Foliar cation contents of laurel forest trees on the Canary Islands

Flora (1996) 191 303-311

© by Gustav Fischer Verlag Jena

Foliar cation contents of laurel forest trees on the Canary Islands K. KOHLI, R. LOSCH I,3, A. M. GONZALEZ-RODlUGUEZ 2, M. S. JIMENEZ 2 and D. MORALES2 1 2

3

Abt. Geobotanik, H. Heine Universitat DUsseldorf, Universitatsstr. 1126.13, D-40225 DUsseldorf Dpto. Biologia Vegetal, Univ. La Laguna, E-38207 La Laguna, Tenerife, Espana Autor for correspondence

Accepted: January 13, 1996

I

Summary The leaf cation contents of 15 different tree species of the laurel forest on the Canary Islands were measured. Different developmental stages of leaves were compared to estimate the effect of leaf age on the cation content. Young expanding leaves showed quite high potassium values when compared with mature leaves from the previous year and with senescing leaves. The opposite trend, accumulation with age, generally occurred with Ca, Mg, Fe, Mn, and AI, but with species-specific intensity. The levels of the investigated elements varied considerably between the species; exceptionally high nutrient levels were found in Viburnum rigidum. The cations in the leaves from laurel forest trees were similar to the reported ion contents in leaves of tropical trees, both being higher than those in leaves of temperate forest trees. In most species of the laurel forest the aluminium contents were between 10 and 30 mmol kg- 1 dw. This was far above the Al contents of most other terrestrial plants and pointed to a high Al availability in the soils and a high foliar Al tolerance. Accumulation of Al- and to a lesser extent Fe and Mn as well - could be considered a consequence of primitive traits of the coriaceous leaf type like longevity and poor transpiration control. Key words: Laurel forest, cation content, aluminium accumulators, coriaceous leaf, temperate evergreens forests

1. Introduction The Macaronesian laurel forest is an evergreen treedominated vegetation under a virtually annual perhumid climate and moderate temperatures. The forest is restricted to NE-oriented mountain slopes of the Canary Islands, Madeira and the Azores (CEBALLOS & ORTUNO 1976; KUNKEL 1987). It has structural relationships with the Mediterranean sclerophyllous vegetation (GooDALL 1983) and with tropical mountain forests (HUBL 1988; LOSCH & FISCHER 1994). Phylogenetically this vegetation dates back to the laurophyllous vegetation which during the Tertiary covered the northern parts of Africa (AXELROD 1975) and the land region Western to the Tethys Ocean (CIFERRI 1962). Approximately two dozen tree and shrub species of systematically quite different location built up this forest. The flora (BRAMWELL & BRAMWELL 1974), structure (e.g. DANSERAU 1968) and phytosociology (e.g. GONZALES-HENRIQUEZ et al. 1986) have been investigated in detail. However, little is known about the

ecophysiology of this ecosystem. Initial sporadic studies about temperature resistances of the trees (LOSCH 1980; LARCHER et al. 1991) and their water relations (HOLLWARTH & KULL 1979; LOSCH 1993) now are followed by in-depth studies of the laurel forest of Agua Garcia! Tenerife (MORALES et al. 1996a, b). They aim to quantify energy and matter fluxes through this forest, which is mainly composed by Laurus azorica, Persea indica and Myrica faya. Leaf nutrient contents were determined monthly in the three main tree species of this forest (GONZALEZ-RODRIGUEZ et al., submitted). From these data and biometric data of leaf numbers per tree, crown structures and related informations (MORALES et al. 1996a), mineral contents of the canopy of this forest were scaled up. The data of Laurus, Persea and Myrica showed obvious species-specific differences of leaf mineral contents. As the detailed study in the Agua Garcia forest included only three tree species at one location, it seemed necessary to extend the studies to most other tree species in laurel forests of Tenerife and to sample at FLORA (1996) 191

303

Table 1. Investigated species including abbrevation (Abbr.) and number of trees sampled for each location (A = Anaga mountains (Cruz del Carmen, El Bailadero and Taganana), 1= Tigaiga (Los Realejos), T = Teno mountains (Barranco de las cuevas negras) and number of leaf samples per age group (Age: y = young, m = mature and s = senescent). Family

Localities

Abbr. Species

A

I

T

y

m

s

Caprifoliaceae Sc Sambucus palmensis CHR. SM. Vr Viburnum rigidum VENT.

0 3

3 3

0 0

3 5

3 6

0 1

Myricaceae Mf Myrica Jaya AITON

5

6

7

0

Rhamnaceae Rg Rhamnus glandulosa AITON

6

0

0

5

6

0

Aquifoliaceae Ic flex canariensis POIRET Ip flex platyphylla WEBB & BERTH.

4 4

1 0

1 0

6 4

6 4

0 0

Rosaceae PI Prunus lusitanica (WILLD.) FRANCO Bc Bencomia caudata (W. & B.) AITON

1 0

0 0

0 1

1 0

Oleaceae Pe Picconia excelsa (AITON) DC.

Age

2

0 0

4

4

Lauraceae La Laurus azorica (SEUE.) FRANCO Of OcoteaJoetens (AITON) BERTH. Ab Apollonias barbujana (CAV.) BORNM.

4 2 7

2 1 0

1 0 3

6 2 6

7 3 10

1 0 2

Myrsinaceae Hb Heberdenia bahamensis SPRAGUE Pc Pleiomeris canariensis (WILLD.) DC.

4 2

0 0

0 8

2 3

4 10

0 0

Theaceae Vm Visnea mocanera L. fil.

3

0

2

4

5

0

different locations. This permits a broad characterisation of leaf ion contents of the laurel forests and comparisons with those of other forests, including evergreen tropical as well as deciduous forests in temperate regions. Furthermore, species-specific patterns of mineral ion accumulation may become evident which could be enigmatic for individual species.

2. Material and methods Expanding (= young), mature (= sprouted during the previous year) and senescent (= more than two years old, with signs of senescence) leaves were sampled in March 1992 from IS different tree and shrub species of the laurel forest in various regions of Tenerife. Species, family relationships, and sampling places are listed in Table 1 ; different bedrocks are found at the different sampling places (CALDAS et al. 1982). At the different localities 10-20 leaves from the available trees/shrubs were sampled and mature leaves and whenever possible young and senescent leaves were collected. 304

FLORA (1996) 191

The leaves were air-dried and stored in paper bags. In the laboratory leaf area (area meter Li3100, LiCor, USA) and dry weight (oven-dry, 60°C) of the samples were determined. The dried material was ground in a ball mill, dried to constant weight (48 h, 60°C) and digested with nitric acid (3 ml 65% HN0 3 p.a. per 100 mg ground material) for 7 h at 180°C under pressure. The residue was diluted with double des tilled water, filtered in volumetric flasks and made up to 50 ml with doubledestilled water. The ion contents were determined by ICP-AES (Plasma II, Perkin Elmer, FRO). The quality of the digestion procedure and the analysis was controlled by digesting and analysing one sample of reference material (Standard reference material No.1573, Tomato leaves, National Bureau of Standards, Washington) and one blank sample per 22 samples.

3. Results Ion contents of leaves at different stages of their ontogenetic development differed considerably in many species. An increase in mineral contents during the life

~

900

"I

800

300

Potassium

Magnesium

u5: 700 CJl 600

~

500

o

~

o

400

300 200 10c)

o D

E

E

~

I I~

young

_

mature

~

I

~

senescent

Fig. 1. Potassium content in leaves of different age groups and different species (All abbreviations are as in Table 1). Mean and standard deviation, number of samples per age glass as in Table 1. 900 'I

Calcium

800

u5: 700 CJl

600

~

500 o 400

E 300 E

'-' 200 o 100 ~

U

o D

Ir young

~

~ _

I

mature

~

r

f

b§J senescent

Fig. 2. Calcium content in leaves of different age groups and different species. Mean and standard deviation, abbreviations and number of samples per age class as in Table 1.

span of the leaves was apparent in nearly all measured cations. Potassium (Fig. 1) was the only exception: as in most other species, in nine species of the laurel forest, potassium content was on average 1.5 times higher in young leaves than in leaves from the previous year. In Pleiomeris canariensis and Sambucus palmensis the potassium content of juvenile leaves was almost 3 times higher than that in mature leaves. Only with Ocotea foetens, Heberdenia excelsa and Prunus lusitanica, there were no obvious differences. While potassium is mobile in phloem, calcium accumulates steadily during the lifetime of a leaf because it is transported to the apex by the transpiration stream through the xylem, but is not retranslocated through the phloem. It follows that in all investigated species from the laurel forest, the calcium contents of mature leaves were higher than those of the young leaves and reached their peak in senescent leaves (Fig. 2).

100

r

o D

young

_

~ I~

I

I

~

mature

I I

I

b§J senescent

Fig. 3. Magnesium content in leaves of different age groups and different species. Mean and standard deviation, abbreviations and number of samples per age class as in Table 1.

An increase in the leaf ion contents with age was shown also for the other cations in most of the investigated species, The magnesium levels (Fig. 3) in the dark-green, mature leaves were about 50 percent higher than in the light-green juvenile leaves. This increase may be due to the rise of the chlorophyll content. The same pattern was obvious for the investigated micronutrients. In many species the ion levels of the mature leaves were much higher than that in young leaves. Exceptions from this generally observed accumulation pattern were recorded for Prunus lusitanica, Ocotea foetens and Picconia excelsa that showed substantially higher iron and manganese contents in young leaves than in mature leaves (Fig. 4). This decrease cannot be explained by leaf or tree individuality, as several young and mature leaves have been collected from the same tree and as (with the exception of Prunus lusitanica) several trees were sampled. In the other species, iron and manganese contents of mature leaves were considerably above the micronutrient levels in young leaves (Fig. 4). A particularly obvious example of Fe accumulation was found in Laurus azorica, in which the iron content increased from 3 to nearly 10 mmol kg- 1 dw in senescent leaves. In Viburnum rigidum, the Fe content rose from 3.5 (young leaves) to 9.5 mmol kg- 1 dw in mature, respective 7.5 mmol kg- 1 dw in senescent leaves. The manganese contents increased from 0.5 to 1.5 mmol kg- 1 dw in Pleiomeris canariensis and from 5 to 12 mmol kg- 1 dw in flex platyphylla, e.g. the manganese content of mature leaves was three times as much as that of the expanding leaves. While in the first species (P. canariens is) the absolute Mn level was quite low, flex attained very high Mn contents by accumulation. Aluminium was also accumulated in a number of the investigated species, particularly in both flex species and FLORA (1996) 191

305

----------

40

Aluminium

,3:

30

-0

a>

-Y

20 0

E E

10

0

u

I c

"-

OJ

UJ :> ;>: ct

,-

u I-j

II Q

I-j

~

Q

U

m

QJ Q

(IJ

-J

~

"- .[J .[J u E' 0

15

Iron

3: -0

a>

10-

-Y

0

E E

,-

5

0 15

Manganese

3: -0

10

a> -Y

o

5

E E O~~~~3LUL~~~~~~UL~~LdLD.u

UC"-OJUQ~UQJ(IJ"-.[J.[JUE' I-j I-j Q Q -J 0

m

UJ :> ;>: ct

D

young

_

mature

r

~

senescent

Fig. 4. Aluminium, iron, and manganese content in leaves of different age groups and different species; note different y-axis in upper graph. Mean and standard deviation, abbreviations and number of samples per age class as in Table 1.

in Viburnum rigidum, the latter being prominent for the Fe levels in the leaves too. The average Al content of leaves was three times higher than that of Fe and five times higher than that of Mn. While the accumulation of individual elements varied considerably between the species and leaf age groups, the sum of the cation contents, i.e. of K, Ca, Mg, Fe, Mn and AI, were less different between species and age groups (s. Table 2). For most species, the total cation content ranged between 500 and 700 mmol kg- 1 dw. The sum of aluminium and the micronutrients iron and manganese reached levels of between 10 and 30 mmol kg- 1 dw. However, some species had distinctly higher total cation contents, i.e. the two Myrsinaceae (Heberdenia and Pleiomeris) and Rhamnus glandulosa approached or exceeded a total of 1 mol kg- 1 dw macronutrient cations in mature leaves. High total cation 306

FLORA (1996) 191

contents were also found in mature and senescent leaves of the Caprifoliaceae Viburnum rigidum. In Viburnum rigidum as well as in the two Aquifoliaceae (!lex spec.), the total of Fe, Mn and Al was about twice as high as in the leaves of the other species. Mature leaves of Myrica faya nearly reached these high levels of micronutrients plus aluminium. However, the total ion contents and Fe+Mn+AI-Ievels in the leaves of the Caprifoliaceae Sambucus palmensis do not deviate from the levels of most of the other species. So, a particularly high cation accumulation does not seem to be a family-specific feature of the Caprifoliaceae. The totals of micronutrient contents of juvenile Picconia excelsa and Ocotea foetens leaves were higher than those of the mature leaves. It may be that the freshly development leaves lack an efficient control for the entrance of these metal ions into their tissues. During maturation, a dilution or an absolute decrease in the initially very high Mn, Fe, and Al ion contents by export may occur. Differences in ion uptake, which depend upon the local supply, were not very marked. In Fig. 5 the leaf cation contents of mature leaves are arranged according to the sampling sites. It may be inferred that stronger accumulators have high ion contents regardless of the origin of sampling. The Al and Fe contents of Viburnum rigidum leaves from the Anaga and the Tigaiga site, for example, exceeded those of the other species, e.g., this was species-specific and not site-specific. On the other hand, the Ca and Mg contents of Laurus azorica leaves were remarkably low in the Anaga and Tigaiga samples. The same was true for the levels of K, Al and Mn contents of Picconia leaves from all three regions of Tenerife when they are compared with co-occuring species. The exceptionally high Ca content in the leaves of this Picconia species from the Anaga mountains, which does not agree with the much lower values measured elsewhere, requires additional measurements, as it was based only on the sample from one tree. Also, there was no apparent relationship between the accumulation pattern of different species and their occurrence on geologically different sampling sites. The mountain areas of Anaga and Teno differ in the chemistry and structure of their bedrocks and soils because they date back to independent volcanic events (CALDAS et al. 1982). Nevertheless, calcium accumulating species like Rhamnus glandulosa, Heberdenia bahamensis and Pleiomeris canariensis grew in the Anaga as well as in the Teno mountains; the same was true for the aluminium accumulators. This suggests that leaf ion contents of trees in the laurel forest of the Canary Island were governed by the species-specific peculiarities of the nutrient turnover, while the local supply was of minor importance for the element composition in the foliage.

'f""

""

Table 2. Total cation contents (means) and sum of AI, Fe and Mn content in mmol kg- 1 dw in young (y), mature (m) and senescent (s) leaves of different species. Species

cation concent

Fe+Mn+AI

m

y

s

y

m

s

1040

9 12

17 40

37

Sambucus palmensis Viburnum rigidum

960 710

560 870

Mf Myricafaya Rg Rhamnus glandulosa

440 920

590 1200

13 11

23 22

Ic Ip

/lex canariensis /lex perado

560 780

580 770

14 14

24 38

PI Bc

Prunus lusitanica Bencomia caudata

520

850 870

14

12 20

Pe

Picconia excelsa

620

670

850

19

11

18

La Laurus azorica Of Ocotea foe tens Ab Apollonias barbujana

580 570 540

502 620 600

710

17 23 9

15 17 15

23

Hb Heberdenia bahamensis Pc Pleiomeris canariensis

800 985

920 990

16 9

20 11

Vm Visnea mocanera

730

670

22

19

Sc Vr

560

13

HOO

., '" en

1200 1000

-0

800

..0{

600

0

JOO

E E

200

i

I

0 AIT

., $

AIT

AIT

AIT

AIT

AIT

AIT

AIT

AIT

AIT

AIT

AIT

AIT

AIT

AIT

II

50

JO

-0

en

30

0

20

..0{

E E

10 0 A I T

A I T

u (fJ

Element

'-

:0-

0

A I T

A I T D>

~~

a:

A I T

A I T

A I T

U I-j

A I T u (1J

A I T IJJ

Q

A I T

A I T ~

o

A I T .0


A I T

A I T

A I T

u

Q

K ~Ca mMg . A l ~Fe ~Mn

Fig. 5. Total cation content (upper graph) and sum of Fe, Mn and Al content (lower graph, 20times spread y-axis) of mature leaves from different species (abbreviations as in Table1) sampled on different localities (A =Anaga, I = Tigaiga, T = Teno). Mean. FLORA (1996) 191

307

4. Discussion The relationship between the content of an essential element in a plant and its growth may be described by an optimum curve: below a (lower) critical content growth is inhibited by deficiency and above an upper critical content toxicity becomes limiting. The interaction between a regulated ion uptake and growth keeps the contents of the elements between certain limits. As these critical contents vary between species, the range of elemental contents considering all cormophytic species is much wider than the range in a certain type of vegetation or for an individual species. The foliar potassium content of laurel forest trees is in the lower third of the very broad range from 80 to 1740 mmol kg- 1 dw in terrestrial plants (LARCHER 1994). Nevertheless, even the lowest potassium contents measured in mature leaves of Myrica faya, Picconia excelsa and Ocotea foetens are well above the critical potassium contents for trees: the critical K content ranges between 30 and 80 mmol kg- 1 dw for broadleaved tropical trees (DRECHSEL & ZECH 1991) and between 80 and 100 mmol kg- 1 dw for Picea abies (ULRICH 1975). Mature leaves of laurel forest trees contained, on average, slightly more potassium than the leaves of broad-leaved deciduous trees from temperate forests (130 to 380 mmol kg- 1 dw; RICKLEFS & MATTHEW 1982). In laurel forests, the potassium content of the foliage is higher than the foliar K contents in spruce of Continental Europe. Spruce needles typically contain less than 200 mmol kg- 1 dw (ULRICH 1975, KAZDA & ZVACEK 1989). This contrasts with the K contents oftropical broad-leaved trees as they fall into the same range as those of the laurel forest trees (DRECHSEL & ZECH 1991). The K contents reported for trees of early succession stages of tropical forests (WILLIAMS-LINERA 1985), on the other hand, average 770 mmol kg- 1 dw and are well above the values measured for the laurel forest trees. In all laurel forest species, the calcium content of mature leaves exceeded the lower critical values of 40-120mmolkg- 1 dw reported for broad-leaved tropical trees (DRECHSEL & ZECH 1991). Only young Prunus lusitanica leaves showed Ca contents below 50 mmol kg- 1 dw. The range of Ca contents in leaves of laurel forest trees resembles that of tropical trees, for which values of between 100 and 870 mmol kg- 1 dw have been reported (WILLIAMS-LINERA 1985, DRECHSEL & ZECH 1991, KAZDA & LOSCH 1992). Broadleaved deciduous trees from temperate regions show lower leaf Ca contents between 25 and 300mmolkg-1 dw (DRECHSEL & ZECH 1991). Spruce needles contain even less Ca than the deciduous trees (ZOTTL 1985, KAZDA & ZVACEK 1989). 308

The decrease in K content and the increase in Ca content during the leaf development results in a corresponding change in the KlCa ratio (KINZEL 1982). While in young leaves KlCa ratios of up to 4 were found, the ratios decrease to values about 1 during maturation. In some species, like Rhamnus glandulosa, Picconia excelsa, Pleiomeris canariensis, a KlCa ratio of less than 0.5 was found due to a considerable Ca accumulation. The low KlCa ratio is comparable to the values found in Crassulaceae, which are considered calciotrophic (KINZEL 1982). However, in the absence of data about the Ca solubility, it cannot be dediced, whether laurel forest trees belong to the calciotrophic physiotype that demands high Ca contents, or whether they precipitate surplus Ca as oxalate-salts. The Ca accumulating species Rhamnus glandulosa contains high amounts of Ca oxalate (HEGNAUER 1973). The range of magnesium contents in mature leaves of tree species in laurel forests resembles that of tropical trees. Broad-leaved deciduous trees show lower magnesium contents. The foliar magnesium contents of spruce (10 to 40mmolkg- 1 dw, KAZDA & ZVACEK 1989) is much lower than that of any of the tree species of the laurel forest. In some tree species of the laurel forest, the micronutrient contents are markedly high in comparison with the values of other terrestrial plants. The iron contents of mature leaves of laurel forest trees ranged between 4 and 9 mmol kg- 1 dw and are high when compared with the iron contents for terrestrial plants, which are reported to be 0.04 to 12 mmol kg- 1 dw (LARCHER 1994). The same was true for manganese: the foliar Mn contents of laurel forest trees - on average about 2 mmol kg-I dw with maximum values over 12 mmol kg-I dw - is higher than in most terrestrial plants (0.05 to 18 mmol kg-I dw, LARCHER 1994). However, high iron contents (0.7 to 12 mmol kg-I dw) and manganese contents (0.5 to 9 mmol kg-I dw) have been found in broad-leaved tropical trees (DRECHSEL & ZECH 1991), which in this respect resemble the laurel forest trees. The small number of tree species in the laurel forest showed a high variability of manganese contents. The highest average value was 12 mmol kg- 1 dw in /lex platyphylla, and the lowest manganese content was 0.2 mmol kg-I dw in Prunus lusitanica. The manganese content of the latter was actually below the critical content of 0.3 mmol kg- 1 dw given for some tropical tree species by DRECHSEL & ZECH (1991). The aluminium contents of the laurel forest foliage are remarkably high in comparison to those in other terrestrial plants, for which leaf contents between 1.5 and 18 mmol kg- 1 dw are given by LARCHER (1994). From the majority of plants, so-called aluminum hyperaccumulators that reach foliar Al levels of more than 1000 ppm = 37 mmol kg- 1 dw are distinguished

r

FLORA (1996) 191 ,

I I

r (CHENERY & SPORNE 1976). The most well known aluminum hyperaccumulator is tea: old leaves from tea reach Al values of 100 mmol kg- 1 dw (BAUMEISTER & ERNST 1978); Foy et aI. (1978) report even higher contents up to and more than 740(!) mmol kg- 1 dw. In the laurel forest, the Al contents of mature tree leaves ranged from 10 to 30 mmol kg- 1 dw, which is distinctively higher than in the majority of plant species. According to DRECHSEL & ZECH (1991), foliar Al contents are below 10 mmol kg- 1 dw in most of the broadleaved tropical and subtropical trees. Very few species (Acacia senegal, Ceiba pentandra, Cordeauxia edulis) have foliar Al contents above 20 mmol kg- 1 dw. Only in Gmelina arborea, the Al level is higher than the defined limit for Al hyperaccumulation, 37 mmol kg-I dw (= 1000 ppm). In lowland rain forests, seasonal dry forests and mangrove swamps of Sri Lanka the average Al content in leaves was generally below 10 mmol kg- 1 dw. In montane cloud forests of Sri Lanka, however, the average foliar Al content is 38 mmol kg-I dw (WERNER & BALASUBRAMANIAM 1992) and therefore exceeds the values found in the laurel forest of the Canary Island. High Al availability is typical for heavily leached soils (laterits) in the humid tropics and for acid soils with a pH below 5 (WOOLHOUSE 1983). AI toxicity together with an increased availability of Fe and Mn are considered the main limiting factors for growth of unadapted plants on acid soils (KINZEL 1982). The most prominent effect of high Al contents on these plants is a reduction of root growth. The exact location of the hazardous impact of Al on the root - apoplast, membran, cytoplasm - as well as the mechanism of Al toxicity is still under discussion (DELHAIZE & RYAN 1995); a key role of a Ca-Al-interaction is supposed (RENGEL 1992, DELHAIZE & RYAN 1995). In Al tolerant plants, the maintenance of root growth is supposed to be due to an Al exclusion from the root apices, which can be related to the efflux of organic acids, especially malate (DELHAIZE & RYAN 1995). However, this tolerance mechanism seems to be of minor importance to laurel forest trees, as they do not exclude but obviously accumulate AI. Similar to the different tolerance mechanism in species adapted to high concentration of other metals (BAKER & WALKER 1990), Al accumulation might be an alternative adaption to high AI availability (Foy et aI. 1978). The high foliar Al contents in upper montane rain forests in Sri Lanka correspond to high available Al contents of about 30 mmol kg-I dry soils of this region (WERNER & BALASUBRAMANIAM 1992). However, on the Canary Islands, analysis of the upper layers of the laurel forest soils (EMMERICH et aI., unpublished results) revealed a pH of 6 and exchangeable Al contents below 1 mmol kg-I dry soil. The highest Al contents were about 6 mmol kg-I dry soil or less than 20% of the cation-

exchange capacity (CEC), which is lower than the values found in soils of many temperate deciduous forests (SCHEFFER & SCHACHTSCHABEL 1984). The CEC ofthe laurel forest soils is obviously not dominated by Al but by basic cations. Therefore, in contrast to the often reported decrease in Ca and Mg content in plants grown under Al stress (HUANG et aI. 1992, RENGEL 1992, NICHOL et aI. 1993), the Ca and Mg supply to laurel forest trees was sufficient in spite of the high Al contents in leaves. Al accumulation in laurel forest trees may be another example for the phenomenon of metal hyperaccumulation by plants, which is also known for other elements, but for which the raison d' etre is still unknown (BOYD & MARTENS, 1992). According to CHENERY & SPORNE (1976), Al accumulation is primarily found in tropical families of dicotyledonous plants and correlates with primitive traits such as arborescent growth. The majority of the tree species in the laurel forest belong to families with a predominantly tropical occurrence. This is consistent with the ancient character of the laurel forest (tertiary relict). However, high Al contents are also present in species from modern families with mainly temperate distribution, e.g. Viburnum rigidum (Caprifoliaceae). The high Al contents (18 mmol kg- 1 dw) in 4-yearold spruce needles (ZOTTL 1985) suggests a relationship between Al accumulation and leaf longevity. Generally, the Al contents in young leaves of laurel forest trees were much lower than those in older leaves. The same relationship has been found in tea (BAUMEISTER & ERNST 1978, Foy et aI. 1978) and spruce. As the difference in Al content between young and mature leaves was higher in some species than in others, the influence of leaf age on Al content has been investigated by observing the seasonal change in foliar Al contents for selected species (GONZALEZ-RODRIGUEZ et aI., unpublished results). The accumulation of Al might depend on the rate of transpiration throughout the lifetime. The high transpiration rates of coriaceous leaves due to a weak stomatal regulation (LOSCH 1993, ZOHLEN et aI. 1995) may contribute to AI accumulation. The various Al accumulation patterns, which have been observed in different species, may be related to differences in transpiration; field measurements are required to test this hypothesis. High foliar Al contents in Al accumulating plants require a high Al tolerance, which is believed to be due to the capacity to chelate Al with organic acids or phenolic compounds (Foy et aI. 1978, TAYLOR 1987, VERKLEIJ & SCHAT 1990). In mature and old tea leaves, most of the Al species have been found in a catechin complex and only minor amounts were chelated by organic acids (NAGATA et al. 1992, 1993). Laurel forest leaves also contain high amounts of phenolic compounds, which in the Aquifoliaceae might accumulate to FLORA (1996) 191

309

10% of the dry weight (HEGNAUER 1964). In flex canariensis and l. platyphylla, stress injured leaves become black due to a release of phenols (LOSCH 1980). These compounds may contribute to Al detoxification in intact leaf tissues. Generally, the phylogenetic ally ancient, coriaceous leaf type in trees of the laurel forest showed nutrient contents which resembled tropical trees more than deciduous trees from temperate climates. The striking accumulation of Al in leaves is probably less due to a high Al availability in the soil but is related to leaf longevity and poor stomatal regulation. Presumeably, Al accumulation is not an ancient feature per se but a consequence of the primitive features of the coriaceous leaf.

5. Acknowledgement The authors thank Dr. U. SANTORE for linguistic improvements and an anonymous referee for helpful comments.

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Buchbesprechung MEUSEL, H., KASTNER, A: Lebensgeschichte der Gold- und Silberdisteln. Monographie der mediterran-mitteleuropiiischen Compositen-Gattung Carl ina. Bd. 2: Artenvielfalt und Stammesgeschichte der Gattung. - Wien, New York: Springer 1994. (Denkschr. Osterr. Akad. Wiss., Math.-Naturw. Kl. 128). - 657 S., 179 Abb., 32 Tafeln, 6 Tab. 4°, Hartkarton, DM 380,00. - ISBN 0-387-865587 Mit einem Abstand von 4 J ahren zum 1. Band haben die Autoren den 2. Band ihrer jetzt insgesamt 951 S. (!) umfassenden monumental en Carlina-Monographie vorgelegt. Darin werden die einzelnen Sippen und ihre Phylogenie behandelt. Vorztige und Besonderheiten des Gesamtwerkes wurden in der Rezension des l. Bandes (Flora 186: 391-392. 1992) bereits ausflihrlich gewtirdigt, so daB hier weitgehend darauf verwiesen werden kann. Auch an diesem Band sind neben den beiden Hauptautoren viele weitere Mitarbeiter, z. T. mit eigenen Beitragen, beteiligt (E. VITEK, K. WERNER, F. EHRENDORFER, I. EBERT & J. GREILHUBER, I. HAGEMANN, V. VOGGENREITER und V. MAYER), und die Namensliste der Kollegen, denen flir Zuarbeit gedankt wird, flillt eine ganze groBformatige Seite.

Auf das Vorwort (4 S.) folgen Hinweise flir den Leser (2 S.) mit einer umfangreichen Liste der verwendeten Abktirzungen und eine sehr pragnante englische Zusammenfassung (7) S. Der Hauptteil ist in die folgenden 5 Kapitel gegliedert: 1. Systematische Stellung und Gliederung der Gattung (15 S.), mit Conspectus und Bestimmungsschltissel. Die 28 Arten (mit vielen Unterarten und Varietaten) verteilen sich auf 5 Untergattungen und eine Reihe von Sektionen und Subsektionen. Gegentiber dem I. Band wird den beiden annuellen Arten der Mitina-Gruppe (c. racemosa, C. lanata) der Rang einer eigenen Untergattung eingeraumt und diese in gr6Bere Nahe des als primitiv angesehenen subgenus Carlowizia gerlickt. 2. Systematische Ubersicht (510 S.), mit Synonymie, Typusangaben, morphologischer Charakteristik, ausflihrlichen Angaben tiber Umgrenzung, Gliederung, Standortbindung, Verbrei tung und Vergesellschaftung und Listen der gesehenen Belege. Dieser Hauptteil der Monographie wird durch eine groBe Zahl guter Zeichnungen (bes. der Blattfolgen und der Wuchsformen), hervorragender SchwarzweiB-Fotos und Farbbilder wichtiger Details, der ganzen Pflanze und der Pflanzen in ihrem Lebensraum sowie durch Arealkarten vorztiglich erganzt. Hinzu kommen zahlreiche, z. T. sehr umfangreiche FLORA (1996) 191

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