Convergent Evolution of Mediterranean-Climate Evergreen...

Convergent Evolution of Mediterranean-Climate Evergreen Sclerophyll ShrubsAuthor(s): Harold A. Mooney and E. Lloyd DunnSource: Evolution, Vol. 24, No. 2 (Jun., 1970), pp. 292-303Published by: Society for the Study of EvolutionStable URL: http://www.jstor.org/stable/2406805 .

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CONVERGENT EVOLUTION OF MEDITERRANEAN-CLIMATE EVERGREEN SCLEROPHYLL SHRUBS

HAROLD A. MOONEY AND E. LLOYD DUNN

Department of Biological Sciences, Stanford University, Stanford, Calif ornia 94305

Received November 5, 1969

It has been long noted that in areas of climatic similarity throughout the world the aspect, or physiognomy, of the vegetation is also similar (Humboldt, 1806; Griesebach, 1872). This relationship of climate to vegetation forms the very foundation of ecological plant geography. There has been a long history of attempts to utilize various nonfloristic, physiognomic characterizations of vegetation and to re- late these to regional, local, and microclimate (DuRietz, 1931; Raunkiaer, 1934; Adamson, 1939; Cain, 1950; Cooper, 1961; Knight, 1965; Knight and Loucks, 1969). Although the details of the different research approaches have been subject to a certain amount of controversy the broad generalities are above dispute. Environ- mental similarity can produce growth form similarity no matter what the evolutionary history of the flora in question.

In California, for example, within the chaparral vegetation, the dominant plants belong to such diverse families as the Eri- caceae, Rhamaceae, and Rosaceae, yet all possess a large degree of growth-form similarity, i.e., deep-rooted, evergreen, sclerophyllous shrubs (Fig. 1). Even more striking than this within-vegetation type homology is the homology found between vegetation growing in similar but disjunct climates.

The mediterranean climatic type (summer drought, winter rain) occurs in Cali- fornia, South Africa, central Chile, southern Australia, as well as in the Mediterranean region. In all five of these areas the native vegetation has a similar appearance: a dense scrub dominated by woody evergreen sclerophyllous species. The isolation of these geographic areas and the almost com-

plete taxonomic dissimilarity between their resident floras indicates that they have had distinct evolutionary histories (Table 1).

The general life strategies of plants found in repiesentative mediterranean climatic areas are similar (Table 2). Drought- evading annuals predominate. Herbaceous perennials, which die back to the ground surface during the drought period, are also common. Similar proportions of these life forms are characteristic of the desert. The dominant life forms of the mediterranean climatic regions are evergreen trees and shrubs which tolerate the drought period. In deserts the tree form is generally absent and the shrubs, for the most part, lose all or a portion of their leaves during the summer drought.

The severity of the mediterranean climate is apparent when the details of the seasonal climatic regime are examined. The seasonal course of temperature and soil moisture are shown for a representative Californian region (Fig. 2). During the summer months when temperatures are generally favorable for growth, moisture is limiting and atmospheric and soil drought prevails. When the rain returns in the winter and the soil moisture profile is re- charged, temperatures are low and growth is further limited. Annuals germinate following the first winter rains but growth is very slow while temperatures remain cold. As temperatures increase in late winter and early spring, and while moisture is still available, growth greatly accelerates. As the upper soil profile dries out the annuals complete their life cycle. They survive the drought period as seeds. Thus there is but a brief period when conditions are favorable for vigorous plant growth (Major,

EVOLUTION 24:292-303. June, 1970 292



CONVERGENT EVOLUTION OF SCLEROPHYLL SHRUBS 293

Chaparra

FIG. 1. Growth-form homology of characteristic dominant plants of the California chaparral. Cha- mise (Adenostoma fasciculatum), scruh oak (Quercus dumosa), chaparral whitethorn (Ceanothus leucodermis), and eastwood manzanita (Arctostaphylos glandulosa) all helong to different families. This figure taken from Hellmers et al., 1955.



294 H. A. MOONEY AND E. L. DUNN

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210 x ~ ~ ~ ~ ~~~I-J0 < C~~~~~~~0

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FIG. 2. The seasonal course of air temperature and soil moisture and relative plant growth activity at a Southern California chaparral site (unpublished data).

1963). The seasonal growth activity cycle of the evergreen shrubs generally corresponds to that of the annuals except that growth commences and ends later for the former since soil moisture is primarily utilized from greater depths in the profile.

In addition to the above combinations of climatic elements which are obviously inimical to plant growth, a number of other features are characteristic of some or all mediterranean regions which are equally restrictive, such as soil mineral deficiencies, periods of violent winds, and frequent fires (see Naveh [1966], for an excellent comparison of the California and Mediter- ranean ecosystems).

We propose that the basis for the high degree of convergence in plant form found in the mediterranean climatic areas is that this environmental types poses a series of limitations to plant growth. Although there are a number of possible evolutionary strategies to surmount any given environmental stress, the adaptive possibilities become in- creasingly limited as the number of stresses compound.

This principle can be illustrated in the following manner for tree and shrub forms. The droughts of the mediterranean climatic regions are somewhat unpredictable as to their time of initiation and their duration. Drought can be avoided, of course, by leaf




TABLE 1. Dominant shrub and tree flora.

Number of:

Common to: Familes Genera Species

California and Chile 9* 2 0 California and

Mediterranean 12 10 0 Mediterranean

and Chile 11 4 0 Chile, California,

Mediterranean 7 0 0 Total Taxa 59 156 352

* These data derived from lists compiled by the authors from personal observations and standard floras.

drop. However, it would not be efficient to couple leaf drop or subsequent leaf initiation to an anticipatory environmental trigger such as photoperiod because of the unpredictability of the timing of the drought. However, a more opportunistic system could operate under such conditions. When water is available, rapid growth could be initiated. This would operate if root systems were superficial and leaf production rapid so that newly fallen rain could be quickly utilized. This actually is a common adaptive mode in the deserts. How- ever, in most desert regions the winter rains are usually accompanied by suitable growing temperatures. This is not the case in mediterranean climates.

The alternate strategy would be to hold leaves the year round and tolerate the

drought in some manner. Evergreenness would solve the problem of unpredictability and would be a competitively advantageous system in terms of production. When conditions are suitable carbon fixation would occur without the energy expense and loss of time of building a production apparatus. Deep root systems could reach water at depth and shorten or eliminate the duration of the drought (such deep water reserves do not exist in deserts except in rare habitats).

Although evergreenness solves certain environmental problems it creates others. When water stress periods arise, as they will, stomatal control can provide temporal drought escape mechanisms. Effective cuticular resistance to water loss is also nec- essary. Restricting water loss in this way also restricts the influx of CO2 and hence production. This occurs during periods of high temperatures when potential loss of energy through respiration would be greatest. Some energy-conserving mechanisms are thus required unless production has been so great as to compensate for this loss. Further, reduction of water loss by stomatal closure also results in the reduction of evaporational cooling and potential overheating of the leaf. This overheating effect is greater the larger the leaf surface. This could lead to a selection for smaller leaf sizes.

The above example considers just a few

TABLE 2. Life form composition of the vegetation of several different climatic regions.

% Species distribution

Chamae- Hemi- Crypto- No. of Vegetation type Trees Shrubs phytes crypto- phytes Annuals species

Mediterranean scrub California (Santa Catalina Island) 9 9 5 27 5 41 391t Chile (Quebrada de La Plata) 2 17 6 20 12 42 319t

Italy (Argentario) 6 6 6 29 2 42 866*

Rain forest (Queensland) - 96 - 2 0 2 0 141*

Arctic tundra (Spitzbergen) 0 1 22 60 15 2 1l0*

Desert scrub (Transcaspian lowlands) 0 11 7 27 14 41 768*

t Data from Thorne, 1967. t Data from Schlegel, 1966. * Data from Cain, 1950.




SOIL MOISTURE

ANNUAL HIGH WINTER AVAILABLE TEMPERATURE HIGH PRECIPITATION FIRE RAINS AT AT DEPTH FAVORABLE EXTENDED SUMMER ATMOSPHERIC SELECTIVE GENERALLY FREQUENCY FAIRLY WARM DURING YEAR DROUGHT TEMPERATURE DROUGHT LIMITING TEMPERATURE DROUGHT ROUND FORCES

(MOST YEARS)

8 r ACTIVE SOIL PHOTOSYNTHESIS SOIL METABOLIC HYDROLYSIS NOT TEMPERATURE MOISTURE INCREASED LIMITATIONS RAPID LITTER LIMITED LIMITING RESPIRATION

DECOMPOSITION MIDDAY STOMATAL

MINERAL CLOSURE COMPETITIVE POOR DURING DROUGHT LIMITATIONS HUMUS - DECREASED

PRODUCTION PHOTOSYNTHESIS CONSEQUENCES

YEAR ROUND --WATER LOSS RESE RVE PRODUCTION jTHROUGH

RERV-, 4 0 l | POSS,IBLE t WATER

DUTIC EPLETION LIMITING DUJRING

ONY THICK GROWTH CELL

Ite8 WALLS l __________ . ______ - -~- MINERALS LEAF

TRAPPING CONSERVING STMULATED BEHAVIOR PLANTLIMITING OVERTEMPERATURE r | - 4- - - -1~~~~- - - - - - - - - MTN

- - - - - - j

WATER WATER FIRE RESPROUTING SECONDARY METABOLISM SCLEROPHYLL TRAPPING CONSERVING STIMULATED BEHAVIOR PLANT N2 FIXING AT HIGH LEAF METABOLIC DEVICES DEVICES GERMINATION SUBSTANCES , EVERGREEN CAPACITY H 0 STRESS r , L CONSERVATION EVOLUTIONARY

a) SMOOTH BARK a) SOIL NON- a) DORMANCY 0)ADVENTITIOUS DEEP 2 SMALL OF RESERVES b) MULTIPLE WETTABILITY MECHANISMS BUDS ROOTS LEAF STRATEGIES

UPRIGHT b) ANTIBIOSIS b) LONG LIVED b) SURPLUS RESERVES- BRANCHES SEEDS REQUIRED

c) HEAVY SEED C) RAPID REGROWTH COAT ll C __ _ -- - ___- -_ ___-_ ___-__

GRAZING PRESSURES YEAR ROUND PREDATION

FIG. 3. An evolutionary model for the mediterranean-climate shrub form.

of the environmental selective forces of the mediterranean climatic areas and the im- plications of alternate adaptive strategies. Any single adaptive strategy must simul- taneously solve all of the various selective stresses which exist in the environment as well as any new ones a solution may create.

A more complete and partially docu- mented model of selective agents and adaptive strategies for shrub and tree forms follows (Fig. 3). This model is most certainly a gross simplification of the actual situation; however, it hopefully provides a framework of understanding. Documenta- tion will be given for blocks of the flow diagram for convenience and brevity. Em- phasis is on the California region. However, these principles are understood to apply generally to all mediterranean climatic regions.

Soil Moisture, which is Available at Depth Throughout Most o f the Year, is

Utilized by Deep Root Systems which makes Year-round Pro-

duction Possible Hellmers et al. (1955) found that the

roots of all Californian chaparral shrub species which they excavated were exten- sive in either their depth or breadth of penetration into the soil mantle, or both. Roots of some species were traced to a depth of 9 meters and were presumed to extend even deeper. Shachori et al. (1967) similarly concluded that maqui (evergreen scrub of the Mediterranean region which corresponds to the "chaparral" of Califor- nia and "matorral" of Chile) shrubs in Israel were obtaining water from depths of greater than 8 meters and thus were able to survive the long drought period. Cali-




-i 30 .

(t) 20- .NE

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F-) LOf LUJ

00

10 U) 7 NEW

MONTHS- LEAVES

LLJ Z /j

~- I

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i J A SON DOJ F M AM J J A S MONTHS

FIG. 4. The seasonal course of photosynthetic capacity of a chaparral shrub, Rhus ovata, as related to soil moisture content at a depth of 2 meters. Unpublished data from a Southern California study site.

fornia oaks have been found to penetrate fractured rock to depths of at least 2 7 meters (Lewis and Burgy, 1964).

We have found that both Chilean matorral and Californian chaparral evergreen sclerophyllous shrubs photosynthesize, in most cases, throughout the year, although at reduced levels during the drought (Fig. 4). Certain species, during severe drought years, may lose their capacity to maintain a positive carbon balance. Drought reduction in photosynthetic capacity of maqui plants has been noted by Guttenberg and Buhr (1935), as well as others (see Grieve, 1955). There are indications that for certain species at least the reduction in physiological activity of chaparral plants during summer is controlled partially by environmental factors other than a reduction in water supply (Hanes, 1965).

It is important to note that in all Medi- terranean areas there is a gradient of plant forms found as one progresses from the center of this climatic type toward the

desert fringes. Shrubs generally become shallower rooted and have smaller evergreen leaves or leaves which drop during the drought. Further, even within the center of the Mediterranean climatic types there are subshrubs with these attributes which are often successional species and occupy arid microsites. Shrubs of these types do not have access to moisture during the drought and they lose or dramat- ically reduce their photosynthetic capacity (Eckhardt, 1952).

The Photosynethetic Process is not Com- pletely Limited by Low Winter or

High Summer Temperatues, thus Permitting Year-round

Production Temperatures are rarely too high or too

low throughout the entire year to completely limit photosynthesis. As an index of the average daytime temperatures during any given month, Larcher (1961)




utilizes the average of the mean monthly temperature and the mean monthly maxi- mum temperature. For a northern outpost of the Mediterranean vegetation at Riva, Italy (45 ?53'N) he indicates that the average daytime temperature of the coldest month, January, is 6.7 C and of the warmest month, July, 26.8 C. In contrast, at Los Angeles, California (34?03'N), near the southern end of the distribution of the homologous Californian chaparral, the average daytime temperature of the coldest month (January also) is only 15.6 and of the warmest month (August), 25.0. These temperatures, which are representative of the cold and warm extremes of distribution of the shrubby sclerophyll type are all well within the range where active photosynthesis is possible.

Measurements of the seasonal metabolic activity of a chaparral shrub near Los Angeles (Fig. 4) indicates that although there is a reduction in photosynthetic capacity during the summer drought, as well as during the cold winter months, production is generally never entirely environ- mentally limited. Larcher (1961) has re- ported similar results for Italian maqui plants.

Thus, the environmental conditions are such that evergreenness is an appropriate strategy in mediterranean-type climates. The cost of maintaining evergreen leaves that can withstand periodic environmental stress is less than that of producing a new photosynthetic system annually which would only be present during completely favorable periods (Mooney and Dunn, 1969).

However, evergreenness introduces certain evolutionary problems. Evergreen leaves offer a year-round food source for herbivores and are thus subject to poten- tially high predation. Selection for sclero- phylly, as well as for distasteful or toxic principals in the leaves would certainly be an advantage in this environment. Indeed, most chaparral, matorral, and maqui shrubby plants have sclerophyllous leaves often containing toxic products.

The Extended Drought Period Induces Physiological Moisture-conserving Mecha-

nisms which have Consequences on Production as well as

Thermal Balance Soil moisture becomes decreasingly avail-

able during the drought period even to deep-rooted plants. During occasional severe drought years soil moisture may be totally unavailable during late summer (Hill and Rice, 1963; Harvey and Mooney, 1964). Plants under a progressive drought stress decrease their transpirational water loss by stomatal control. During the early stages of a drought the stomates may close only during the midday period when vapor pressure deficits are greatest. As the drought progresses the plant tissues are under water stress for longer periods each day and stomatal closure becomes increas- ingly important in maintaining a favorable water balance (Fig. 5). It is during these times that effective means of controlling cuticular water loss are important. Sclero- phyll plants generally have very thick cuticular layers (Cooper, 1922).

As a result of restriction in water efflux, CO2 influx is also reduced and photosynthesis declines. Temperatures are warmest during the drought period so that the potential for reserve loss through high rates of respiration is also great. Thus, all factors at this time lead toward a depletion of energy reserves. However, there are indications that as the summer drought period progresses, respiration rates at any given temperature are declining (Fig. 5). This phenomenon may be a simple feedback system where respiration rate is dependent on the level of substrate present.

A further consequence of a reduction in transpiration is a modification of the energy balance of the leaf which could result in an overtemperature, particularly since the drought period occurs during periods of high radiation loads (Lange and Lange, 1963). The smaller leaf sizes would have a less pronounced overtemperature effect (Gates et al., 1968) and would thus be of selective advantage in this environ-




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10 6

2-

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HOURS C\

0I z | < , > _ _ FEB 28,FEB 196196

2-)

0-

0 20 30 40 TEMPERATURE ?C

FIG. 5. The daily course of photosynthesis and temperature-related respiration rates of Lithraea caustica near Santiago, Chile during early and middle drought. Both photosynthetic capacity and the intensity of respiration declines as the drought progresses (unpublished data).

ment. An additional selective advantage to small leaves is that they have a low ratio of blade tissue to water conduction tissue and thus they are less subject to desicca- tion injury (Eckardt, 1952). In this regard, there is a clear gradient of decreasing sizes of evergreen leaves progressing toward the more arid parts of the mediterranean climates (Mooney and Dunn, 1970).

The Mediterranean-type Climate Produces Nutrient-deficient Soils which have

an Evolutionary Impact on the Native Vegetation

Nutrient deficiencies, particularly nitrogen, phosphorous, and sulphur, are com-

mon in the mediterranean-climate soils of California (Jenny et al., 1950; Martin, 1958; Walker and Williams, 1963). In similar climates in Israel nitrogen is also often deficient (Reifenberg, 1947). The comparatively warm temperatures which prevail during the rainy season lead to active hydrolysis of soil minerals and consequent leaching. These conditions also promote a rapid decomposition of litter (loc. cit.). In California, at least, the high erodibility of the soils accentuate the nutrient deficiencies. These deficiencies are of a magnitude that they restrict the growth of native chaparral shrubs (Hellmers et al.,




1955) as well as herbs (Woolfolk and Duncan, 1962).

Such widespread nutrient limitations could provide an evolutionary stimulus. For example, a remarkable number of chaparral shrubs have nitrogen-fixing ca- pability (Hellmers and Kelleher, 1959; Vlamis et al, 1964). These shrubs belong to such diverse families as the Rhamnaceae (Ceanothus), Rosaceae (Cercocarpus) and Leguminoseae (Pickeringia).

In addition to the evolution of nutrient- gathering devices there may be selection for nutrient conservation and low use by mediterranean-climate shrubby plants. It has been proposed that hard evergreen leaves promote the conservation of minerals in the ecosystem. Water-soluble minerals would be slowly released from the leaves on the plant to the soil rather than in a massive flush if they all dropped simul- taneously. Further, the hard leaves are slowly broken down by soil microorganisms in the litter, again leading to a gradual release of minerals (Monk, 1966).

In time, however, in these mineral-poor environments the total pool of nutrients may become tied up in the roots, fruits, and dead leaves of the evergreen plants and the whole system stagnates. Fire, or death, releases these minerals and the system re- generates (Specht et al., 1958).

Fire is a Natural Part of the Environment of Mediterranean-climate Areas and

is a Major Evolutionary Force Fire burns intensively through the medi-

terranean brushland at generally frequent intervals. For example, Biswell et al. (1952) estimated that the chaparral in the northern part of its distribution burns at least once every 20 years. DeBano et al. (1967) give an average fire interval of 25 years in Southern California chaparral. Vogl (1967) concluded that fire has in- fluenced the evolution of all of the major vegetation types of Southern California. Even prior to European settlement fires burned with regular frequency (Biswell, 1967).

Succession following intense fire is very rapid in the California chaparral. Re- sprouting may occur as fast as 10 days following fire and within 2 months new branches may exceed 24 inches in length (Plumb, 1963). Within periods as short as 6 to 8 years the shrubs completely recover the ground surface (Sweeny, 1967). A large number of studies on fire succession and fire adaptations have established that the chaparral shrubs either resprout vigorously after fire or have stimulated germination of seeds which have lain long dormant in the soil, or both (see, for example, Cooper, 1922; Sampson, 1944; Stone and Juhren, 1951; Sweeny, 1956; Hadley, 1961). This is true in other mediterranean climate areas (Walter, 1968).

The fire itself evidently releases minerals which are bound up in the woody vegetation and thereby increases the soil fertility for a brief period (Vlamis and Gowans, 1961). This could explain in part the lush initial successional growth. However, the loosened surface soil is subject to high erosion (Biswell et al., 1952) and much of this increase in mineral availability can be lost if the first winter rains are intense. Fire may also deactivate accumulated toxins in the soil and result in growth stimulation (McPherson and Muller, 1969).

These shrubby species which resprout are often subjected to heavy grazing predation by mammals. For example, in Cali- fornia the opening of heavy stands of chaparral by fire results in a subsequent increase in the deer populations. Resprouting chaparral may have an entirely different successional history, depending on the intensity of grazing. Light grazing suppresses growth and heavy grazing by deer may kill the resprouts (Biswell et al., 1952; Biswell, 1961). Such pressures could select against palatability. A comparison of evergreen versus deciduous species of oaks in Cali- fornia showed that although both generally fire-sprout, the evergreen species are more prolific in this regard. Further, the evergreen oaks had a resprout pattern which




was more resistant to heavy grazing (Long- hurst, 1956).

The degree of resprouting is quite dependent on the timing of the fire. For example, resprouting will be reduced if fire in California chamise (Adenostoma fasciculatum) stands occurs in early spring as compared to fall (Doman, 1967). The apparent reason for this is that the spring growth flush is partially dependent on stored carbohydrate reserves. These reserves are not replenished until fall (Jones and Laude, 1960). The moisture status of the soil in any given year may complicate this pattern (Plumb, 1963).

Thus, there must be adequate reserves in these shrubs, not only to promote vigorous regrowth in the event of fire whenever it occurs, but also to initiate even further growth if grazed. Resprouting plants in the spring must be in a very delicate balance.

There are Indications tkat Sclerophyll Shrubs have Evolved Forms and

Products which Serve to Trap and Conserve Moisture

In a detailed study of the disposition of rainfall in the chaparral vegetation in Cali- fornia, Hamilton and Rowe (1949) found that a relatively large portion of the precipitation reached the soil surface as stem- flow (almost a third in certain species). Species with stiff upright branches and smooth bark have the greatest stem-flow. Since stem-flow yields a slow and steady supply of moisture right at the base of the shrub where the soil has the highest infiltration capacity, this is a premium source of moisture. Specht (1957) has doc- umented how stem-flow in nanophyllous shrubs of the heath vegetation in South Australia can result in striking patterning of soil moisture. These shrubs intercept a large percentage of the rainfall and direct most of it to the ground by stem-flow. This results in a "rain shadow" at the periphery of the plant. Smaller-leaved shrubs would have greater precipitation arriving as through fall (Bauer, 1936).

A rather puzzling recent finding is that a number of important chaparral species produce organic substances which accumu- late on the soil surface under these shrubs and render them more or less non-wettable (DeBano et al., 1967). This leads to high surface runoff and consequent erosion and mineral depletion. However, although these soils have a very low infiltration rate they also have a low evaporation rate. Is the moisture that enters the soil at the base of the plant through stem-flow, and is con- served by a protective covering, of a greater amount than that lost by runoff because of the effects of the non-wetting agents? Once into the profile and below the non- wettable layer moisture would not be readily lost through evaporation. Fire acts to push the non-wettable layer to lower depths in the soil, leaving a thin wettable layer (loc. cit.). This phenomenon is evidently widespread in the California chaparral regions.

Toxic products of certain sclerophyll shrubs have a detrimental effect on herbaceous under-story plants (Muller et al., 1968; McPherson and Muller, 1969). Such antibiotic substances may be of benefit to the water relationships of the shrub which produces these compounds. An analysis of the energy costs of production of these compounds versus the benefit in growth derived from any increase in water intake is needed for an assessment of these relationships.

SUMMARY

The vegetations found in the mediterranean climate regions of the world are highly convergent in form, although they have had distinct evolutionary histories. There are numerous selective forces in this climatic type, such as fire, drought, high temperatures, rainfall unpredictability, and mineral deficiencies. These multiple selective forces are often interacting and involve the biotic as well as the physical compo- nents. A solution to any one of these selective forces in turn creates new adaptive challenges. Thus, the total possible solu-




tions to the various combinations of forces becomes limited, leading to convergence in form and function.

ACKNOWLEDGMENTS

We express our appreciation to the NSF (GB 5223, GB 8184) for the support of this work, as well as the Ford Foundation's University of California-University of Chile Convenio. The senior author was a NSF senior postdoctoral fellow and the junior author a Turtox Fellow during the tenure of this study.

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