Aboveground biomass allocation, leaf growth, and photosynthesis

Oecologia (Berl.) 24, 159-173 (1976)

Oecologia 9 by Springer-Verlag 1976

Aboveground Biomass Allocation, Leaf Growth, and Photosynthesis Patterns in Tundra Plant Forms in Arctic Alaska Douglas A. Johnson* and Larry L. Tieszen Department of Biology,Augustana College, Sioux Falls, South Dakota 57102, USA

Summary. Tundra plant growth forms can generally be characterized as consisting predominantly of low-growing perennial grasses and sedges, perennial herbaceous forbs, dwarf deciduous shrubs, and dwarf evergreen shrubs. Gross aboveground carbon allocation, leaf growth, and photosynthesis pattern studies were initiated to develop a quantitative understanding of the functional importance of these particular tundra growth forms. Photosynthetic capacities of 13 species were determined under standardized exposure conditions using a 14CO2 field system and ranged between 5 and 47 mg CO2.g dry wt-~.h -1. These results, in conjunction with detailed leaf growth determinations, support the generalization that species with an evergreen growth form have lower photosynthetic capacities than species with a perennial graminoid, forb, or deciduous shrub growth form. However, these low photosynthetic capacities in evergreen shrubs are associated with relatively extended leaf longevities. Conversely, deciduous shrub forms exhibited high photosynthetic capacities, but were offset by relatively short leaf longevity periods. The perennial grasses, sedges, and forbs showed patterns intermediate to these. As a result, it appears that among tundra species of different growth form, photosynthetic capacity is inversely related to leaf longevity.

Introduction The composition and structure of plant communities through evolutionary time are dependent upon the action of abiotic and biotic variables on the available biological pool. In arctic Alaska these interactions are expressed as: 1) communities in which perenniality and vegetative reproduction predominate (Billings and Mooney, 1968; Johnson and Tieszen, 1973), 2) a decrease in numbers * Present address and address for offprint requests: Crops Research Laboratory, Utah State University, UMC 63, Logan, Utah 84322, USA

160

D.A. Johnson and L.L. Tieszen

of species from the south [e.g., 250 species at Umiat (Britton, 1957)] to the arctic coast [e.g., 120 species in the Barrow area (Dennis and Johnson, 1970)], and 3) a decrease in community stature as the arctic coast is approached. Even though arctic tundra areas may differ in the number and kinds of species which make up their vegetation, tundra plant growth forms can generally be characterized as consisting predominantly of low-growing perennial grasses and sedges, perennial herbaceous forbs, dwarf deciduous shrubs, and dwarf evergreen shrubs. The degree to which these differences in growth form are related to functional differences is not thoroughly documented; however, work of Bliss (1962), Hedberg (1964), and Mooney and Dunn (1970b) has suggested that these growth form distributions have functional importance and that each growth form is characterized by a particular allocation strategy which presumably allows successful exploitation of particular habitats. Indeed, distributions of major growth forms have recently been quantitatively related to specific environmental complexes in tundra (Wielgolaski and Webber, 1976). In addition Lewis and Callaghan (1975) and Tieszen and Wieland (1975) have described distinct growth strategies of meadow grasses, sedges, and fellfield plants. These specific growth form strategies likely involve allocation differences for carbon and nutrients (Mooney and Dunn, 1970b; Chapin et al., 1975; Mooney et al., 1975). The allocation patterns of these important plant resources to growth, storage, maintenance, and protection likely represent the means by which plants meet the demands of their environment (Mooney and Chu, 1974) and suggest differences in physiological processes and primary production. This present study was initiated to develop a quantitative understanding of the relationships between gross aboveground carbon allocation, leaf growth, and photosynthesis patterns in different tundra growth forms.

Methods The research was conducted at the Meade River Field Station (70~ 157~ 15 m elev.) of the Naval Arctic Research Laboratory located on the Meade River on the Alaskan North Slope and is approximately 110 k m SSW of Barrow, Alaska. Climate at Meade River is somewhat more continental than the strongly maritime dominated climate at Barrow. For example, the 1975 s u m m e r mean wind velocity and air temperature at 1 m was 11.4 k m .h ~ and 5 ~ C, respectively, while total precipitation during this same period was 80 ram. A detailed geological as well as pedological description of the area can be found in Rickert and Tedrow (1967). The vegetational communities of the area have been described by Tieszen and Johnson (1968) and generally consist of low perennial grasses, sedges, shrubs, and herbs. Plant nomenclature follows Hultbn (1968). For the leaf growth study, 20 plants each of Salix pulchra, Betula nana, Eriophorum vaginatum, and Ledum palustre were selected in June and followed during leaf elongation and senescence throughout the growing season. For Salix and Betula, both deciduous shrub species, leaf groups produced at a given node were followed with as m a n y as 6 leaves produced per leaf group. For these species leaves within a leaf group were consecutively numbered from the most basal to the most distal leaf within the leaf group. Eriophorum, a tussock graminoid, consists of collections of individual tillers comprising a tussock. Individual tillers produced as m a n y as 5 leaves during the growing season. Leaves in Eriophorum were consecutively numbered from the first green leaf to emerge and thus the oldest living leaf on the plant to the last leaf to emerge. Because of -the evergreen growth form of Ledum, leaves of different age classes were followed throughout the growing season.

Allocation, Leaf Growth, and Photosynthesis in Tundra Plants

161

Individual leaf maps of each plant allowed the same leaf to be followed throughout the growing season. Leaf length was measured from the point of petiole attachment to the leaf blade, or in the case of Eriophorum from the base of the plant, to the most distal green portion of the outstretched leaf. Leaf area to length and leaf dry weight to area observations using similar leaf material from the same vicinity as the selected plants were collected three times during the growing season and consisted of 200 observations for each species, except for Salix which consisted of 400 observations. Regression equations were determined for each species and enabled leaf lengths to be converted to their respective leaf dry weights. Leaf photosynthetic measurements were determined using a portable 1~CO2 field system. The field system and procedure have been described by Tieszen et al. (1974). The field system consists of a plexiglass leaf chamber which is temperature controlled by a Peltier thermoelectric stage. The procedure involves exposing an intact leaf to a t4COz air mixture, immediately cooling the leaf sample in the field following exposure, drying the exposed leaf sample, combusting this sample, and radioactivity counting using liquid scintillation techniques. According to studies by Ludwig and Canvin (1971), net 14CO2 uptake in a leaf is maximum at exposure periods less than 30 s. This initial period may approximate gross CO2 influx. After 30 s, net CO2 uptake decreases as the exposure time in 14CO2 is extended and after 10 min approaches normal net CO 2 exchange rates. This decrease in 14CO2 uptake is associated with a4cQ2 evolution from the leaf. Thus, with exposure times of 1 min used in this study, COz uptake measured was likely closer to gross CO2 uptake than net photosynthesis. Air temperatures within the leaf exposure chamber were measured using a shielded copperconstantan fine wire thermocouple. Leaf temperatures were also monitored and were within 1~ C of chamber air temperatures. Artificial irradiation was provided by a high intensity incandescent aircraft landing lamp (100 W) and was monitored with a Lambda Co. Model LI-190S quantum sensor. In addition, the incoming chamber air stream was bubbled through water which yielded a high relative humidity within the plexiglass leaf chamber. A period of 5 min was allowed for the leaf to attain a steady state gas exchange rate before 14CO2 exposure. Only leaves which were the longest, most recently fully expanded on the plant were used during these field experiments. Areas of the exposed leaf samples were outlined on paper with subsequent leaf area (one side) determinations made in the laboratory using a Lambda Model LI-3000 portable area meter.

Results A s an i n d i c a t i o n o f a b o v e g r o u n d c a r b o n u ti li zat i o n by the m a j o r species present in the M e a d e R i v e r C a m p area, dry weight analyses o f i n d i v i d u a l a b o v e g r o u n d p l an t p a r t c o m p o n e n t s were o b t a i n e d at p e a k season a n d are p r esen t ed in F i g u r e 1. Th es e results for Betula nana, Salix pulchra, an d Ledum palustre represent p l a n t p a r t dry weights for an a v e r a g e a b o v e g r o u n d i n d i v i d u a l or in the case o f Eriophorum vaginatum f o r an i n d i v i d u a l tiller. A n a b o v e g r o u n d i n d i v i d u a l was d efi n ed in this study as all a b o v e g r o u n d b i o m a s s issued f r o m a stem w h i ch goes b e l o w g r o u n d . A l t h o u g h a b o v e g r o u n d individuals m a y h a v e been interconnected b e l o w g r o u n d , each a b o v e g r o u n d s h o o t issued f r o m a stem w h i ch goes b e l o w g r o u n d was c o n s i d e r e d an individual. E v e n t h o u g h n o n - c u r r e n t stems c o m p r i s e d the largest standing a b o v e g r o u n d b i o m a s s c o m p a r t m e n t for Betula, Salix, a n d Ledurn, the bulk o f c u r r e n t a b o v e g r o u n d p r o d u c t i o n was c o m p r i s e d o f l e a f p r o d u c t i o n as was the case for Eriophorum. Values p r e s e n t e d in F i g u r e 1 w o u l d likely v a r y f r o m year to year d e p e n d i n g o n e n v i r o n m e n t a l conditions. L e a f length m e a s u r e m e n t s o f i n d i v i d u a l leaf p o si t i o n s were t a k e n t h r o u g h o u t the 1975 g r o w i n g season f o r these same t u n d r a species. Th ese leaf lengths were c o n v e r t e d to their respective areas using regression e q u a t i o n s p r e s e n t e d in T a b l e 1. T h es e regression r e la ti o n s h i p s were significant at the 0.995 level an d correla-


162 1.2 Bet ula

nano

(Q)

Solix

1.2

pulchra

Q8 O.4

Eriophorum Voginotum (e)

&

Ledum polustre

(d) Q2

0.1

Current Leaves

Current Stems

Lost Year Leaves

Lost Year Stems

2-6 Year Leaves

2 -I0 Year Stems

> I0 Year Stems

Fruits

Fig. l a - d , Average dry weight biomass allocation for different plant compartments of an aboveground individual for Betula nana (a), Salix pulchra (b), Eriophorum vaginatum (c), and Ledum palustre (d). Average compartment allocations were estimated from a sample of 40 aboveground individuals for Betula, Salix, and Ledum and 50 tillers for Eriophorum. Determinations were obtained 21-26 July

Table 1. Leaf area to leaf length regression relationships for Betula nana, Eriophorum vaginatum, Ledum palustre, and Salix pulehra

Species

Equation

Betula Eriophorum Ledum Salix

loge(Area) = - 0 . 3 3 8 + 1 . 8 5 2 loge(Area) = - 2 . 4 6 6 + 1 . 0 6 5 loge(Area) = - 1.856 + 1.621 loge(Area) = - 1.413 + 1.811

[loge(length)] [loge(length)] [loge(length)] [loge(length)]

r

F

0.94 0.90 0.79 0.94

1,946"* 1,513"* 521 ** 4,042 **

** All regression relations are significant at P~0.995 and were determined using 200 observations for Betula, Eriophorum, and Ledum and 400 observations for Salix


163

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9

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I

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9

9 _~_~

, Ledum

x

Betula

3'0

o June

2'0 July

i

I

30

9

i

,9

xl

29

August

Fig. 2. Mean leaf dry weight seasonal progressions for Salix pulchra, Eriophorum vaginatum, Ledum palustre, and Betula nana with 220, 77, 140, and 240 leaves, respectively, followed for leaf length throughout the growing season. Dry weights were derived from these seasonal leaf length measurements using leaf area to length and leaf dry weight to area relationships determined three times during the growing season

tion coefficients, r, were 0.94, 0.90, 0.79, and 0.94 for Betula, Eriophorum, Ledum, and Salix, respectively. Leaf dry weights were then estimated using leaf dry weight and area relationships determined three times during the growing season for each species. Seasonal progressions of mean leaf dry weights for each species are presented in Figure 2. More detailed individual leaf growth dynamics depicted in Figures 3 and 4 show that Betula and Salix, deciduous shrubs, produce a sudden burst of leaf material; whereas, Eriophorum, a tussock cottongrass, exhibits a sequential pattern of leaf development. Ledum, an evergreen shrub species, has leaves which persist for more than one growing season. Estimates of leaf mortality for Ledum suggest that 70% of leaves initiated the same season have abscised two growing seasons after their initiation and that over 96% have abscised after four growing seasons. The response of CO2 uptake to light and temperature was also investigated for these four tundra species (Figs. 5 and 6). These relationships were determined the 1st week in August and even though some degree of senescence was present, these relative responses may still be representative of responses earlier in the growing season. Photosynthesis was light saturated in Salix, Betula, and Ledum at 1,380 ~teinsteins-m- 2. s- 1 or approximately 60% of maximum solar noon intensity measured directly into the solar beam at Meade River, while Eriophorum became light saturated at 740 ge- m - 2 S- 1 Temperature optimum for Salix and Ledum was at 15~ while optimum for Betula and Eriophorum was 10~

164


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. Solix

IO

,

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A

(o)

Leaf

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Position

8 6 4

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x

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fo

,

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30

20 June

. . . . . I0

0,/", 20 July

50

, 9

19 August

,

29

Fig. 3a and b. Leaf dry weight seasonal progressions for different leaf positions for Salix pulchra (a) and Eriophorum vaginatum (b). For Salix and Eriophorum 220 and 77 leaves, respectively. were followed for leaf length throughout the growing season. Dry weights were derived from these seasonal leaf length measurements using leaf area to length and leaf dry weight to area relationships three times during the growing season

Photosynthetic capacities for a number of tundra species are presented in Table 2. These determinations were obtained using a standardized irradiation and temperature of 2,300 g e . m - 1. s- 1 and 15~ Data from Figure 5 as well as work of Mooney and Billings (1961) and Tieszen (1973) suggest that photosynthesis at this light intensity was likely saturated in all species. However, since temperature optima in Figure 6 were between 10-15~ and since 10-20~ optima have been found for other tundra species (Tieszen, 1973; Gerasimenko and Zalenskiy, 1974; Tieszen and Wieland, 1975), 15~ was not likely the temperature optimum for all species examined. Leaf photosynthetic capacity for Ledum was highest in one year old leaves and declined in capacity with increasing age. For Vaccinium highest photosynthetic capacities were exhibited in current leaves with characteristically lower, but constant photosynthesis through 3-year old leaves. Additionally, photosyn-


Leaf Type . Current x Last Year 9 2 Year Old 9 3, Year Old

x_

Ledurn

165

palustre

(a)

.x

.x

.x. _

_

. IXA

iX&

/ i

,

t i/

i

i

i

i

i

i

I

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i

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Betula nana

Position

Leaf 9

I

9

3

(b)

x 2 E3

-g

A 4

o 5 a 6

,_1

i

20

30 ' June

a ---"~"L

h

I0

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e

20 duly

i

t

30

e

9

e

19 August

29

Fig. 4a and b. Leaf dry weight seasonal progression for different leaf types for Ledum palustre (a) and different leaf positions for Betula nana (b). For Ledum and Betula 140 and 240 leaves, respectively, were followed for leaf length throughout the growing season. Dry weights were derived from these seasonal leaf length measurements using leaf area to length and leaf dry weight to area relationships determined three times during the growing season

thetic capacity appears quite consistent within a growth form. The highest photosynthetic capacities were found in deciduous shrubs and grasses and sedges. Evergreen shrub species typically exhibited the lowest photosynthetic capacities with perennia! forbs having intermediate CO2 uptake rates. Estimates of leaf growth phase duration, leaf longevity, and photosynthetic capacity for typical leaves of different growth forms are presented in Table 3. Photosynthetic capacities were highest for species with shortest periods of leaf longevity. For example, Betula and Salix, both deciduous shrubs, have leaf longevities of 61 and 64 days, respectively, and exhibited CO2 uptake rates of 37 and 40mg CO2.g dry w t - l . h -1, respectively. Conversely, species with typically long leaf longevities had leaves with relatively low photosynthetic capacities. For example, Ledum and Vaccinium, evergreen shrubs, have leaf longevity periods of 200 and 360 days, respectively, and exhibited CO2 uptake capacities of 14 and 6mg CO2.g dry w t - l . h -1, respectively. Carex and Eriophorum, both sedges, have intermediate leaf longevities and photosynthetic capacities.

166


40

l

,,-x

! j J

c ~o

Betula

/

~ T

I

i

i'/i

0

20 *E o

I I

o_ I0 ///

k,..._ Ledum I

I

500

I

I

I000 1500 Light, pe .m'2.s "l

2OOO

Fig. 5. Photosynthesis of Betula nana, Salix pulchra, Eriophorum vaginatum, and Ledum palustre as a function of light at a leaf temperature of 15~ a n d a high relative humidity. Measurements were taken 8-11 August between 0,900-1,800 solar time. Each point represents the mean of six replicates with associated + one standard error

/~- Salix

40

7"

30

o~ z0

l

, , i ~ /I/''-

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/-: ~'~-