceous sub-shrub heather (Calluna vulgaris). A limited distribution of this species post-glacially expanded dramatically With deterioration in climate and early destruction of forest. Although it is a remarkably successful species, able to maintain itself for fifty years in the soil seed bank, its dominance in the heathlands is due to land-use practices, particularly periodic rejuvenation by burning to enhance its nutritional value as browse for sheep and grouse. On heathlands in low rainfall area, it forms extensive, almost pure stands, while in wetter conditions on peaty soils, it occurs in mixture with Eriophorum and other bog adapted plants. After drainage of these peaty soils, Calluna soon assumes dominance here too.

In early trials of afforestation on the heathlands it was noted that spruces and other species virtually ceased growth when planted in Calluna swards, whereas pioneer species such as the pines and larches did not suffer this 'check' to their growth. Where a spruce stand was planted alongside a pine or larch stand, the trees adjacent to the pioneer species began to grow again as soon as the pioneer species had suppressed the heather and it was discovered that the spruce had sent out long unbranched roots which branched profusely under the pioneer canopy. There then developed a technique of introducing spruce or other Calluna sensitive species into mixtures with pine or occasionally larch on Calluna dominated sites or where it was thought Calluna would become dominant. Clearly there was a competitive effect which was not one of shading of the young trees. 'Checked' trees remain alive, grown very slowly and are uniformly yellow green in colour, suggesting severe N deficiency. Trees planted on cultivated ground grow normally for several years until the Calluna returns, whereupon they 'check' as though newly planted into the sward. Intensive investigation in the 1950's demonstrated that any treatment that killed the heather, even mulching with cut heather, removed the inhibition to growth. Later Handley and Robinson at Oxford showed that root washings of active live heather inhibited the growth of a number of ectomycorrhizal fungi normally associated with spruce, whereas they did not affect those commonly associated with pines or larches. Partially shaded, weak, old Calluna or that which was growing on less acid soil (pH > 5) was also a less effective competitor.

The objective of mixed plantings was to obtain early Calluna suppression, release of spruce from 'check' and its eventual domination of the pioneer species leaving a pure or almost pure stand of the desired species. Large areas were planted in this way with greater or less success depending on the density of Calluna or the selection of appropriate provenances of the 'nurse' species. The difficulties of managing these mixtures and the introduction of herbicides, especially 2,4-D and 2,4,5-T, led some silviculturalists again to advocate pure spruce stands with Calluna control when it reappeared after the initial cultivation, the hope being that the spruce would then close canopy, eliminating the problem. Although this could be readily achieved in research trials, it is much more difficult on rough terrain in normal forest management with a limited period for application to avoid adverse weather, damage to trees and with the Calluna still active. Glyphosate is now the commonest herbicide used for this purpose, although 'check' now is relieved also by aerial application of urea-N.

Pure stands of spruce thus can be established on sites that are dominated by Calluna before planting. However, this success can be short-lived as the effect of killing Calluna, with the attendant release of N, proves insufficient to maintain spruce development to canopy closure on the poorest heath or peat soils and the trees again demonstrate N deficiency symptoms. Growth rates can be restored by further N fertilization but this has to be repeated at three-year intervals as first demonstrated in Northern Ireland in the early 1970's and later on a series of Scottish sites.

The mixture effect
The better growth of spruce observed in mixed plantings initially was attributed to the suppression of Calluna but it was not until the 1970's that it was realised that the availability of nitrogen to the spruce was involved. First noted in Ireland, the same effect began to appear in properly designed experiments at about eight years from planting. In these experiments, designed to examine the effect of and need for repeated N application, the control plots had been planted as larch:spruce and pine: spruce mixtures on the supposition that pure spruce would 'check' in the Calluna sward. Some trials included Calluna control as split treatments and it was the performance of these treatments, which became N deficient and slowed in growth, compared to accelerating growth of spruce in mixture that emphasised the low N availability on both the heath and peat sites.

The effect was first noted in the periodic height measurements (Fig. 1) but was also obvious in foliage colour and in foliar analyses (Table 1). The uniformity in the age at which the effect becomes apparent across the widely different sites is striking and probably has to do with the mineralisation rate of N in the upper soil organic layers becoming limiting in the pure spruce stands. The foliar N concentrations in the mixed stands is equivalent to that in pure stands which have received 640 kg N ha^-1 up to that time.

The improved N status of the mixture is clearly apparent but the question arose as to whether it was simply a result of the lower stocking rates of spruce in the mixtures. In each experiment measured, the above ground N in either the foliage or in the total stand was estimated (Table 2). In each case the mixed stand had as much or more accumulated N above ground. The mixtures could, therefore, gain access to N at a rate at least equal to and generally greater than the pure stands. This was particularly interesting in the larch: spruce stands, because the larch was by no means vigorous, still had a well developed ground flora and in one case was severely browsed by deer.

Possible mechanisms for the mixture effect
When starting to investigate the nursing phenomenon in these mixtures, it was clear that a number of mechanisms might be involved. The field experiments which were suitably replicated and had had consistent treatment were all by 1980 about 15-years-old, so that some treatments had begun to close canopy and the effect of mixing had developed six or so years before. The initial hypotheses to be tested, therefore, attempted to take into account many of the usual ecosystem concepts of nutrient cycling. Among the comparative measurements which have been made, in paired and replicated field plots, are:

1. Canopy leaching, stemflow and throughfall chemistry.

2. Litter inputs.

3. Mineralisation rates in the upper soil horizons.

4. Fine root distribution and dynamics.

5. Microbial activity in upper soil horizons.

The study of throughfall was based on the idea that in the better supplied mixture stands, either or both of the species might release N and other nutrients from their greater foliar mass. The better developed crowns of the mixed stands, of course, did intercept a higher proportion of the precipitation and also could be expected to collect greater quantities of minerals in dry deposition than the pure stands. The results showed that larch components of mixtures on poor peat absorbed N rather than released it and this proved also to be the case for both pure spruce and when mixed with pine or larch at another site. However, the third site in the lowest rainfall did release small quantities of N while in an equivalent Irish trial, rather more N reached the forest floor. The inconsistency between sites and the generally small values of added N in throughfall that did occur, do not suggest enriched throughfall as the means of the enhanced growth in mixture.

As larch is deciduous and pine only retains 1-2 years of needles, the litterfall in mixed stands is bound to be greater than pure spruce which retains its needles for 5-7 years. In addition, the unchecked growth of the pioneer species means that they increase their foliage mass more quickly than the spruce early on. The possibility that this litter input could provide a source of readily available N, thus looked attractive as a mechanism. However, the poor stature of many of the larches in the mixed plots illustrated that this quantity was likely to be small. It was shown that although larch may have 2.5% N in its foliage before senescence, by abcission this has decreased to less than 0.7% though still greater than pine and spruce on these sites. Furthermore that N is not immediately available and is only released after about 9-12 months. The total inputs from this source were, thus, 3.3-6.7 kg N ha^-1 for larch in mixture, 6.8 for Scots pine and 6.3 for lodgepole pine. These values compared with values of 1.1-2.4 kg N ha^-1 yr^-1 for pure spruce plots. The addition of N through litterfall, even when accumulated over some years, does not indicate a significant contribution to the mixture effect.

The above ground inputs of N being relatively insignificant, interest soon turned to the below ground effects. The mineralisation of N in the upper 10 cm of the soil was determined in both spruce:pine and larch:spruce mixtures. The upper horizon was separated into shallow (3-5 cm) layers and incubated both in the field and laboratory. In these studies, samples were taken monthly so that seasonal differences could be determined. These showed that although immobilization of nitrogen occurred on several occasions in the pure spruce, it was rare in the mixed stand. To check on the possibility that the microclimate beneath the different canopy types could result in differential net mineralization, samples from pure spruce were incubated in the mixed stand and vice versa. No significant differences were found indicating that neither the consistently drier conditions of the mixed stand or any temperature differences were affecting the results. The amounts of net mineralization of N (all NH₄, no NO₃ being detected) estimated on an annual basis, were 28 and 60 kg N ha^-1 yr^-1 in the pure spruce and spruce:larch mixtures respectively. In the mixture, about 60 per cent of this mineralised N was found in the top 0-3 cm layer, whereas in pure spruce mineralization was more evenly spread through the top 10 cm. In the pine:spruce mixture which was located in a much harsher climate, in situ estimates of net N mineralization over the year reached 51 kg ha^-1 in pure spruce and 66 kg ha^-1 in mixture. In the case the organic soil underlying the litter layers proved to be the most active zone of mineralization accounting for 90 and 60 per cent of the total in pure and mixed stands respectively. Laboratory incubations supported these results and in some cases gave very similar values when adjusted for temperature differences while the uptake of N by Betula seedlings in a bioassay gave strikingly similar results.

Clearly the mixed stands create an environment, possibly through litterfall and changes in the physical conditions of the upper soil layers, that permits higher rates of mineralization than can occur in the pure spruce stands. Although the differences between the stand types vary in different localities and on different soil types, they are of a sufficient order largely to explain the differences in above ground accumulated N.

In searching for explanations of the mixture effect, it was important not to neglect the litter input below ground, that is the contribution from fine roots. Detailed excavation of major root systems in a pine:spruce mixture demonstrated the differential vertical distribution of the species, where the tope-like laterals of spruce were confined to the upper layers of the soil above the less extensive and more deeply penetrating pine roots. When the fine root distribution was ex4Tjned in the 0-3, 3-6 and 6-9 cm depth layers, the pure spruce biomass averaged 54, 38 and 8 per cent respectively, whereas the spruce in mixture was 38, 42 and 20 percent for these layers. Pine in mixture gave more uniform values of about 37, 35 and 28 per cent. The mixture fine root systems then are more evenly distributed through the upper layers and must, therefore, have access to a greater volume of mineralizable organic matter as well as avoiding the climatic extremes of the surface layers to which the pure spruce is largely confined. Another marked difference between the pure and mixed stands became apparent when the mean monthly standing crop of the top 10 cm of soil was calculated. The total live weight of fine roots was 135 g m^-2 in pure spruce and only 86 g m^-2 in the mixture (56 spruce, 30 pine). When expressed in terms of the active root tips m^-2, the differences were even more marked, there being 1113 × 101 in pure spruce and only 361 × 103 spruce and 236 x 103 (total 597 × 103) in the mixture. Variation in the nutrient content of the live root tips resulted in rather smaller differences between treatments such that the mean standing crop of N in fine roots was 13.6 kg ha^-1 in pure spruce and 13.2 kg ha^-1 in the mixture (spruce 7.4).

There is no doubt, therefore, that the pure spruce stand in this instance allocates a much higher proportion of its carbon resources to fine root production in the upper few centimetres of the soil, where climatic stresses give rise to higher mortality. The mixed stand avoids this problem through more uniform distribution and has a production rate about 30 per cent lower than the pure spruce, thus releasing resources for above ground growth.

The conditions for microbial activity in the soil are likely to be better in the mixed rather than in the pure stand as noted before. When this was checked by estimating total microbial biomass from amended respiration data collected on a monthly basis over two years, the pure spruce stand had a mean value of 77 g m^-2 compared to 113 g m^-2 in the spruce:pine mixture in the LFH layer while both stands had 73 g m^-2 in the underlying organic horizon. Studies of live and dead fungal hyphae showed that there was no real difference in live hyphal length in the LFH of pure spruce or mixed stands and that the greater total fungal length found in spruce must be due to a build up of dead hyphae. Additionally, the rate of decomposition of hyphae was greater in the mixed stand and this, together with the larger microbial biomass, may account for much of the greater availability of mineral N in these mixtures.

As a result of these investigations the nursing effect of pioneer species on spruce in mixture has been thoroughly described. It is clear that a number of factors are involved and that the effect is related to the changes in the upper soil induced in the mixed stands. Here there is greater mineralization and availability of nitrogen than in pure spruce on these difficult sites but the mechanisms are not yet clear. The source of the additional available N must be from the native organic matter of the site. The lack of differences in atmospheric inputs between treatments within sites and the inability to demonstrate more than marginal amounts of fixation of dinitrogen by free-living organisms confirms the source of the N as being the indigenous organic matter.

A major difficulty in elucidating the mechanisms involved has been the age of the field experiments available to work in. All of these were about 15 years from planting, whereas the effect begins to materialise at about 8-10 years. Once the effect has started, these mixtures are clearly aggrading systems within which the ecological interactions of different organisms and micro-environments stimulate more favourable conditions. It is difficult, therefore, to pinpoint the initial mechanisms. What are the basic differences between these pure and mixed stands at age 6-8?

Current hypotheses consider the case of access to intractable native N by different tree species, possibly through symbioses with different mycorrhizal fungi; the limitation of available carbon to mineralization processes and the influence of the differential rooting patterns on aeration and exploration of the uppermost soil horizons. Experimentation on these and other aspects is in progress.

Conclusions and silvicultural implications
Enhanced growth of tree species in mixture has frequently been claimed in the silvicultural literature, particularly from studies of mixed conifer-leaf tree stands in Europe. Very few of these references to the benefits have been based on detailed comparative studies of equivalent pure and mixed stands and have largely arisen from observation, often with an element of bias. The nursing effect of pine on spruce was observed as early as the 1890's in Danish plantations and then repeatedly in upland Britain as afforestation practice attempted to extend spruce on to ever more difficult sites. Initially the effect was thought to be a matter of overcoming competition from Calluna but the extension of spruce onto degraded sites of low N turnover has demonstrated that the 'nurse' species is able to enhance the nutritional status sufficiently to permit adequate growth of spruce. The effect is not dissimilar to he changes taking place in natural stands through succession.

Even if the mechanisms involved are not yet fully understood, it is clear that there is a benefit from mixing pioneer species with spruce. The benefit silviculturally is in successfully raising more productive stands without the need for continued inputs of expensive N fertilizer and possibly the avoidance of chemical control of Calluna. The realisation that the nursing effect did exist has led to large areas of oligotrophic sites being planted with Sitka spruce in mixture with lodgepole pine on deep peats and Scots pine on heathland. These areas are often relatively remote and often prone to windthrow, so are unlikely to be thinned. The aim is, therefore, to achieve high growth rates of spruce, suitably pruned by the pine until the spruce suppresses the pine to leave a mainly pure spruce stand at the end of the rotation. To achieve this will involve careful assessment of the early growth rate of the 'nurse' and its spatial distribution in the mixture.

Finally, these studies have illustrated the principle that to understand the productive functions of a forest, it is necessary to investigate the whole system and not simply to rely on mensurational data, necessary though these are. By attempting to resolve the mechanisms of the mixture effect, these studies have improved our general understanding of the forest ecosystem and opened up questions which might not otherwise have been asked.

Acknowledgements
This paper has drawn on the results of several investigators, much of whose work is still unpublished and I am grateful to Clare Alexander, John D. Miller and Berwyn Williams of the Macaulay Institute for Soil Research, Professor Hugh Miller of Aberdeen University and Helen McKay, Mark Campbell and John Morgan of my own Department; also to Clive Carlyle now of CSIRO, Australia. The Forestry Commission freely made available their field experiments for the work described.

Table 1 Foliar N concentrations in Sitka spruce in pure and mixed stands in Scotland (% ODW)

Location	Age	SS-N	SS + larch	SS + pine
Mabie	13	0.98	1.41
Inchnacardoch	15	0.80	1.48	1.79
Culloden	13	0.77		1.20

Larch was Hybrid larch at Mabie and Japanese at Inchnacardoch
Pine was lodgepole at Inchnacardoch and Scots at Culloden

Table 2 Above-ground accumulated N in pure and mixed stands in Scotland

		kg ha-1
		Foliar		Total a.g. N
Location	Age	pure	S mixed	pure	S mixed
Mabie	13	54	87	n.d.	n.d.
Inchnacardoch	17	38	JL 72 LP 132	68	JL 148 LP 228
Culloden	15	15	103	24	169

Mixture species as in Table 1.