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2Universidad de Concepcion, Campus Chillan, Chillan, Chile; and 3Department of Evolution, Ecology, and Organismal Biology, Ohio State University, Columbus, Ohio 43210-1293
Received for publication December 9, 1997. Accepted for publication July 31, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Chile • Fagaceae • nitrogen • Nothofagus • phosphorus • resorption
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; see also Lesquereux, 1892
; Berry, 1914
; Kulp, 1961
). Because of their short leaf lifespan, deciduous angiosperms are thought to have been preadapted to the cool seasonal environments that increasingly characterized the midlatitudes of the northern hemisphere from the Cretaceous onward (Axelrod, 1966
), and this may explain the dominance of deciduous forests in cool seasonal northern environments today. In contrast, during the Cretaceous the southern hemisphere was less strongly seasonal in precipitation and slower to cool than the northern hemisphere. As a result much of the temperate southern hemisphere is still dominated by broad-leaved evergreen forests. The primary exception to this is the temperate and subantarctic forest region of Chile, where combinations of mixed deciduous broad-leaved forests and evergreen broad-leaved forests dominate low- and midelevations, with deciduous trees at the higher elevations up to treeline.
Aerts and van der Peijl (1993)
and Berendse (1994)
have hypothesized that the key to evergreen success in nutrient-poor areas is not nutrient use efficiency, but nutrient conservation. The proposed mechanisms of nutrient conservation are high rates of resorption, high C : element ratios in leaves and roots, and low litter production. Although some studies have shown deciduous and evergreen species to have similar nutrient resorption rates (as a percentage of initial nutrient content), others have demonstrated lower rates in evergreens than deciduous species (e.g., Chapin and Kedrowski, 1983
; Aerts, 1990
). Yet other studies (e.g., Small, 1972
; Gray, 1983
; Berendse and Jonasson, 1992
) have found deciduous species to have lower N and P resorption than co-occurring evergreen species.
Most comparative studies of the ecophysiological differences have been confounded by two factors. First, many have relied on comparisons between taxonomically distant or ecologically distinct evergreen vs. deciduous species. As a result, interpretation of the differences among taxa in terms of leaf lifespan may be confounded by differences in evolutionary history and selection environments that these taxa have experienced. Second, there exists on a global basis a relatively continuous range of leaf lifespans from several days to decades, and deciduous species with leaf lifespans of 4–5 mo may resemble evergreens with leaf lifespan of 1–1.5 yr more than deciduous species with leaf lifespans <1 mo (Reich, Wallers, and Ellsworth, 1992
). As a consequence, broad comparisons of deciduous vs. evergreen species' characteristics may be weakened by the broad range of leaf lifespans that are averaged into each of the two groups.
In this study, we sought to avoid these confounding factors by comparing the leaf lifespan, specific leaf mass, and foliar nutrient dynamics of three congeneric, sympatric Chilean species of southern beech (Nothofagus), which differ dramatically in leaf lifespan and elevational limits: Nothofagus obliqua, N. pumilio, both of which are deciduous and have leaf lifespans of 4–5 mo, and the evergreen N. dombeyi, which has a 3–4 yr leaf lifespan. These three species are major constitutents of the Valdivian temperate rain forest found in Chile and parts of Argentina from 37° to 43° S latitude (Veblen et al., 1996
). This forest type is characterized by the deciduous species N. obliqua and N. alpina at lower elevations, the evergreen N. dombeyi, N. nitida, and N. betuloides at intermediate elevations, and N. pumilio (deciduous) and N. antarctica (evergreen) at higher elevations up to treeline, with some overlap in the altitudinal ranges of the various species (Veblen et al., 1996
). Within the range of the Valdivian rain forest, N. betuloides becomes more dominant to the south and the deciduous species to the north. Chusquea bamboos are common in the understory of the Valdivian forests (Veblen et al., 1996
). In our study area, N. dombeyi has an elevational distribution intermediate between those of the deciduous N. obliqua and N. pumilio and occurs in mixed stands with both.
As these species are distributed widely in the active volcanic landscape of central Chile with its strong relief and relatively young soils, we anticipate that they have experienced strong elevational gradients of soil fertility, with nutrient availability, water-holding capacity and, actual evapotranspiration (AET) all decreasing with increasing elevation. For example, in our study area total inorganic N and available P in the soil both decreased by 31% along an elevation gradient of 
330 m (Decker, unpublished data). Thus, because these three tree species differ not only in their leaf lifespan but also in their ecological distribution along this elevation gradient, we erected separate sets of hypotheses for how foliar nutrient dynamics might vary among these three species in relation to leaf lifespan and to variations in nutrient availability/microclimate along the elevation gradient.
Hypotheses based upon leaf lifespan
H1L
Foliar nutrient concentrations will be significantly greater in the deciduous species than in the evergreen species. Similarly, the greater investment of carbon in structure in the evergreen leaves will result in the evergreen leaves having greater specific leaf mass (measured as milligrams dry leaf mass per square decimetre leaf area) than will leaves of the deciduous species.
H2L
In the deciduous species, foliar nutrient concentrations will remain relatively constant from the time of full leaf expansion to the beginning of senescence, then decrease rapidly as N and P are resorbed prior to abscission. In the evergreen species, resorption will occur prior to the emergence of the new leaf cohort each spring; however it is not clear whether resorption will be from all prior leaf cohorts or just the oldest leaf cohort. Over the full leaf lifespan of each species, proportional and actual resorption will be greater in the evergreen species than the deciduous species.
H3L
Based on the nutrient patterns described for evergreen black spruce (Picea mariana) in the northern United States by Tyrrell and Boerner (1987)
, we hypothesized that mean foliar nutrient concentrations and proportional resorption would decrease with increasing leaf age in the evergreen N. dombeyi, as resources are progressively reallocated from older to younger leaf tissues. (Note: there is no elevational counterpart to this hypothesis.)
Hypotheses based upon distribution along the elevation gradient
H1E
Foliar nutrient concentrations will decrease and specific leaf mass increase with increasing mean elevation of the species distribution (i.e., N. obliqua–N. dombeyi–N. pumilio).
H2E
Resorption of nutrients prior to abscission will increase with increasing elevation (i.e., with decreasing soil nutrient availability).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Nothofagus obliqua occurs in areas where mean annual precipitation is 1500–3000 mm, typically with dry summers. This species is tolerant of a wide variety of soil types and textures (Veblen et al., 1996
). Leaves of N. obliqua are ovoid with dentate/lobate margins, with average dimensions of 8–10 cm length and 4–6 cm width (Rodriquez, Matthei, and Quiezada, 1983
). Nothofagus obliqua has the largest leaves of the three species we studied. Pure stands of N. obliqua are now uncommon due to clearing of the valleys and lower foothills for agriculture (Veblen et al., 1996
).
Nothofagus pumilio (Poepp. et Endl.) Krasser (common name: lenga) is a deciduous species that occurs at higher elevations up to treeline from Talca in central Chile south to Tierra del Fuego (Rodriquez, Matthei, and Quiezada, 1983
). It most commonly forms dense, even-aged stands on old tectonic surfaces, but may also occur in mixed stands with a variety of other Nothofagus species (Armesto, Casassa, and Dellenz, 1992
; Veblen et al., 1996
). Nothofagus pumilio is found in areas with as much as 5000 mm of annual rainfall, but is also tolerant of both dry soils and low temperatures; this species is the most cold tolerant of the three species in this study (Veblen et al., 1996
) The leaves of N. pumilio are simple, ovoid, and average 2–3 cm in length and 1–3 cm in width (Rodriquez, Matthei, and Quiezada, 1983
).
Nothofagus dombeyi (Mirb.) Oerst. (common name: coigue) is a widespread evergreen dominant of the Andes, Coastal Cordillera, and moist lowlands of central and southern Chile from 35° to 47° S latitude (Rodriquez, Matthei, and Quiezada, 1983
). This species is also found in areas that receive up to 5000 mm of rainfall per year, but is relatively less tolerant of both drought and freezing than N. pumilio (Veblen et al., 1996
). For example, during a drought in 1979 in the Chilean Lake District, N. obliqua responded by prematurely abscising its leaves, whereas N. dombeyi responded by dying (Veblen et al., 1996
). The leaves of N. dombeyi are typically ovoid-lanceolate or lanceolate-rhomboid, with length ranging from 2 to 3 cm and width from 1 to 2 cm (Rodriquez, Matthei, and Quiezada, 1983
). In our study area, the leaf lifespan of N. dombeyi is 3.5–4.0 yr.
All three of these species are typically ectomycorrhizal (Singer and Moser, 1960
, 1965
). In fact, the diversity of fungi that form ectomycorrhizae appears to be dependent on the diversity of Nothofagus species and density of Nothofagus stems in a given forest stand (Singer and Moser, 1965
).
Sample site
All three Nothofagus species were sampled in a single mixed-species stand near the village of Las Trancas, at 37° S in Region VII of Chile. This site lies in the foothills and lower slopes of the Andes at an elevation range of 1400–1800 m. Nothofagus dombeyi and N. obliqua occur near the upper elevation limits of their respective range, but N. pumilio occurs sparsely at lower elevations (down to 1300 m). We chose to sample these three species in a mixed stand to minimize the differences in microclimate and nutrient availability to which the individual trees were exposed, thus allowing us to test more robustly the long-term evolutionary differences in ecophysiology.
The soils of the site were derived from volcanic deposits. Analysis of soil samples taken in November 1995 indicates that these soils are slightly acidic (mean pH = 5.4) and low in inorganic N (mean 6.1 mg/kg) and P availability (mean 0.04 mg/kg) (data from Decker and Boerner, 1997
). These data are consistent with what one would expect in a young, weakly developed soil if the parent material had high P fixation capacity (cf. Walker and Syers, 1976
).
Field methods
Three canopy individuals of each species were chosen at random from a larger pool of trees being used for a more comprehensive study of the ecophysiology of this genus in this region. All were between 40 and 60 cm diameter at breast height of 1.45 m (dbh) and showed no sign of recent injury or disturbance. On 17 dates from mid-November 1995 to mid-May 1996, ten sun leaves from each N. obliqua and N. pumilio tree, and ten sun leaves from each annual leaf cohort of each N. dombeyi tree were removed and returned to the laboratory in paper bags. Brown, dry, presumably fully senesced, sun leaves were harvested at the end of the growing season (May 1996) for the deciduous species and for the evergreen species as the oldest leaf cohort senesced (March and April 1996).
Laboratory methods
The area of each leaf was determined; then each leaf was dried to constant mass at 60°–70°C and weighed. Leaves from individual trees were then pooled, ground, and digested with concentrated H2SO4 and 30% H2O2. Nitrogen concentration was determined by Kjeldahl distillation and phosphorus concentration by the ascorbic acid method, using U.S. Bureau of Standards Pine Needles and Orchard Leaves as standards. Specific leaf mass was expressed as milligrams per square decimetre area, and foliar N and P as both milligrams per gram dry leaf mass (concentration basis sensu Aerts, 1996
) and milligrams per decimeter leaf area (content or pool basis sensu Aerts, 1996
).
Repeated-measures, one-way analysis of variance (ANOVA) was used to compare foliar N and P concentrations among species and among N. dombeyi leaf cohorts during the period during which both specific leaf mass and nutrient concentrations were relatively constant, on both on area and mass basis. Least square means were used to post-test differences among species' means. Resorption was calculated as the difference between N and P content of leaves at the time of full leaf expansion and that of senesced leaves. One-way ANOVA was used to compare resorption rates among species. In addition, for N. dombeyi foliar N and P concentrations and annual resorption were calculated for each annual leaf cohort as the difference between November and May leaves for that cohort. Resorption proficiency (sensu Killingbeck and Whitford, 1996
) was estimated from fresh litter nutrient concentrations. The Ryan-Einot-Gabriel-Welsch Modified F test was used to post-test differences among resorption means (SAS, 1995
). This test was chosen for its strength against Type I errors. All statistical analyses were performed using SAS for Windows, Version 2.1 (SAS, 1995
), and all significant differences were at P < 0.01, except where otherwise noted.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Foliar N Dynamics
On a mass basis, foliar N concentration during the period of the growing season in which foliar nutrient concentrations were relatively stable varied significantly among species, and decreased in the order: N. obliqua > N. pumilio > N. dombeyi (Table 1). However, when differences in SLM were taken into account by expressing foliar N concentration on an area or content basis (i.e., milligrams N per decimetre leaf area), the pattern changed. On an area basis, foliar N decreased in the order: N dombeyi > N. pumilio > N. obliqua (Table 1). On both mass and area bases, foliar N was significantly lower in the 4th-yr leaf cohort (the 1992 cohort) of N. dombeyi than in the three younger annual leaf cohorts (Table 1)
The temporal pattern of foliar N in leaves of N. dombeyi varied across the leaf lifespan. During the 1st yr, foliar N first decreased rapidly during December and January, then returned to November levels by March, and remained constant the rest of the 1st yr and the 2nd yr. There were significant and positive differences between foliar N in leaves at the end of their 2nd yr and those at the beginning of their 3rd (Fig. 1), and between those at the end of their 3rd and beginning of their 4th yr. During both the 3rd and 4th yr, foliar N decreased consistently through the growing season.
There was no significant resorption of foliar N from N. dombeyi leaves during the first two growing seasons, either on a mass (= concentration) or area (= content) basis (Table 2); however, resorption of N from older leaves was significant, and averaged 30 and 50% during the 3rd and 4th growing seasons, respectively. There was a significant and positive relationship between leaf age and N resorption in N. dombeyi (Fig. 3).
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Nothofagus dombeyi foliar P concentrations varied little through the leaf lifespan. During the 1st yr foliar P decreased through the first half of the growing season, then remained relatively constant until 1–2 mo before abscission (Fig. 2). In contrast to the pattern of resorption of foliar N, there was no statistically significant difference in resorption of P among N. dombeyi leaf cohorts on either mass or area bases (Table 2).
The seasonal pattern of foliar P concentrations in the two deciduous species differed somewhat from those for foliar N. Nothofagus obliqua leaves changed little in foliar P from the time of full leaf expansion in mid-December through abscission in May (Fig. 1). In contrast, foliar P concentration in N. pumilio leaves was much more variable, with alternating periods of increase and decrease throughout the growing season.
The resorption of P differed greatly between the two deciduous species (Table 2). Nothofagus pumilio resorbed 39–48% of its foliar P prior to leaf abscission, and P resorption by N. pumilio during its 7 mo leaf lifespan was as high, or higher, as that observed in N. dombeyi over a 4-yr leaf lifespan (Table 2). In contrast, there was no significant resorption of P from leaves of N. obliqua prior to abscission (Table 2). There were no significant differences among species in P resorption proficiency (Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). In N. dombeyi, there was a steady increase in specific leaf mass through the first half of the first growing season, after which specific leaf mass remained relatively constant until abscission.
The specific leaf masses of the two deciduous species were similar to those reported for the deciduous Quercus rubra in the northern United States (Jurik, 1986
), but 20–25% lower than that reported for N. pumilio on Tierra del Fuego (Gutierrez et al., 1991
). Although the leaves of the two deciduous species differed by a factor of five in average leaf area, they did not differ significantly in specific leaf mass. We observed an increase in specific leaf mass at the beginning of the growing season in the deciduous N. obliqua, similar to what we observed in N. dombeyi. In contrast, we observed no distinct early-season change in specific leaf mass in N. pumilio, nor did we observe in any of the three species the strong seasonality in specific leaf mass that Jurik (1986)
reported.
The foliar N and P concentrations we observed in the deciduous Nothofagus species (N. pumilio and N. obliqua) were similar to or slightly below those reported for N. pumilio on Tierra del Fuego (Gutierrez et al., 1991
; Richter and Frangi, 1992
) and for north-temperate members of the Fagaceae (Boerner, 1984a
). Similarly, the evergreen N. dombeyi had similar concentrations of foliar N to those reported for the evergreen N. betuloides on Tierra del Fuego (Gutierrez et al., 1991
), and greater foliar N but lower foliar P concentration than the evergreen N. truncata from New Zealand (Miller, 1963a
).
On a concentration basis, foliar N and P were significantly lower in the evergreen N. dombeyi than in either deciduous species, as was also the case in a comparison of evergreen N. betuloides and deciduous N. pumilio on Tierra del Fuego (Gutierrez et al., 1991
). The deciduous N. pumilio and N. obliqua also differed significantly from each other in foliar N and P, with N. obliqua leaves having significantly higher foliar N and lower foliar P. However, when foliar nutrient concentrations were calculated on a content or area basis, the leaves of the two lower elevation species (N. dombeyi and N. obliqua) had significantly greater N than did those of the N. pumilio. On an area basis, N. dombeyi leaves had significantly greater foliar P than did the leaves of the two deciduous species, which did not differ from each other.
With respect to our alternate hypotheses, the foliar N concentrations of these three species were consistent with the leaf lifespan hypothesis when the concentrations were calculated on a mass basis (i.e., N. dombeyi < N. pumilio < N. obliqua). However, when foliar N was expressed on a per unit leaf area basis, the evergreen species had the greatest foliar N and the pattern among species was not consistent with either of our hypotheses. The reversal in the ranking of N. dombeyi foliar N on mass vs. area bases was the consequence of the difference in specific leaf mass among species. As light interception is a function of leaf area, not leaf mass, the greater foliar N per unit leaf area in N. dombeyi might be expected to translate into greater photosynthetic capacity. However, preliminary estimates of photosynthesis under field conditions (N. obliqua: 140.2 nmol C·g-1·s-1 vs. N. dombeyi 35.8 nmol C·g-1·s-1; K. L. M. Decker, unpublished data) suggest that the greater structural mass and complexity of the N. dombeyi leaves may offset the greater foliar N in this species. Based on the differences in specific leaf mass among species, the carbon cost of a N. dombeyi leaf is approximately twice that of a leaf of either of the deciduous species; however, even on an area basis, the foliar N content of N. dombeyi leaves exceeds that of the deciduous species by only 9–31%. Thus, it is clear that it will take considerably more time for a leaf of N. dombeyi to pay its carbon cost through net photosynthesis than will be the case for the deciduous species.
On a mass basis, the pattern of differences in foliar P (i.e., N. dombeyi < N. obliqua < N. pumilio) was consistent with our leaf lifespan hypothesis. Once again, when foliar P was calcuated on an area basis, the rankings changed such that the two deciduous species had lower foliar P than the evergreen species, a pattern not consistent with either of our hypotheses. Knops and Koenig (1997)
also observed lower foliar N and P and greater specific leaf mass in leaves of evergreen species than deciduous species of Quercus in central California.
In the evergreen N. dombeyi, foliar N concentration increased through much of the growing season in the 1st-yr leaf cohort and remained fairly constant throughout the growing season in the 2nd-yr cohort. In contrast, there were significant decreases in foliar N through the growing season in the two oldest leaf cohorts. Overall, foliar N in N. dombeyi was significantly lower in the oldest leaf cohort than in the younger three. Nothofagus dombeyi foliar P followed a different temporal pattern: a gradual decrease in foliar P through the growing season in the first cohort combined with relatively constant foliar P in the older three cohorts. There were no significant differences in foliar P among N. dombeyi leaf cohorts. Thus, the pattern of foliar N and P among and within leaf cohorts we observed in N. dombeyi did not fit the pattern of progressive and steadily decreasing concentrations described in the evergreen Picea mariana by Tyrrell and Boerner (1987)
. Unlike P. mariana, in which the majority of the photosynthetic carbon gain occurs during a given leaf's first growing season, the temporal patterns of foliar N and P among N. dombeyi leaf cohorts appear to be most optimal for carbon gain during the leaf's second growing season
On a mass basis, we observed neither significant differences in proportional N resorption among species nor were there significant differences among species in N resorption proficiency, despite our relatively modest sample size. On an area basis, in contrast, we did observe that proportional resorption of N was significantly lower in N. dombeyi than in N. pumilio or N. obliqua. Both measures of proportional P resorption indicated that N. dombeyi and N. pumilio resorbed more P than did N. obliqua, and P resorption proficiency was greater in N. dombeyi than in the two deciduous species. Proportional P resorption by N. obliqua was not significantly different from zero. In N. dombeyi, there was a significant and positive relationship between N resorption and leaf cohort age; however, there were no differences in P resorption among N. dombeyi leaf cohorts.
Knops and Koenig (1997)
, in their study of evergreen and deciduous Quercus species from a range of community types, substrates, and aspects in central California, concluded that N resorption did not differ among "sympatric" species of Quercus while P resorption did. They also concluded that there was no compelling evidence for any relationship between N conservation and N availability. However, as the Quercus species examined by Knops and Koenig (1997)
were only sympatric in the broad, geographical sense and did not co-occur within individual communities to any great extent, the degree to which their conclusions can be applied to species that do co-occur at the community scale is uncertain. We submit that studies that compare different taxa from different sites cannot resolve the relationships, if any, between soil nutrient availability and nutrient conservation. Only studies that compare closely related evergreen and deciduous species that occur naturally in the same stands can clarify this issue.
The rates of resorption we observed in these Chilean Nothofagus species were similar to those reported for deciduous and evergreen species in the Fagaceae in New Zealand and the northern hemisphere (Table 3). In addition, the rates of resorption we observed for the Chilean deciduous species were similar to those reported for the deciduous conifer Larix laricina by Tyrrell and Boerner (1987)
. However, the rates of resorption that Tyrrell and Boerner (1987)
reported for the evergreen conifer Picea mariana (58–69% N and 73–81% P) were considerably higher than what we and others have observed for evergreens in the Fagaceae (Table 3).
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concluded that there was no consistent evidence for a difference in resorption between evergreen and deciduous species, our more restricted comparison of resorption within a single family suggests such a pattern does exist, at least for N. The evergreen species listed in Table 3 had six of the eight lowest N resorption rates and averaged among them 41% N resorbed; in contrast, the six highest N resorption rates were from deciduous species, which averaged 59% N resorbed. There appears to be no such pattern for P resorption, however. P resorption averaged 55% in the evergreen Fagaceae species and 49% in the deciduous species. Furthermore, if the negative P resorption by N. obliqua is disregarded, the mean P resorption of the deciduous species rises to 58%. Once again, these results differ from those of Knops and Koenig (1997)
who postulate strong P feedback between plant and soil but no such strong feedback for N.
Reich, Walters, and Ellsworth (1992)
present an approach for integrating individual leaf ecophysiological traits to allow extrapolation of such data to the understanding of whole-plant and forest stand dynamics. They emphasize the interrelation among leaf traits and the unique combination of traits necessary to persist in a given environment. Thus, our results concerning specific leaf mass and nutrient dynamics can only contribute to the understanding of the controls on the distribution of these species along an elevation gradient from a broader ecophysiological context; however, among-species differences in nutrient relations do not suffice to explain the elevational distribution of these three species.
Nothofagus pumilio occupies the highest elevations and is important as a mid-to-late successional tree as far south as Tierra del Fuego (Veblen et al., 1996
). As such, it must be adapted to both generally extreme climate conditions (especially low AET) and highly variable weather patterns. This species makes maximal use of the short growing season and relatively low temperatures by producing small, structurally inexpensive leaves with moderately high nutrient concentrations. Such leaves can be produced at relatively modest carbon cost and brought to maximum photosynthesis quickly when spring arrives. At the end of the short growing season, the combination of small leaf mass and high N resorption minimizes the losses of key nutrients, and the relatively high tolerance to freezing allows N. pumilio to extend the photosynthetic season beyond what might otherwise be expected (Veblen et al., 1996
). Armesto, Casassa, and Dollenz (1992)
have hypothesized that the ability of N. pumilio to colonize recent disturbances is related to its combination of the ectomycorrhizal habit and the presence of associated, N-fixing herbs to supply N. Our data suggest that strong N conservation may play as great a role as these N supply sources in the success of N. pumilio on such substrates.
Because of the permanence of the stress-tolerating mechanism of longer leaf lifespan, N. dombeyi must take a conservative approach to growth and survival. This, we feel, is reflected in the low leaf nutrient concentrations we observed, more so than in resorption. However, a high rate of resorption may not be critical to broad-leaved evergreens because the relatively low nutrient demand of trees with low relatively growth rate and low leaf nutrient concentrations may still be met if nutrient turnover in the forest floor and soil is relatively slow and gradual. The N and P resorption profiencies of N. dombeyi, as measures by nutrient concentrations in fresh litter, are greater than those reported for both deciduous (Carlisle, Brown, and White, 1966
; Boerner, 1984a
) and evergreen (Miller, 1963a
) species in this family. The resultant high C:N and C:P ratios will, in turn, result in strong nutrient immobilization in the litter (cf. Boerner, 1984b
) and slow release of nutrients during decomposition. Thus, the success of N. dombeyi may be the consequence, to some degree, of its ability to maintain its foliar nutrient economy in the warmer and wetter midelevations. The inability of this nutrient-conservative species to persist at higher elevations, where the advantages of the evergreen habit might be expected to be even greater may be due to its having a lower freezing tolerance than N. pumilio (Veblen et al., 1996
). Differences in summer soil moisture levels and P availability may also affect the local distribution of N. dombeyi and N. pumilio in their overlap zone, much as they are thought to do where N. pumilio and the evergreen N. betuloides overlap farther to the south (Gutierrez et al., 1991
). Differences in antiherbivore defenses may also help account for these distribution patterns. Coley, Bryant, and Chapin (1985)
suggested that low resource availability in a given site should lead to the dominance of evergreen species with low growth rate, large investments in antiherbivore defenses, and relatively low levels of herbivore damage. Although one is tempted to erect such an hypothesis for the dominance of N. dombeyi over N. pumilio at midelevations, Veblen et al., (1996)
report that the evergreen N dombeyi commonly suffers greater herbivore damage than does N. pumilio in stands where the two species co-occur.
Nothofagus obliqua is near its upper elevational limit in our study area and may well exhibit foliar nutrient behavior patterns more suited to lower, warmer elevations. For example, because of downslope transport of soil materials to the valley bottoms resulting from the heavy precipitation loads on the west slopes of the Andes, a constant supply of fresh, weatherable mineral materials should make P availability relatively high, at least as compared to upper and midslope elevations. Thus, one might anticipate that the available N:P ratio in the soil solution in the areas occupied by N. obliqua would be lower than those occupied by N. dombeyi and N. pumilio. As a consequence, one might anticipate selection pressure for greater N conservation, relative to P, in the topographic positions occupied by N. obliqua. We feel this is reflected in the resorption behavior of N. obliqua, which has the highest N resorption rate and lowest P resorption rate among both the three Chilean Nothofagus species we examined and the larger suite of fagaceous species (Table 3).
Although we found relatively little support for our initial hypotheses concerning the comparative foliar nutrient dynamics and resorption by these three species, we believe we have demonstrated a role for foliar nutrient dynamics and conservation in the elevational distribution of these three species. Ongoing studies of nutrient supply rates (via ectomycorrhizae and litter decomposition) and carbon acquisition, when combined with existing models for the relationships among leaf lifespan, phenology, carbon acquisition, and herbivore defense, will yield a more complete picture of the ecophysiological strategies underlying the distributions and post-establishment interactions among these species.
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FOOTNOTES |
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4 Author for correspondence (e-mail: boerner.1@osu.edu, phone: 614-292-8280, fax: 614-292-2030).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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———. 1996 Nutrient resorption from senescing leaves of perennials: are there general patterns? Journal of Ecology 84: 597–608.[CrossRef]
——— and M. J. van der Peijl. 1993 A simple model to explain the dominance of low-productivity perennials in nutrient-poor habitats. Oikos 66: 144–147.[CrossRef][ISI]
Armesto, J. J., I. Casassa, and O. Dollenz. 1992 Age structure and dynamics of Patagonian beech forests in Tierra del Paine National Park, Chile. Vegetatio 98: 13–22.[CrossRef][ISI]
Axelrod, D. I. 1966 Origin of deciduous and evergreen habits in temperate forests. Evolution 20: 1–15.
Berendse, F. 1994 Competition between plant populations at high and low nutrient supply. Oikos 71: 253–260.[CrossRef][ISI]
———, and S. Jonasson. 1992 Nutrient use and nutrient cycling in northern ecosystems. In F. S. Chapin III, R. L. Jeffries, J. F. Reynolds, G. R. Shaver, and J. Svoboda [eds.], Arctic ecosystems in a changing climate, 337–356. Academic Press, London.
Berry, E. W. 1914 The upper Cretaceous and Eocene floras of South Carolina and Georgia. United States Geological Survey Professional Paper 84, Washington, DC.
Boerner, R. E. J. 1984a Foliar nutrient dynamics and nutrient use efficiency of four deciduous tree species in relation to site fertility. Journal of Applied Ecology 21: 1029–1040.[CrossRef][ISI]
———. 1984b Nutrient fluxes in litterfall and decomposition in four forests along a gradient of soil fertility in southern Ohio. Canadian Journal of Forest Research 14: 794–802.[CrossRef]
Carlisle, A., A. H. F. Brown, and E. J. White. 1966 Litter fall, leaf production, and the effects of defoliation by Tortrix viridiana in a sessile oak (Quercus petraea) woodland. Journal of Ecology 54: 65–85.[CrossRef]
Chapin, F. S., III. 1980 The mineral nutrition of wild plants. Annual Review of Ecology and Systematics 11: 233–260.
———, and R. A. Kedrowski. 1983 Seasonal changes in nitrogen and phosphorus fractions and autumn retranslocation in evergreen and deciduous taiga trees. Ecology 64: 376–391.[CrossRef][ISI]
Coley, P. D., J. P. Bryant, and F. S. Chapin, III. 1985 Resource availability and plant antiherbivore defenses. Science 230: 895–899.
Decker, K. L. M., and R. E. J. Boerner. 1997 Soil nitrogen status under three Nothofagus species in pure and mixed stands in central Chile. Proceedings of the II Southern Connection Congress, Valdivia, Chile, 147 (Abstract). University of Southern Chile, Valdivia, Chile.
Escudero, A., J. M. del Arco, and M. V. Garrido. 1992 The efficiency of nitrogen retranslocation from leaf biomass in Quercus ilex ecosystems. Vegetatio 99–100: 225–237.
Gray, J. T. 1983 Nutrient use by evergreen and deciduous shrubs in southern California. I. Community nutrient cycling and nutrient-use efficiency. Journal of Ecology 71: 21–41.[CrossRef]
Gutiérrez, E, V., R. Vallejo, J. Romañá, and J. Fons. 1991 The subantarctic Nothofagus forests of Tierra del Fuego: distribution, structure, and production. Oecologia Aquatica 10: 1–14.
Jurik, T. W. 1986 Temporal and spatial patterns of specific leaf weight in successional northern hardwood tree species. American Journal of Botany 73: 1083–1092.[CrossRef][ISI]
Killingbeck, K. T., and W. G. Whitford 1996 High foliar nitrogen in desert shrubs: an important ecosystem trait or defective desert doctrine? Ecology 77: 1716–1727.[CrossRef]
Knops, J. M. H., and W. D. Koenig. 1997 Site fertility and leaf nutrients of sympatric evergreen and deciduous species of Quercus in central coastal California. Plant Ecology 130: 121–131.[CrossRef][ISI]
Kulp, J. L. 1961 Geologic time scale. Science 133: 1105–1114.
Lesquereux, L. 1892 The flora of the Dakota group. United States Geological Survey Monograph 17, Washington, DC.
Mayor, X., and F. Rodá. 1992 Is primary production in holm oak forests nutrient limited? Vegetatio 99–100: 209–217.
Miller, R. B. 1963a Nutrients in hard beech. I. The immobilisation of nutrients. New Zealand Journal of Science 6: 365–377.
———. 1963b Nutrients in hard beech. II. Seasonal variation in leaf composition. New Zealand Journal of Science 6: 378–387.
———. 1963c Nutrients in hard beech. III. The cycle of nutrients. New Zealand Journal of Science 6: 388–413.
Ostman, N. L., and G. T. Weaver. 1982 Autumnal nutrient transfers by retranslocation, leaching, and litter fall in chestnut oak forest in southern Illinois. Canadian Journal of Forest Research 12: 40–51.
Reich, P. B., M. B. Walters, and D. S. Ellsworth. 1992 Leaf life span in relation to leaf, plants, and stand characteristics among diverse ecosystems. Ecological Monographs 62: 365–392.[CrossRef]
Richter, L. L., and J. L. Frangi. 1992 Bases ecologicas para el manejo del Bosque de Nothofagus pumilio de Tierra del Fuego. Revista de la Facultad de Agronomia, La Plata 68: 35–52.
Rodriquez, R. R., O. Matthei S., and M. Quezada M. 1983 Flora Arbórea de Chile. Editorial de la Universidad de Concepcion, Concepcion, Chile.
SAS. 1995 Statistical analysis system user's guide: statistics. SAS Institute, Cary, NC.
Sabaté, S., A. Sala, and C. A. Gracia. 1995 Nutrient content in Quercus ilex canopies: Seasonal and spatial variation within a catchment. Plant and Soil 168–169: 297–304.
Singer, R., and M. Moser. 1960 Ectotropic forest tree mycorrhiza and forest communities. Ecology 41: 549–551.[CrossRef][ISI]
———, and ———. 1965 Forest mycology and forest communities in South America. I. The early fall aspect of the mycoflora of the Cordillera Pelada (Chile), with a mycogeographic analysis and conclusions regarding the heterogeneity of the Valdivian floral district. Mycopathologia et Mycologia Application 26(2–3) 9–189.
Small, E. 1972 Photosynthetic rates in relation to nitrogen recycling as an adapatation to nutrient deficiency in peat bog plants. Canadian Journal of Botany 50: 2227–2233.
Staff, H. 1982 Plant nutrient changes in beech leaves during senescence as influenced by site characteristics. Oecologia Plantarum 3: 161–170.
Tyrrell, L. E., and R. E. J. Boerner. 1987 Larix laricina and Picea mariana: relationships among leaf life-span, foliar nutrient patterns, nutrient conservation, and growth efficiency. Canadian Journal of Botany 65: 1570–1577.
Walker, T. W., and J. K. Syers. 1976 The fate of phosphorus during pedogenesis. Geoderma 15: 1–19.
Veblen, T. T., C. Donoso, T. Kitzberger, and A. J. Rebertus. 1996 Ecology of southern Chilean and Argentinian Nothofagus forests. In T. T. Veblen, R. S. Hill, and J. Read [eds.], The ecology and biogeography of Nothofagus forests, 293–353. Yale University Press, New Haven, CT.
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