Oecologia (Berlin) (1988) 75: 625-632 Oecologia Springer-Verlag1988 The effect of canopy gaps on growth and morphology of seedlings of rain forest species J. Popma 1, 2 and F. Bongers 1, 2 a Department of Plant Ecology, University of Utrecht, Lange Nieuwstraat 106, NL-3512 PN Utrecht, The Netherlands 2 Laboratorio de Ecologia, Facultad de Ciencias, Universidad Nacional Aut6noma de M+xico, 04510 Mexico D.F. Summary. Growth and morphology of seedlings of ten trop- ical rain forest species were studied at Los Tuxtlas, Mexico. Seedlings were grown in three environmental conditions: the shaded forest understorey (FU, receiving 0.%2.3% of the daily photosynthetic photon flux, PF, above the can- opy), a small canopy gap of approx. 50 m 2 (SG, receiving 2.1-6.1% of daily PF), and a large canopy gap of approx. 500 m 2 (LG, receiving 38.6-53.4% of daily PF). The growth of all species was enhanced in gaps, and in LG the effect was stronger than in SG. Plants grown in LG had a sun- plant morphology, with a high root-shoot ratio (R/S), a high specific leaf weight (SLW) and a low leaf area ratio (LAR). Plants grown in SG or FU showed a shade-plant morphology, with a low R/S, a low SLW and a high LAR. Growth responses varied from species unable to grow in the shade but with strong growth in the sun, to species with relatively high growth rates in both shade and sun conditions. Shade tolerant species were able to grow in the shade because of a relatively high unit leaf rate. The pioneer Cecropia had a high growth rate in LG because of a high LAR. Most species showed a complex growth response in which they resembled the shade intolerant extreme in some aspects of the response, and the shade tolerant extreme in other aspects. Key words: Biomass allocation- Canopy gaps - Seedling growth - Shade tolerance - Tropical rain forest Many tropical rain forest tree species seem to depend on, or benefit from, canopy gaps for successful regeneration (Hartshorn 1978, 1980; Whitmore 1984). Current hypothe- ses concerning maintenance of the high species richness of tropical rain forest state that ecological differentiation in growth response to canopy gaps of different sizes may play an important role (Brokaw 1985 a; Connell 1978 ; Denslow 1980; Pickett 1983; Ricklefs 1977; Whitmore 1982). With respect to this ecological differentiation, species are generally classified in two main ecological groups: spe- cies which require light gaps for their regeneration and those which do not. This latter group is referred to as shade tolerant (Denslow 1980; Hartshorn 1980; Martinez-Ramos 1985 ; Whitmore 1984). The first group can be further subdi- vided in species which are truly shade intolerant and only regenerate in large gaps, where they complete their entire Offprint requests to: J. Popma, Utrecht life cycle (pioneer species or large gap specialists), and spe- cies which may be shade tolerant in some stage(s) of their development, but which require gaps to reach maturity (small gap specialists, gap species, nomad species; Denslow 1980; Hartshorn 1980; Martinez-Ramos 1985). As many species cannot easily be classified into one of these groups, such a classification can only serve as a working model. The assignment of a species to one ecological group or another is usually based on the spatial and temporal distribution of species over microhabitats (Brokaw 1985b; Denslow 1980; Hartshorn 1980; Martinez-Ramos 1985; Whitmore 1984), on size/age class analyses (Knight 1975), on germination and growth characteristics of seedlings or juvenile individuals (Ng 1978; Garwood 1983), or on growth rates and life span of adult trees (Lieberman et al. 1985; Primack et al. 1985). Detailed knowledge on the growth response of species to canopy gaps of different sizes should lead to a further understanding of the degree of differentiation which appar- ently exists among species (Bazzaz and Pickett 1980). This differentiation may be found in several aspects of a species' growth response, among which total plant light compensa- tion point (the daily photosynthetic photon flux, PF, at which relative growth rate is 0) and light saturation point (daily PF above which no further increase in relative growth rate occurs), morphological and physiological acclimation potential, and allocation of resources to defence against herbivores and pathogens (Augspurger 1984a; Dirzo 1984; Fetcher et at. 1983; Oberbauer and Strain 1985; Okali 1971, 1972; Sasaki and Mori 1982; Whitmore and Bowen 1983). In this paper the differentiation between tropical rain forest species with respect to shade tolerance, enhancement of growth in gaps of different sizes, and effects of these gaps on plant morphology, will be evaluated. Seedlings were chosen because: (1) Seedlings are expected to respond stronger to environmental differences than plants at a later stage in their life cycle. (2) Many tropical tree species form a seedling bank shortly after seed release. Survival and growth then depends for a large part on the environmental conditions at the site. (3) Seedlings are relatively easy to work with experimentally. Study site The study was carried out at the tropical biological station "Los Tuxtlas", Veracruz, Mexico (18~ 95~ Climatological data available from the nearby station of