© 1999 Kluwer Academic Publishers. Printed in the Netherlands. 197
Leaf size zonation pattern of woody species along an altitudinal gradient on Mt. Pulog, Philippines
I. E. Buot, Jr.
1& S. Okitsu
21Institute of Biological Sciences, University of the Philippines at Los Banos, College, Laguna, 4031 Philippines,
2Laboratory of Forest Ecology, Chiba University, 648 Matsudo, Matsudo City, Chiba 271, Japan
Accepted 21 January 1999
Key words: Leaf size classes, Montane zone, Philippines, Phytogeographical transition region, Raunkiaer–Webb classification, Tropical mountain
Abstract
Leaf size zonation along an altitudinal gradient from 2000–2700 m a.s.l. on Mt. Pulog, Cordillera mountain range, Luzon Is., Philippines was examined using the Raunkiaer-Webb classification system. The entire altitudinal range studied was dominated by the small leaf size classes which possess thick, lustrous, pubescent leaves adapted to high evapotranspiration, cold temperature and other stressful conditions. Altitudinal leaf size zonation was identified as follows: (1) pure needle leaved zone from 2000–2300 m a.s.l., (2) mixed needle leaved/microphyllous zone from 2300–2400 m a.s.l., (3) microphyllous zone from 2400–2600 m a.s.l. and, (4) microphyllous/nanophyllous zone from 2600–2700 m a.s.l., coinciding with the altitudinal vegetation zonation. This pattern is different from that in other tropical mountains, which usually show a gradual shift from a mesophyllous zone in the lowland to a nanophyllous zone in the upper subalpine. Stressful conditions such as steep topography (1200–2300 m a.s.l.), cloud cover, decrease of temperature, strong winds (2600–2700 m a.s.l.) could have influenced the altitudinal leaf size zonation on Mt. Pulog. The complex phytogeographical position of Mt. Pulog as a transition region between the tropics and subtropics have also influenced leaf size zonation as in eastern Himalaya, southwestern China and Taiwan.
Introduction
Tree leaf size and other leaf morphological char- acteristics generally vary with climatic and edaphic conditions and with altitude and latitude (Box 1981;
Chabot & Hicks 1982; Givnish 1987; Grubb 1974;
Liu 1993; Ohsawa & Ozaki 1992; Ohsawa 1993a, b, 1995; Reich et al. 1992; Richards 1996; Webb 1959;
Whitmore 1984; Woodward 1987). Identifying the leaf size zonation pattern of major tree species along altitudinal gradients could be the first step toward un- derstanding the environmental conditions causing leaf size zonation.
A particularly appropriate site for such studies is Mt. Pulog (2924 m a.s.l.), Cordillera mountain range, Luzon Is., Philippines. Mt. Pulog is the high- est mountain on Luzon Is. Phytogeographically, Mt.
Pulog occupies an important position in east Asia. It
is situated in the northernmost massif of the tropics in the Philippines and represents the transition zone between the tropics and the subtropics. Many con- stituent genera have northern affinities (Pinus, Ilex, Skimmia, Rhododendron, Vaccinium, etc.). It is also a convergence zone of floristic elements originating from Gondwanaland and Laurasia. Environmentally, it has a steep topography in the lower zones (ca. 1200–
2200 m a.s.l.) with dry and rocky substrate. This is rather unusual in the tropics where the lower zones are usually mesic. The higher altitudes have strong winds and are frequently covered with clouds. As a result, the altitudinal vegetation zonation had been re- ported to be unique in the tropics (Buot & Okitsu 1997, 1998). Pinus dominates in the lower altitudes instead of the tropical dipterocarp flora which climbed up to 1800 m a.s.l. on Mt. Apo, Philippines (Schoenig et al.
1975). On Mt. Pulog, dipterocarps are absent. Such
peculiar environmental conditions, floristic composi- tion and altitudinal vegetation zonation on Mt. Pulog together may influence leaf size zonation along alti- tudinal gradients. These factors might lead to a leaf size zonation different from the usual pattern in the Philippines and in the tropics. However there are no published reports on leaf size zonation on Mt. Pulog.
The aim of this paper is, first to establish the leaf size zonation pattern along the altitudinal range of Mt.
Pulog. Secondly, the paper discusses the environmen- tal factors influencing leaf size zonation. Thirdly, it explains the relation of the leaf size zonation to the phytogeography of Mt. Pulog.
Study area
Environmental conditions
The study area was Mt. Pulog (2924 m a.s.l., 16◦300 3600 N, 120◦500 2000 E), Cordillera mountain range, Luzon Is., Philippines (Figure 1). The slopes of Mt.
Pulog are generally quite steep and the soil is fairly deep. The rocks at least in the summit proved to be an- desitic (Merrill & Merritt 1910). However, at the lower zones, the topography is very steep and rugged with a very dry and rocky substrate. Cloud cover and frequent rains are observed at mid and high altitudes. At the forest limit, strong wind is common. The mean an- nual precipitation (1974–1994) is 3910 mm in Baguio City (ca. 1500 m a.s.l.), south of Mt. Pulog (Figure 2).
Annual mean temperature (AMT) for 1974–1994 from the same weather station is 19.3◦C (Figure 2). The coldest month, January, has a mean temperature of 18◦C. Using the lapse rate of 0.6◦C per 100 m rise in altitude (Barry & Chorley 1976; Sarmiento 1985), the annual and January mean temperature at ca. 2000 m a.s.l. would be 16.4◦C and 15◦C, respectively. At the lowest elevation of Mt. Pulog, ca. 1200 m a.s.l. the AMT would be 21◦C while for the coldest month it would be 20◦C. The expected temperature reading in the forest limit at ca. 2600–2700 m a.s.l., would be 12◦C and 11◦C for the annual mean and the coldest month, respectively.
Altitudinal vegetation zones
There are four altitudinal vegetation zones on Mt.
Pulog (Buot & Okitsu 1997, 1998): (I) pure
Pinus forest (2000–2300 m a.s.l.), (II) Mixed Pinus-evergreen broadleaved forest (2300–2400 m a.s.l.), (III) the Lithocarpus-Dacrycarpus-Syzygium- Leptospermum forest (2400–2600 m a.s.l.) and (IV) the Rhododendron-Clethra-Eurya forest (2600–
2700 m a.s.l.). The zones are determined by a cluster analysis. The names of the zones are after the common dominants determined by the dominance analysis of Ohsawa (1984).
Zone I. Pure Pinus forest zone (2000–2300 m a.s.l.) In the Cordillera mountain range, the Pinus forest nor- mally occupies the main bulk of the mountain slopes from 1000 m a.s.l. to the summits of most peaks that do not attain an altitude of more than 2000 m (Merrill & Merritt 1910). On Mt. Pulog, pure Pinus forest reaches up to 2300 m a.s.l. Pinus kesiya is practically the only tree to be found in this zone. It forms an open park-like forest, sometimes associated with very few individuals of Ageratina adenophora (Asteraceae), Coriaria intermedia (Coriariaceae) and Imperata cylindrica (Poaceae).
The maximum recorded DBH value obtained from the three plots is 57 cm while the maximum height is 30 m. The pine was the only tree species in the plot, hence the RBA recorded is 100%.
Zone II. Mixed Pinus-evergreen broadleaved forest zone (2300–2400 m a.s.l.)
This forest zone is the lower limit of the mossy forest.
Mosses, liverworts and lichens cover abundantly the ground, and the trunks and crowns of trees. This zone is the transition between the pure Pinus forest below 2300 m a.s.l. and the mossy forest. It has 17 species and the maximum recorded DBH is 177 cm while the maximum height is 25 m.
The single dominant species of this zone is Pi- nus kesiya Royle ex Gordon with RBA of 67%. The pine, together with the evergreen broadleaved trees such as Deutzia pulchra Vid. and Schefflera oblongi- folia Merr., occupy the canopy layer. The dominant pine trees in this zone however, differ from those found in the pure pine forest in being richly laden with bryophytes and lichens on their trunks and crowns.
Shrubby and herbaceous vegetation crowd the under- story of this steep zone. A few stands of Rhododen- dron subsessile Rendle and Clethra luzonica Merr., which usually dominate in higher altitudes, grow in this zone.
Figure 1. The study area, Mt. Pulog, northern Philippines. Sampling areas (closed squares) are along the ascent trail (broken line) towards the peak (open triangle). Contour lines are at intervals of 500 m. The inset shows the location of Mt. Pulog in the Philippines.
Figure 2. Climograph for Baguio City, the nearest weather station to Mt. Pulog.
Zone III. Lithocarpus-Dacrycarpus-Syzygium-Lepto- spermum forest zone (2400–2600 m a.s.l.)
This zone is characterized by having large and tall trees covered with lichens and bryophytes on their trunks and branches. The highest number of species in this zone is 20. The maximum DBH is 95 cm while the maximum height is 35 m.
There are seven tree species dominating this for- est zone. The most common are Lithocarpus woodii (Hance) A. Camus, Dacrycarpus steupii de Laub., Syzygium besukiense (Miq.) Masamune and Lep- tospermum flavescens Sm. with RBA of 52%, 37%, 35% and 20%, respectively.
The other dominants are the lauraceous Neolitsea microphylla Merr. with RBA of 7.8%, the fagaceous Lithocarpus sp. with RBA of 19% and the myrtaceous Eurya nitida (Korth.) Dyer with RBA of 14.5%.
This lauro-fagaceous, podocarpaceous and myrta- ceous mossy forest zone is unique in being composed of tall and large trees with prominent crowns.
Zone IV. Rhododendron-Clethra-Eurya forest zone (2600–2700 m a.s.l.)
This zone is the forest limit on Mt. Pulog. This is also the upper limit of the mossy forest region.
Above this zone is the grassland summit with scattered Rhododendron subsessile and Vaccinium sp. in steep slopes.
Vegetation is observed to be composed of few species with small individuals having a maximum DBH of 21 cm. The height tends to be very low with a maximum recorded height of 8 m.
There are four dominant species, namely Rhodo- dendron subsessile, Clethra luzonica Merr., E. nitida, and S. besukiense. Clethra luzonica has an RBA of 48% while R. subsessile has an RBA of 41.6%.
It is noteworthy that the dominants of this zone (C. luzonica and R. subsessile particularly) are com- ponents of the lower zones as understory elements especially in site 4 (Figure 1).
Methods
Leaf collection and measurement
To clarify the changes in leaf size of major tree species, collection of leaf samples were done at the ten sampling sites (Figure 1) ranging from 2000–2700 m above sea level corresponding to the study sites of Buot & Okitsu (1997, 1998). These sites were selected
because the tree vegetation in the altitudinal range was quite intact and therefore could represent the forest zone. Below 2000 m a.s.l., there were very few nat- ural communities of pines, which were in extremely precipitous slopes, hence the sampling commenced at 2000 m a.s.l. The sampling sites were composed of transects and quadrats. Quadrats were used in sam- pling sites 1–3, being composed of only one tree species. Likewise quadrats were used in sampling sites 9–10 since the vegetation was so dense and shrubby.
The density of sites 9–10 was aggravated by the oc- currence of the dwarf bamboo in the understorey of this site, making it difficult to use the point centered quarter method. The sizes of the quadrats were 720 m2 (24×30 m) for 2000 m and 2100 m a.s.l. sampling sites and 400 m2 (20×20 m) for the 2200 m a.s.l sampling site. In sampling sites 9–10, the quadrat size was 25 m2 each (5 ×5 m). In sampling sites 4–8, the samples were taken at 20 m interval on a 400 m long transect traversing horizontally or diag- onally across the sampling site depending on the slope and the topography.
The elevation (m), the slope, exposure and the area (m2) were recorded in every sampling area. The elevation was measured with an altimeter.
In this study, the leaflet of the compound leaf was the basis in classifying the leaf size class. Raunkiaer (1977) stressed that if the whole compound leaf is measured, it will be too large to compare with the av- erage simple leaf. However, the leaflet approaches the average size of a simple leaf. Hence, Raunkiaer (1977) emphasized, that it is more natural to regard the leaflet of the compound leaf as the basis in leaf size classifi- cation. Besides, leaflets have significant independence from each other in heat and gas exchange and are being abscised individually in many species
Five to fifty mature leaves from each tree species were selected as samples. For very tall trees, fallen leaves on the ground were collected. In unfavorable situations such as extremely dense undergrowth only a few samples were taken for measurement.
The leaf samples were dried to avoid deterioration as it was difficult to measure in the field. Leaf size re- duction due to drying was insignificant since the leaves were not succulent. Besides that, in the tropics, leaf measurements are almost always based on dried mate- rial. The one-sided leaf area was obtained by using the formula of Cain & Castro (1959) as follows:
Leaf area=2/3(l×w)
Table 1. Leaf size classes by Raunkiaer–Webb clas- sification (Shimwell 1971).
Leaf size class Size range (sq. mm) (Raunkiaer 1977, Webb 1959) Leptophyll <25
Nanophyll 25–225 Microphyll 225–2025 Notophyll 2025–4500 Mesophyll 4500–18225 Macrophyll 18225–164025 Megaphyll >164025
where, l refers to the full length of the leaf and w to the width of the leaf at its widest portion.
This formula is widely used by several workers in the tropics where leaves of plants have usually en- tire margins and are not lobed (Dolph 1977). In the present study, leaves were not lobed and the shapes and general outline were not significantly different;
thus using the equation may not alter the results sig- nificantly. However, using this equation in leaves with non-entire margins or which are lobed, would not yield good results (Dolph 1977).
For cylindrical or needle leaves, the formula of Nasrullah et al. (1994) was used as follows:
Leaf area=circumference of the leaf×length of the leaf
2 .
Mean leaf area obtained for each angiospermous species was then compared with the leaf size classes of Raunkiaer–Webb (Shimwell 1971) to determine the leaf size class for each species (Table 1).
Leaves of gymnosperms were designated as nee- dle leaved conifer and broadleaved podocarpaceous conifer. The leaf of the tree fern was referred to as the Cyathea type.
Cluster analysis of leaf size classes
The total RBA of the tree of each leaf size class was subjected to cluster analysis using CLUSTAN partic- ularly the standardized squared Euclidean distance of Tanaka & Tarumi (1995).
A dendrogram was then constructed using the single-linkage cluster or the nearest neighbor method (Sneath & Sokal 1973). Each distinct zone was named after the dominant leaf size class of that zone. The dominant leaf size class is defined as the leaf size with the highest RBA value. When two zones have similar dominant leaf size, the name of the second dominant
leaf size, was combined with the top dominant to avoid confusion; e.g., microphyllous/nanophyllous zone in- stead of microphyllous zone, to differentiate from a purely microphyllous zone. Nomenclature was that of Merrill (1923a, 1925, 1926).
Results
Leaf size classes and leaf size spectra
Table 2 shows the mean leaf area and leaf size classes of the woody plant species determined on the southern and eastern slopes of Mt. Pulog.
There is a clear indication that the small leaf size classes were prevalent in all the altitudinal zones of Mt. Pulog while the larger leaf size classes were sup- pressed. The small leaf size classes composed of the nanophyll, and the microphyll constituted 11%, and 56% respectively of the total species. The larger leaf size classes composed of the notophyll and the mes- ophyll constituted only 19% and 6%, respectively.
Other leaf types such as the needle leaved conifer, the broadleaved conifer and the Cyathea type (fern), constituted 3% each, respectively.
Table 3 shows spectra of leaf size classes based on RBA. It is apparent that leaf size classes were more di- verse at mid-altitudes (2300–2600 m a.s.l.) (Table 3).
At both the lower (1200–2300 m a.s.l.) and the higher (2600–2700 m a.s.l.) elevations, leaf size classes were few, only 1 for the lower altitude and 2 for the higher altitude.
Leaf size zonation
Four leaf size zones were identified along the altitu- dinal range of 2000–2700 m a.s.l., at a dissimilarity level of 6 (Figure 3). The zones were: (1) needle leaved zone (2000–2300 m a.s.l.), (2) mixed needle leaved/microphyllous zone (2300–2400 m a.s.l.), (3) microphyllous zone (2400–2600 m a.s.l.) and (4) mi- crophyllous/nanophyllous zone (2600–2700 m a.s.l.).
The needle leaved zone extending down to the lowest altitude of Mt. Pulog, ca. 1200 m a.s.l., was composed of pure needle leaved Pinus. The mixed needle leaved/microphyllous zone was dominated by the needle leaved Pinus. The microphyllous zone was dominated by Lithocarpus, Syzygium and others. The microphyllous/nanophyllous zone was dominated by the microphyllous Clethra and Eurya. Nanophyllous broadleaves such as Rhododendron and Vaccinium sp.
were also very prominent in this zone.
Table 2. Mean leaf area and leaf size classes of the tree species on the eastern and southern slopes of Mt. Pulog. Leaf size classes were based on Raunkiaer–Webb classification (Shimwell 1971).
Name of species Mean leaf area Leaf size class Region of greatest
(Family) (sq. mm) development∗
(a) Angiosperms
Leptospermum flavescens Sm. (Myrtaceae) 41 nanophyll Australia
Rhododendron subsessile Rendle (Ericaceae) 83 nanophyll Asia
Diplycosia sp. (Ericaceae) 98 nanophyll Asia
Vaccinium sp. (Ericaceae) 199 nanophyll Asia
Eurya nitida (Korth.) Dyer (Theaceae) 274 microphyll –
Neolitsea microphylla Merr. (Lauraceae) 331 microphyll –
Syzygium besukiense (Miq.) Masamune (Myrtaceae) 414 microphyll Malesia
Lithocarpus sp. (Fagaceae) 498 microphyll Asia
Decaspermum puniculatum (Lindl.) Kurz (Mytaceae) 824 microphyll Malesia
Persea philippinensis Elm. (Lauraceae) 880 microphyll –
Ilex crenata Thumb. fa luzonica (Rolfe) Loes. (Aquifoliaceae) 939 microphyll Asia
Daphniphyllum glaucescens Bl. (Euphorbiaceae) 1114 microphyll Asia
Rapanea philippinensis (A.DC.) Mez (Myrsinaceae) 1159 microphyll Malesia
Skimmia japonica Thumb. (Rutaceae) 1288 microphyll Asia
Helicia robusta Wall. (Proteaceae) 1293 microphyll –
Drimys piperita I look.f. (Winteraceae) 1339 microphyll Australasia
Psychotria crispipila Merr. (Rubiaceae) 1442 microphyll –
Saxifragaceae sp (Saxifragaceae) 1480 microphyll –
Clethra luzonica Merr. (Clethraceae) 1495 microphyll Malesia
Vaccinium indutum Vidal (Ericaceae) 1522 microphyll Asia
Gaultheria cumingiana Vid. (Ericaceae) 1555 microphyll Asia
Vernomia benguetensis Elm. (Asteraceae) 1574 microphyll –
Polyosma philippinensis Merr. (Saxifragaceae) 1629 microphyll –
Lithocarpus woodii (Hance) A. Camus (Fagaceae) 1764 microphyll Asia
Deutzia pulchra Vid. (Saxifragaceae) 2264 notophyll Asia
Eurya coriacea Merr. (Theaceae) 2303 notophyll –
Meliosma multiflora Merr. (Sabiaceae) 3076 notophyll Malesia
Maesa denticulata Mez (Myrsinaceae) 3119 notophyll Malesia
Euodia reticulata Merr. (Rutaceae) 3292 notophyll –
Neolitsea megacarpa Merr. (Lauraceae) 3400 notophyll –
Viburnum odoratissimum Ker. (Caprifoliaceae) 3517 notophyll Asia
Schefflera oblongifolia Merr. (Araliaceae) 6560 mesophyll –
Macaranga dipterocarpifolia Merr. (Euphorbiaceae) 12800 mesophyll –
(b) Gymnosperms
Dacrycarpus steupii de Laub. (Podocarpaceae) 5 broadleaved podocarpaceous Australasia conifer
Pinus Kesiya Royle ex Gordon (Pinaceae) 145 needleleaved conifer Asia
(c) Fern
Cyathea fuliginosa (Christ.) Copel. (Cyntheaceae) 937 Cyathea type Malesia
∗As indicated by Merrill & Merritt (1910); – no clear indication of the region of greatest development
At a dissimilarity level of 7 (Figure 3), three main groups could be identified. The first group from 2000-2300 m a.s.l., represents the pure needle leaved coniferous zone. The second group from 2300-2400 m a.s.l., represents the mixed needle leaved conifer-
broadleaved zone which serves as the transition zone between the pine and the mossy forest. The third group from 2400–2700 m a.s.l., represents the mossy forest dominated by microphyllous trees.
Figure 3. Dendrogram of ten sample sites obtained by the nearest neighbor method using the standardized squared Euclidean distance (Tanaka & Tarumi 1995). The four leaf size zones are: Zone I), needle leaved conifer zone at 2000–2300 m a.s.l.; Zone II), mixed needle leaved conifer-microphyllous zone at 2300–2400 m a.s.l.; Zone III), microphyllous zone at 2400–2600 m a.s.l. and Zone IV), microphyllous-nanophyllous zone at 2600–2700 m a.s.l.
At a dissimilarity level of about 13 (Figure 3), two big divisions of leaf zones could be distinguished re- flecting natural boundaries on Mt. Pulog. These are the needle leaved zone below 2000 m a.s.l., and the mixed needle leaved/microphyllous zone from 2300–2700 m a.s.l. The former is the pure pine region. The latter is the mossy forest of Mt. Pulog composed of trees with twisted branches profusely covered with a thick mat of bryophytes and other epiphytes.
Discussion
Characteristics of the leaf size zonation on Mt. Pulog The leaf size zones were apparently similar to the altitudinal vegetation zones of Mt. Pulog (Table 4).
The needle leaved conifer zone (Zone I) at ca. 1200–
2300 m a.s.l., coincided with the pure Pinus for- est. The mixed needle leaved conifer/microphyllous zone (Zone II) at ca. 2300–2400 m a.s.l., coin- cided with the mixed Pinus-evergreen broadleaved forest, a unique and anomalous zone, being the
ecotone between the pure needle leaved pine and the microphyll-dominated mossy forests. The mi- crophyllous zone (Zone III) at ca. 2400–2600 m a.s.l., corresponded to the Lithocarpus-Dacrycarpus- Syzygium-Leptospermum forest. The microphyl- lous/nanophyllous zone (Zopne IV) at ca. 2600–
2700 m a.s.l., concurred with the Rhododendron- Clethra-Eurya forest.
It can be noted that Zone II at 2300–2400 m a.s.l., is quite unique and anomalous. It is a mixed needle leaved conifer and microphyllous broadleaved forest.
It is a transition forest zone between the pure pine and the mossy forest. The environmental conditions in this zone are quite anomalous too. It has precipitous topography promoting pine growth. The cloud cover, a characteristic feature of the mossy forest in the higher altitudes of Mt. Pulog, has its lower limit in this zone.
Hence, the pine here is covered with thick bryophytic layer in contrast to the pine in the pure pine zone. With the aforementioned anomalies in environmental condi- tions and vegetation composition, the leaf zonation is subsequently anomalous.
Table 3. Leaf size spectra in the altitudinal zones of Mt. Pulog. Top dominant leaf size class is indicated by asterisk (∗). Dominance is based on the relative basal area (RBA) calculated from diameter at breast height (DBH) values. RBA is expressed in %.
Sampling site 1 2 3 4 5 6 7 8 9 10
Altitude 2000 2100 2200 2325 2400 2485 2520 2585 2615 2700
Exposure N 40◦W S 60◦E N 10◦E N 58◦E N 22◦E S 47◦E S 85◦E S 60◦W N 10◦W N 80◦W
Slope (degree) 90 70 80 70 50 45 55 55 60 45
Plot size (sq. m.) 720 720 400 157 101 416 256 136 25 25
Name of place Tuliling Tuliling Kinabonkilop Tuliling Taltalpoc Taltalpoc Taltalpoc Mogawan Pulag Pulag
Number of species 1 1 1 17 15 20 15 12 8 8
Number of leaf size class 1 1 1 4 5 5 5 5 2 2
Leaf size classes RBA RBA RBA RBA RBA RBA RBA RBA RBA RBA
(a) Angiosperms
nanophyll 0 0 0 1 0 + 0.4 20.3 37.5 42.2
microphyll 0 0 0 12.3 85.7∗ 64.3∗ 53.8∗ 58.8∗ 60.6∗ 58∗
notophyll 0 0 0 11 5.6 6 4.2 4.1 0 0
mesophyll 0 0 0 6.3 0.4 0.2 1.1 3.3 0 0
(b) Gymnosperms
needle leaved conifer 100∗ 100∗ 100∗ 67∗ 0 9.3 0 0 0 0
broadleaved podocarpaceous 0 0 0 0 4.4 18 37 13 0 0
conifer (c) Fern
Cyathea type 0 0 0 1.7 4.3 2.7 3.6 0.7 2.4 0
The most important observation was the unique pattern of leaf size class zonation along the altitudinal gradient on Mt. Pulog (Table 4). Small leaves domi- nate throughout the altitudinal range of the mountain (Tables 3 and 4, Figure 3). The trend is from the smaller leaf size class (needle leaved conifer) on the lower altitudes, ca. 1200–2400 m a.s.l., to the next larger leaf size class (microphyll) on the higher al- titudes, ca. 2400–2700 m a.s.l. (Figure 3, Tables 3 and 4). This is the opposite of the general trend of the leaf size class distribution along altitudinal gradi- ents in other tropical mountains. In the tropics, with increasing altitudes leaf size usually decreases from mesophyllous in the lowland to notophyllous in the lower montane then microphyllous in the upper mon- tane and lower subalpine and to nanophyllous in the upper subalpine, e.g., in Mt. Makiling, Philippines (Brown 1919), in New Guinea (Grubb 1974) and Mt. Kerinci, Indonesia (Ohsawa & Ozaki 1992). This change in leaf size from a larger size to a smaller size, generally follows a gradient from a more favorable habitat to a more stressful one towards the higher al- titudes. This pattern is true not only in large massifs but also in small mountains frequently covered with clouds (Whitmore 1984).
The observed difference on the leaf size zonation pattern on Mt. Pulog, could be due to variations in temperature, topography, cloud cover, rainfall, soils, floristic composition and the possible influence of phytogeography.
Influence of temperature on leaf size zonation Richards (1996) and Whitmore (1984) discussed the relation of the trends of leaf size zones to annual mean temperature (AMT) on tropical mountains. Usually in the mesophyllous forest in the lowland the AMT is ca.
20–26◦C. The notophyllous forest in the lower mon- tane has an AMT of ca. 13–20◦C. The microphyllous forest in the upper montane and lower subalpine has a usual AMT of ca. 9–13◦C. The nanophyllous forest in the upper subalpine has an AMT of ca. 6–9◦C.
On Mt. Pulog, temperature ranges (Figure 2) sim- ilar to those of the foregoing discussion resulted in a different leaf size zonation pattern. At 1200–2300 m a.s.l., with AMT of ca. 15–21◦C, pure needle leaved pine trees dominated the zone in contrast to the noto- phyllous trees of the usual tropical leaf size zonation.
At 2300–2400 m a.s.l., with AMT of ca. 14–15◦C, needle leaved pine trees dominated in contrast to the usual notophyllous trees in the tropical leaf size zona-
Table 4. The leaf size classes of the dominant species on the altitudinal zones on Mt. Pulog and the altitudinal leaf zones.
Altitude (m) Dominants RBA (%) Leaf size of dominants Dominant leaf size Altitudinal leaf size zone (Buot & Okitsu 1998)
2600–2700 Rhododendron subsessile 41.6 nanophyll microphyll microphyllous/nanophyllous Clethra luzonica 48.8 microphyll
Eurya nitida 15.6 microphyll
2300–2600 Lithocarpus woodii 52 microphyll microphyll microphyllous
Dacrycarpus steupii 37 broadleaved podocarpaceous conifer
Syzygium besukiense 35.5 microphyll Leptospermum flavescens 20.3 nanophyll
Lithocarpus sp. 19 microphyll
2200–2300 Pinus kesiya 67 needle leaved conifer needle leaved conifer needle leaved conifer/microphyllous 1200–2200 Pinus kesiya 100 needle leaved conifer needle leaved conifer needle leaved conifer
tion. At 2400–2700 m a.s.l. with AMT of 12–14◦C, microphyllous trees occurred as in the usual tropical leaf size zonation.
The microphyllous and the microphyllous/nano- phyllous zones in the higher altitudes on Mt. Pulog are similar to the usual tropical leaf size zonation. This suggests that temperature mainly controlled the leaf size zonation in the higher altitudes of Mt. Pulog as in the usual pattern. However the unique zonation pattern in the needle leaved and mixed needle leaved /micro- phyllous zones in the lower altitudes of Mt. Pulog (Tables 3 and 4) clearly indicates that temperature is not the only factor influencing the leaf size zonation in these zones. Other factors, together with temperature, must influence the peculiar leaf size zonation at lower elevations of Mt. Pulog.
Influence of topography and other environmental factors on leaf size zonation
Ohsawa (1995) emphasized that the change of leaf size is also caused by various stress factors other than tem- perature. In the subtropical warm temperate forests in southern Japan, the shift from notophyllous to micro- phyllous evergreen broadleaves was observed along stress gradients such as between ridges and valleys or from a windy mountain top to a foothill within fairly similar temperature regimes (Ohsawa 1993b).
In the pure needle leaved forest on Mt. Pulog, ca.
1200–2300 m a.s.l., the topography is rugged and very steep, up to 90◦C slope (Table 3). Subsequently, many stressful conditions are confounded possibly influenc- ing leaf size. The rocky substrate is dry though the
rainfall is high (Figure 2), since the water holding ca- pacity is expectedly low on account of the precipitous slopes. This condition enhances the growth and domi- nance of the needle leaved pine, one of the two gym- nosp erms in Table 2. Mesophyllous and notophyllous trees are susceptible to wilting in dry conditions with high evapotranspiration (Chabot & Hicks 1982; Genin 1994; Gifford & Foster 1989; Grime 1979; Larcher 1995), and so are unable to colonize steep slopes.
In the mixed needle leaved/microphyllous forest zone, ca. 2300–2400 m a.s.l., some sites are dry, rocky and precipitous while other patches are moist and mesic. The dry and rocky substrate could have pro- moted the dominance of the needle leaved pine. The mesic condition in some parts of Mt. Pulog promote the growth of notophyllous and mesophyllous trees such as Deutzia and Schefflera respectively. Cloud cover is observed in this zone and could have pro- moted the mesic condition and stressful low temper- ature condition for a tropical biota.
The cloud zone which is common in tropical mountains (van Steenis 1972; Leigh 1975; Sug- den 1982; Whitmore 1984; Kitayama 1992; Ohsawa 1993a) usually coincides with the mossy forest and has its lower limit at 2300 m a.s.l. on Mt. Pulog. The leaves of some trees were observed to possess mor- phological adaptations such as pubescence (Clethra, Rhododendron) and lustrous appearance (Eurya, Syzy- gium) implying cutinization (Bowes 1996; Buot &
Okitsu 1995; Fahn 1990; Wilkinson 1979), indicators of severe stress. However, experimental studies on the influence of cloud cover, rainfall,soil, and wind have
not yet been done on Mt. Pulog. Dolph & Dilcher (1980) have looked into the relation of leaf size and climate, and they have recognized four foliar belts on the basis of leaf size in the tropics of the western hemisphere. The foliarbelts demonstrate the impor- tance of climate on leaf size but more data are needed to establish the precise relationship. Hence there is a need to conduct such experimental studies on Mt. Pu- log to further confirm the relationship of leaf size and climate.
Leaf size zonation and phytogeography of Mt. Pulog Besides environmental conditions, the phytogeograph- ical nature of a locality could possibly influence leaf size zonation. Ohsawa (1993c) reported extremely complex vegetation zonation pattern in the phytogeo- graphical transition between the tropical and temper- ate regions in eastern Himalaya, southwestern China and Taiwan at 20–30◦N. Consequently, the leaf size zonation in the transition region was significantly al- tered from the usual microphyll (in the tropical up- per montane) to a gymnosperm leaf type in eastern Himalaya, southwestern China and Taiwan (Ohsawa 1993c).
From the point of view of phytogeography, Mt. Pu- log and the vicinity in the Cordillera mountain range, being the northernmost massif of the tropics in the Philippines, represent the transition region between the tropics and the subtropics. Transition regions al- ways present peculiarities (Ohsawa 1993c; Tang &
Ohsawa 1997), due to geographical and unstable en- vironmental conditions.
Many constituent species have northern affinities (Table 2). Buot & Okitsu (1998) described this region as the convergence zone of plant species of diverse leaf morphologies originating from Asia (Pinus, Ilex, Skimmia, Rhododendron, Vaccinium, Yushania, etc.), Australasia (Dacrycarpus, Drimys, Leptospermum) and Malesia (Clethra, Cyathea, Maesa, Syzygium, etc.) (Table 2). The existence of the five land bridges (Figure 4) connecting the Philippines with the neigh- boring floristic regions in the past (Dickerson 1924, 1928; Taylor & Hayes 1980; McCabe et al. 1982), allowed plant species originating from diverse floristic regions with diverse leaf morphologies to migrate into Mt. Pulog.
The Asiatic types on Mt. Pulog (Table 2), migrated to the Philippines through the Taiwan-northern Luzon land bridge (Figure 4A). This land bridge was during the early Tertiary (54.8 million years B. P.) when Tai-
Figure 4. Former land connections (broken line) which were the possible pathways of plant migration in the past.
(A) Taiwan-northern Luzon, (B) Indo-China/Hainan and Palawan/Mindoro, (C) Palawan-Borneo, (D) Sulu-Borneo and, (E) Sarangani Is.-Celebes land bridges. Modified from Dickerson (1928), Taylor & Hayes (1980), and McCabe et al. (1982).
wan was still connected with the mainland Asia and Luzon was separated from the rest of the Philippines (Dickerson 1928). In addition, the Asiatic types of Mt. Pulog could have also traversed through the Indo- China/Hainan area and Palawan/Mindoro land bridge (Figure 4B)during the early Tertiary (Taylor & Hayes 1980; McCabe et al. 1982).
The Malesian species on Mt. Pulog (Table 2), have crossed the Palawan-Borneo (Figure 4C) and Sulu-Borneo (Figure 4D) land bridges in the late Pliocene (2.30 million years B.P.) to early Pleistocene (1.77 million years B. P.) when the rest of the Philip- pines was already united with Luzon. The dipterocarps must have come along with this migration as evi- denced by the Pliocene dipterocarp fossil in northern Luzon (Dickerson 1928). However, maybe because of
edaphic/topographic reasons, the dipterocarps did not survive on Mt. Pulog. Other tropical taxa common in other mountains in the Philippines such as Pittospo- rum pentandrum Merr., Ficus hauili Blanco, F. nota Merr., Premna odorata Blanco among others, survive on Mt. Pulog in depressions at 1500 m a.s.l. (Merrill &
Merritt 1910; Buot & Okitsu 1998). More tropical taxa included in this study such as Rapanea, Decasper- mum, Meliosma, Maesa, Macaranga and others, reach up to 2500 m a.s.l. generally with decreasing RBA (Buot & Okitsu 1998). The tropical Syzygium, how- ever, is an exception as it has higher abundance at 2700 m a.s.l. (15.6%) than at 2300 m a.s.l. (3.1%) (Buot & Okitsu 1998).
Some Australasian elements represented by Lep- tospermum, Dacrycarpus and Drimys, among others are also present on Mt. Pulog (Table 2). These gen- era however are also well-developed in the northern hemisphere and must have reached Australia through Malesia(Merrill & Merritt 1910). Their occurrence on Mt. Pulog, however, had not yet been explained.
In a phytogeographical study of mountain plants, van Steenis (1962) found out that plants which are well-developed both in Asia and Australia must have passed through the Taiwan-northern Luzon land bridge (Figure 4A) in extending their distribution over the tropical mountains. The Australasian species on Mt. Pulog therefore must have traversed the Taiwan- northern Luzon and the Sarangani Is.-Celebes (Fig- ure 4E) land bridges during their migration from the north to the Australasia or vice-versa. Apparently, these Australasian species, found the high massif a suitable habitat for establishment.
The past land connections of the Philippines as out- lined above is also partly responsible for the unique and diverse floristic composition of Mt. Pulog as com- pared with the rest of the Philippine Islands. The diversity of the flora subsequently enhances the unique vegetation and leaf size zones, different from the other Philippine mountains. The phytogeographical position of Mt. Pulog, as transition zone between the tropics and subtropics, must be related to the stressful condi- tions promoting the anomalies in the zonation of the massif. More experimental studies need to be con- ducted to allow detailed and precise analysis of the relationship.
Acknowledgements
The authors are grateful to Forester G. S. Fianza of the Department of Environment and Natural Re- sources (DENR), Benguet, Philippines for giving the permit to do some fieldwork and access to some park records. Mr H. Cayat, Mr R. Lupos and Mr S.
Nayosan of DENR facilitated the field survey. The discussions with Dr N. O. Aguilar of the University of the Philippines at Los Baños Herbarium (CAHP), Dr Franz Seidenschwarz of the University of San Car- los (Philippines) Herbarium (CEBU) and Dr Elizabeth A. Widjaja of Herbarium Bogoriense (BO), Indonesia were greatly appreciated. Mr B. F. Hernaez of CAHP, helped in plant identification. Dr J. F. Veldkamp, ed- itor of the Flora Malesiana Bulletin, Rijksherbarium, Leiden, The Netherlands, confirmed the validity of the botanical names listed in Table 2 and gave encourage- ment through his fruitful discussions. The Japanese Ministry of Education, Science, Culture and Sports awarded the senior author a graduate scholarship.
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