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50 EUEClRGEN: Quercus suberNEtWClRK •completely felled after the civil war. Populations with older trees - Cuenca del rioUlla, Potes and Hoyo de Manzanares - are forests in which firewood extraction wasless intense or more ancient. Other circumstances, such as cork exploitation orinaccessibility, also allow for the preservation of old trees.Figure 2 represents the demographic structure of each stand. Withoutconsidering regeneration, all of them show a trend to ageing, with the density ofdiameter class I being lower than the density of older trees. In some populations,such as Haza del Lino, Foncastin and Potes, this trend is counteracted by a go'odregeneration of cork oak. An analysis of survival dynamics would be needed. Inother cases, the situation is worsened by very low reproduction rates - Sierra deCarrascoy, Hoyo de Manzanares and Pinet. In these populations the density of corkoak regeneration is lower than the density of trees in the lower diameter classes,and thus the replacement of adult trees is not assured. Absolute data show thesituation more clearly: in the stand Pinet, 11 young cork oaks were found(4 seedlings and 7 sprouts) on a total sampled surface of 2512 m 2 • In Hoyo deManzanares, 3 seedlings and 1 sprout appeared on 3140 m 2 • In Carrascoy, only oneyoung cork oak individual was found. Besides, our data generally overestimate thenumber of cork oaks, since the sampling was made around the Q. suber stands,where seedlings are more abundant than in plots without the species.Different reasons explain the low reproduction rate of cork oak in eachpopulation. In Hoyo de Manzanares, adult trees were healthy and had goodfructification, but germination was limited by the edaphic conditions: granites andsands, with steep slopes where soil evolution is difficult. In Pinet, oaks are rathershort (less than 10 m), and have suffered from recent fires (the..last one in 1991);trees showed a great number of new shoots, but acorn productioh is very scarce. InCarrascoy, where only 10 cork oaks appeared, all the trees "were affected bydrought, with tip decay and symptoms of a Hypoxylon mediterraneum attack. Smallquantities of acorns are produced but, even if some of them germinate, they can notresist the drought.In Table I, all studied populations are classified according to the rate density ofregeneration/ density of mature trees.Degree of dominance of cork oak (Table 2) is explained on the basis of differentfactors. In general, when cork is exploited, the species is favoured. When cork oakis dominated by other species, it can be due to a lack of interest in management (alow number of oaks, for example), or to limiting environmental conditions. Recentfires lead to the dominance of Q. suber because of its resistance to this event whilethe rest of the trees are burned.The percentage of Q.sllber seedlings compared with other species showed atrend in the decrease of cork oak dominance in all populations, with this trendbeing most pronounced in the populations Sierra de Carrascoy and Hoyo deManzanares, where the ratio is about 2% (Table 2). Percentage of decrease alsopoints to these two populations as the most regressive ones.Table 3 shows the tree species in each population with regard to diversity. Theproportions of the number of trees of each species are presented in Figure 3. In thisfigure, values of Margalef index for diversity are also indicated. In general, highfloristic diversity was found, since environmental conditions allow not only theexistence of cork oak, but also the presence of other species, some of them havingmarginal character: for example, Q. pyrennica and Q. ilex in Potes, Q. jaginea inSestrica, or Pinus pinaster in Bozoo and Pinet.
PRESENTATIONS 51Table 1. Density of regeneration of cork oak (Or), density of mature cork oaks (Oa) andratio between them (R) (for abbreviations relating to the stands studied, see Fig. 1)GR VA S CS BU C/PO Z M MU VDr 1175.16Da 420.38R 2.79391.7 748.41 436.31 429.94 171.18 95.54226.1 545.38 544.59 553.34 242.83 3281.73 1.37 0.8 0.77 0.7 0.2912.73 7.96101.9 63.690.12 0.1243.79453.820.096Table 2. A: Percentage of mature cork oak trees with respect to the number of maturetrees of ali species in the plot. B: percentage of juvenile cork oak trees with respect toregeneration of all species in the plot. (A-B): difference of the % rates. P: loss ofdominance in % expressed as: [(A-B)/A] x 100 (for abbreviations relating to the standsstudied, see Fig. 1)GR S VA BU V Z C/PO CS MU MA 100 428 73.7 30A-8 26.3 12P 26.3 28.547.726.421.344.636.4 85.717.6 3818.8 47.751.6 55.667.6 38.429.2 13.438.4 2556.8 65.189.5 24.230.5 259 22.265.9 91.728.32.126.292.5Table 3. Tree species present in the plots of each population, according to theirdominance80zoo (BU) Q. suber (OS), Pinus sylvestris (PS), P. nigra (PN), Q. ilex (01),P. pinaster (PR), Arbutus unedo (AU), Q. pyrenaica (OPy), Q.faginea (OF), Q. petraea (OP)Cuenca del Ulla (C/PO)Q. robur(OR), Q. suber(OS), P. pinaster(PR), Q. pyrenaica(OPy), Arbutus unedo (AU), Pyrus cordata (PC), Castaneasativa (CS) -Potes (S)S. Carrascoy (MU)Foncastin (V A)Sestrica (2)Hoyo de Manzanares (M)Pinet (V)S. Calderona (CS)Haza del Lino (GR)Q. suber (OS), Arbutus unedo (AU), Q. ilex (air, Prunus avium(PA), /lex aquifolium (lA)'"Q. ilex (01), Q. suber (OS), Pinus halepensis (PH)P. pinea (PP), Q. suber(OS), Q. iJex(OI)Q. suber(OS), Q. ilex (01), Q. faginea (OF), P. pinaster(PR)Q. ilex (01), Q. suber (OS), Juniperus oxycedrus (JO)Q. suber (OS), P. pinaster(PR), Q. ilex (0/), Arbutus unedo(AU)Q. suber (OS), P. pinaster(PR), Arbutus unedo (AU), Q. ilex(0/)Q. suber (OS)The degree of tree diversity in each stand is explained by climatic conditions andby management of the forest. Northern populations - Bozoo, Cuenca del UUa,Potes - were most diverse, because they gather Mediterranean species together withdeciduous trees. Besides, hvo of them (Bozoo and Cuenca del Ulla) were afforestedwith pines in the past: P. pinaster in Cuenca del Ulla and P. nigra and P. sylvestris(already present before afforestation) in Bozoo. In both populations, the highestnumber of species was found in plots with these pines. The effect of reforestationresults in an increase of other trees, either due to the establishment of suitableconditions for their growth, or to the measures aimed at protecting the plantation(mainly from cattle grazing). The latter measures also favoured spontaneousvegetation in the observed plots.
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