gentian is unlikely to be a mere incidental by-product of apical dominance. ... Key words: apical dominance, Gentianella, grazing tolerance, herbivory, meristem ...
Evolutionary Ecology 14: 373±392, 2000. Ó 2001 Kluwer Academic Publishers. Printed in the Netherlands.
Tolerance of Gentianella campestris in relation to damage intensity: an interplay between apical dominance and herbivory ARI-PEKKA HUHTA1*, TOMMY LENNARTSSON2, JUHA TUOMI1, PASI RAUTIO1 and KARI LAINE1 1
Department of Biology, University of Oulu, Oulu, Finland; 2Department of Conservation Biology, Section of Conservation Botany, Swedish University of Agricultural Sciences, Uppsala, Sweden (*author for correspondence, fax: +358-8-553-1061; e-mail: Ari-Pekka.Huhta@oulu.®) Received 7 July 2000; accepted 20 December 2000 Co-ordinating editor: T. Juenger Abstract. Meristem allocation models suggest that the patterns of compensatory regrowth responses following grazing vary, depending on (i) the number of latent meristems that escape from being damaged, and (ii) the activation sensitivity of the meristems in relation to the degree of damage. We examined the shape of compensatory responses in two late-¯owering populations (59°20¢N and 65°45¢N) of the ®eld gentian. Plants of equal initial sizes were randomly assigned to four treatment groups with 0, 10, 50 and 75% removal of the main stalk. The plants were clipped before ¯owering, and their performance was studied at the end of the growing season. The northern population showed a linear decrease in shoot biomass and fecundity with increasing biomass removal, while the response in the southern population was quadratic with maximum performance at the damage level of 50% clipping. This nonlinear shape depended upon the activation sensitivity of dormant meristems in relation to their position along the main stem. The highest plant performance was achieved by in¯icting intermediate damage which induced regrowth from basally located meristems. In contrast, the topmost branches took over the dominance role of the main stem after minor apical damage (10% clipping). Consequently, the breakage of apical dominance is a necessary precondition of vigorous regrowth in this species. However, compensation in the ®eld gentian is unlikely to be a mere incidental by-product of apical dominance. The ability to regrow from basally located meristems that escape from being damaged by grazing may well be a sign of adaptation to moderate levels of shoot damage. Key words: apical dominance, Gentianella, grazing tolerance, herbivory, meristem allocation, overcompensation, shoot architecture
Introduction Plants have several ways of avoiding the negative eects of herbivory. Flowering phenology may enable plants to escape in time from grazing (Crawley, 1997; Lennartsson et al., 1997). Thorns and speci®c secondary metabolites
374 may serve as defences against herbivores (e.g. Karban and Baldwin, 1997; Tollrian and Harvell, 1999). Plants may also tolerate damage by compensating for the loss of biomass and thereby recovering from damage either partly or completely (e.g. McNaughton, 1983; Belsky, 1986). Monocarpic herbs which reproduce only once often have a relatively high compensatory capacity. Escarre et al. (1996) found that cutting of 50% of the above-ground biomass early in the season had no signi®cant eect on the ®nal vegetative biomass of Picris hieracioides or Crepis foetida. In the same experiment, C. pulchra produced more vegetative biomass when defoliated, but its reproductive output decreased. Hendrix and Trapp (1989) found Pastinaca sativa to compensate fully after herbivory in terms of the total seed mass produced. The weight of individual seeds, however, was lower among the defoliated plants. Partial and full compensation has also been observed in Cynoglossum ocinale, Verbascum thapsus and Senecio jacobaea (Verkaar et al., 1986; Prins et al., 1989), Tragopogon dubius (Reichman and Smith, 1991), Melampyrum sylvaticum and M. pratense (LehtilaÈ and SyrjaÈnen, 1995), and Ambrosia artemisiifolia and Melilotus albus (Irwin and Aarssen, 1996). Some studies have even yielded evidence of overcompensation, showing that damaged plants produce more biomass and/or seeds than undamaged control plants. Overcompensatory regrowth of vegetative organs after damage is well documented among perennial grasses (McNaughton, 1979; Oesterheld and McNaughton, 1988; Georgiadis et al., 1989; Oesterheld, 1992; Alward and Joern, 1993). Indications of overcompensatory reproduction have been found in a few monocarpic herbs, e.g. Thlaspi arvense (Benner, 1988), Ipomopsis aggregata (Paige and Whitham, 1987; Paige, 1992, 1994, 1999), and Gentianella campestris (Lennartsson et al., 1997, 1998). Other studies have failed to detect overcompensation (e.g. Bergelson and Crawley, 1992a, b; Bergelson, et al., 1996). Most studies of overcompensation have evaluated the degree of compensation (under-, exact- and over-compensation; sensu Belsky, 1986) in relation to a ®xed level of damage. However, according to meristem allocation models (Tuomi et al., 1994; Nilsson et al., 1996a, b; for a review, see LehtilaÈ, 1999), the variation of compensatory responses in relation to dierent levels of damage intensity may often be biologically more informative of the tolerance mechanisms. The shape of the compensatory response curves may vary from a linear decline along with an increasing grazing intensity to a unimodal curve with maximum performance at weak or moderate damage levels and with undercompensation for very high damage levels (McNaughton, 1983; Belsky, 1986; Tuomi et al., 1994; Nilsson et al., 1996b). Because the amount of photosynthetic tissue as well as the number of meristems available for regrowth will be reduced more or less linearly by increasing levels of damage, a
375 linear decrease in plant performance along with increasing damage intensity would be an obvious expectation. However, the results obtained by a number of researchers (Oesterheld and McNaughton, 1988; Paige, 1992; Escarre et al., 1996; Wegener and Odasz, 1997) suggest that a humped or unimodal response to herbivory may occur among a variety of species. There is no general explanation why unimodal compensatory responses should be common. However, theoretical models have generated unimodal response curves in relation to damage intensity. For instance, Tuomi et al. (1994) and Nilsson et al. (1996b) have shown that the shape of compensatory responses may vary as a function of the magnitude of dormant meristems which can be activated by damage, and of the activation sensitivity of the buds in relation to damage intensity. If the pool of dormant buds is suciently large and the buds are activated by the slightest damage, overcompensation will result even after the removal of the shoot apex. If the bud activation is more restrained, plant performance will reach a maximum at moderate or high levels of damage. We studied the relationship between the damage level and the compensatory responses of the biennial grassland herb G. campestris in two dierent populations and climatic zones. The species is known to have a very good compensatory capacity, including overcompensation in some populations (Lennartsson et al., 1997, 1998; Juenger et al., 2000). Overcompensation in G. campestris is restricted to certain populations with a long history of management by grazing and/or mowing (Lennartsson et al., 1997) and to certain damage timing relative to ¯owering phenology (Lennartsson et al., 1998). However, whether overcompensation is also restricted to certain damage levels has not been examined earlier. For this reason, we determined the pattern of compensatory responses across a range of damage levels for the two populations of the ®eld gentian. In particular, we tested how extensive shoot damage should be to induce full-intensity regrowth and, on the other hand, how well plants tolerate very high levels of damage. We also studied the shoot architecture and branching patterns after apical damage and after more severe damage, in order to determine the degree of compensation allowed by apical dominance, meristem reserves and nutrient resources. Finally, we brie¯y discuss possible evolutionary histories of tolerance and overcompensation in monocarpic herbs, and particularly whether overcompensation in the ®eld gentian may be an indicator of adaptation to herbivory, or whether it is a byproduct of adaptations to other environmental factors favouring unbranched vertical growth, such as light competition (cf. Aarssen and Irwin, 1991; Aarssen, 1995).
376 Material and methods Study species and populations The ®eld gentian, G. campestris ssp. campestris (L.), BoÈrner, is a strictly biennial herb (Lennartsson et al., 1997; cf. Kelly, 1989). The seeds germinate in spring. The plant forms a rosette and an overwintering taproot during the ®rst summer, and develops into a ¯owering plant the second summer. The ®eld gentian is phenologically variable and includes early- and late-¯owering ecotypes. The ®eld gentian is highly sel®ng and obtain c. 80±95% seed set in absence of pollinators (Lennartsson et al., 2000). Both populations in the present study represent the late-¯owering type. The plants usually reach 10± 30 cm in height during the ¯owering phase and have 4±7 internodes (HaÈmetAhti et al., 1998). Branches typically sprout from the second to fourth nodes and produce 10±20 ¯owers. Late-¯owering gentians possess an ability to recover from various types of damage, e.g. mowing and cattle grazing and simulated grazing by cutting (Lennartsson et al., 1998; A-P. Huhta, personal observation). The ®eld gentian is a grassland species that grows in northern and central Europe, as far east as northwest Russia and central Austria and southwards down to eastern Spain and central Italy. In northern Finland, the species reaches the northeastern limit of its distribution (HulteÂn and Fries, 1986). The habitat preference of the species is semi-natural meadows and pastures varying from dry to mesic. In most part of its distribution, the ®eld gentian is an archaeophyte, and it has greatly suered from the cessation of traditional farming. Both in Finland and in Sweden, the species has declined during the past 50 years, mainly due to the current lack of management of unfertilised pastures and hay meadows (Rassi et al., 1991; Aronsson et al., 1995; Lennartsson and Svensson, 1996). The BjoÈrnvad population is located in the province of SoÈdermanland in central Sweden (59°20¢N, 16°51¢E). The grassland is dominated by Avenula pratensis±Festuca rubra and herb-rich Agrostis capillaris meadow (cf. PaÊhlsson, 1994). The area has been used for grazing since at least the 17th century up till today (Lennartsson et al., 1998). The Keminmaa population is located in the Finnish community of Keminmaa in the province of Lapland on the northern coast of Gulf of Bothnia (65°45¢N, 24°30¢E). The small patchy meadow in Keminmaa resembles common bent meadow (cf. PaÊhlsson, 1994), being dominated by grasses A. capillaris and Poa pratensis and herbs Achillea millefolium, Galium verum and Pimpinella saxifraga. Previously, the meadow was used as a sheep pasture, but it has been abandoned since the late 1950s. Slight overgrowing by shrubs and tree saplings is clearly visible (Huhta and Rautio, 1998). The Swedish population is known to show strong
377 overcompensatory responses to clipping (Lennartsson et al., 1997, 1998). The compensatory capacity of Finnish ®eld gentians has not been studied earlier. Experimental design The experiments were carried out using overwintered plants that had passed their rosette stage and started to develop into ¯owering stage. The clippings were executed on the 16th of July 1997 in BjoÈrnvad and on the 28th of July in Keminmaa, about 3 and 1.5 weeks before ¯owering, respectively. Prior to the clipping, plants within similar size classes were measured and labeled. In BjoÈrnvad, the average initial height (average SE) before clipping was 10.80 0.18 cm, the minimum size being 8 cm and the maximum size 13 cm. The corresponding values for Keminmaa were 13.16 0.76 cm, 8 cm (minimum) and 20 cm (maximum). Thereafter the plants were randomly assigned to one of the four treatment groups: 0% (control), 10, 50 or 75% of the main stem being removed. After clipping plants had 7, 7, 4 and 2 remaining main stalk internodes for 0, 10, 50, and 75% clippings, respectively. Additionally, the immediate surroundings (c. 0.25 m2) of each plant were mown at the height of about 5 cm to give all replicate plants equal starting conditions in relation to above-ground competition. It should be noted that also the unclipped vegetation was rather low, c. 7 cm average height. Therefore, the cutting of vegetation did not imply any major changes in the plants' nearest neighborhood. In BjoÈrnvad, 23±25 plants were initially marked for each treatment, but mainly due to trampling by horses, the ®nal samples sizes were smaller: 19, 16, 21 and 12 plants for 0, 10, 50, and 75% clippings, respectively. Because the Keminmaa population was very small in size, only 28 individuals were initially included in the experiments (seven per treatment). Due to trampling by humans, some plants were lost, with the consequence that the ®nal sample sizes for the four treatment groups were 5, 4, 5 and 5, respectively. No natural herbivory on experimental plants was observed in either of the populations. The plants were collected when the fruits were fully ripened in late September. From the collected plants, we measured height, above- and belowground biomasses, number of branches, number of fruits, number of seeds per fruit, number of unfertilized ovules per fruit and seed weight. For biomass estimates, the plants were dried for 2 days at +80 °C. For the estimation of the number of seeds and ovules and seed weight, we collected two fruits per plant, one from the upper and one from the lower part of the plant. Since part of the seeds had already fallen out of the capsules, the number of seed scars was used as an estimate of the total number of seeds per capsule (Lennartsson et al., 1997). Unfertilized ovules can easily be counted because they remain at the bottom of the capsule (Lennartsson et al., 1997). One hundred seeds per plant were weighed to estimate the average seed weight for each individual plant. The
378 number of seeds per plant was estimated by multiplying mean number of seeds per fruit by the total number of fruits per plant. Finally, the total seed mass per plant was calculated using the average seed weights of individual plants and total number of seeds per plant. We quanti®ed the degree of compensation, following Belsky (1986), by dividing the performance of clipped plants with that of control plants. Undercompensation means that the clipped plants perform less well. If the clipped and control plants perform equally well, there is exact or full compensation. Finally, overcompensation occurs when the clipped plants perform better than the control plants. Statistical analysis The data were tested with two-way factorial ANCOVA with origin (BjoÈrvad in Sweden and Keminmaa in Finland) and treatment (0, 10, 50 and 75% biomass removal) as factors and initial (before cutting) height as a covariate. If the assumption of the homogeneity of variances was not met (Levene's test), the data were either log-transformed (X log Y + 1) in order to achieve homogeneity or, if log transformation did not improve homogeneity, rank-transformed prior to analysis. The rank transformation was done following the method presented by Conover and Iman (1982), where both the dependent variable and the covariate are replaced by their ranks, and also with the hybrid method, where only the dependent variable is replaced by ranks (Stephenson and Jacobson, 1988). Because these rank transformation procedures produced similar results, we only present the results from the ®rst procedure. If transformations did not alter the results compared to the original untransformed values, the ANCOVA results from the original values are reported (Montgomery, 1984; p. 118). If the transformations changed the ANCOVA results, the transformation is indicated in the ANCOVA table: lg for log and rt for rank transformation (Table 1). Covariate by factor interactions were nonsigni®cant (indicating homogeneity of regression slopes) and consequently were not included in the ®nal models (Table 1). The means reported in Table 1 were calculated from the original data and were not adjusted for the covariate. Multiple comparisons between the treatments were performed for the original untransformed variables by using the LSD procedure (Saville, 1990). Explicit tests to study the shape of compensatory responses were done for the total number of seeds per plant and the above-ground biomass by ®tting up to the term of the third degree (linear [y a + bx], quadratic [y a + bx + cx2] and cubic [y a + bx + cx2 + dx3] with x clipping intensity). This was done by decomposing the treatment eect (clipping intensity) sum of squares into single-degree-of-freedom polynomial contrasts (Sokal and Rohlf, 1995). Only statistically signi®cant terms (F-test, p < 0.05)
ANCOVA Initial height (cov) Origin Treatment Interaction
75%
50%
10%
Cutting intensity Control
F1,78 F1,78 F3,78 F3,78
3.93, 0.08, 0.69, 4.15,
0.21a (0.10) 0.69a (0.18) 1.43b (0.27) 1.67b (0.38) p p p p
= = = =
0.051 0.782 0.562 0.009
1.00a (0.55) 0.75a (0.48) 0.60a (0.40) 0.20a (0.20) F1,78 F1,78 F3,78 F3,78
0.40, 1.43, 8.92, 2.98,
4.79a (0.39) 7.38b (0.53) 6.43b (0.41) 3.17c (0.42) p p p p
= = < =
0.530 0.236 0.001 0.037
6.00a (0.84) 4.75a (0.25) 5.00a (0.63) 2.80b (0.49) ±
±
±
±
18.12a (1.08) 19.22a (1.45) 17.12a (1.19) 11.78b (1.82)
10a (1) 15a (1) 22b (3) 9a (1)
35.29, p < 0.001 F1,80 13.16, p < 0.001 F1,80 19.33, p < 0.001 F3,80 1.36, p = 0.261 F3,80
12.66a (0.56) 13.56a (0.58) 13.75a (0.66) 8.85b (1.00)
F1,63 0.97, p = 0.327 F1,80 ± F1,80 F3,63 7.32, p < 0.001 F3,80 ± F3,80
0.11a (0.07) 0.19ab (0.10) 2.00c (0.54) 1.33bc (0.38) 1.53, 1.74, 3.53, 3.34,
p p p p
= = = =
14a (2) 9b (1) 10ab (1) 7b (1) 0.220 0.191 0.019 0.023
35.00a (5.28) 43.00a (4.75) 46.40a (5.59) 38.60a (5.89) F1,58 4.80, p = 0.032 F1,58 14.74, p < 0.001 F3,58 4.96, p = 0.004 F3,58 1.46, p = 0.236
41.80a (4.68) 56.11a (4.56) 77.47b (6.54) 45.06a (3.55)
2nd order branches Height (cm) Number of nodesb Root biomass (mg) Measured parameter Low node branchesa 1st order branches Locality BjoÈrnvad Keminmaa BjoÈrnvad Keminmaa BjoÈrnvad Keminmaa BjoÈrnvad Keminmaa BjoÈrnvad Keminmaa BjoÈrnvad Keminmaa
Table 1. Eects of increasing clipping intensity on vegetative and reproductive characters (mean SE) in BjoÈrnvad and Keminmaa. rt = rank transformed, lg = log transformed values used in ANCOVA. Means denoted by the same letter do not dier signi®cantly from each other ( p > 0.05 with LSD correction for multiple comparisons)
379
a
F1,79 F1,79 F3,79 F3,79
0.936 0.738 0.002 0.006
24.40a (3.17) 20.40a (2.32) 18.20ab (3.73) 11.00b (1.70) F1,75 F1,75 F3,75 F3,75
0.55, 7.41, 5.68, 1.71,
72.5ab (2.85) 76.27b (2.67) 71.00ab (1.91) 65.18a (1.83) p p p p
= = = =
0.462 0.008 0.001 0.171
73.40a (0.68) 63.00ab (5.70) 59.80ab (3.66) 54.67b (8.69) F1,75 F1,75 F3,75 F3,75
2.37, 9.61, 1.38, 0.61,
11.6a (1.6) 20.1b (3.8) 14.0ab (1.8) 11.7a (2.7) p p p p
= = = =
0.128 0.003 0.255 0.611
3.6a (1.2) 9.4a (3.6) 11.6a (5.7) 4.7a (1.4) F1,73 F1,73 F3,73 F3,73
0.140a (0.063) 0.172b (0.0073) 0.192b (0.0092) 0.192b (0.0136) 5.93, p = 0.017 64.32, p < 0.001 3.74, p = 0.015 1.14, p = 0.339
0.091a (0.005) 0.098a (0.0045) 0.096a (0.0032) 0.129b (0.0125)
F1,68 F1,68 F3,68 F3,68
0.12, 4.57, 3.53, 3.30,
94.99a (9.19) 166.89bc (21.52) 216.19b (28.13) 122.85ac (20.57) p p p p
= = = =
0.727 0.036 0.019 0.026
254.51a (46.64) 256.88a (62.06) 208.42ab (39.89) 101.26b (14.96)
Keminmaa
Total seed mass (mg)
Keminmaa BjoÈrnvad
Seed weight (mg) (rt)
Keminmaa BjoÈrnvad
Unfertilized ovules
Keminmaa BjoÈrnvad
Number of seed scars
Keminmaa BjoÈrnvad
0.006, p = 0.112, p = 5.40, p = 4.46, p =
13.21a (0.96) 22.00b (1.53) 30.55c (3.48) 12.08a (1.62)
BjoÈrnvad
Number of fruits
The c. 3 lowest nodes; b Nodes on main stem plus nodes on branches.
ANCOVA Initial height (cov) Origin Treatment Interaction
75%
50%
10%
Cutting intensity Control
Measured parameter Locality
Table 1. Continued
380
381 were included in the equations presented in Figure 2. In these tests the original untransformed data without covariate was used. Note that, in spite of the small sample sizes in the Keminmaa population, the power of the statistical tests was generally good; in the case of seed production (cf. Fig. 2), for example, the power of the test for treatment eect (clipping intensity) was over 0.8 for both populations (for power calculation cf., e.g. Zar, 1996; p. 195). Results Architectural changes Undamaged ®eld gentians had a single main stem and 4±6 primary (1st order) branches (Table 1), which mostly developed at the upper nodes of the main stem (Fig. 1). Shoot damage stimulated branch growth along the main stem. In BjoÈrnvad, clipping induced the growth of new branches, especially from the lower stem nodes (Fig. 1). This response was most conspicuous after 50% clipping (Fig. 1). The highest damage levels (50±75%) also induced the growth of secondary (2nd order) branches (Table 1). In Keminmaa, only primary
Figure 1. Schematic presentation of responses to clipping in the southern (BjoÈrnvad) and northern (Keminmaa) populations of G. campestris. Examples of primary (1st order) and secondary (2nd order) branches and a low-node branch are indicated by thin arrows. The clipping heights for 50 and 75% cut plants are indicated by thick arrows.
382 branches were produced, and mainly at the upper stem nodes, in both undamaged and damaged plants (Table 1, Fig. 1). In both populations, plants subjected to the 50% damage level had more branches by the end of the season than plants with a lower or higher damage level, thus showing a nonlinear response in relation to damage level (Table 1). Biomass compensation In general, the southern ®eld gentians (BjoÈrnvad) showed considerably better compensation than the northern population. In BjoÈrnvad, all damage levels induced full or overcompensation (Fig. 2a). The lowest damage level of 10 and 50% increased the above-ground biomass by 1.6- and 2.8-fold, respectively, compared to the control plants. Even after 75% clipping, the plants were able to compensate almost fully for the lost above-ground biomass (Fig. 2a). The Keminmaa population showed full compensation for all but the highest damage levels (Table 1, Fig. 2a). In conclusion, the compensatory response curve of the above-ground biomass in relation to the damage level showed a nonlinear pattern with a maximum at around 50% damage in BjoÈrnvad and a decreasing linear pattern in Keminmaa (Fig. 2a). Interestingly, root biomass was not adversely aected by clipping. In BjoÈrnvad, the damage level of 50% even signi®cantly increased (1.85 times) the ®nal root biomass as compared to the control plants (Table 1). Consequently, shoot regrowth seems not to have taken place at the cost of root growth. Fruit and seed production In BjoÈrnvad, the response of reproductive performance to clipping was parallel to that of the vegetative biomass. Overcompensation in the number of fruits and seeds was induced by both 10 and 50% biomass removals (Table 1, Fig. 2b), with a strong nonlinear relationship between the damage level and compensation. The 10 and 50% damage levels increased the number of fruits 1.7- and 2.3fold, respectively, as compared to the control plants, whereas 75% biomass removal resulted in full compensation (Table 1). The number of seeds and the seed mass per plant at the end of the season showed a similar nonlinear pattern (Table 1, Fig. 2b). In Keminmaa, no overcompensation was observed, and the number of fruits and seeds decreased with increasing damage intensity. The 10 and 50% damage levels induced full compensation, while the highest damage level of 75% strongly reduced fecundity (Table 1, Fig. 2b). In BjoÈrnvad, none of the damage levels had any adverse eect on the number of seeds per fruit, whereas in Keminmaa, the highest damage level reduced this measure signi®cantly (Table 1). The average weight per seed was signi®cantly increased by all damage levels in Keminmaa, and by the highest damage level in BjoÈrnvad (Table 1).
383
Figure 2. The shape of compensatory responses in the two study populations: (a) above-ground biomass and (b) number of seeds per plant. Mean and SE of the original data are also shown.
The compensatory responses of the number of seeds per plant in relation to damage level showed a quadratic pattern in BjoÈrnvad and a decreasing linear pattern in Keminmaa (Fig. 2).
384 Discussion Field gentians in the northern population of Keminmaa compensated at best fully, whereas plants in the southern population of BjoÈrnvad showed overcompensation in most performance measurements. Thus, the response of the Keminmaa population to an increasing level of damage is consistent with the general view that herbivory is invariably detrimental to plants. This geographic dierence in compensatory responses supports the assumption that overcompensation should be less frequent in northern populations, due to the shorter growing season (Lennartsson et al., 1998). However, the dierences in the timing of clipping between the two populations may also be responsible for the dierences, and the results should therefore be interpreted with caution. Although the northern ®eld gentians were able to compensate fully for vegetative losses caused by up to 50% clipping, their fecundity decreased linearly along with the increasing level of damage. In BjoÈrnvad, in contrast, the compensation responses were strongly nonlinear, with the most pronounced overcompensation (both in vegetative and fecundity performance measures) taking place at an intermediate damage level. This pattern agrees nicely with McNaughton's (1983) suggestion of a unimodal compensation curve. He expected plants to overcompensate in response to low levels of herbivory and above some threshold level of damage the performance of damaged plants collapses in relation to undamaged plants. A unimodal curve is also generated by meristem allocation models (Tuomi et al., 1994; Nilsson et al., 1996b). When most of the dormant meristems are activated to grow by minor apical damage, tolerance is highest at low damage levels. On the other hand, when a high damage level is required to activate the latent meristems, maximum tolerance is obtained at relatively high levels of damage. The present results agree better with the latter situation, because plant performance in BjoÈrnvad was highest after 50% clipping. Apparently, in the ®eld gentian, the probability that latent meristems are induced to grow depends on the degree of damage as well as on the position of meristems along the main stem. Below, we will ®rst discuss the fate of meristems in relation to damage intensity and then brie¯y outline a perspective for the evolution of tolerance as an interplay between light competition or other selective forces favouring vertical growth and herbivory favouring tolerance traits. Tolerance and fate of meristems in relation to damage intensity In general, for compensatory regrowth to take place, two preconditions are required: (1) a sucient amount of undierentiated meristems which can initiate the development of branches, and (2) sucient resource pools to support the initial growth of branches and, eventually, the maturation of fruits and
385 seeds. Abundant compensatory fruit production requires a third precondition, namely that (3) reproductive eort should have a priority in the resource allocation of the damaged plants, i.e. the developing fruits and seeds must provide suciently strong sinks for the limiting resources. Sucient meristem supply (1) is an absolute prerequisite for compensatory growth. While a shortage of meristems will reduce the compensatory capacity (Tuomi et al., 1994), a reduction of photosynthetic tissue and above-ground sources (2) may be compensated for by the increased allocation of stored reserves from the roots (e.g. McNaughton, 1979; Bryant et al., 1983; Oesterheld, 1992). The importance of source supplies is indicated by the fact that overcompensation has most frequently been demonstrated in conditions where competition is low and there is a surplus of nutrients and water (e.g. Benner, 1988; Maschinski and Whitham, 1989; Whitham et al., 1991; Mabry and Wayne, 1997). Moreover, in the case of most of the species tested, large plants tend to compensate better than small ones (P. sativa: Hendrix and Trapp, 1989; I. aggregata: Bergelson et al., 1996). The high priority of reproductive investment (3) is likely to be satis®ed in monocarpic plants, while perennial species, when damaged, are able to postpone their reproductive eort to the next growing period (Verkaar et al., 1986). This may be one reason why studies on perennial herbs, such as Hypericum perforatum (Irwin and Aarssen, 1996) and Lythrum salicaria (Venecz and Aarssen, 1998), have generally shown low compensation in terms of reproductive output. Once triggered, compensation should increase along with the amount of resources available for the development and growth of new sinks. Hence, larger resource removal should result in lower compensation. In the BjoÈrnvad population of G. campestris, however, that was not the case. The most abundant overcompensation was not induced by very low levels of damage, but about half of the main stem had to be removed before compensatory growth was induced with its full intensity. Accordingly, minor apical damage stimulated branch growth only at the upper nodes, whereas intermediate damage level could also induce regrowth from basally located meristems. Mere removal of the shoot tip was not sucient to induce branching from the basal nodes and maximum compensation, possibly because apical dominance is exerted not only by the shoot tip of the main stem but, to some degree, also by other highpositioned nodes. Consequently, the lateral branches closest to the apex could continue to inhibit the meristems at lower stem nodes (see Zieslin and Halevy, 1976; Suzuki, 1990). Branching from the basal nodes, in contrast, resulted in a multiple-stem architecture, where none of the stems seemed to reach advantage over the others. In BjoÈrnvad, this branched architecture of 50% clipped plants was the most conspicuous feature of the compensatory response. Generally, such rejuvenation (Moorby and Wareing, 1963) or reiteration of plant architecture (HalleÂ, 1986) requires regrowth to start from basally located meristems.
386 As a consequence of the need for high damage levels in G. campestris, basally located meristems become a general prerequisite for overcompensation. Also in I. aggregata it has been observed that symmetrical regrowth from basal meristems is related to high tolerance and asymmetrical regrowth to low tolerance (T. Juenger, personal communication). It may therefore be reasonable to assume that the presence of basal meristems may well be an adaptation to grazing (Owen and Wiegert, 1981). On the other hand, other monocarpic herbs, such as Rhinanthus minor (Scrophulariaceae), which lack basal meristems may compensate well for low and even moderate damage, although they are sensitive to high damage levels (50±75% clipping, Huhta et al., 2000). In general, branching may be regarded as a visible morphological expression of compensatory capacity. Architectural changes will be further re¯ected in vegetative and reproductive performance, depending on the fate of activated meristems (Geber, 1990). Fecundity may thus correlate with architecture (e.g. Preston, 1998), provided that the activated meristems produce ¯owers or in¯orescences and that there are sucient source supplies to support the development and maturation of seeds. Obviously, low- and intermediate-level damage caused no source limitation in the BjoÈrnvad population, as both fruit and seed production were increased by clipping. However, removal of 75% of the main stem resulted in lower performance than a damage level of 50%. This may be caused by a de®cit of nutrients and photosynthates and/or a de®cit of undierentiated meristems. A de®cit of meristems is unlikely because removal of 50% of the main stem induced regrowth from basal meristems situated lower than the clipping height of 75% damage. The ®rst alternative may be a plausible explanation if the remaining meristems did not act as strong enough sinks for stored resources (cf. Whitham et al., 1991). Meristems may need initial support from photosynthetic tissue to develop into growing shoots large enough to function as a sink, or they may be hormonally depressed when the shoot biomass drops below a certain threshold level. In conclusion, the damage level of 50%, which was also used by Lennartsson et al. (1997, 1998) in earlier experiments on G. campestris, is presumably close to an optimal compromise which, on one hand, leaves a sucient number of meristems and other resources for regrowth and, on the other hand, is ecient enough in removing the apical suppression of basal meristems, thereby triggering the growth of the remaining meristems. This pattern is consistent with the `node counting' hypothesis that the number of nodes on a lateral branch is related to its nodal position along the main stem (Sachs, 1999). Lateral meristems close to the apex of the main stem produce only few nodes before ¯owering, whereas meristems located at a lower position tend to produce a greater number of nodes when triggered to grow. Consequently, the fate of an apex and the resulting branching pattern are related to the distance of the apex from the roots (Sachs, 1999). We expect that this may well be a fundamental
387 principle which determines architectural changes associated with regrowth responses in ¯owering plants. Evolutionary pathways for increased tolerance In G. campestris, and also in many other plants, compensatory regrowth requires apical suppression of meristems before damage as well as breakage of apical dominance after damage. Clearly, apical dominance may have evolved for reasons other than grazing tolerance, such as an adaptation to the competition for light. The question that has been debated (e.g. Aarssen and Irwin, 1991; Aarssen, 1995; JaÈremo et al., 1996; LehtilaÈ, 1999) is whether overcompensation and compensation in general are mere by-products of selective forces favouring vertical growth and unbranched architecture among undamaged plants (pathway 1 in Fig. 3), or whether they represent evolution of tolerance due to high risk of damage (pathway 2 in Fig. 3). Which of the two alternative evolutionary pathways may provide the best explanation of the compensatory response curve and architectural changes in G. campestris? If unbranched vertical growth is the main advantage of apical dominance, one would expect a single shoot to develop under most damage levels. Increased branching following damage is thus expected only if the damaged plants happen to grow in conditions with weak competition for light
Figure 3. Two evolutionary pathways modifying tolerance. The ®rst (1) represents the evolution of vertical growth through apical dominance. The second (2) represents the evolution of tolerance traits, of which some are aected by apical dominance. These two evolutionary pathways are not independent, as herbivory may in¯uence the degree of apical dominance through indirect selection and by favouring the presence of reserve meristems for regrowth.
388 (JaÈremo et al., 1996). If, in contrast, suppression of meristems in order to maintain a high compensatory capacity is the major advantage, the hormonal control of apical dominance should allow a release of suppressed meristems after both minor and major damage (but see, Nilsson et al., 1996b). The results of this study support both assumptions: low levels of damage induce the development of a single high shoot from the highest node, while higher damage levels induce multiple branching from basal meristems. Similar patterns have been shown for other species, such as I. aggregata (Polemoniaceae), which is reported to require major damage to induce overcompensation, whereas low levels of damage induce weaker compensation (Paige, 1994; but see, Bergelson et al., 1996). In the experiments by Paige and Whitham (1987), even 95% removal of above-ground tissue resulted in overcompensation. Consequently, our results indicate that both vertical growth and compensatory capacity are favourable, but presumably under dierent environmental conditions. Because mere apical damage did not induce vigorous branching, ®eld gentians are presumably subject to selection for vertical growth and unbranched shoot architecture under some environmental conditions. Such vertical growth is likely to be favourable under a low grazing pressure when the surrounding vegetation remains high and dense and the risk of damage to the plant is low. In contrast, vigorous branching after higher levels of damage may indicate that compensatory branching is more important than vertical growth under other environmental conditions. Such conditions are likely to prevail when grazing pressure is high on the surrounding vegetation, thus decreasing the intensity of competition for light and increasing the probability that individual plants experience more severe damage. It can be noted that a common garden experiment with 14 populations of G. campestris only revealed overcompensation in populations with a long history of grazing and/or mowing (Lennartsson et al., 1997; Juenger et al., 2000). This strongly suggests that the evolution of overcompensation in the ®eld gentian is associated with a high probability of damage. Clearly, the evolution of tolerance in herbaceous plants involves the selection of several traits, particularly apical dominance, vertical position of meristems, general plant architecture and patterns of resource allocation. Of these traits, apical dominance is favourable for the plant both in terms of light competition (i.e. vertical growth) and compensatory capacity (a reserve of suppressed meristems). In highly tolerant plants, apical dominance may therefore have evolved by the simultaneous action of two evolutionary pathways, although the primary pathway in most plants has probably been the competition for light (Aarssen and Irwin, 1991) and other growth conditions modifying shoot architecture (Aarssen, 1995). Other tolerance traits, such as basal positioning of meristems and the ability to allocate resources to secondary growth, are less advantageous for the plants in the absence of damage
389 (cf. Juenger and Bergelson, 1997, 2000), and are therefore more likely to have evolved through direct selection for tolerance. To conclude, we argue that the evolution of the grazing tolerance of herbaceous plants involves two evolutionary pathways: (1) evolution of vertical growth through apical dominance, and (2) evolution of tolerance traits. The relative importance of the two evolutionary pathways may well vary for different tolerance traits. It is likely that both pathways must have been involved in the evolution of tolerance in the ®eld gentians and presumably also in other grazing-tolerant species.
Acknowledgements We are grateful to M. HyvaÈrinen, T. Juenger, M.-M. KytoÈviita, P. Rinne, H. VaÈre and two anonymous referees for their constructive criticism of the manuscript. We appreciate the cooperation of the workers in Keminmaa parish, and also wish to thank the Anduri family at BjoÈrnvad. The work was ®nancially supported by the Academy of Finland (Project No. 40951) and by the Swedish Council for Forestry and Agricultural Research.
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