Responses of Fagus engleriana Seedlings to Light and Nutrient ...

Acta Botanica Sinica 植 　　物 　　学 　　报

2004, 46 (5): 533－541

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Responses of Fagus engleriana Seedlings to Light and Nutrient Availability GUO Ke1,2*, Marinus J. A. WERGER2 (1. Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China; 2. Department of Plant Ecology and Evolutionary Biology, Utrecht University, Utrecht, The Netherlands)

Abst rac t : The responses of field-grown Fagus engleriana Seem. seedlings to light and soil nutrient availability were investigated. Two-year-old seedlings were grown for two growing se asons unde r six treatment conditions, including three light levels (L1: 1%-2% of full sunlight; L2: 18% of full sunlight; L3: 100% of full sunlight), with and without fertilizer addition (F1 and F0) for each light level. The results showed that light and nutrients had significant effect on seedling growth as measured in terms of shoot height, stem basal diameter and biomass; the mean increments of shoot height over two growing seasons were significantly less in L1 than in L2 and L3 (P ＜0.001), and in L3 than in L2 (P ＜0.01), but the increments during the first growing season were not significantly different among the treatments; the increments of stem basal diameter and biomass components were significantly less in L1 than in L2 and L3 (P ＜0.001); the increments of stem basal diameter and whole plant biomass were not statistically different between L2 and L3; adding fertilizer did not affect the seedling growth under closed forest canopy, but had effect in the environments with more sunlight. The results suggest that (1) two-year-old F. engleriana seedlings could survive the conditions of closed forest canopy, but their growth might be se ver ely inhibit ed; (2 ) t he see dlings could grow as well as or even better in small forest gaps than in open sites; and (3) fertile soil might e nhanc e see dling growth in fore st gaps an d ope n sit es, but no t under closed forest canopy. Ke y wo rds: Fagus engleriana ; light; nutrient; shoot height; biomass; stem basal diameter; relative growth rates (RGR) Beech forest is one of the most common forest types in the temperate regions of North America, Europe, and Japan (Chabot and Mooney, 1985; Polunin, 1985; Ellenberg, 1988; Vankat, 1990; Peters, 1997). In those forests beech species are consid ered as tolerant t o shade or deep-sh ade conditions (Watt, 1923; Baker, 1950; Loach, 1970; Nakas hizuka an d Nu mata, 1982b ; Hara, 1987; Canh am, 1988; 1990; Ellenberg, 1988; Poulson and Platt, 1989; Grubb et al., 1996; Peters, 1997). In China, however, beech forests occur only in the mountains of the su btropical region from 23o to 34o N, and contain many evergreen tree species and bamboo species (Wang, 1965; Ts ien et al., 1975; Wu, 1980; Hong and A n, 1993; Zhou and Li, 1994; Ban and Qi, 1995; Cao, 1995; Peters, 1997; Guo, 1999). Occurrence of dense bamboo shoots in the understory is considered a limiting factor in th e est ablis hment of beech t rees (Nakas hizuka and Numata, 1982a; Hara, 1983; 1985; 1987; Nakashizuka, 1987; Pet ers an d Ohkubo, 1990; Cao, 1995; Peters, 1997). This typ e of forest h as been named montane d ecidu ous and evergreen broad-leaved mixed forests (Wu, 1980), but usually looks like a deciduous broad-leaved forest in the northern part of its lat itudinal range or in th e upper part of its

altitudinal distribution range (Ban and Qi, 1995; Cao, 1995). In t he beech forests of China, especially in the mixed forests, beech seedlings and saplings are h ardly found in the pat ches where th e canopy is closed or the bamboo grows vigorously (You, 1962; Tsien et al., 1975; Ban and Qi, 1995; Cao, 1995; Peters, 1997). Those seedling s occur infrequently in small gaps and in patches where the canopy is spars e, or on steep slop es wh ere t he canopy is o pen (Cao, 1995). So il n utrient co ndit ion is an impo rtan t facto r for increased photosynthetic prod uction and improved chance for survival of seedlings suppressed under shade (Peters, 1997). On t he mo untain s lopes where t he beech forests occur, soil thickness and nutrients often vary markedly with topography, inclination and aspects of slopes, concave or convex slopes, altitude, etc. (Ban and Qi, 1995). Therefore, in beech forests n ot only the light, but also nutrien ts are distributed in patchiness. The patchy distribution of beech seedlings or saplings leads to the following hypotheses addressed in this study: (1) beech seedlings do not tolerate the shade conditions in the understory of closed forests and they cannot survive

Received 30 Jun. 2003 Accepted 22 Sept. 2003 Supported by the Knowlage Innovation Project of The Chinese Academy of Sciences (KSCX1-08), the State Key Basic Research and Development Plan of China (G1999043507) and the Dutch Royal Academy of Arts and Sciences. * Author for correspondence.

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those conditions for long. For successful regeneration, they need more favorable light conditions; and (2) soil nutrient availability affects beech seedling establishment and vigorous growth. We tested these hypotheses in an experiment conducted in the field during two growing seasons, and evaluated the responses of Fagus engleriana seedlings in terms of shoot height, stem basal diameter, biomass components, relative growth rate, leaf area, and root to shoot ratio.

1 Materials and Methods 1.1 Climate and vegetation of the study site The experiment was carried out in Daba Forest Farm, a remo te mo un tain area in no rt heast ern Sichu an, China (elevation 1 420 m, 32.5o N, 107o E). Mean ann ual temperature is 9.6 o C, mean an nual p recipitation 1 450 mm, mean annual potential evaporation 776 mm, and mean annual relativ e humidity 83%. The rainy s eason coin cides with the gro wing seas on, and p eaks at 270 mm in June. Growing season lasts about 170 d fro m late Ap ril t o mid -Octo ber (Data from Daba Forest Farm). The major ty pes of natural vegetat ion fro m 1 400 m to 2 000 m in altitude are deciduous broad-leaved forests. They consist of mosaics of patches that are usually dominated or co-dominated by Fagus eng leriana Seem., F. ha yatae su bsp. pa sh an ica, Quercu s a liena var. acut eserrat a, Carpinus cordata var. chinensis, and C. fargesiana. Some pat ches are co -dominated by an y of three birch species (Betula spp.), which are commonly considered as pioneers in t hose fo rests (Zhu, 1983). In the valleys that incis e to altitu des below 1 400 m some evergreen species, s uch as Cyclobal anopsis sp p. and Lithocarpus sp p., are able to reach the forest canopy. 1.2 Seedlings and their treatments One thousand two-year-old seedlings of F. engleriana gro wing in a tree nu rsery of the forest farm were transplan ted int o 1 000 3.5-L pots containing sand y loam soil obtained from the local forest at 5－20 cm depth. They were returned to the nursery (with full sunlight) to acclimate to the new growth conditions and to let the roots (especially the fine roots) to recover for a period from autumn to next spring. One week before bud flush in spring, 30 seedlings were randomly selected from among 763 healthy seedlings and harvested to obtain initial values of growth variables. The remaining seedlings (n = 733) were divided randomly into six g roups, and moved t o the experimental environments in the beech forest: two groups for each of the three light levels (see below), with one group receiving fertilizer treatment.

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Three light levels were: L1, shaded by a closed forest canopy with 1%－2% of full sunlight during growing season, but 60% of fullsunlight when leaves were shed; L2, shaded by a bamboo screen with 18% of full sunlight year round and some s mall sun flecks of rather short durat ion due to the open structure of the screen; L3, exposed to full sunlight in the forest clearing. Two fertilization treatments were: F0, with out fertilizer; F1, with fertilier addition as nutrient solution at two-month intervals. The rate of fertilizer application was 100 kg nitrogen (N), 20 kg phosphorus (P) and 85 kg potassium (K) per hectare per year. Apart from the normal rain, the seedlings were well watered during the growing seasons. 1.3 Harvests, measurement and calculations Harvest s an d measuremen ts were performed in April (t = 0) and Octob er (t = 1) in the first year, and in October (t = 2) in the following year. Measu rements were made of shoot height (H, mm), stem basal diameter (Bd, mm), leaf size s caled by len gth (Ll, mm) and wid th (Lw, mm) of the blades, and biomass (mg) of ro ots, main stem, branches and leaves (Mr, Ms, Mb and Ml, res pectively) after oven dry ing for 24 h at 80－ 90 o C. Leaf areas (LA, mm2 ) were calculated from a linear regression in the form of: LA =－18.3 + 0.664 × Ll × Lw ( r2 = 0.995, P < 0.001 ) The above regres sion equatio n was based on len gth and width measurements of 110 randomly selected leaves. At t = 0, sho ot heights and st em basal diameters of all seedlings were measured (n = 763). Thirty seedlings were randomly selected and harvested. Based on the values of s h oo t h eig h t, s t em b asal diameter, ro o t mas s an d abovegroun d mass (Ma, mg) of the 30 seedlings, the following regression equations were established: Mr = 257 + 1.27 × H × Bd ( r2 = 0.643, P < 0.001 ) Ma =－279 + 1.37 × H × Bd ( r2 = 0.821, P < 0.001 ) The initial root mass and aboveground mass of the other 733 seedlings were estimated by these equations. In each group, ten seedlings were randomly selected at t = 1 and 20 s eedling s at t = 2 and were h arv ested and measured. Leaf mass ratio (LMR), leaf area ratio (LAR) and root to shoot ratio (R/S) were calculated by the following formulas: LMR = Ml / M LAR =LA/M (mm2 /mg) R/S = Mr/Ma where M is the whole plant biomass. Relative growth rates (RGR, growing season －1 ) of shoot, root, and the whole plant were calculated by the fo rmula given by Hunt (1978): RGR = [Ln(M2 ) － Ln (M1 )] / (T2 －T1 )

GUO Ke et al.: Responses of Fagus engleriana Seedlings to Light and Nutrient Availability

3.2 Stem basal diameter growth and biomass production The growth in stem basal diameter was continuous over the growing season and was proportional to total biomass increment, which differed among treatment s. Differences were already statistically significant during the first growing season: Although the mean increments of bio mass in L1F0 and L1F1 did not differ s tatistically from zero in the first year, they were hig hly significantly s maller than the mean increments of treatment L2 and L3 over one and two growing seasons. This makes the increments in stem basal diameter and in total biomass a more effective parameter to measure g rowth respo nse in these deciduous seedlings than height growth. All this suggest s that t he growt h of seedlings beneath the closed forest canopy was inhibited severely, and the low light in tensity of 2% of full sunlight at the forest floor must be considered a factor limiting carbohydrate production and seedling growth. Nevertheless, the positiv e increment o f plant biomas s (even exclu ding the leaf mass that is largely shed at the end of each growing season) over two growing seasons in L1 suggests that the seed lin gs are able t o gro w slo wly b en eat h t he clos ed canopy. Popma and Bongers (1988) in their stu dy on the seedlings of tropical rain forest species found that leaf area ratio decreased and unit leaf area rate (assimilation rate) increased with increase in light intensities. The beech seedlings in this experiment exhibited similar responses. Relatively larger leaf areas and leaf area rat ios were the main factors that enabled the seedlings in L2 to grow as well as the seedlings in L3. 3.3 Allocation of biomass It has been known for a lon g time that growth rates of various parts of a plant are under mutual control and they are growing in balance (Kozlowski, 1971; Cannell and Dewar, 1994). The seedlings invested proportionally less dry mass to roots (lower R/S ratio) and more to leaves (higher LMR) in L1 than in L2 and L3. In L2 and in L3 fertilized seedlings allocated relat ively less dry mass to roots t han non-fertilized seedlings (Table 2; Fig.5). This demonstrated that the seedlings under the forest canopy tended to increase their capacity to cap ture ligh t under low light conditions , and the non-fertilized seedlings tended to enhance their capacity for nutrient uptake under low nutrient conditions. High transpiration rates in L3 might be another important factor that led seedlings to partition more dry mas s to roots: the seedlings in L3F0 partitioned more than half of their assimilates to roots. These seedlings had to meet their needs for water and nutrients resulting from their high transpiration rates and high growth rates at the open site.All these demon strate t hat allomet ry o f th e s eedlings in the different

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treatments could optimize their potential growth. 3.4 Light and nutrient availability Many shade-tolerant deciduous tree species reach their maximum photosynthetic rates or growth at approximately o ne-th ird t o o n e-h alf o f full s u nligh t (Loach , 1970; Kozlowski, 1971; Canham, 1988). For the two-y ear-old F. engleriana seedlings, the low light intensity under the forest canopy seems to be the limiting factor for their growth. A moderate light intens ity in small g aps as ben eath the bamboo screen can greatly enhance their growth, and further increase in gap size and illumination had little effect on growth. Altho ugh the fertilized seedlin gs in the open sit e and und er bamboo s creen grew significan tly b etter than the non-fertilized seedlings in this experiment, an d nutrients have been considered beneficial to improve the growth and the chance for surv ival of beech seedlings supp ress ed under the forest canopy (Peters, 1997), fertilization in this experiment did not result in a difference in growth, as measured in terms of height, biomass and stem basal diameter, between the two treatments beneath the forest canopy. 3.5 Survival and suppression beneath the forest canopy Canham (1990) found that Fagus grandifolia suffered periods of suppression prior to recruitment in to can opy and the average total length of su ppression even ranged from 45 to 52 years. Poulson and Platt (1989) point ed out that the juveniles could survive in shade for more than 100 years. Fagus crenata was considered to be able to survive about 10 years in the understory of mature forests in Japan (Nakashizuka and Numata, 1982b; Hara, 1987; Nakashizuka, 1987; 1988). Other beech species were also considered to be able to surv ive a number of y ears at t he fores t floor (Watt , 1923; Zh u and Yan g, 1985; Wen and Cao, 1993; Peters, 1997). In this experiment abou t 10% o f the s eedlin gs un der t he fo rest canop y died an d the top parts of more seedlings died du ring the first year, but mos t of the seedlings survived the conditions. The positive increment of plant biomass (even excluding the leaf mass that is largely shed at the end of each growing season) over two growing seasons in L1 d emonst rates that t he two -year-old seedlings are able to slowly grow beneath the canopy, and survive under those conditions. This suggests that there are other reas ons why beech seedlings are rare b eneath the closed canopy. The results of this experiment also showed that small gaps in the forest strongly improve the regeneration of F. engleriana. Acknowledgements: GUO Ke thanks Prof. LI Bo-Sheng, Prof. WANG Xian-Pu, Prof. ZHANG Xin-Shi and Dr. CAO Kun-Fang for their coordination and valuable suggestions,

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and Mr. YUE Wei-Yuan, ZHOU Jun and many other persons in the Daba Forest Farm for their h elp in this study. We are grateful to Dr. Heidrun Huber and Dr. Josef F. Stuefer for th eir help in data analysis. Financial support from the Dutch Royal Acad emy of Arts and Science an d the Chinese Ministry of Personnel are gratefully acknowledged. References:

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