The growth, production and nutrition of Rubus fruticosus L. agg. in woodlands

This study examined patterns of growth shown by Rubus fruticosus L.agg. andndash; the bramble andndash; and looked at factors which controlled its growth. Literature on bramble ecology that was relevant to British conditions was reviewed. The study took place in Wytham Woods, a mixed, mainly deciduous wood. The soils varied from shallow limestone soils to heavy clays. Rubus vestitus Weihe and Nees was the most common bramble species.
Three sites were chosen ranging from one where bramble growth was very poor to one of the most productive bramble areas in Wytham. The changing distribution of dry weight within a bramble stand over a year was followed by splitting the plant into its components; leaves, laterals, canes of different ages.
The bramble stands were sampled on ten occasions during 1974 at four to six week intervals. All above ground material was removed from ten one metre-square plots from each area, at each sample time.
The overall pattern of dry weight change was the same for the three areas. Total biomass showed a summer peak, the result of three processes:-
The dry weight increase by first-year canes.
The initial increase and subsequent decline in weight of the second-year cane system because of growth and die-back of the laterals.
The final die-back of the cane system that had flowered the previous year. Standing dead, dead canes still attached to the rootstock and separate from the litter-layer andndash; showed no seasonal change in dry-weight.
The dry weight increase by the laterals was of the same magnitude as that by canes. The main cane growth however was 5-6 weeks later than that of the laterals. Thus lateral leaves formed the bulk of the early summer canopy while cane leaves became more important in autumn and over winter.
Leaf area per plot was estimated from direct measurement of a sample of known dry weight. The winter bramble canopy was only half that in summer. Summer leaf-area-index were 0.8 in the poorest bramble site measured, 2.1 in the best.
The main growth of canes and literals was by increase in length. Total stem length per plot showed a summer peak.
Variations in both plant size and plant density (numbers/sq.m) caused the dry weight differences between the areas. Tree competition appeared to control these differences, the best growth occurring under an ash canopy, the worst under an oak/sycamore one.
Bramble biomass in Wytham was up to seven times greater than that reported for other sites in Britain. Published values summer biomass of bramble in Britain range from 30-450 g/sq.m and annual turnover of dry weight from 50-300 g/sq.m/yr. This annual dry weight turnover is less than that for open-grown, herb communities, but similar to that for herb and shrub layers in British woodlands. Not all of this dry weight turnover represents currant photosynthesis because there are transfers between roots, canes and laterals.
Over the year the bulk of the biomass was in the second-year cane system. The turnover time for the standing dead was 1-2 years. Cycling of material in a bramble stand was thus faster than for woody shrubs.
A few rootstocks were excavated. The sizes of the root crowns matched the differences in shoot growth between areas. Crowns ranged from 5-60mm in diameter and may act as storage organs. The size and spread of the major roots, 4-6 per crown was also greatest in the areas of greatest shoot growth.
Records of flowering and fruiting were kept both per unit area and per cane. Flowering was less in areas of low vigour of growth, which were usually the most shaded. A direct correlation between length of laterals and flowering success was found by comparing flowering on regrowth canes, or defoliated canes, with the control canes for that area. Length of lateral and flowering success depended on the overall vigour of the cane and the laterals' position on it. Laterals on side branches and the terminal portions of canes were shorter and flowered less.
The process of tip-rooting was investigated. Root-boss formation increased when the tips of growing canes were covered by black fabric bags, but not when clear polythene bags were used. This confirmed the results of others that root-boss formation is increased if the stem apex is in the dark. Although defoliating canes reduced their growth it did not increase the number that tip-rooted.
The growth of the daughter plants was followed. The parent cane was cut at different times after the original tip-rooting and leaving different lengths of cane attached to the daughter plant. Transfer of material to the daughter plant was shown to start in autumn and continue until the following July. The material was largely derived from the metre of cane nearest to the daughter plant. Reverse transfer, from daughter rootstock to parent cane can occur. Sections of parent cane separated from their parent root remained alive and bore laterals if attached to the daughter plant. Competition for cane reserves between laterals and daughter plants may occur, as in one experiment lateral growth was greater on canes separated from their daughter plants than where a daughter plant was present.
Tip rooted canes were more abundant in areas of poor bramble growth than in vigorous areas. Such low vigour areas had a poorly developed stand structure such that cane tips were more likely to touch the ground.
Vegetative and floral reproduction were complementary within the areas considered. Flowering increased and tip-rooting decreased as the canes became more vigorous.
Non-destructive measurements WPTB investigated as a means of recording bramble growth. Some measurements were made on individual stems, but the main emphasis was on plot based measurements. These were calibrated against destructive sampling.
Point-quadrats were used for leaf-assessment. Changes in leaf-hits per plot agreed well with variations in measured leaf weight and area. agreement was better for new leaves than for those which survived over winter, because the former were more horizontal.
Mean canopy height, derived from the heights of leaf hits rose in the summer during lateral growth and declined in autumn, partly because of lateral die-back from the tips, partly because of settling of canes under their own weight.
A dense bramble stand was found to have three layers; an upper leafy zone containing the bulk of the current canopy; a layer in which are the second year canes and the remains of the previous years' canopy; the leafless lower layer containing most of the standing dead. Each year's canes grow through the lower layers and come to lie horizontally as a result of their own weight and that of their laterals the subsequent year. In following years these canes are forced lower down the structure.
Intersection values were used to estimate stem quantities. These were a variant of Buffon's Needles technique, adapted by Newman (1966) for root length determinations. The number of stem intersections with rods placed across a plot was related to total stem length per plot. Stems differed greatly in-angle which caused variation in the relationship particularly with the taller bramble stands.
Intersection values multiplied by height were well correlated with plot dry-weight, r2 = 0.8. Regressions of dry weight on adjusted intersection values showed little difference for different times of the year.
These non-destructive methods were used to assess bramble stands which were too limited in area for repeated destructive sampling.
Part of the growth of first-year canes is based on rootstock reserves part on photosynthesis built up by the cane leaves. To separate these two, leaves on first year canes were removed as they formed. This reduced cane growth by 20-50% relative to the controls. The reduction was greatest in areas of low vigour, which had the smallest rootstocks. The relatively smell surface area of the canes made it unlikely that stem photosynthesis made a large contribution to growth of the defoliated canes. A large part of their growth was presumably therefore on rootstock reserves.
Autumn defoliation of first-year canes caused the same reduction of subsequent lateral growth as defoliation from the start of cane growth. This implied that the main build up of cane reserves was from September onwards. Defoliation of the laterals themselves reduced lateral growth by about 50%, the least reduction being found on the thickest canes, which had the highest reserves.
Regrowth of bramble, after removal of top growth, was measured in two 100 sq.m areas. In first-year regrowth cane lengths were about 65% of those in the second-year regrowth which were equal to those in uncleared bramble. Lateral and leaf production in second-year regrowth were less than in the uncleared bramble, but by the third year the differences were very small.
Variations in bramble dry weight over the sampling areas were related to the presence of different tree species in the overstorey. Two surveys of the vegetation, by Field and Dawkins (In prepn.) and Lloyd (unpubl.), allowed further investigation of the relation between the nature of the overstorey and the bramble growth beneath it. Greatest growth in Wytham occurred under ash, least under sycamore or beech. Oak was intermediate. A shrub layer reduced bramble growth under an ash canopy, but had no effect on the amount of bramble under sycamore.
Non-destructive methods were used to compare growth of bramble under gaps with that in the surrounding shade. Under old gaps growth was 5-6 times that under the tree canopy. Gaps 7m across were created to determine the rate of response to a change in the overstorey. The dry-weight of bramble under the gaps, estimated by non-destructive methods, almost doubled in the year following the creation of the gap. A similarly large increase occurred in the next year.
The concentrations of magnesium, calcium, phosphorus, potassium and nitrogen were determined for leaf and stem material collected over the year. Potassium, magnesium and calcium were measured using a spectrophotometer after dry-aching to remove organic material. Phosphorus was determined by the molybdo-vanadate yellow test. Nitrogen was determined, after micro-keldahl digestion of samples, by the indophenol blue test.
Calcium concentrations were higher, potassium and nitrogen levels lower, in material from the most productive of the three bramble sites. This area had the highest soil pH and so probably also the greatest calcium availability. The lower potassium and nitrogen concentrations were probably caused by greater carbohydrate production "diluting" the minerals present. Absolute weights of all five elements in the bramble biomass were greatest in the most productive bramble area.
Falls in the concentrations of nitrogen, phosphorus and magnesium in the second-year canes, during lateral growth, indicated transfer of material to the laterals. The bulk of the mineral requirement for the laterals came however from fresh transfer to the second-year cane system from the roots. Total transfer of minerals from the roots to the second-year cane system was similar to that taken up by first-year canes, but about 60% of first-year uptake occurred after the maximum was reached in second-year canes. Total weight of minerals in the above-ground biomass reached a peak in the summer.
Falls in the mineral concentrations during die-back indicated that some minerals were withdrawn from stems and leaves prior to their death. The weight of minerals in the standing dead was small compared to the amount in the live shoots so that return of minerals to the rootstocks or to the litter must have been rapid.
The starch concentration of the dried stem material was determined by using the iodine-complex formation. Starch built up in first-year canes in the autumn after their extension had ended. There was a further rapid rise in starch content just prior to lateral growth beginning. This indicated that reserves were laid down in the autumn, but also that photosynthesis by the cane leaves in the spring was important. The starch content of the canes fell during lateral growth.
A similar spring peak of starch content was found in the third-year canes and their laterals. This declined as die-back proceeded, probably because of withdrawal of material to the rootstocks.
This study was the first to measure growth of bramble on an example of the productive sites found in many woods in Britain. The growth pattern found was similar to that for the less productive sites described elsewhere. The partitioning of resources within the plant was considered in various ways: first-year cane growth vs. lateral growth; transfer growth vs. current photosynthesis; floral vs. vegetative reproduction. Quantitative measurements were made of the bramble's response to tree competition particularly the nature of the tree canopy. From information on the internal and external factors which control growth an attempt was made to explain both the differences in growth between sites in Wytham and those between Wytham and other woods.