Vegetatio 79: 33-39, 1989 © 1989 Kluwer Academic Publishers. Printed in Belgium. 33

Stratification in a New Zealand rain forest

Mark W. Bee, J. Bastow Wilson 1 & Alan E Mark Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand; IAuthor for correspondence

Accepted 14.10.1988

Keywords: Epiphyte, Forest, Liane, Synusia, Understorey

Abstract

An undisturbed podocarp-broadleaved rain forest was sampled in eight strata (synusiae): canopy, small tree, shrub, low shrub, herb, ground, epiphyte and liane. Independent classifications of the quadrats were performed using the eight strata, and the classifications were correlated. Relations between strata were significant, but weak. Using records of plants of all ages, the relations largely reflected the presence of seedlings of woody species in the ground and herb strata. Using only records of species present as adults, the relations were quite different, and at least some seemed to reflect sociological interactions. Relations between a tree and a herb stratum was lower than in McCune & Antos' (1981) investigation, probably because of the narrow ecological range of the present study area.

Nomenclature: Flora of New Zealand, Vol. I, Allan, H. H. 1961. Vol. II.Moore, L. B. & Edgar, E. 1970. Govern- ment Printer, Wellington; Connor, H. E. & Edgar, E. 1987. Name changes in the indigenous New Zealand Flora, 1960-1986 and Nomina Nova IV, 1983-1986. N. Z. J1 Bot. 25:115-170.

Introduction

Forest plant communities are often seen as compris- ing a number of strata: canopy, small tree stratum, shrubs, herbs etc. (Cain 1934; Smith 1973), even though the boundaries between strata may be ar- bitrary (Popma et al. 1988). Strata are a special case of the 'synusiae' of Gams (1918), life-form groupings within a community, and the synusial concept can include also epiphyte and liane groupings which cannot be described as strata.

Such physiognomic categories are a guide to syn- morphology (Rejm~lnek 1977). There has been argu- ment whether these strata/synusiae are indepen- dent. Du Rietz (1930) suggested that the strata/synusiae had 'very little relationship to each other', i.e. the distribution of the communities of one

stratum/synusia was not correlated with the distribu- tion of those of other strata/synusiae. Lippmaa (1933), Cain (1936), and Hosokawa (1951) made similar interpretations. Others have suggested that plant interactions are generally vertical rather than horizontal (Gleason 1936), giving communities comprising all strata/synusiae (Webb et al. 1967). Although stratification has been considered most often in tropical rain forests (Richards 1952), the concept applies also to temperate forests (e.g. Rogers 1980) and especially to temperate rain forest (Dawson & Sneddon 1969).

Previous objective studies on the relation between forest strata have been made on the temperate/bore- al forests of northern North America (Rogers 1980; Carleton & Maycock 1980, 1981; McCune & Antos 1981). The temperate rain forests of New Zealand

34

have greater stratal complexity than these northern

temperate forests, and several investigators have

compared their structure to tropical rain forests (e.g.

Cockayne 1928; Cockayne & Turner 1928; Dawson

& Sneddon 1969; Dawson 1988).

Over a wide geographic/ecological range of sites, relations between strata must be found; to take an ex-

treme case, a comparison of tropical rain forest with

boreal coniferous forest will give the trivial result

that the tropical understories are found under tropi-

cal canopies, and boreal understories under boreal

canopies. The main determinant of species composi-

tion over such an ecological range will be the physi-

cal environment, not interactions between species.

Therefore, the relations between strata are best ex-

amined over a narrow ecological range.

We examined stratum/synusia relations in a uni-

form area of rain forest in the Catlins Ecological Re- gion of southern New Zealand.

Methods

Study site

The 'Podocarp-broadleaved forest' (Cockayne 1928)

or 'Conifer-broadleaf forest' (Wardle 1983) of

Papatowai Scenic Reserve (Table 1; Allen 1978) is

typical of the lowland rain forest of the area.

It is dominated by the gymnosperms Prumnopitys ferruginea and Dacrydium cupressinum, (occasion-

al Prumnopitys taxifolia and Podocarpus cunning- hamii), sometimes emergent from a more continu- ous canopy of the angiosperm Weinmannia racemo- sa. There is regeneration of all these except P. taxifo- lia. The small-tree stratum is especially rich in

species, the most important being Griselinia littora- lis, Pseudowintera colorata, Myrsine australis, Pseu- dopanax erassifolius, P. eolensoi and Coprosma luc- ida, and several of these species occasionally reach

the canopy. Small individuals of these species are often found among the shrubs, with other species

that grow no larger, e.g. Coprosma rhamnoides. The herb stratum is dominated by ferns, such as

Asplenium bulbiferum and Blechnum discolor. Spe- cies restricted to the ground (< 0.1 m) stratum are few, the most common being Nertera aft.

Table 1. The frequencies (10 m × 10 m) of major species over the sampled area. Codes: E = normally an epiphyte, L = normally a liane.

Category Species Code Frequency (%)

Tree ferns Dicksonia squarrosa 46 Dicksonia fibrosa 34 Cyathea smithii 19

Other ferns Asplenium flaccidum E 72 Phymatosorus diversifolius 70 Asplenium bulbiferum 65 Blechnum discolor 62 Cyathea colensoi 55 Grammitis billardieri E 25 Hymenophyllum scabrum E 25 Hymenophyllum rarum E 24 Polystichum vestitum 18 Hymenophyllum flabellatum E 18

Gymno- Prumnopitys ferruginea 73 sperms Dacrydium cupressinum 60 Woody Weinmannia racemosa 79 angiosperms Griselinia littoralis 70

Pseudowintera colorata 65 Myrsine australis 53 Podocarpus cunninghamii 50 Parsonsia heterophylla L 48 Rubus cissoides L 48 Pseudopanax crassifolius 47 Coprosma lucida 43 Aristotelia serrata 42 Fuchsia excorticata 41 Pseudopanax colensoi 40 Metrosideros diffusa L 37 Carpodetus serratus 25 Melicytus ramiflorus 23 Pseudopanax simplex 19 Ripogonum scandens L 18 Neomyrtus pedunculata 17 Pittosporum tenuifolium 13 Coprosma feotidissima 70 Coprosma rhamnoides 53 Coprosma rotundifolia 49

Angiosperm Nertera aff. dichondraefolia 21 herbs Earina mucronata E 16

dichondraefolia. Other occupants of this stratum are seedlings.

Epiphytes are frequent. They include both species that grow also on the ground, e.g. seedlings of the tree Weinmannia racemosa, and quite large plants of

the tree Griselinia littoralis, as well as species found almost always as epiphytes, e.g. the ferns Grammitis

billardieri, Hymenophyllum rarum and Asplenium flaccidum.

Climbers are also frequent, especially Parsonsia heterophylla, Rubus cissoides, Metrosideros diffusa and Ripogonum scandens.

The area sampled is sheltered and shows no signs of storm disturbance, and the forest is in practically pristine condition (Allen 1978).

FieM methods

A 400 m × 2 km band across the reserve was sam- pled. Within this band, eighty 10 m x 10 m quad- rats were placed at random, and in each the pres- ence/absence of vascular plant species was recorded in seven strata/synusiae:

Canopy (defined as reaching the upper 1/3 of the canopy, where the canopy is the more or le~s con- tinuous upper limit of most tree foliage)

Small tree (reaching the mid 1/3 of the canopy) Shrub (reaching more than 1 m above ground, but

no higher than the mid 1/3 of the canopy), Herb (taller than 0.1 m) Ground (shorter than 0.1 m) Epiphytes Lianes.

Analysis methods

The quadrats were classified using the quadrat/spe- cies data by Cluster Analysis, using Flexible sorting strategy (beta = -0.25) and the complement of Jaccard's coefficient as estimate of dissimilarity. This is the combination of methods recommended by Williams et al. (1973). Agreement between clas- sifications was measured by coefficient R (Rajski 1961). Significance of the coefficient was tested by the log likelihood ratio test recommended by Orl6ci (1968).

Such cluster analyses were performed using all records, and also using only records of species pres- ent in a particular quadrat as adults. For the latter

35

analyses, some strata were missing from some quad- rats, making it impossible to calculate R, these quad- rats were therefore omitted, leaving 54 quadrats.

Where a species was present in more than one stra- tum, the association between occurrences in the two strata was examined by Fisher's Exact Test (Fisher 1938).

Detrended Correspondence Analysis (Hill & Gauch 1980) was used to estimate the range of vegetational composition in the samples.

Results and interpretation

Relations between strata are quite different depend- ing whether juveniles are included (all-ages), or only adults (Fig. 1).

Many of the relations in the all-ages analysis (Fig. 1) can be taken as relations between the pres- ence of a species in, say, the canopy and, as seedlings, in lower strata. Thus the ground stratum shows strong relations with the small tree, shrub, herb and liane strata/synusiae in the all-ages analysis because of the presence of small tree, shrub and iiane seedlings in the herb and ground strata. Epiphytes show no strong relation to any other stratum- synusia; seedlings of epiphytic species are rarely found on the ground. The canopy stratum is particu- larly isolated in the all-ages analysis, mainly because seedlings of the canopy species are relatively sparse.

Considering only adult plants (Fig. 1), the pic- ture changes markedly. Many of the strong relations previously seen that involved the ground and herb strata weaken or disappear. This confirms that those relations were based on the presence in the ground and herb strata of seedlings of tree, shrub and liane species. There are relatively strong relations in the adults analysis between the canopy and the small tree and liane strata/synusiae, and to a lesser degree with the herb and shrub strata. This can best be inter- preted as an effect of the canopy on species under- neath. The group of strata: canopy / small tree / shrub / liane are generally linked in the adults analy- sis. The canopy / shrub relation is not significant, perhaps due to the intervening small tree stratum, and neither is the small tree / liane relation. The canopy / herb relation is strong, presumably due to

36

ALL AGES Canopy

E p l p h y t ~ m a l l tree

Llene ~ Shrub

Key i

Ground Herb

R:0.60+ Solid line: P<O.05 R--0.55-0.59 Dashed lines: R..D.50-0.49 not significant

(not R<0.50 shown)

ADULTS Canopy

Eplphyte ~" i- " ~ ~ Small tree - i %~-~

Lie n e / ~ # ~ ~ " ~ / ~ / ~ S hr u b x\ "~'4. xX\l

Ground Herb Fig. 1. Relations between strata, estimated by using Rajski's R to compare classifications of rain forest quadrats using the separate strata.

the effect o f the c a n o p y on l ight level a n d / o r litter.

Epiphytes show strong relations to small trees and

shrubs. This is surpr is ing, since mos t are rarely

f o u n d on shrubs, bu t much more c o m m o n l y on

c a n o p y trees.

A s s o c i a t i o n be tween ind iv idua l s o f the same spe-

cies in different strata gives insight into regeneration.

Twenty three ou t o f 232 such assoc ia t ions were sig-

n i f ican t (Table 2), more t h a n the 5% Type I errors

expected. In o rde r to achieve an overall Type I e r ror

rate o f 0.05, a s igni f icance level o f

0 .05/232 = 0.000215 shou ld be used for ind iv idua l

compar i sons . A t this level there are two s igni f icant

associa t ions .

Table2 . Association between individuals of the same species oc- curring in different strata, with the sign of the association and probability (P) by Fisher's Exact Test.

Species Strata compared Sign P

Carpodetus serratus small tree epiphyte + 0.0296 small tree herb + 0.0252 small tree shrub + 0.0296 canopy ground + 0.0093

Coprosma rotundifolia shrub ground + 0.0187 Dacrydium cupressinum canopy small tree + 0.0095 Dacrycarpus dacrydioides canopy herb + 0.0003 Dicksonia fibrosa small tree shrub - 0.0164 Dicksonia squarrosa small tree shrub - 0.0195 Griselinia littoralis shrub ground - 0.0133 Melicytus ramiflorus ground epiphyte + 0.0250

canopy epiphyte + 0.0497 canopy ground + 0.0250

Myrsine australis canopy shrub + 0.0094 Pittosporum tenuifolium small tree herb + 0.0252 Pseudopanax colensoi herb epiphyte + 0.0117

canopy ground + 0.0482 Pseudopanax crassifolius shrub ground + 0.0008

Pseudopanax simplex ground epiphyte + 0.0427 Pseudowintera colorata canopy shrub - 0.0040 Weinmannia racemosa herb epiphyte + 0.0308

small tree epiphyte - 0.0007

small tree herb - 0.0374

Discussion

Other s tudies o f this type have been o f t empera te -

borea l forests in N o r t h Amer i ca , all less complex

s t ruc tura l ly t han the rain forests o f New Zea land .

Those s tudies have genera l ly ind ica ted l i t t le re la t ion

be tween strata . Rogers (1980) found li t t le re la t ion

be tween canopy and unders to rey in species compos i -

tion of mesic forests in north-west USA. There were no unders torey species relat ively a b u n d a n t in the

he mloc k s tands tha t were no t also a b u n d a n t in o ther

types o f mesophy t i c forest. H e concluded: ' commu-

ni ty o rgan i sa t ion is h a p h a z a r d ' . Taylor e t al. (1987)

also con lcuded tha t there was ' surpr i s ing ly p o o r '

co r re la t ion be tween tree canopy cover and under-

s torey vegeta t ion. The lack o f re la t ion found by

these two workers is pa r t i cu la r ly surpr i s ing since,

unl ike the workers c i ted below, they inc luded juve-

nile plants of tree species in their definition of under-

storey. Similarly, Ca r l e ton & M a yc oc k (1981) found

that only t21 out of 410 understorey taxa showed specificity to any canopy vegetational class, and McCune & Antos (1981) found only weak correla- tion between strata. Carleton & Maycock (1980) found that wet bog forests had a characteristic over- storey and a characteristic understorey, as did up- land mesic forests, but otherwise there was little rela- tion between overstorey composition and understorey composition. This is some confirmation that stratal correlations will be seen when the ecolog- ical range examined is great enough. Correlations can also be seen when the environment differs be- cause of successional change. Thus, Everett et al.

(1983) examined successional Pinus monophylla -

Juniperus osteosperma woodland, and concluded that the species in the understorey were determined by the microenvironment created by the canopy trees.

It is not possible to compare all details of our results with previous work, since stratification is simpler in most northern temperate forests. In those forests it is not possible to identify liane or vascular epiphyte synusiae.

Different investigators of between-strata relations have used different methods, so it is not generally possible to compare their results directly. We at- tempted to make a comparison with the results of McCune & Antos (1981). The latter workers com- pared the tree and herb strata by calculating dis- similarity coefficients between all possible pairs of quadrats, and correlating the coefficients found us- ing the tree stratum with those found using the herb stratum. Their field records differed somewhat from ours, but we applied their method as closely as possi- ble to our New Zealand data. We used their defini- tions of the tree and herb strata, and their method of correlating dissimilarities between all possible pairs of quadrats, but not their method of classifica- tion, since the correlation coefficient they used is in- appropriate with qualitative data. Our data, using their method, gave correlation between the tree and herb strata with r=+0.0502, compared to their reported value of +0.28 for the same comparison. The difference probably reflects the greater ecological range of McCune & Antos' data. When generalisa- tions are made about relation (or lack of it) between strata, the ecological / vegetational range should be

37

indicated. In our data, the length of the first DCA axis is 2.6 s.d.

The separate analyses of all-ages and adult rela- tions made here, have not been made in most previ- ous work. The relations are clearly different between the two analyses.

The number of significant associations between individuals of the same species in different strata are rather few (Table 2), emphasising the homogeneity of the vegetation. Most of the associations are posi- tive ones. These could be caused by soil differences

- juveniles and adults of a species congregating where the soil was more favourable for that species. It is also possible that they are the result of limited dispersal, seedlings tending to congregate beneath adults. Since many of the associations involve the epiphyte synusia, isolated from the soil, the dispersal explanation is likely to be important. All the species involved seem to be dispersed by birds; however the behaviour of the birds is not known, whether they fly away with the fruit and deposit the seed at a dis- tance, or whether they eat the fruit on the bearing tree and deposit the seed underneath. The latter has been suggested for Olea cunninghamii (not present in our area) and Prumnopi tys ferruginea (Herbert 1986). The negative association between small tree and shrub occurrences for two species of Dicksonia

tree fern could represent a tendency for groups of tree fern plants to occupy tree gaps, and therefore be even-aged. The negative associations of small tree and epiphyte Weinmanniaracemosacould be related to this; IV.. racernosa often establishes on tree ferns (Wardle 1966), and if the tree ferns are present be- cause the area is a canopy gap, by the time W. racemosa plants reach small tree size the gap will have closed and the tree ferns died, removing the main niche of W. racemosa seedlings.

The previous quantitative work referred to above has not included the epiphyte synusia. In both ana- lyses (Fig. 1), there is a moderately strong correlation between the epiphyte and small tree synusiae, proba- bly reflecting that some tree species have a greater tendency to harbour epiphytes, for example some of the tree ferns (Page & Brownsey 1986). The epiphyte synusia is the only one not to show any R value higher than 0.6, supporting the comment of Bark- man (1978) that epiphyte synusiae are often poorly

38

correlated with terrestial synusiae. It is usually difficult to see clear guilds (sensu Root

1967) in plants since almost all Embryophyta are au- totrophic, and so use the same resource supply and so in a sense form one guild (Schoener 1986; Harper 1979; Goldberg & Werner 1983). The strata/synusiae of forests form one example of guild differences in plant communities, since they may be regulated by different environmental factors (Dunn & Stearns 1987). Our results do not show these guilds/strata/synusiae to be independent, even over a narrower ecological range than other workers have examined, since there are many significant relations in our data. However, the relations are not strong. This supports the suggestion by Cain (1936) that: 'The inferior layers are dependent on the superior layers and the general complex for modification and amelioration of the habitat, but, in many instances, are not dependent on the specific floristic assem- blages', a concept related to that of 'equivalence of competitors' (Goldberg & Werner 1983).

Acknowledgements

We thank Graeme Bee for assistance with field work, and Bill Lee and Ralph Allen for comment on a

draft.

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