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The relative share of graminoid and forb life‐forms in a natural gradient of herb layer productivity

The relative share of graminoid and forb life‐forms in a natural gradient of herb layer productivity Kull, O and Aan, A 1997 The relative share of graminoid and forb life forms in a natural gradient of herb layer productivity - Ecography 20 146-154 The proportional share of graminoid and forb hfe-form in the herbaceous layer was lnvesugated along a productivity gradient at Laelatu, westem Estonia With an mcrease in the herbaceous layer standing crop from 43 5 to 723 7 g m~^ the graminoid life-form became dominant in total above-ground mass and in species number Three hypotheses to better explain competitive ability of graminoids were tested 1) graminoids are able to form higher foliage, 2) they are able to distnbute foliage nitrogen in a more beneficial way, 3) they have better nitrogen use efficiency 21 sample plots 50 x 50 cm were harvested All above-ground parts of vascular plants were removed by two canopy layers Vertical separation of layers were made according to the height of half hght interception A species list was compiled, totai and leaf masses and leaf nitrogen content of both life-forms were measured by layer ANOVA showed that there were no significant differences in vertical distnbution of foliage or foliage nitrogen between life-forms in the productivity gradient, and hypotheses 1) and 2) are not supported by our data-set Hypothesis 3) is approved partly as the nitrogen concentration in graminoid foliage was 20% less than in forbs If one supposes that nitrogen retention time is equal in both life-forms then graminoids must have higher nitrogen use efficiency when compared to forbs Although the influence of life-form x productivity interaction on leaf nitrogen concentration was not sigmficant, there was a tendency that difference in leaf mass to nitrogen ratio of the two life-forms increased with increasing incident hght Thus, we can hypothesize that graminoid species dominate m high productive plots where the incident light mtensity is also higher due to their better nitrogen use efficiency when compared to forb species O Kull (olevt@park tartu ee) and A Aan, Inst of Ecology, Estonian Academy of Sctences, Lat 40, EE-2400 Tartu, Estonia Grouping plants into growth form classes in plant community studies provides a logical link between physiological strategies and ecosystem or global processes because species of a given growth form often share a similar physiology and traits by which growth forms are classified can have important ecosystem consequences (Chapin 1993) It is well known that in an herbaceous community the graminoid life-form wiU often dommate in high productivity conditions (Tilman 1988, Kull and Zobel 1991, Tamm 1991) This dominance has usually been explamed by the abihty of Accepted 22 May 1996 Copynght © ECOGRAPHY 1997 ISSN 0906-7590 Pnnted in Ireland - all nghts reserved ] 46 grasses to grow higher foliage than forbs, leadmg to their supenor competitive ability for exploiting light resources (Caldwell 1987, Tilman 1988, Tamm 1991) With increasing herbaceous layer productivity the average height of the canopy also mcreases, but species diversity decreases rapidly (Gnme 1979, Tilman 1985, Schulze and Chapin 1987, Taylor et al. 1990). Studies m the last decade have shown that plant nitrogen economy may be also an important predictor of competitive ability (Radm 1983, Poorter et al 1990, Chiba and Hirose 1993) To attain high growth rates in ECOGRAPHY 20 2 (1997) a nitrogen limited environment, plants should have either a high nitrogen uptake rate or high nitrogen-use efficiency (NUE) or both (Chiba and Hirose 1993) It has been shown, in companson with some agncultural crop plants, that grasses use nitrogen more efficiently than broad-leaved species (Radin 1983) Nitrogen-use efficiency depends on mtrogen productivity and nitrogen residence time in the plant tissue (Berendse and Aerts 1984) In the case where the nitrogen residence time is similar, dry mass to nitrogen ratio can be used to assess the NUE (Schimel et al 1991) Due to the dose relationship between leaf photosynthetic capacity and leaf nitrogen content (Van Keulen et al 1989), the ability to distribute nitrogen in foliage under a strong light gradient m a canopy may also be an important predictor of plant competitive ability It has been shown that plants tend to distnbute nitrogen between their leaves in a way which maximizes whole plant photosynthesis (Hirose and Werger 1987) In the case of this optimal nitrogen distnbution, nitrogen concentration in leaves follows the light gradient in the canopy (Hirose et al 1988) The aim of our study was to investigate growth-form distnbution along a natural gradient of herb layer productivity and to understand whether changes in relative share of grammoid and forb hfe forms are associated with 1) differences in vertical distnbution of foliage, 2) difTerences in ability to distribute nitrogen in foliage, or 3) differences in nitrogen-use efficiency 1964) Three shots were taken above each plot From every photograph the relative amounts of canopy gaps were measured with respect to zenith angle and relative amount of diffuse solar radiation to honzontal surface, a,,, was calculated No correction for possible differences in direct solar radiation was made An a^ value of 1 corresponds to full light (no overstory) and an a^ value of 0 corresponds to complete shade Light interception m the herb layer was measured with a 30 cm line-sensor pyranometer Interception was calculated as the average of five measurements above and below the herb layer and was evaluated as the fraction of full light Height where half of the total herb layer interception occurs was also predicted dunng the light measurements Foliage measurements All the above-ground parts of the herbaceous layer were removed within the 50 x 50 cm sample plot as two canopy layers Vertical separation of layers was made according to the height of half hght interception A species list was compiled for both layers Plant matenal of both layers was then soned into grammoids and forbs, and then into leaves and stems plus generative organs Soned matenal was dned and weighed Nitrogen analysis Materials and methods The study area is at Laelatu, on the west coast of Estonia The mean temperature for July is 17°C, and for January - 5 ° C The mean annual precipitation is 500 mm The soil is mainly a rendzic leptosol with a pH around 7 The parent matenal is limestone shingle mixed with vanous coastal sediments Overstory, where present, is species nch, consisting of Quercus robur, Betula pendula, Fraxtnus excelstor, Populus tremula, Pinus sylvestris, Juntperus communis, Corylus avellana, Comus sangmneus, Vtburnum opulus among others The field layer is mainly perennial herbs and grasses with C3 type of photosynthesis, and is extremely species rich Up to 63 vascular plant species per m^ have been counted m some parts of the study area (KuU and Zobel 1991). All field measurements were made m July, 1991 21 sample plots of 0 25 m^ (50 x 50 cm) were distributed over the area, from closed broad-leaf forest through wooded meadow to almost open meadow, to obtain a natural productivity gradient of the herbaceous layer Three samples from upper and lower canopy leaves of graminoids and forbs were taken separately from every sample plot Nitrogen content was measured by the standard Kjeldahl method Statistical analysis Two-way ANOVA was used to evaluate productivity level (two levels, < 175 g m ~ - of foLage, 11 sample plots, and > 175 g m~^ of foliage, 10 sample plots), life-form (forb or graminoid) and productivity x life-form interaction effects on the vertical distribution of foliage and nitrogen The least square linear regression techmque was used to fit curves Results Herbaceous layer productivity in studied plots differs greatly Standing crop (total aboveground mass) changes from 43 5 to 723 7 g m " - and mass of foliage from 30.5 to 306 4 g m~^. Differences in incident to herbaceous layer hght are partly responsible for vanations in productivity (Fig 1), but other factors are probably important as well When considenng the plots receiving almost full hght (>0.8 of full hght), then a Light measurements Relative mcident hght was measured usmg the hemisphencal ("fish-eye") photograph technique (Anderson ECOGRAPHY 20 2(19977 Relative incident light fig 1 Relationship between relative incident light (measured as proportion of full light) and herbaceous layer standing crop Linear regression has an R- value 0 371 positive relationshtp between statidmg crop and foliar nitrogen content appears (Fig 2) Species diversity differed greatly in the studied plots Total number of species of vascular plants vanes from 3 to 51 in a 0 25 m- plot There were two plots under broadleaf forest without any graminoid species whereas maximal number of grammoid species per plot was 19 Maximal species diversity was measured at standing crop values between 250 and 300 g m ' - (Fig 3) TTie ^ bell-like shape of the curve appears clearly on species number versus intercepted light relationship (Fig 4) But decrease of total sp>ecies number in dense canopies of the herbaceous layer is mainly due to a decrease in forb species number, leading to a steady mcrease of grammoid species share in the total species number with mcreasing light interception in the herbaceous layer (Fig 5) Dominance of grammoid species m high-productivity sites also appears in foliage dry mass (Fig 6) With increasing herbaceous layer productivity, foliage dry mass of grarmnoids increases steadily whereas foliage dry mass of forbs has a maximum at intermediate values of total herbaceous foliar mass. This different behaviour of two growth-forms in a productivity gradient IS statistically sigmficant, as shown by the two-way ANOVA (Table 1) 148 Grammoid species do not have more foliage in upper canopy when compared with forb species (Table 2) There is even a reverse tendency that forbs have relatively more foliage in the upper canopy at high productivity sites if compared to low productivity sites, but these differences are not significant (Table 1) Leaf nitrogen concentration is ca 1 3 times higher in leaves from the upper canopy if compared to leaves in the lower canopy This ratio is slightly bigger m dense canopies of productive sites if compared with less productive plots (Table 2), but this factor, as well as growth-form or growth-form x site productivity mteraction have no statistically significant effect on the nitrogen concentration ratio (Table 1). There is a significant difference between average fohar nitrogen concentration in forbs and graminoids (Table 1) Grammoid species have ca 20% less nitrogen per dry weight than forb species (Table 2) The site productivity X growth-form mteraction has no statistically significant effect on leaf nitrogen concentration (Table 1) Consequently, the hypothesis that dry mass to nitrogen ratio m the two life-form groups behaves differently along a productivity gradient is not confirmed by this analysis There is a tendency that difference in leaf dry mass to nitrogen ratio of the two life-form groups mcreases with mcreasing incident light (Fig 7). ECOGRAPHY 20J (1997) 700 - 600 - 500 T a S 400 Ol C 300 Foliage nitrogen concentration [% DW] Fig 2 Relationship between average foliar nitrogen concentration and herbaceous layer standing crop m plots receiving > 80% of full light Regression has an R- value 0 573 Discussion The studied herbaceous layer productivity gradient is formed mainly by the light gradient above the herbaceous layer, l e due to differences in tree canopy coverage Differences in nutntional status of the sites also have some importance when supposing that tissue nitrogen concentration and site fertility are positively correlated (Fig 2) In natural communities these two factors, site fertility and incident to herbaceous layer light intensity, are probably negatively correlated with each other, as m case of natural succession, where relative amotmt of trees in the commumty and site fertility mcrease simtiltaneously (Tilman 1990) But in a semi-natural community, such as a wooded meadow, these two factors are more independent of each other (KuU et al 1995) With increasing herbaceous layer productivity the graminoid life-form will clearly dominate m our plots This dominance appears in the proportion of leaf mass as well as in species number Between standmg crop and species number a well-known relationship appeared with maximum species diversity m mtermediate standing crop values (Gnme 1979) Standing crop value of 280 g m~^ in the case of the highest species diversity (51 species in 0.25 m^) falls mto the range of other investigations from different commumties (e g KJinkhamer and Jong 1985, Garcia et al 1993). There exist several explanations for the decrease in species diversity with lncreasmg standing crop (Gough et al. ECOGRAPHY 20 2 (1997) 1994) Competition for light may be an important predictor of species diversity in the investigated communities Increased competitive pressure, measured as intercepted canopy light, was associated with a gradual decrease in species number in the herbaceous layer (Fig 4) But, as shown by our data, this decrease in species number is only due to forb species Number of graminoid species per plot does not decrease with increasing compwtition for light and the relative share of graminoid sjjecies in the community increases gradually On the basis of our data we can conclude that species numbers of graminoid and forb life-forms behave differently m a productivity gradient despite the responsible mechanism If to assume that increased competitive pressure is the reason leading to a decrease in species diversity with increasing productivity, then the differential behaviour of forb and graminoid species numbers confirms the assumption that graminoid species are better competitors than forb species Our data do not support the hypothesis that dominance of graminoids in an herbaceous canopy at high productive sites is caused by the ability to grow higher foliage. According to our results changes of forb foliage along the productivity gradient do not differ m the upper and lower canopy This does not mean that dommant sp)ecies do not have higher foliage, but does mean that forb growth-form is replaced by grammoid growth-form simultaneously m both canopy layers Several sp)ecies level studies have shown that foliage of 60 • 50- 40 J II "5 30 - 20 I 400 Standidng crop [g/m^] Fig 3 Relationship between herbaceous layer standing crop and number of vascular plant species in a 0 25 m- plot dominant species form upper layer m the canopy and capture most of the available light suppressing with this other species (e g Hirose and Werger 1995) But our data on foliage distnbution show that we cannot use this explanation m growth-form to growth-form companson Vertical distnbution of foliar nitrogen in the studied herbaceous canopies follows the known pattem Relatively more nitrogen is invested into upper leaves Our data conform with the finding that a vertical gradient in foliar nitrogen concentration depends on total canopy dry mass (Hirose et al 1987) Schimel et al (1991) found that in tallgrass praine the ratio of nitrogen concentration in the top layer to the nitrogen concentration in the bottom layer changes between 1 and 2 and was bigger m high biomass sites and correlated with total intercepted PAR In this study we also found that a nitrogen gradient in the canopy was slightly steeper in high productive sites Difference between the life-forms was not statistically significant, thus a differential strategy in vertical distnbution of foliar nitrogen cannot be responsible for dommance of graminoids in high productivity sites The most pronounced difference between grammoid and forb hfe-forms found in this study was in average fohar nitrogen concentration. Graminoids had ca 20% less mtrogen in their leaf biomass when compared with 150 forb species In general, lower nitrogen concentration may denote two things less efficient nitrogen uptake or higher nitrogen use efficiency McJannet et al (1995) have shown an example of wetland plant community that plants with higher above-ground biomass had lower tissue nitrogen concentration than smaller plants in the community and explained this with high growth rate of bigger plants It is likely that low tissue nitrogen concentration is the result of high NUE The reciprocal of nitrogen concentration m plant tissue can be used as an assessment of differences in NUE in conditions of equal nitrogen retention time m plants (Berendse and Aerts 1987) We can assume that at this time it did not differ between graminoids and forbs in the studied plots as almost all species identified are perennials in both life-form groups Forb spwcies were relatively more successful m shady habitats when compared with graminoids as m some plots, under a dense tree canopy, graminoid spwaes were missing entirely. Some other studies have also shown that graminoids tend to be very sensitive to shading (Chapin et al 1995) High leaf nitrogen content probably allows forbs to build up a powerful hght-captunng apparatus needed to photosynthesize under a low light environment (Evans 1989) There are also other possible mechamsms what can add to the domination of grammoids in high producuve sites For instance, it has been shown that grass litter ECOGRAPHY 20 2 (1997) Relative intercepted iight Fig 4 Relationship between light intercepted by the herbaceous layer (measured as fraction of full light) and total vascular plant species number ( • ) , or number of graminoid species (D) in a 0 25 m- plot Relative intercepted light Fig 5 Relationship between light intercepted by the herbaceous layer (measured as fracuon of full light) and relauve share of graminoid species number in relation to total number of vascular plant species Regression has an R^ value 0 631 E C C X J R A P H Y 20-2 (1997) Total foliage dry mass [g/m^] Fig 6 Relationship between total herbaceous layer foliage dry mass and foliage dry mass of forbs ( • ) and graminoids (D) Curves are fitted with 2nd order polynomials Table 1 F values and significance levels for two-way ANOVA of dry mass and nitrogen data "' = P > 0 05, *** P < 0 001 * There are two productivity levels (<175 gm"-^ and > 175 g m"-^) and two life-forms (graminoid and forb) Productivity Foliage dry weight per ground area of one growth-form Dry weight ratio of foliage in upper canopy to lower canopy Ratio of N concentration in upper canopy to lower canopy Mean N concentration in foliage (%) 19 14"* 0 03"' 2 29"' 0 03"' Source of vanation* Growth-form 3 14"' 0 99"' 1 16"' 14 14*'* Produc X Gr -f 13 3 1 ' " 0 48"' 0 35"' 0 08"' Table 2 Distnbution of leaf dry mass and nitrogen between canopy layers in Ufe-form and productivity groups Group means and SD values are given Life-form Forb Graminoid Productivity group <175gm-^ >175gm-^ <175 g m-^ >175gm-= Rauo of foliage in upper canopy to lower canopy 0.821 +0 197 1.136 + 0601 0 712 + 0 163 0 521+0 084 Ratio of N concentration in upper canopy to lower canopy 1 225 ± 0 047 1.316 + 0 042 1 297 + 0 046 1 337 ± 0 028 Foliar N concentration (% DW) 2 12 + 0 10 2 07 ± 0 14 163 ± 0 13 1 64±0.09 ECOGRAPHY 20-.2 (1997) 09 Relative incident light Fig 7 Relationship between relative incident light (measured as proportion of full light) and foliage dry mass to leaf nitrogen ratio in forbs ( • ) and graminoids (D) Regressions have an R^ values 0 005 (forbs) and 0 112 (grammoids) the suppression of forb seedling emergence by grass {Poa pratensis) litter - Funct Ecol 9 635-639 Caldwell, M M 1987 Plant architecture and resource competition - In Schulze, E -D and Zwolfer, H (eds). Potentials and limitations of ecosystem analysis Ecol Stud 61 164-179 Chapin, F S III 1993 Functional role of growth forms in ecosystem and global processes - In Ehlennger, J R and Field, C B (eds). Scaling physiological processes leaf to globe Academic Press, pp 287-312 - , Shaver, G R , Giblin, A E , NadelhofTer, K J and Laundre, A J 1995 Resposes of Arctic tundra to expenmental and observed changes in climate - Ecology 76 694-711 Chiba, N and Hirose, T 1993 Nitrogen acquisition and use in three perennials in the early stage of pnmary succession Acknowledgements - This work was supported by the Esto- Funct Ecol 7 287-292 nian Sciences Foundation We are the most grateful to Laun Evans, J R 1989 Photosynthesis and nitrogen relaUonships in Oksanen for his constructive comments leaves of C, plants - Oecologid 78 9-19 Garcia, L V , Maranon, T , Moreno, A and Clemente, L 1993 Above-ground biomass and species nchness in a Mediterranean salt marsh - J Veg Sci 4 417-424 References Gough, L , Grace, J B and Taylor, K L 1994 The relationship between species nchness and community biomass the Anderson, M C 1964 Studies of the woodland light climate importance of environmental vanables - Oikos 70 2 7 1 I The photographic computation of light condiuons - J 279 Ecol 52 27-41 Berendse, F and Aerts, R 1987 Nitrogen-use-efBciency a Gnme, J. P 1979. Plant strategies and vegetation processes John Wiley biologically meaningful definition' - Funct Ecol 1 2 9 3 Hirose, T and Werger, M J A 1987 Maximizing daily 296 canopy photosynthesis with respect to the leaf nitrogen J L and Reader, R J 1995 Mechanisms underlying may suppress forb seedling emergence (Bosy and Reader 1995) On the basis of our data we may hypothesize that graminoid species dominate in high productivity plots where the incident light intensity is also high, due to their better nitrogen use efficiency when compared with forb species But a specially designed exp)enment is needed to show whether an mcrease in difference of NUE between granunoids and forbs with increasing hght resource availability (Fig 7) is sufficient to be responsible for a change in dominance ECOGRAPHY 20-2 (1997) allocation pattem in the canopy - Oecologia 72 520526 - and Werger, M J A 1995 Canopy structure and photon flux partitioning among species in a herbaceous plant commumty - Ecology 76 466-474 - , Werger, M J A , Pons, T L and van Rheenen, J W A 1988 Canopy structure and leaf nitrogen distnbution in a stand of Ly^imachia vulgaru L as influenced by stand density - Oecologia 77 145-150 Keulen. H van, Goudnaan. J and Seligman, N G 1989 Modelling the efTects on nitrogen on canopy development and crop growth - In Russell, G , Marshall, B and Jarvis, P G (eds). Plant canopies their growth form and function Soc Exper Biol Sem Ser Vol 31 Cambndge Umv Press, pp 83-104 Klinkhamer, P G L and de Jong, T J , 1985 Shoot biomass and species richness in relation to some environmental factor in a coastal dune area m The Netherlands - Vegetatio 63 129-132 KuU, K and Zobel, M 1991 High species nchness m an Estonian wooded meadow - J Veg Sci 2 711714 Kull, O , Aan, A and Soelsepp, T 1995 Light interception, nitrogen and leaf mass distnbution in a multilayer plant community - Funct Ecol 9 589-595 McJannet, C L , Keddy, P A and Pick, F R 1995 Nitrogen and phosphorus tissue concentrations in 41 wetland plants a companson across habitats and functional groups Funct Ecol 9 231-238 Poorter, H , Remkes, C and Lambers, H 1990 Carbon and nitrogen economy of 24 wild species diffenng m relative growth rate - Plant Physiol 94 621-627 Radin, J W 1983 Control of plant grovrth by nitrogen differences between cereals and broadleaf species - Plant Cell Environm 6 65-68 Schulze, E - D and Chapin, F S III 1987 Plant specialization to environments of different resource availability - In Schulze, E -D and Zwolfer, H (eds). Potentials and limitations of ecosystem analysis Ecol Stud 61 120-148 Schimel, D S , Kittel, T G F , Knapp, A K , Seastedt, T R , Parton, W J and Brown, V B 1991 Physiological interactions along resource in a tallgrass praine - Ecology 72 672-684 Tamm, C O 1991 Nitrogen in terrestnal ecosystems - Ecol Stud 81 Taylor, D R , Aarssen, L W and Loehle, C 1990 On the relationship between r/K selection and environmental carrying capacity new habitat template for plant life histor> strategies - Oikos 58 239-250 Tilman, D 1985 The resource-ratio hypothesis of plant succession - Am Nat 125 827-852 - 1988 Plant strategies and the dynamics and structure of plant communities - Pnnceton Univ Press - 1990 Constraints and tradeoffs toward a predictive theory of competition and succession - Oikos 58 3-15 - and Cowan, M L 1989 Growth of old field herbs on a nitrogen gradient - Funct Ecol 3 425-438 ECOGRAPHY 2<h2 (1997) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Ecography Wiley

The relative share of graminoid and forb life‐forms in a natural gradient of herb layer productivity

Ecography , Volume 20 (2) – Apr 1, 1997

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Wiley
Copyright
Copyright © 1997 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0906-7590
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1600-0587
DOI
10.1111/j.1600-0587.1997.tb00357.x
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Abstract

Kull, O and Aan, A 1997 The relative share of graminoid and forb life forms in a natural gradient of herb layer productivity - Ecography 20 146-154 The proportional share of graminoid and forb hfe-form in the herbaceous layer was lnvesugated along a productivity gradient at Laelatu, westem Estonia With an mcrease in the herbaceous layer standing crop from 43 5 to 723 7 g m~^ the graminoid life-form became dominant in total above-ground mass and in species number Three hypotheses to better explain competitive ability of graminoids were tested 1) graminoids are able to form higher foliage, 2) they are able to distnbute foliage nitrogen in a more beneficial way, 3) they have better nitrogen use efficiency 21 sample plots 50 x 50 cm were harvested All above-ground parts of vascular plants were removed by two canopy layers Vertical separation of layers were made according to the height of half hght interception A species list was compiled, totai and leaf masses and leaf nitrogen content of both life-forms were measured by layer ANOVA showed that there were no significant differences in vertical distnbution of foliage or foliage nitrogen between life-forms in the productivity gradient, and hypotheses 1) and 2) are not supported by our data-set Hypothesis 3) is approved partly as the nitrogen concentration in graminoid foliage was 20% less than in forbs If one supposes that nitrogen retention time is equal in both life-forms then graminoids must have higher nitrogen use efficiency when compared to forbs Although the influence of life-form x productivity interaction on leaf nitrogen concentration was not sigmficant, there was a tendency that difference in leaf mass to nitrogen ratio of the two life-forms increased with increasing incident hght Thus, we can hypothesize that graminoid species dominate m high productive plots where the incident light mtensity is also higher due to their better nitrogen use efficiency when compared to forb species O Kull (olevt@park tartu ee) and A Aan, Inst of Ecology, Estonian Academy of Sctences, Lat 40, EE-2400 Tartu, Estonia Grouping plants into growth form classes in plant community studies provides a logical link between physiological strategies and ecosystem or global processes because species of a given growth form often share a similar physiology and traits by which growth forms are classified can have important ecosystem consequences (Chapin 1993) It is well known that in an herbaceous community the graminoid life-form wiU often dommate in high productivity conditions (Tilman 1988, Kull and Zobel 1991, Tamm 1991) This dominance has usually been explamed by the abihty of Accepted 22 May 1996 Copynght © ECOGRAPHY 1997 ISSN 0906-7590 Pnnted in Ireland - all nghts reserved ] 46 grasses to grow higher foliage than forbs, leadmg to their supenor competitive ability for exploiting light resources (Caldwell 1987, Tilman 1988, Tamm 1991) With increasing herbaceous layer productivity the average height of the canopy also mcreases, but species diversity decreases rapidly (Gnme 1979, Tilman 1985, Schulze and Chapin 1987, Taylor et al. 1990). Studies m the last decade have shown that plant nitrogen economy may be also an important predictor of competitive ability (Radm 1983, Poorter et al 1990, Chiba and Hirose 1993) To attain high growth rates in ECOGRAPHY 20 2 (1997) a nitrogen limited environment, plants should have either a high nitrogen uptake rate or high nitrogen-use efficiency (NUE) or both (Chiba and Hirose 1993) It has been shown, in companson with some agncultural crop plants, that grasses use nitrogen more efficiently than broad-leaved species (Radin 1983) Nitrogen-use efficiency depends on mtrogen productivity and nitrogen residence time in the plant tissue (Berendse and Aerts 1984) In the case where the nitrogen residence time is similar, dry mass to nitrogen ratio can be used to assess the NUE (Schimel et al 1991) Due to the dose relationship between leaf photosynthetic capacity and leaf nitrogen content (Van Keulen et al 1989), the ability to distribute nitrogen in foliage under a strong light gradient m a canopy may also be an important predictor of plant competitive ability It has been shown that plants tend to distnbute nitrogen between their leaves in a way which maximizes whole plant photosynthesis (Hirose and Werger 1987) In the case of this optimal nitrogen distnbution, nitrogen concentration in leaves follows the light gradient in the canopy (Hirose et al 1988) The aim of our study was to investigate growth-form distnbution along a natural gradient of herb layer productivity and to understand whether changes in relative share of grammoid and forb hfe forms are associated with 1) differences in vertical distnbution of foliage, 2) difTerences in ability to distribute nitrogen in foliage, or 3) differences in nitrogen-use efficiency 1964) Three shots were taken above each plot From every photograph the relative amounts of canopy gaps were measured with respect to zenith angle and relative amount of diffuse solar radiation to honzontal surface, a,,, was calculated No correction for possible differences in direct solar radiation was made An a^ value of 1 corresponds to full light (no overstory) and an a^ value of 0 corresponds to complete shade Light interception m the herb layer was measured with a 30 cm line-sensor pyranometer Interception was calculated as the average of five measurements above and below the herb layer and was evaluated as the fraction of full light Height where half of the total herb layer interception occurs was also predicted dunng the light measurements Foliage measurements All the above-ground parts of the herbaceous layer were removed within the 50 x 50 cm sample plot as two canopy layers Vertical separation of layers was made according to the height of half hght interception A species list was compiled for both layers Plant matenal of both layers was then soned into grammoids and forbs, and then into leaves and stems plus generative organs Soned matenal was dned and weighed Nitrogen analysis Materials and methods The study area is at Laelatu, on the west coast of Estonia The mean temperature for July is 17°C, and for January - 5 ° C The mean annual precipitation is 500 mm The soil is mainly a rendzic leptosol with a pH around 7 The parent matenal is limestone shingle mixed with vanous coastal sediments Overstory, where present, is species nch, consisting of Quercus robur, Betula pendula, Fraxtnus excelstor, Populus tremula, Pinus sylvestris, Juntperus communis, Corylus avellana, Comus sangmneus, Vtburnum opulus among others The field layer is mainly perennial herbs and grasses with C3 type of photosynthesis, and is extremely species rich Up to 63 vascular plant species per m^ have been counted m some parts of the study area (KuU and Zobel 1991). All field measurements were made m July, 1991 21 sample plots of 0 25 m^ (50 x 50 cm) were distributed over the area, from closed broad-leaf forest through wooded meadow to almost open meadow, to obtain a natural productivity gradient of the herbaceous layer Three samples from upper and lower canopy leaves of graminoids and forbs were taken separately from every sample plot Nitrogen content was measured by the standard Kjeldahl method Statistical analysis Two-way ANOVA was used to evaluate productivity level (two levels, < 175 g m ~ - of foLage, 11 sample plots, and > 175 g m~^ of foliage, 10 sample plots), life-form (forb or graminoid) and productivity x life-form interaction effects on the vertical distribution of foliage and nitrogen The least square linear regression techmque was used to fit curves Results Herbaceous layer productivity in studied plots differs greatly Standing crop (total aboveground mass) changes from 43 5 to 723 7 g m " - and mass of foliage from 30.5 to 306 4 g m~^. Differences in incident to herbaceous layer hght are partly responsible for vanations in productivity (Fig 1), but other factors are probably important as well When considenng the plots receiving almost full hght (>0.8 of full hght), then a Light measurements Relative mcident hght was measured usmg the hemisphencal ("fish-eye") photograph technique (Anderson ECOGRAPHY 20 2(19977 Relative incident light fig 1 Relationship between relative incident light (measured as proportion of full light) and herbaceous layer standing crop Linear regression has an R- value 0 371 positive relationshtp between statidmg crop and foliar nitrogen content appears (Fig 2) Species diversity differed greatly in the studied plots Total number of species of vascular plants vanes from 3 to 51 in a 0 25 m- plot There were two plots under broadleaf forest without any graminoid species whereas maximal number of grammoid species per plot was 19 Maximal species diversity was measured at standing crop values between 250 and 300 g m ' - (Fig 3) TTie ^ bell-like shape of the curve appears clearly on species number versus intercepted light relationship (Fig 4) But decrease of total sp>ecies number in dense canopies of the herbaceous layer is mainly due to a decrease in forb species number, leading to a steady mcrease of grammoid species share in the total species number with mcreasing light interception in the herbaceous layer (Fig 5) Dominance of grammoid species m high-productivity sites also appears in foliage dry mass (Fig 6) With increasing herbaceous layer productivity, foliage dry mass of grarmnoids increases steadily whereas foliage dry mass of forbs has a maximum at intermediate values of total herbaceous foliar mass. This different behaviour of two growth-forms in a productivity gradient IS statistically sigmficant, as shown by the two-way ANOVA (Table 1) 148 Grammoid species do not have more foliage in upper canopy when compared with forb species (Table 2) There is even a reverse tendency that forbs have relatively more foliage in the upper canopy at high productivity sites if compared to low productivity sites, but these differences are not significant (Table 1) Leaf nitrogen concentration is ca 1 3 times higher in leaves from the upper canopy if compared to leaves in the lower canopy This ratio is slightly bigger m dense canopies of productive sites if compared with less productive plots (Table 2), but this factor, as well as growth-form or growth-form x site productivity mteraction have no statistically significant effect on the nitrogen concentration ratio (Table 1). There is a significant difference between average fohar nitrogen concentration in forbs and graminoids (Table 1) Grammoid species have ca 20% less nitrogen per dry weight than forb species (Table 2) The site productivity X growth-form mteraction has no statistically significant effect on leaf nitrogen concentration (Table 1) Consequently, the hypothesis that dry mass to nitrogen ratio m the two life-form groups behaves differently along a productivity gradient is not confirmed by this analysis There is a tendency that difference in leaf dry mass to nitrogen ratio of the two life-form groups mcreases with mcreasing incident light (Fig 7). ECOGRAPHY 20J (1997) 700 - 600 - 500 T a S 400 Ol C 300 Foliage nitrogen concentration [% DW] Fig 2 Relationship between average foliar nitrogen concentration and herbaceous layer standing crop m plots receiving > 80% of full light Regression has an R- value 0 573 Discussion The studied herbaceous layer productivity gradient is formed mainly by the light gradient above the herbaceous layer, l e due to differences in tree canopy coverage Differences in nutntional status of the sites also have some importance when supposing that tissue nitrogen concentration and site fertility are positively correlated (Fig 2) In natural communities these two factors, site fertility and incident to herbaceous layer light intensity, are probably negatively correlated with each other, as m case of natural succession, where relative amotmt of trees in the commumty and site fertility mcrease simtiltaneously (Tilman 1990) But in a semi-natural community, such as a wooded meadow, these two factors are more independent of each other (KuU et al 1995) With increasing herbaceous layer productivity the graminoid life-form will clearly dominate m our plots This dominance appears in the proportion of leaf mass as well as in species number Between standmg crop and species number a well-known relationship appeared with maximum species diversity m mtermediate standing crop values (Gnme 1979) Standing crop value of 280 g m~^ in the case of the highest species diversity (51 species in 0.25 m^) falls mto the range of other investigations from different commumties (e g KJinkhamer and Jong 1985, Garcia et al 1993). There exist several explanations for the decrease in species diversity with lncreasmg standing crop (Gough et al. ECOGRAPHY 20 2 (1997) 1994) Competition for light may be an important predictor of species diversity in the investigated communities Increased competitive pressure, measured as intercepted canopy light, was associated with a gradual decrease in species number in the herbaceous layer (Fig 4) But, as shown by our data, this decrease in species number is only due to forb species Number of graminoid species per plot does not decrease with increasing compwtition for light and the relative share of graminoid sjjecies in the community increases gradually On the basis of our data we can conclude that species numbers of graminoid and forb life-forms behave differently m a productivity gradient despite the responsible mechanism If to assume that increased competitive pressure is the reason leading to a decrease in species diversity with increasing productivity, then the differential behaviour of forb and graminoid species numbers confirms the assumption that graminoid species are better competitors than forb species Our data do not support the hypothesis that dominance of graminoids in an herbaceous canopy at high productive sites is caused by the ability to grow higher foliage. According to our results changes of forb foliage along the productivity gradient do not differ m the upper and lower canopy This does not mean that dommant sp)ecies do not have higher foliage, but does mean that forb growth-form is replaced by grammoid growth-form simultaneously m both canopy layers Several sp)ecies level studies have shown that foliage of 60 • 50- 40 J II "5 30 - 20 I 400 Standidng crop [g/m^] Fig 3 Relationship between herbaceous layer standing crop and number of vascular plant species in a 0 25 m- plot dominant species form upper layer m the canopy and capture most of the available light suppressing with this other species (e g Hirose and Werger 1995) But our data on foliage distnbution show that we cannot use this explanation m growth-form to growth-form companson Vertical distnbution of foliar nitrogen in the studied herbaceous canopies follows the known pattem Relatively more nitrogen is invested into upper leaves Our data conform with the finding that a vertical gradient in foliar nitrogen concentration depends on total canopy dry mass (Hirose et al 1987) Schimel et al (1991) found that in tallgrass praine the ratio of nitrogen concentration in the top layer to the nitrogen concentration in the bottom layer changes between 1 and 2 and was bigger m high biomass sites and correlated with total intercepted PAR In this study we also found that a nitrogen gradient in the canopy was slightly steeper in high productive sites Difference between the life-forms was not statistically significant, thus a differential strategy in vertical distnbution of foliar nitrogen cannot be responsible for dommance of graminoids in high productivity sites The most pronounced difference between grammoid and forb hfe-forms found in this study was in average fohar nitrogen concentration. Graminoids had ca 20% less mtrogen in their leaf biomass when compared with 150 forb species In general, lower nitrogen concentration may denote two things less efficient nitrogen uptake or higher nitrogen use efficiency McJannet et al (1995) have shown an example of wetland plant community that plants with higher above-ground biomass had lower tissue nitrogen concentration than smaller plants in the community and explained this with high growth rate of bigger plants It is likely that low tissue nitrogen concentration is the result of high NUE The reciprocal of nitrogen concentration m plant tissue can be used as an assessment of differences in NUE in conditions of equal nitrogen retention time m plants (Berendse and Aerts 1987) We can assume that at this time it did not differ between graminoids and forbs in the studied plots as almost all species identified are perennials in both life-form groups Forb spwcies were relatively more successful m shady habitats when compared with graminoids as m some plots, under a dense tree canopy, graminoid spwaes were missing entirely. Some other studies have also shown that graminoids tend to be very sensitive to shading (Chapin et al 1995) High leaf nitrogen content probably allows forbs to build up a powerful hght-captunng apparatus needed to photosynthesize under a low light environment (Evans 1989) There are also other possible mechamsms what can add to the domination of grammoids in high producuve sites For instance, it has been shown that grass litter ECOGRAPHY 20 2 (1997) Relative intercepted iight Fig 4 Relationship between light intercepted by the herbaceous layer (measured as fraction of full light) and total vascular plant species number ( • ) , or number of graminoid species (D) in a 0 25 m- plot Relative intercepted light Fig 5 Relationship between light intercepted by the herbaceous layer (measured as fracuon of full light) and relauve share of graminoid species number in relation to total number of vascular plant species Regression has an R^ value 0 631 E C C X J R A P H Y 20-2 (1997) Total foliage dry mass [g/m^] Fig 6 Relationship between total herbaceous layer foliage dry mass and foliage dry mass of forbs ( • ) and graminoids (D) Curves are fitted with 2nd order polynomials Table 1 F values and significance levels for two-way ANOVA of dry mass and nitrogen data "' = P > 0 05, *** P < 0 001 * There are two productivity levels (<175 gm"-^ and > 175 g m"-^) and two life-forms (graminoid and forb) Productivity Foliage dry weight per ground area of one growth-form Dry weight ratio of foliage in upper canopy to lower canopy Ratio of N concentration in upper canopy to lower canopy Mean N concentration in foliage (%) 19 14"* 0 03"' 2 29"' 0 03"' Source of vanation* Growth-form 3 14"' 0 99"' 1 16"' 14 14*'* Produc X Gr -f 13 3 1 ' " 0 48"' 0 35"' 0 08"' Table 2 Distnbution of leaf dry mass and nitrogen between canopy layers in Ufe-form and productivity groups Group means and SD values are given Life-form Forb Graminoid Productivity group <175gm-^ >175gm-^ <175 g m-^ >175gm-= Rauo of foliage in upper canopy to lower canopy 0.821 +0 197 1.136 + 0601 0 712 + 0 163 0 521+0 084 Ratio of N concentration in upper canopy to lower canopy 1 225 ± 0 047 1.316 + 0 042 1 297 + 0 046 1 337 ± 0 028 Foliar N concentration (% DW) 2 12 + 0 10 2 07 ± 0 14 163 ± 0 13 1 64±0.09 ECOGRAPHY 20-.2 (1997) 09 Relative incident light Fig 7 Relationship between relative incident light (measured as proportion of full light) and foliage dry mass to leaf nitrogen ratio in forbs ( • ) and graminoids (D) Regressions have an R^ values 0 005 (forbs) and 0 112 (grammoids) the suppression of forb seedling emergence by grass {Poa pratensis) litter - Funct Ecol 9 635-639 Caldwell, M M 1987 Plant architecture and resource competition - In Schulze, E -D and Zwolfer, H (eds). Potentials and limitations of ecosystem analysis Ecol Stud 61 164-179 Chapin, F S III 1993 Functional role of growth forms in ecosystem and global processes - In Ehlennger, J R and Field, C B (eds). Scaling physiological processes leaf to globe Academic Press, pp 287-312 - , Shaver, G R , Giblin, A E , NadelhofTer, K J and Laundre, A J 1995 Resposes of Arctic tundra to expenmental and observed changes in climate - Ecology 76 694-711 Chiba, N and Hirose, T 1993 Nitrogen acquisition and use in three perennials in the early stage of pnmary succession Acknowledgements - This work was supported by the Esto- Funct Ecol 7 287-292 nian Sciences Foundation We are the most grateful to Laun Evans, J R 1989 Photosynthesis and nitrogen relaUonships in Oksanen for his constructive comments leaves of C, plants - Oecologid 78 9-19 Garcia, L V , Maranon, T , Moreno, A and Clemente, L 1993 Above-ground biomass and species nchness in a Mediterranean salt marsh - J Veg Sci 4 417-424 References Gough, L , Grace, J B and Taylor, K L 1994 The relationship between species nchness and community biomass the Anderson, M C 1964 Studies of the woodland light climate importance of environmental vanables - Oikos 70 2 7 1 I The photographic computation of light condiuons - J 279 Ecol 52 27-41 Berendse, F and Aerts, R 1987 Nitrogen-use-efBciency a Gnme, J. 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Plant strategies and vegetation processes John Wiley biologically meaningful definition' - Funct Ecol 1 2 9 3 Hirose, T and Werger, M J A 1987 Maximizing daily 296 canopy photosynthesis with respect to the leaf nitrogen J L and Reader, R J 1995 Mechanisms underlying may suppress forb seedling emergence (Bosy and Reader 1995) On the basis of our data we may hypothesize that graminoid species dominate in high productivity plots where the incident light intensity is also high, due to their better nitrogen use efficiency when compared with forb species But a specially designed exp)enment is needed to show whether an mcrease in difference of NUE between granunoids and forbs with increasing hght resource availability (Fig 7) is sufficient to be responsible for a change in dominance ECOGRAPHY 20-2 (1997) allocation pattem in the canopy - Oecologia 72 520526 - and Werger, M J A 1995 Canopy structure and photon flux partitioning among species in a herbaceous plant commumty - Ecology 76 466-474 - , Werger, M J A , Pons, T L and van Rheenen, J W A 1988 Canopy structure and leaf nitrogen distnbution in a stand of Ly^imachia vulgaru L as influenced by stand density - Oecologia 77 145-150 Keulen. H van, Goudnaan. J and Seligman, N G 1989 Modelling the efTects on nitrogen on canopy development and crop growth - In Russell, G , Marshall, B and Jarvis, P G (eds). Plant canopies their growth form and function Soc Exper Biol Sem Ser Vol 31 Cambndge Umv Press, pp 83-104 Klinkhamer, P G L and de Jong, T J , 1985 Shoot biomass and species richness in relation to some environmental factor in a coastal dune area m The Netherlands - Vegetatio 63 129-132 KuU, K and Zobel, M 1991 High species nchness m an Estonian wooded meadow - J Veg Sci 2 711714 Kull, O , Aan, A and Soelsepp, T 1995 Light interception, nitrogen and leaf mass distnbution in a multilayer plant community - Funct Ecol 9 589-595 McJannet, C L , Keddy, P A and Pick, F R 1995 Nitrogen and phosphorus tissue concentrations in 41 wetland plants a companson across habitats and functional groups Funct Ecol 9 231-238 Poorter, H , Remkes, C and Lambers, H 1990 Carbon and nitrogen economy of 24 wild species diffenng m relative growth rate - Plant Physiol 94 621-627 Radin, J W 1983 Control of plant grovrth by nitrogen differences between cereals and broadleaf species - Plant Cell Environm 6 65-68 Schulze, E - D and Chapin, F S III 1987 Plant specialization to environments of different resource availability - In Schulze, E -D and Zwolfer, H (eds). Potentials and limitations of ecosystem analysis Ecol Stud 61 120-148 Schimel, D S , Kittel, T G F , Knapp, A K , Seastedt, T R , Parton, W J and Brown, V B 1991 Physiological interactions along resource in a tallgrass praine - Ecology 72 672-684 Tamm, C O 1991 Nitrogen in terrestnal ecosystems - Ecol Stud 81 Taylor, D R , Aarssen, L W and Loehle, C 1990 On the relationship between r/K selection and environmental carrying capacity new habitat template for plant life histor> strategies - Oikos 58 239-250 Tilman, D 1985 The resource-ratio hypothesis of plant succession - Am Nat 125 827-852 - 1988 Plant strategies and the dynamics and structure of plant communities - Pnnceton Univ Press - 1990 Constraints and tradeoffs toward a predictive theory of competition and succession - Oikos 58 3-15 - and Cowan, M L 1989 Growth of old field herbs on a nitrogen gradient - Funct Ecol 3 425-438 ECOGRAPHY 2<h2 (1997)

Journal

EcographyWiley

Published: Apr 1, 1997

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