Photosynthetic capacity, integrated over the lifetime

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1 Photosynthetic capacity, integrated over the lifetime Blackwell Publishing Ltd. of a leaf, is predicted to be independent of leaf longevity in some tree species Sonia Mediavilla and Alfonso Escudero Departamento de Ecología, Facultad de Biología, Universidad de Salamanca, Salamanca, Spain Summary Author for correspondence: Sonia Mediavilla Tel: Fax: ecomedv@usal.es Received: 1 February 2003 Accepted: 28 March 2003 doi: /j x x The relationships between leaf longevity and the average photosynthetic capacity of the different leaf age-classes present in the crown were studied in several tree species to understand the effects of the differences in leaf longevity on the final C budget of the leaves. Photosynthetic capacity per unit leaf mass (A/mass) was measured in leaves of all age classes present in the crown. Demographic analyses were conducted to establish the age structure of the leaf populations. With these data we estimated the average A/mass of the whole leaf population, weighted by the mass of leaves present in each age class. A/mass decreased in all the evergreen species as the leaves aged. As a result, the rates averaged over all leaf age classes in species with greater leaf longevity were much lower than rates of young foliage, and the average rate of decreased with the increase in leaf life span of the different species. The reduction in the average rates with the increase in leaf life span was proportional to the increase in leaf longevity. With these results it may be predicted that CO 2 integrated over the lifetime of the leaf should be independent of leaf longevity. Key words: CO 2, leaf age, leaf life span, leaf nitrogen, specific leaf area, woody species. New Phytologist (2003) 159: Introduction Many studies have postulated that there is a trade-off between the instantaneous CO 2 rate (A) and leaf life span (Chabot & Hicks, 1982; Reich et al., 1999), which has been explained as the result of compromises between productivity and the leaf traits conferring persistence to the foliage (Field & Mooney, 1986; Reich et al., 1991a, 1997; Gower et al., 1993; Poorter & Evans, 1998; Warren & Adams, 2000). As a consequence of these compromises, there is a log-linear negative relationship between leaf longevity and the instantaneous of young leaves, which has been shown to be very similar in form in a wide range of biomes (Reich et al., 1999). The lower instantaneous CO 2 rate of the leaves with a long life span has been assumed to be more than compensated by a longer duration of the leaves, so that the cumulative CO 2 may be even greater in species with a longer leaf life span (Chapin, 1980; Chabot & Hicks, 1982; Gower et al., 1993; Hiremath, 2000; Cordell et al., 2001). However, since the pioneering work of Small (1972), there have been very few attempts to estimate the integrated over the entire lifetime of the leaf, as a function of instantaneous CO 2 and leaf life span. Most calculations that predict a compensation between A and the persistence of the foliage are based on the slope of the log-linear regression between the instantaneous rate and leaf life span (Reich et al., 1992; Westoby et al., 2000). However, in most of these analyses the variation in A with the age of the leaf has not been taken into account, because the study of the relationships between leaf life span and A has usually been based on the mean rates New Phytologist (2003) 159:

2 204 in young mature leaves (Gower et al., 1993; Reich et al., 1995). Nevertheless, many studies have shown that A tends to decline as the leaves age (Field et al., 1983; Hom & Oechel, 1983; Sobrado, 1994; Kitajima et al., 1997; Oleksyn et al., 1997), and this reduction with leaf age must be taken into account when calculating integrated A over the lifetime of the leaf (Hiremath, 2000; Cordell et al., 2001). Furthermore, evergreen trees with a long leaf life span may have a great mass of old leaves, and their contribution to total CO 2 by the crown can be considerable (Schulze et al., 1977). Thus, the effect of the age of the leaves on A must be taken into account in order to calculate the long-term C budget of a leaf and of the whole canopy (Kitajima et al., 1997). The relationships between leaf longevity and the instantaneous rate of the young leaves have been studied in a wide range of biomes (Reich et al., 1999). However, to our knowledge, there are no studies of the relationship between leaf longevity and A averaged over the lifetime of the leaf in evergreen species. This is partly due to the fact that a proper calculation of average A requires knowledge of leaf demography, as well as measurements of the changes in individual leaf mass and in CO 2 rates over the total life of the leaf. However, data sets of this type are scarce (Kitajima et al., 1997; Kikuzawa & Ackerly, 1999). In this study we measured the potential instantaneous CO 2 rates in the leaves of the different age classes of tree species that differ in leaf life span, with a view to determining the rate of decline in A with the age of the leaf. At the same time we measured the variations in leaf number and in leaf mass along the life of the leaves in order to estimate the potential rate averaged over the lifetime of the leaf and to explore the relationships between leaf longevity and the average instantaneous rate. We anticipated that the slope of the log-linear regression between leaf life span and A is more negative when it is based on average A per unit mass of evergreen species than when it is only based on mean A per unit mass of young leaves and that this makes the effects of leaf longevity on instantaneous rates more negative than has been previously reported. Materials and Methods Study species and area The species selected for the experimental work (Table 1) were as follows: Acer monspessulanum L., Quercus faginea Lam., Q. pyrenaica Willd., Q. coccifera L., Q. suber L., Q. rotundifolia Lam., Ilex aquifolium L., Pinus halepensis Miller, P. pinaster Aiton, P. pinea L., P. sylvestris L. and Taxus baccata L. They thus include evergreen species with a mean leaf life span over c. 3 years (Pinus spp. and T. baccata), evergreen species with a leaf life span between c. 1 and 2 years (Q. coccifera, Q. rotundifolia, Q. suber and I. aquifolium), and deciduous species (A. monspessulanum, Q. faginea and Q. pyrenaica). Table 1 List of the tree species studied and mean (± SE) leaf longevity Species All these species occurred at three sites close to Salamanca (central-western Spain) between latitudes N and N and longitudes between 5 20 W and 6 25 W. Altitudes ranged between 700 and 1500 m above sea level. Climate in the study area is cold Mediterranean. Winter and spring are usually relatively cold, and late frosts can appear even in May. Mean annual rainfall ranged from 300 to more than 1000 mm on the sites situated at the greatest altitude. All sites, however, underwent a summer drought period. The sites consisted of sparse populations (between 50 and 100 specimens ha 1 ) of mature (> 100 years old) individuals. Mean heights were about 4 10 m. All specimens selected for the study were fully sun-exposed. Sampling methods At each site, between three and five specimens of each species were randomly selected on each sampling date. From these individuals, a composite sampling of branches with leaves from different crown positions of each canopy was performed at monthly intervals over 3 years (from 1997 to 1999). The samples were immediately taken to the laboratory and separated into annual segments (shoots) of different age classes. Only one flush of leaf growth was observed in all species. Accordingly, all the leaves born in one particular year were considered to belong to the same age class. Subsamples of shoots per sampling date and per age class were used for demographic analyses and for measurements of leaf biomass. The number of leaves or needles per shoot was counted each month for each age class. The samples were oven-dried at 70 C to constant mass and the mean dry mass per leaf was determined. Gas-exchange measurements Mean leaf longevity (months) Quercus pyrenaica 5.3 ± 0.8 Acer monspessulanum 5.9 ± 1.0 Quercus faginea 6.7 ± 1.3 Quercus suber 15.0 ± 2.7 Quercus coccifera 15.6 ± 4.4 Quercus rotundifolia 23.7 ± 2.8 Ilex aquifolium 25.0 ± 7.3 Pinus pinea 35.5 ± 7.2 Pinus halepensis 36.1 ± 2.5 Pinus sylvestris 48.8 ± 5.4 Pinus pinaster 51.2 ± 3.5 Taxus baccata 62.1 ± 4.6 Data are means of three years of sampling. In Mediterranean climates, leaf ageing within a growing season is parallel to a dramatic increase in the intensity of New Phytologist (2003) 159:

3 205 drought stress. Under these conditions, obviously, the effects of leaf ageing are masked by the effects of drought stress. For this reason, the only way of studying the effect of ageing isolated from other factors is to compare the maximum rates achieved at the beginning of the growth season (photosynthetic capacity in field conditions) by each leaf age class. Net maximum photosynthesis of the different leaf age classes was measured with a portable photosynthesis system (Li-6200, Li-Cor Inc., Lincoln, NE, US) under ambient CO 2 concentrations, air temperature, relative humidities and at saturating ambient irradiances. Since the aim was to measure gas-exchange rates under near optimal ambient conditions, all measurements were conducted from 07:00 to 09:00 h solar time on sunny days during late spring and early summer from 1996 to Only sunlit branches were selected. Owing to the low LAI typical of these Mediterranean woodlands (data not shown), even the oldest leaves receive full sunlight, at least during part of the day. We measured the photosynthetic rates of between 20 and 30 leaves of each age class from four to six individuals per species. Leaves selected for the measurements were fully expanded, but not senescent. The measurements were extended from the end of the leaf expansion period to the start of the drought stress in order to estimate the potential rate as a mean of the rates obtained during the 2 or 3 months with the highest assimilatory potential. Immediately after the gas-exchange measurements, each leaf was harvested, taken to the laboratory, and the projected leaf and needle area (with a Delta-T Image Analysis System, Delta-T Devices LTD, Cambridge, UK), dry mass, and N concentration (with a CE-Instruments NA-2100 autoanalyzer, ThermoQuest, Milan, Italy) of each leaf were determined. From the data thus obtained, the specific leaf area (SLA), the N content per unit leaf mass (N/mass), the stomatal conductance (g s ), the per unit leaf mass (A/mass) and the per unit leaf area (A/area) were estimated. Calculations The mean number of leaves per shoot of a given age on each census date was used to construct static life tables (Begon & Mortimer, 1986), which made it possible to estimate the mean leaf life span for each species according to standard methods. Due to the gradualness of leaf fall, maximum leaf life span was much longer than mean leaf life span and in all species we were able to measure photosynthesis in leaves older than the estimated mean leaf life span. Leaf mass per shoot was calculated each month over the 3-year sampling period by multiplying the mean number of leaves per shoot by the average dry mass per leaf. Total maximum instantaneous CO 2 per shoot of a given age was estimated by multiplying the leaf biomass per shoot at the time of the year at which the maximum gas exchange rates were attained ( June and July) by the mean instantaneous A/mass estimated for the leaves of the same age class. By summing the results obtained for all leaf age classes found in each species, an estimation of the total instantaneous CO 2 per branch in the most favourable period of the year was obtained. By dividing this figure again by the total leaf biomass per branch, it was possible to calculate an average instantaneous A/mass weighted by the mass of leaves accumulated in each age class. This figure may also be taken as an estimate of the potential photosynthetic rate averaged over the lifetime of the leaf, weighted by the fraction of the total leaf lifetime that the leaf spends in each life stage. Statistics Linear regression analyses were used to examine relationships between leaf age and A/mass. The test for significantly different slopes in an analysis of covariance (ANCOVA) was used to determine when the slopes of the decline in A/mass with advancing leaf age were significantly different between the different species. The relationships among leaf longevity and the different leaf traits and gas-exchange parameters estimated were described using linear regression analysis. We used logarithmic (base 10) transformations of the data to linearize the regression functions. All the statistical analyses were performed using the SPSS statistical package (SPSS Inc., Chicago, IL, USA). Results All species displayed a single flush of leaf production, and the number of leaves per shoot increased rapidly during the first weeks of the growth season (data not shown). After this initial increase, in the deciduous species the number of leaves underwent slight variations over a few months and then declined rapidly during autumn (Fig. 1). In the evergreens, the decline in leaf number per shoot was usually more gradual and lasted longer. In most evergreen species leaf mortality occurred mainly during the summer. However, in Q. suber and Q. coccifera most leaf abscission occurred during spring, at the time of emergence of the new leaves. Leaf life span varied between 5.3 months in Q. pyrenaica and 62.1 months in T. baccata (Table 1). Obviously, in the species with a long leaf life span the old leaves represented a large proportion of the total leaf biomass. In T. baccata, at the time of year when the total leaf mass is greater, immediately after the emergence of the new flush, only 19% of the total leaf biomass was formed by current-year leaves. For pines and I. aquifolium current-year foliage comprised between c. 30 and 39% of the total leaf biomass. For Q. rotundifolia it rose to c. 45%, because the number of surviving 3-year-old leaves was already very low during this time of the year. Finally, in the evergreens with the shortest leaf longevity (Q. suber and Q. coccifera) the new leaves represented > 60% of the total leaf biomass. New Phytologist (2003) 159:

4 206 Fig. 1 Survivorship curves for leaves of the different species. In all the evergreen species, maximum A/mass was observed in current leaves and then declined approximately linearly with the age of the leaf (Fig. 2). The decline in A/mass with leaf age was paralleled by a decrease in N/mass and in SLA (Fig. 3). The slopes of the linear regressions fitted to the data for decline in A/mass with leaf age of the different species were significantly different according to an analysis of covariance (P < ). The slopes tended to be steeper for short-lived foliage, with the exception of Q. coccifera. The rates of A/mass decline with advancing leaf age in the different species varied between approximately 19 nmol g 1 s 1 per year of leaf age in Q. suber and 2.2 nmol g 1 s 1 per year in T. baccata. For the rest of the species the decline slopes were between 2.7 and 7.2 nmol g 1 s 1 per year for pine species, around 8 nmol g 1 s 1 per year in I. aquifolium and rose to almost 16 nmol g 1 s 1 per year in Q. rotundifolia. When the A/mass of the different age classes was expressed as a percentage of the maximum values found in young tissue, there was still a significant negative relationship (P = ) between leaf life span and the absolute value of the slope of decline of relative A/mass with leaf age. However, there were no significant differences between the regression slopes fitted to the data of most of the species, the only exceptions being T. baccata and P. pinaster, which showed significantly (P = ) shallower regression lines than the other species (see legend in Fig. 2). On average, the photosynthetic capacities of the different leaf cohorts New Phytologist (2003) 159:

5 207 Fig. 2 Pattern of decline in mean (± 1 SE, n = 20 30) photosynthetic capacity with leaf age in evergreen species with different leaf longevities. In the legend for each species, the results of the linear regressions between A/ mass (expressed as percentage of the maximum in the youngest leaf cohort) and leaf age are indicated. Fig. 3 Mean (± 1 SE, n = 20 30) nitrogen concentrations (N/mass) and specific leaf area (SLA) as a function of age class in leaves of evergreen species with different leaf life spans. New Phytologist (2003) 159:

6 208 Table 2 Summary of regressions relating gas-exchange and leaf traits to leaf life span for current-year foliage and for average values Current Average Relationship R 2 P-value Relationship R 2 P-value Log (SLA) = log (leaf life-span) Log (SLA) = log (leaf life-span) Log (N/mass) = log (leaf life-span) Log (N/mass) = log (leaf life-span) Log (g s ) = log (leaf life-span) Log (g s ) = log (leaf life-span) Log (A/area) = log (leaf life-span) Log (A/area) = log (leaf life-span) Log (N/mass) = log (SLA) Log (N/mass) = log (SLA) Log (A/mass) = log (SLA) Log (A/mass) = log (SLA) Log (A/mass) = log (N/mass) Log (A/mass) = log (N/mass) Units; life span (months), SLA (cm 2 g 1 ), N/mass (mg g 1 ), g s (mmol m 2 s 1 ), A/area (µmol m 2 s 1 ), A/mass (nmol g 1 s 1 ). decreased at a rate of between 17 and 30% per year in most of the species, and at a rate of 11 13% per year in T. baccata and P. pinaster. Consequently, after experiencing a long process of decline, in species of long leaf life span the A/mass of the oldest cohort was very low in comparison to the values of the younger tissues, although in all of them there were still positive rates of (at least during favourable periods in the growth season). Obviously, species with shorter leaf longevity did not experience such an intense decrease in A/mass, owing to the shorter duration of the process of decline. For example, in Q. suber and Q. coccifera the oldest leaf cohort still retained between 75 and 78% of the A/mass of the youngest cohort, whereas in T. baccata, P. sylvestris and P. pinea the A/mass of the oldest cohort was only around 23 29% of the maximum, and for the remaining species it varied between 33 and 40%. All the regressions between leaf life span and the different leaf traits and gas-exchange parameters were significant (Table 2 and Fig. 4). Relationships tended to be linear when the variables were log-transformed. As has frequently been reported, the relationships between instantaneous rates and leaf longevity were negative. The reason for this lies in the negative effects of leaf life span on N/mass and on SLA. In turn, A/mass was positively related to SLA and N/ mass. The area-based relationships were much weaker (data not shown) although A/area still showed a negative relationship with leaf longevity (Table 2), thus suggesting that the decrease in A/mass as leaf longevity increased was not merely due to the increase of leaf mass per unit area with leaf longevity. All the significant regressions on leaf life span were stronger, in terms of percentage of variance explained, when calculated with values averaged over the lifetime of the leaf than when they were only based on the data of current-year leaves. The regression slopes between leaf life span and N/mass, g s, A/area, SLA and A/mass were significantly greater in absolute value (i.e. more negative) when they were based on average values than when they were based on current-year foliage. The slope of the A/mass-leaf longevity relationship was significantly greater than 1 for current-year foliage, but it was close to 1 for the average-based regression (Fig. 4). The log log relationship between A/mass and leaf longevity was clearly linear and all the species followed the fitted line. Consequently, the same trend was observed when only evergreen species were included in the comparisons. The slope of the regression fitted for evergreens alone was Discussion Leaf survivorship curves had a shape similar to type I survivorship curves (Begon & Mortimer, 1986). Leaf mortality was concentrated at the end of the maximum leaf life span, which suggests that leaf senescence is under the control of the plant and that it is the result of a reallocation of the resources that limit photosynthesis (Field & Mooney, 1983). Despite the significant decrease in the CO 2 rate with leaf age (Fig. 2), no resorption of N was observed from old leaves until the end of leaf life, since the decline in N/mass with leaf age was compensated for by the increase in leaf mass per unit area (Fig. 3), resulting in an approximately constant absolute N content per leaf. The reduction in A/mass with leaf age was probably due to the decrease in N/mass and in SLA, as well as to a lower N use efficiency of older leaves as N concentrations and SLA decrease (Field & Mooney, 1986; Reich et al., 1992). It has been argued that leaves with a longer life span retain a larger proportion of the initial photosynthetic capacity as they age than shorter-lived leaves (Field & Mooney, 1983; Reich et al., 1991a; Kikuzawa, 1995), and that this helps to justify the retaining of older leaves in species with long leaf life span (Kikuzawa, 1991). This was true in absolute terms for the species we studied. Given that the A/mass of the young tissue was in general higher in the species with shorter leaf longevity (Fig. 4), the absolute decline rate in A/mass with the age of the leaf was also greater in general in leaves with a short life span. However, when calculated as a percentage of the maximum A/mass of each species, the differences in the decline rate of A/mass with leaf age of the different species were less marked. Consequently, in species of long leaf life span the A/mass of the oldest cohort was very low in comparison with the values of the younger tissues. This suggests that New Phytologist (2003) 159:

7 209 Fig. 4 Relationships between mean photosynthetic capacity (A/mass) and leaf life span of the different species for: (a) young (current-year) foliage, and for (b) A/mass values averaged over the lifetime of the leaf. the mean resource use efficiency of evergreen species would be much greater if the old foliage were discarded and the available resources were concentrated in the new leaf biomass. Furthermore, in species with a long leaf life span, the old foliage represented a high percentage of the total leaf biomass. Consequently, the old foliage in the species with long leaf life span contributed in a large extent to determining the average photosynthetic characteristics of the whole canopy. The species with long leaf life span showed much smaller average A/mass than might have been expected from the values observed for young leaves. As a consequence, the slopes of the log-linear relationships between leaf life span and the photosynthetic characteristics were steeper when calculated with average values instead of current-year foliage data (Table 2 and Fig. 4). The slopes of the relationships between leaf longevity and A/mass for the younger leaves were indeed almost identical to the values reported by Reich et al. (1991a, 1992) and Reich (1993), thus confirming the similarity of this type of relationships in different biomes (Reich et al., 1997, 1999). By contrast, the slopes calculated for the average values were much steeper than the values reported by other authors. The changes in slopes for the relationship between CO 2 and leaf life span observed in the present study, in relation to the values reported by other authors, are relevant in the comparisons between species with different leaf life spans because the value of the slope determines whether a low instantaneous CO 2 rate is or is not really compensated for by a long leaf duration. A slope equal to 1 in the log linear relationship between leaf longevity and A would mean that the product of instantaneous rate leaf life span is independent of leaf life span (Westoby et al., 2000). A slope greater than 1 would imply that cumulative C increases with leaf life span. Unlike the conclusions of Westoby et al. (2000), when data for all leaf age classes of evergreens are included in the calculations, the slope for the A/mass-leaf longevity regression suggests that long-term C would be independent of leaf life span instead of increasing with leaf life span. New Phytologist (2003) 159:

8 210 Obviously, our estimates of instantaneous CO 2 rate, based on measurements made on all leaf age classes, but only during periods of maximum photosynthetic activity, cannot be taken as true estimates of the mean CO 2 rate throughout the leaf life. The mean CO 2 of a leaf along its life can only be estimated by a very detailed and time-consuming series of gas-exchange measurements carried out at different times in the growth season, including estimates of respiration during nonassimilatory periods. However, if mean annual CO 2 were correlated to the photosynthetic capacity, our estimates of photosynthetic capacity could be used as an estimate of mean annual in comparative terms. Nevertheless, this assumption may not be valid because of a number of factors. The comparison between evergreen and deciduous species may be affected by the existence of a leafless period in deciduous species. In our cold Mediterranean climate, winter is rather cold and it is not likely that photosynthetic activity during this period would contribute to increasing the total annual of evergreen species to any significant extent. This has also been suggested by other authors in other environments (Schulze et al., 1977). Very little CO 2 was measured on sunny days in winter at a temperature of around 10 C for Q. rotundifolia and Q. suber at the same sites as those used in the present study (Mediavilla & Escudero, unpublished results). Under these circumstances, a large part of the lifetime of evergreen leaves corresponds to seasons with low assimilatory potential. By contrast, the short leaf life span of deciduous species to a large extent coincides with periods favourable for CO 2, at least during the coolest hours of each day. Multiplying mean A/mass in spring by total leaf duration would probably exaggerate the total production of evergreens in comparison with the deciduous species. On the other hand, part of the decline in CO 2 of deciduous leaves during the summer could be attributed to an age-related decline in assimilatory capacity, similar to that seen for the different leaf age classes of evergreen species, and not only to drought stress. This was not taken into account in our approach, because it is very difficult to separate both effects in our Mediterranean climate. The lack of measurements in old deciduous leaves would tend to exaggerate the mean production of deciduous species. However, in moist climates, light-saturated CO 2 of deciduous leaves has seen to be almost constant until senescence (Reich et al., 1991b; Morecroft & Roberts, 1999). Consequently, the agerelated decline in CO 2 in deciduous species is probably negligible in comparison with the decline associated to drought stress, which is also experienced by the evergreens. In any case, as can be seen in Fig. 4, the different species follow the same trend of decline in average A/mass with increasing leaf life span. The slope of the log-linear regression fitted for the evergreens alone was also close to 1, and, accordingly, in this group of species the increase in leaf life span was also compensated for by the decrease in A/mass. Besides the decline in photosynthetic capacity with leaf age, two other factors may contribute to the decrease in the CO 2 rate during leaf ageing: self-shading among leaves, as new leaves are produced in upper-canopy positions and, in dry environments, the increase in drought stress during the summer. Photosynthetic capacity in spring may not be an adequate predictor of the mean annual rate of the different species if there are interspecific differences in the intensity of the deterioration of leaf function due to shading and/or drought stress. It is usually assumed that the effects of self-shading within the canopy are stronger in leaves of longer life span (Gower et al., 1993), and that photosynthesis by older leaves in evergreen species is limited by shading (Beyschlag et al., 1994). With respect to the decrease in A during the summer drought, if all the species studied were more or less similar as regards stomatal sensitivity to drought, the differences in integrated CO 2 should parallel the differences in photosynthetic capacity. Few comparative studies of stomatal sensitivity have been made using species of different leaf longevities. Although some of them report no differences between species differing in leaf longevity (Damesin et al., 1998; Kloeppel et al., 2000), other authors have found a higher stomatal sensitivity of species of longer leaf life span to increases in soil and/or atmospheric drought (Tretiach, 1993; Nardini et al., 1999; Fotelli et al., 2000; Kolb & Stone, 2000). In the same sites of the present study, Q. rotundifolia showed a higher stomatal sensitivity to atmospheric drought than Q. suber (Mediavilla & Escudero, unpublished results). This suggests that a longer leaf life span could be associated with a more conservative water use and with stronger reductions in CO 2 during periods of drought. However, more research is clearly needed to clarify this. The effects of a more intense self-shading within the crown of species of long-lived foliage, as well as a possible higher stomatal sensitivity to drought in evergreen species, would contribute to worsening the final C budget of the leaves with a long life span. This could reduce the slope of the A vs leaf longevity relationship, perhaps to a value of less than 1, which would imply that cumulative C gain decreases with increasing leaf life span. Another issue that could contribute to worsening the final C budget of the leaves with a long life span involves time-discounting effects (Westoby et al., 2000). In view of these negative effects of a long leaf life span on productivity, it is clear that further research is necessary to understand the adaptive significance of a long leaf life span. Acknowledgements This paper has received financial support from the Spanish Ministry of Education (Project No. AMB ) and from the Regional Government of Castilla-León (Project nos SA 47/95 and SA 72/00B). New Phytologist (2003) 159:

9 211 References Begon M, Mortimer A Population ecology. Oxford, UK: Blackwell Scientific Publications. Beyschlag W, Ryel RJ, Dietsch C Shedding of older needle age classes does not necessarily reduce photosynthetic primary production of Norway spruce. Trees 9: Chabot BF, Hicks D The ecology of leaf life spans. Annual Review of Ecology and Systematics 13: Chapin FS The mineral nutrition of wild plants. Annual Review of Ecology and Systematics 11: Cordell S, Goldstein G, Meinzer FC, Vitousek PM Regulation of leaf life-span and nutrient-use efficiency of Metrosideros polymorpha trees at two extremes of a long chronosequence in Hawaii. Oecologia 127: Damesin C, Rambal S, Joffre R Co-occurrence of trees with different leaf habit: a functional approach on Mediterranean oaks. Acta Oecologica 19: Field C, Merino J, Mooney HA Compromises between water use efficiency and nitrogen use efficiency in five species of California evergreens. Oecologia 60: Field C, Mooney A Leaf age and seasonal effects on light, water, and nitrogen use efficiency in a California shrub. Oecologia 56: Field C, Mooney A The photosynthesis-nitrogen relationship in wild plants. In: Givnish TJ, ed. On the economy of plant form and function. Cambridge, UK: Cambridge University Press, Fotelli MN, Radoglou KM, Constantinidou H-IA Water stress responses of seedlings of four Mediterranean oak species. Tree Physiology 20: Gower ST, Reich PB, Son Y Canopy dynamics and aboveground production of five tree species with different leaf longevities. Tree Physiology 12: Hiremath AJ Photosynthetic nutrient-use efficiency in three fast-growing tropical trees with differing leaf longevities. Tree Physiology 20: Hom JL, Oechel WC The photosynthetic capacity, nutrient content and nutrient use efficiency of different needle age-classes of black spruce (Picea mariana) found in interior Alaska. Canadian Journal of Forest 13: Kikuzawa K A cost-benefit analysis of leaf habit and leaf longevity of trees and their geographical pattern. American Naturalist 138: Kikuzawa K Leaf phenology as an optimal strategy for carbon gain in plants. Canadian Journal of Botany 73: Kikuzawa K, Ackerly D Significance of leaf longevity in plants. Plant Species Biology 14: Kitajima K, Mulkey SS, Wright SJ Decline of photosynthetic capacity with leaf age in relation to leaf longevities for five tropical canopy tree species. American Journal of Botany 84: Kloeppel BD, Gower ST, Vogel JG, Reich PB Leaf-level resource use for evergreen and deciduous conifers along a resource availability gradient. Functional Ecology 14: Kolb TE, Stone JE Differences in leaf gas exchange and water relations among species and tree sizes in an Arizona pine-oak forest. Tree Physiology 20: Morecroft MD, Roberts JM Photosynthesis and stomatal conductance of mature canopy Oak (Quercus robur) and Sycamore (Acer pseudoplatanus) trees throughout the growing season. Functional Ecology 13: Nardini A, LoGullo MA, Salleo S Competitive strategies for water availability in two Mediterranean Quercus species. Plant, Cell & Environment 22: Oleksyn J, Tjoelker MG, Lorenc-Plucinska G, Konwinska A, Zytkowiak R, Karolewski P, Reich P Needle CO 2 exchange, structure and defense traits in relation to needle age in Pinus heldreichii Christ a relict of Tertiary flora. Trees 12: Poorter H, Evans JR Photosynthetic nitrogen use efficiency of species that differ inherently in specific leaf area. Oecologia 116: Reich PB Reconciling apparent discrepancies among studies relating life span, structure and function of leaves in contrasting plant life forms and climates: the blind men and the elephant retold. Functional Ecology 7: Reich PB, Ellsworth DS, Walters MB, Vose JM, Gresham CH, Volin JC, Bowman WD Generality of leaf trait relationships: a test across six biomes. Ecology 80: Reich PB, Kloeppel BD, Ellsworth DS, Walters MB Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species. Oecologia 104: Reich PB, Uhl C, Walters MB, Ellsworth DS. 1991a. Leaf lifespan as a determinant of leaf structure and function among 23 Amazonian tree species. Oecologia 86: Reich PB, Uhl C, Walters MB, Ellsworth DS. 1991b. Leaf age and season influence the relationships between leaf nitrogen, leaf mass per area and photosynthesis in maple and oak trees. Plant, Cell & Environment 14: Reich PB, Walters MB, Ellsworth DS Leaf life-span in relation to leaf, plant and stand characteristics among diverse ecosystems. Ecological Monographs 62: Reich PB, Walters MB, Ellsworth DS From tropics to tundra: global convergence in plant functioning. Proceedings of the National Academy of Sciences, USA 24: Schulze ED, Fuchs M, Fuchs M Spatial distribution of photosynthetic capacity and performance in a mountain spruce forest of northern Germany. Oecologia 30: Small E Photosynthetic rates in relation to nitrogen recycling as an adaptation to nutrient deficiency in peat bog plants. Canadian Journal of Botany 50: Sobrado MA Leaf age effects on photosynthetic rate, transpiration rate and nitrogen content in a tropical dry forest. Physiologia Plantarum 90: Tretiach M Photosynthesis and transpiration of evergreen Mediterranean and deciduous trees in an ecotone during a growth season. Acta Oecologica 14: Warren CR, Adams MA Trade-offs between the persistence of foliage and productivity in two Pinus species. Oecologia 124: Westoby M, Warton D, Reich P The time value of leaf area. American Naturalist 155: New Phytologist (2003) 159: