THE DIFFERENTIATION AND FATE OF THE BRACKEN (PTERIDIUM AQUILINUM) FROND AND THEIR RELATION TO THE AGE‐STRUCTURE OF THE SHOOT AND FROND POPULATION

Abstract

On an acid, dry infertile soil in a frost pocket, there are more short shoots (potential fronds) in bracken than differentiated fronds, and more differentiated fronds than emerge from the soil, and more emergent fronds than live in August.Factors affecting the inception, differentiation and fate of fronds are examined. The inherent tendency of fronds from young shoots to develop and to emerge before those from old renders them more liable to lethal winter and spring frosts and to crippling spring frosts. Drought acts mainly on the late emerging fronds. The normal sequence of emergence may be upset.Through their destruction of reserves and their effect on the means of replenishing them, annually fluctuating frost and drought influence the number of shoots initiated yearly, and hence the age-structure of the shoot population. A senescent population of bracken is deflned as one in which the proportion of young to old shoots is lovi-, and fronds from young shoots to fronds from old is also low. Frost and drought are the causes (not necessarily exclusive) of senescence in this area.The validity of the generalizations by Braun (1851) and by Hofmeister (1862) on the number of fronds produced per annum on the shoot of the mature plant, and the duration of the period from the first visible appearance of the frond to its emergence from the soil, have already been called in question (Watt, 1940). Further evidence on these matters and on departures from the 'normal' behaviour of bracken is provided by data from a plot on Lakenheath Warren, on a very acid podsolized sand (pH 3.6-4.0) (area E in Watt, 1940). METHODIn a marked plot 2 x 2 ft (0.37 m^) the fronds, on emergence or at convenient times thereafter, were charted (Fig. i) and identified by numbered rings in three successive years, 1956, 1957 and 1958. The plot was then dug up on 7 August 1958 and the shoots and their number of nodes counted. In this technique, despite the care exercised, there is the possibility of mistakes in identification, where marked fronds, killed by frost whilst still young, then decayed and left the ring above the remains but separated from them. There is also the purely physical difficulty of disentangling from the grass-rootpermeated soil the ultimate shoots without detaching them from their parent short shoots. RESULTSThe data in Table i substantiate earlier findings (Watt, 1950): the small number of live fronds in August (16.7 per 10 ft^: 18.0 per m^-numbers which are close to the mean 75

CONTRIBUTIONS TO THE ECOLOGY OF BRACKEN (PTERIDIUM AQUILINUM)

“…But in other parts of the plant and particularly in the distal, the high correlation between the sectional lengths of the lamina of the previous year and the lengths of the current petiole points to restricted movement of food. This is supported from general experience in the distal region and the hinterland, where treatment of various kinds, the addition of litter, protection by cages from frost, pits dug and refilled, localized trampling and, of course, the abrupt change from short to tall fronds in frost pockets, in rings and in 'depressed' areas (Watt, 1956) all suggest localization of effects. We may thus conclude that the effects of frost in the distal region are not transmitted to the proximal and vice versa, and similarly in the distal region the effects are not transmitted outside the branch system to which the affected fronds belong.…”

“…Thus, shallower soil in the distal region could result in a fall in the height of the frond. Tests at 3 m intervals alongside each transect showed no essential change in soil depth, a mean of i m in area D with a minimum of 0.8 m, and in area E a variation in depth to the B horizon from 0.48 to 0.58 m-a range insufficient to account for a fall to half the height (Watt, 1964). Also in the distal region there is a fall in length of petiole irrespective of age, i.e.…”

“…In the contrast between areas D and E (Watt, 1956) frost as a differentiating factor emerged as important. The greater damage in area E was related to earlier emergence and this in turn to the smaller amount of litter produced.…”

“…The data on these relationships with age are accommodated by the hypothesis already adumbrated (Watt, 1967) on the mechanism of control of the age-structure of the frond population and factors affecting it. It depends on the priority of emergence of fronds from young shoots over those from old, and factors like frost and drought operating selectively against them at different stages in their development.…”