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Geographic variation in seed traits within and among forty-
two species of Rhododendron (Ericaceae) on the Tibetan
plateau: relationships with altitude, habitat, plant height,
and phylogeny
Yongji Wang
1,2
, Jianjian Wang
1,2
, Liming Lai
1
, Lianhe Jiang
1
, Ping Zhuang
1
, Lehua Zhang
3
,
Yuanrun Zheng
1
, Jerry M. Baskin
4
& Carol C. Baskin
4,5
1
Key Laboratory of Resource Plants, Beijing Botanical Garden, West China Subalpine Botanical Garden, Institute of Botany, Chinese Academy of
Sciences, Beijing, Xiangshan, China
2
University of Chinese Academy of Sciences, Beijing 100039, China
3
Lushan Botanical Garden, Jiangxi Province and Chinese Academy of Sciences, Jiujiang, Jiangxi Province 332900, China
4
Department of Biology, University of Kentucky, Lexington, Kentucky 40506
5
Department of Plant and Soil Sciences, University of Kentucky, Lexington, Kentucky 40546
Keywords
Altitude, geographic variation, habitat, plant
height, Rhododendron, seed mass, seed
morphology.
Correspondence
Yuanrun Zheng, Key Laboratory of Resource
Plants, Institute of Botany, Chinese Academy
of Sciences, No. 20 Nanxincun, Xiangshan,
Beijing 100093, China. Tel/Fax: +86 10
62836508; E-mail: zhengyr@ibcas.ac.cn
Funding Information
This work was funded by the Ministry of
Science and Technology of China
[2012GB24910654].
Received: 18 December 2013; Revised: 7
March 2014; Accepted: 11 March 2014
Ecology and Evolution 2014; 4(10): 1913–
1923
doi: 10.1002/ece3.1067
Abstract
Seed mass and morphology are plant life history traits that influence seed dis-
persal ability, seeding establishment success, and population distribution pat-
tern. Southeastern Tibet is a diversity center for Rhododendron species, which
are distributed from a few hundred meters to 5500 m above sea level. We
examined intra- and interspecific variation in seed mass and morphology in
relation to altitude, habitat, plant height, and phylogeny. Seed mass decreased
significantly with the increasing altitude and increased significantly with
increasing plant height among populations of the same species. Seed mass dif-
fered significantly among species and subsections, but not among sections and
subgenera. Seed length, width, surface area, and wing length were significantly
negative correlated with altitude and significantly positive correlated with plant
height. Further, these traits differed significantly among habitats and varied
among species and subsection, but not among sections and subgenera. Species
at low elevation had larger seeds with larger wings, and seeds became smaller
and the wings of seeds tended to be smaller with the increasing altitude. Mor-
phology of the seed varied from flat round to long cylindrical with increasing
altitude. We suggest that seed mass and morphology have evolved as a result of
both long-term adaptation and constraints of the taxonomic group over their
long evolutionary history.
Introduction
The seed is the most important stage in the life cycle of
plants (Baskin and Baskin 2001), and seed traits, includ-
ing mass, dormancy and dispersal, are central compo-
nents of plant life histories (Thompson 1987), whose
importance to plant fitness is widely appreciated (Moles
et al. 2005; Moles et al. 2007; Bolmgren and Cowan 2008;
Hallett et al. 2011; Turnbull et al. 2012). Traditionally,
seed mass within species was considered to be a remark-
ably constant characteristic (Bu et al. 2007). However, if
resources are limited, a plant may allocate them into
many, smaller seeds or into fewer, larger seeds (Moles
and Westoby 2003; Pluess et al. 2005; Bu et al. 2007; Guo
et al. 2010; Wu et al. 2011). Therefore, seed mass within
a species or even an individual plant can vary significantly
(Hendrix 1984; Martijena and Bullock 1997; Hodkinson
et al. 1998; Guo et al. 2010; Turnbull et al. 2012). Seed
mass can vary over 10 orders of magnitude among plant
species, and even within a plant community (Leishman
and Westoby 1994). Such variation in seed mass often is
effected by environmental factors. Both within and among
ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited.
1913
species, a smaller seed mass has been associated with
more disturbed habitats, an increase in altitude (Bu et al.
2007) and with an increase in latitude (Pluess et al.
2005).
Numerous recent studies have found it reasonable to
expect that seed traits within a species, such as seed mass,
could be affected by phylogenetic constraints and develop-
mental allometries (Lord et al. 1995; Tautenhahn et al.
2008; Queenborough et al. 2009; Munzbergova and Plack-
ova 2010; Turnbull et al. 2012). Usually, the variation in
seed mass mainly is among seeds within genera or even fami-
lies (Wolfe 1995); however, differences in seed mass of the
same plant are small (Mazer 1990; Lord et al. 1995). The
reason for low variation in seed mass is phylogenetic con-
straints or niche conservatism (Lord et al. 1995). Adaptive
changes may be restricted by species’ evolutionary history,
that is, complex patterns of covariation among functionally
related traits (Baker 1972; Pigliucci 2003; Pluess et al. 2005).
Generally, variation in seed traits and its causes is unclear
(Bu et al. 2007).
We examined the relationship between elevation, plant
height, habitat, phylogeny, and seed traits among 59 pop-
ulations representing 42 species of Rhododendron in the
southeast Tibetan Plateau. Further, we selected three spe-
cies that occur over a wide altitudinal gradient and have
large differences in seed mass and seed dispersal capacities
(R.thomsonii R.cerasinum, and R.aganniphum var.
flavorufum) to investigate variation in seed traits along
the elevation gradients. In general, ripe seeds of Rhodo-
dendron are oval, flat, and reddish brown, but they vary
with environment (Fig. 1).
We hypothesized that the seeds of Rhododendron spe-
cies would vary greatly in seed traits as a result of their
adaptation to extremely different environments. Specifi-
cally, we addressed the following question: Are seed mass
and morphology related to and vary with elevation,
plant height, and phylogeny? It is necessary to answer
this question in order to understand how plants are
adapted the extreme and unique environments in Hima-
laya.
Materials and Methods
Study sites and sampling methods
Rhododendron (Ericaceae) is one of the largest genera of an-
giosperms, and it includes nine subgenera and more than
1000 species. The genus is widely distributed in Asia, Eur-
ope, and north America (Fang et al. 2005), ranging from
65°north to 20°south latitude in the tropical, temperate,
and boreal zones. Altitudinally, it occurs in vegetation
zones that range from a few hundred meters to about
5500 m above sea level, including subtropical mountain
evergreen broad-leaved forest, coniferous and mixed
broad-leaved forest, coniferous forest, the open-like conif-
erous forest, elfin forest, and Rhododendron shrub. The
morphology of Rhododendron plants and seeds varies sig-
nificantly across this environmental complex.
The Himalaya is the highest mountain chain in the world
and has a complex of ecological environments. From low to
high elevation, the vegetation consists of four types: tropical,
subtropical, temperate, and alpine. The Himalayan region is
the distribution center of Rhododendron (Fang and Lu
1981), and in China, there are 351 species, including three
subgenera, six sections, and 41 subsections containing 36%
of the species in the genus. In addition, the region also is the
diversification center of the genus, where it is taxonomically
very complex (Fang & Lu 1981). In this region, Rhododen-
dron species grow over an altitudinal range from a few hun-
dred meters to about 5500 m, which is an ideal situation for
studying variation in size and morphology of seeds.
The study sites are located on the southeastern Tibet pla-
teau (27.239°–29.996°, 88.5–97.287°) near the Himalayan
and Hengduan Mountains (Fang et al. 2005). The altitude
ranges from 2280 to 4540 m, and the region includes Milin,
Motuo, Bomi, Cuona, Longzi, Yadong, Linzhi, and Chayu
counties (Fig. 2). In 2010, we investigated the seeds of 42
Rhododendron species (59 populations) in three subgenera,
three sections, and 23 subsections (Table 1). Habitats of the
sampling sites included alpine shrub (AS), rocky slope (RS),
and forest (F).
Seeds were collected by hand from more than ten indi-
vidual plants randomly selected from three to five subpop-
ulations at each altitude in late September and early
October 2010. Seeds of each species and subpopulation
were pooled, and mean seed mass of each species within a
Figure 1. Seed of Rhododendron delavayi var. peramoenum.
1914 ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Variation in Seed Traits of Rhododendron Species Y. Wang et al.
population at each altitude was determined. Three to five
mature but unopened fruits were collected from each infr-
uctescence on a plant. To reduce variation among individ-
uals due to potential effects of fruit position on seed mass,
we collected fruits at basal, middle, and distal positions on
each sampled infructescence. These fruits were dissected,
and the seeds were removed and air-dried until used. Seeds
were divided into batches of 1000 air-dried under ambient
laboratory condition seeds, and three batches per popula-
tion site were weighed to the nearest 0.0001 g on an elec-
tronic balance.
Plant height of each sampled individual was measured
to the nearest decimeter. Seed morphology, including
seed length, seed width, seed thickness, seed wing length
at hilium, seed wing length at chalaza, and seed wing
length on lateral sides, was measured for three replicates
of 30 seeds each.
To assess variation in seed traits along the altitudinal
gradient for a single species, we selected three species that
differ in seed size and grow in different alpine habitats,
but with a similar distribution over a large altitudinal
gradient. The altitudinal gradient extended ca. 980 m for
R. thomsonii, 540 m in R. aganniphum var. flavorufum,
and 868 m for R. cerasinum (Table 2). Seed wing length
and seed surface area were calculated using the following
equations:
Seed wing length ¼ðseed wing length at hilium
þseed wing length at chalaza
þseed lateral wing lengthÞ=3;
and seed surface area
¼seed length seed width:
Data analysis
The relationship between altitude and seed traits for every
population was determined with a parametric Pearson’s
product moment (r) test. An ANOVA analysis was used
to test the effects of habitat, subgenus, section, subsection,
and species. When a dataset was unbalanced or included
categorical variables, GLM was used for variance analysis.
Coefficient of variation (CV) of seed mass was calculated
as standard deviation of seed mass (SD) 9100/mean seed
mass (Pluess et al. 2005). All statistics analyses were per-
formed with the Statistical Package for the Social Sciences
version 18.0 (SPSS, Inc., Chicago, IL).
Results
Variation in seed traits among populations
Generally, seed mass, seed length, seed width, ratio of
seed width to thickness, seed surface area, and seed wing
length were significantly negative correlated with altitude,
and seed thickness and ratio of seed length to width were
significantly positive correlated with altitude (Table 2,
Figs 3A, 4A, 5A, 6A, 7A, 8A, 9A, and 10A).
Seed mass, seed length, seed width, ratio of seed width to
thickness, seed surface area, and seed wing length were sig-
nificantly positive correlated with plant height, and ratio of
seed length to width was significantly negative correlated
with plant height. Seed thickness was not significantly corre-
lated with plant height (R=0.155, P=0.259) (Figs 3B, 4B,
5B, 6B, 7B, 8B, 9B, and 10B).
Habitat, subgenera, section, subsection, and species had
various effects on seed traits.
Figure 2. Map of China showing location of
sampling sites on the Tibetan Plateau.
ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 1915
Y. Wang et al. Variation in Seed Traits of Rhododendron Species
Table 1. Environment variables and seed mass (Mean SD, n=3, 1000 seeds for each replicate) of 59 populations of 42 congeneric Rhododen-
dron species.
Population Species Altitude Habitat
Mean height of
plant (m) Subgenus Section Subsection Seed mass (g)
1Rhododendron vellereum 4220 1 2 1 1 10 0.0713 0.0006
2Rhododendron tsariense 4170 1 1.3 1 1 22 0.0550 0.0002
3Rhododendron faucium 3940 1 0.5 1 1 13 0.1275 0.0024
4Rhododendron faucium 3830 3 1.5 1 1 13 0.0684 0.0032
5Rhododendron faucium 3570 3 2.5 1 1 13 0.0658 0.0008
6Rhododendron faucium 4200 3 1.3 1 1 13 0.0748 0.0013
7Rhododendron faucium 3220 2 1.6 1 1 13 0.0775 0.0023
8Rhododendron catacosmum 4130 3 3 1 1 2 0.1161 0.0005
9Rhododendron calvescens 3600 3 4 1 1 6 0.1491 0.0031
10 Rhododendron principis 3760 3 3.5 1 1 10 0.0772 0.0017
11 Rhododendron trichocladum 3660 3 1 3 0.0757 0.0006
12 Rhododendron hookeri 3680 2 1 1 1 13 0.0829 0.0017
13 Rhododendron megalanthum 3210 2 2.5 1 1 13 0.0536 0.0019
14 Rhododendron megalanthum 2600 2 2 1 1 13 0.0560 0.0066
15 Rhododendron maddenii subsp.
Crassum
2650 2 2 2 2 14 0.1257 0.0031
16 Rhododendron maddenii subsp.
Crassum
2670 3 2.5 2 2 14 0.1499 0.0008
17 Rhododendron arboreum var.
roseum
2510 3 4 1 1 17 0.0207 0.0047
18 Rhododendron setiferum) 3570 3 3 1 1 6 0.0940 0.0005
19 Rhododendron coriaceum 3260 3 3 1 1 12 0.0543 0.0021
20 Rhododendron kongboense 4450 1 0.1 2 3 0.0817 0.0009
21 Rhododendron keysii 2900 3 4 1 1 8 0.0703 0.0007
22 Rhododendron lacteum 4040 2 1 1 1 10 0.0918 0.0096
23 Rhododendron lacteum 4540 1 0.4 1 1 10 0.1152 0.0005
24 Rhododendron lacteum 4000 3 1.5 1 1 10 0.0670 0.0034
25 Rhododendron grande 2760 2 3 1 1 1 0.1764 0.0106
26 Rhododendron heliolepis 3420 3 1.5 2 2 16 0.0522 0.0017
27 Rhododendron heliolepis 3570 3 1 2 2 21 0.0858 0.0027
28 Rhododendron lulangense 3170 2 1.5 1 1 10 0.1222 0.0006
29 Rhododendron bainbridgeanum 4000 1 1 1 1 6 0.0764 0.0006
30 Rhododendron trichostomum 4490 1 0.3 2 3 0.0828 0.0014
31 Rhododendron agastum 3570 3 1.7 1 1 7 0.0685 0.0011
32 Rhododendron erosum 3140 3 4 1 1 5 0.0993 0.0062
33 Rhododendron triflorum 3150 3 2 2 2 21 0.2845 0.0048
34 Rhododendron arboreum 3140 3 6 1 1 17 0.0718 0.0001
35 Rhododendron pruniflorum 4170 1 1.1 1 1 20 0.0263 0.0015
36 Rhododendron sinogrande 2640 2 5 1 1 1 0.1980 0.0224
37 Rhododendron pendulum 2870 3 1.2 2 2 15 0.0607 0.0007
38 Rhododendron mekongense 3940 1 0.4 3 0.0418 0.0011
39 Rhododendron mekongense 3690 1 1.5 3 0.0793 0.0013
40 Rhododendron campylogynum 4350 1 0.1 2 2 23 0.0193 0.0009
41 Unnamed species 3690 1 2.8 1 1 0.0726 0.0005
42 Rhododendron delavayi var.
peramoenum
2280 2 2.5 1 1 17 0.0758 0.0009
43 Rhododendron erythrocalyx 3490 3 1.5 1 1 6 0.0755 0.0001
44 Rhododendron calostrotum var.
calciphilum
4150 1 0.2 2 2 19 0.0235 0.0006
45 Rhododendron kyawi 3210 2 3 1 1 4 0.1069 0.0017
46 Rhododendron kyawi 2920 3 3.5 1 1 4 0.0978 0.0055
47 Rhododendron aperantum 4010 1 0.3 1 1 2 0.0393 0.0008
48 Rhododendron nivale 4450 1 0.1 2 2 9 0.1046 0.0005
49 Rhododendron aganniphum 4530 1 1.4 1 1 10 0.0689 0.0003
1916 ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Variation in Seed Traits of Rhododendron Species Y. Wang et al.
Seed mass
Habitat, subsection, and species had significant effects on
seed mass, but subgenera and section did not (Table 2).
Seed mass was highest for rocky slope and lowest for
alpine shrub (Table 2, Fig. 3C).
Seed length
Habitat, section, and species had significant effects on
seed length, but subgenera and subsections did not
(Table 2). Seed length was highest for rocky slope and
lowest for alpine shrub (Table 2, Fig. 4C).
Seed width
Habitat, subsection, and species had significant effects on
seed width (Table 2). Seed width was highest for rocky slope
and lowest for alpine shrub (Table 2, Fig. 5C).
Seed thickness, ratio of seed length to width, and
ratio of seed width to thickness
None of these three seed traits differed significantly
among habitats, subgenera, sections, subsections, or spe-
cies (Table 2, Fig. 6C, 7C, and 8C).
Seed surface area
Seed surface area differed significantly among habitats,
subsections, and species, but not among subgenera or sec-
tions (Table 2). Seed surface area was highest for rocky
slope (Table 2, Fig. 9C).
Seed wing length
Seed wing length differed significantly among habitats,
sections, subsections, and species (Table 2). Seed wing
length was highest for rock slope and lowest for alpine
shrub (Table 2, Fig. 10C).
Variation in seed traits among populations
of same species
Seed mass was significantly correlated with altitude in R.
aganniphum var. flavorufum (R=0.474, P<0.05, Fig. 11B)
but not in R. thomsonii (R=0.363, P=0.083, Fig. 11A)
or R. cerasinum (R=0.195, P=0.068 Fig. 11C).
Variation in seed mass among populations was rela-
tively high, with CVs of 23.1% for R. aganniphum var.
flavorufum, 21.3% for R. thomsonii, and 15.44% for R.
cerasinum (Table 3).
The effect of plant height and habitat on other seed traits
was not significant among populations within a single species.
Discussion
Seed trait responses to altitude
Seed mass variation among congeneric species
We detected a significant negative correlation between seed
mass and altitude among the 59 Rhododendron populations
Table 1. Continued.
Population Species Altitude Habitat
Mean height of
plant (m) Subgenus Section Subsection Seed mass (g)
50 Rhododendron aganniphum 4170 1 1 1 1 10 0.1201 0.0500
51 Rhododendron stewartianum 3720 2 1.2 1 1 13 0.0666 0.0020
52 Rhododendron stewartianum 2900 3 2.5 1 1 13 0.0422 0.0015
53 Rhododendron stewartianum 3210 3 1 1 1 13 0.0710 0.0023
54 Rhododendron stewartianum 3260 3 2.5 1 1 13 0.0887 0.0001
55 Rhododendron hirtipes 4130 3 2 1 1 6 0.1002 0.0008
56 Rhododendron campanulatum 4040 1 2 1 1 3 0.0972 0.0016
57 Rhododendron campanulatum 3570 3 2 1 1 3 0.1060 0.0006
58 Rhododendron uvarifolium 3150 3 2 1 1 18 0.0967 0.0058
59 Rhododendron uvarifolium 3600 3 3 1 1 18 0.1182 0.0009
Habitat: 1, alpine shrub; 2, rocky slope; 3, forest.
Subgenus: 1, Subgen. Hymenanthes (Blume) K. Koch; 2, Subgen. Rhododendron; 3, Subgen. Pseudazalea Sleumer.
Section c: 1, Sect. Ponticum G. Don; 2, Sect. Rhododendron; 3, Sect. Pogonanthum G. Don.
Subsection: 1, subsect. Grandia Sleumer; 2, subsect. Neriiflora Sleumer; 3, subsect. Campanulata Sleumer; 4, subsect. Parishia Sleumer; 5, subsect.
Barbata Sleumer; 6, subsect. Selensia Sleumer; 7, subsect. Irrorata Sleumer; 8, subsect. Cinnabarina (Hutch.) Sleumer; 9, subsect. Lapponica (Balf.
F.) Sleumer; 10, subsect. Taliensia Sleumer; 11, subsect. Barbata Sleumer; 12, subsect. Falconera Sleumer; 13, subsect. Thomsonii Sleumer; 14,
subsect. Maddenia (Hutch.) Sleumer; 15, subsect. Edgeworthia (Hutch.) Sleumer; 16, subsect. Heliolepida (Hutch.) Sleumer; 17, subsect. Arborea
Sleumer; 18, subsect. Fulva Sleumer; 19, subsect. Saluenensia (Hutch.) Sleumer; 20, subsect. Glauca (Hutch.) Sleumer; 21, subsect. Triflora
(Hutch.) Sleumer; 22, subsect. Lanata Chamb; 23, subsect. Campylogyna (Hutch.) Sleumer.
ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 1917
Y. Wang et al. Variation in Seed Traits of Rhododendron Species
and a negative correlation between elevation and seed mass
among populations of the same species. These results were
in accordance with two previous studies (Baraloto et al.
2005). A negative relationship also was found between alti-
tude and seed mass in a flora of the eastern Tibetan Plateau
(Bu et al. 2007). A decrease in seed mass with altitude may
be due to plastic responses induced by the environment
caused by a decline in resource availability. Low tempera-
tures at higher altitudes may reduce photosynthetic rates,
and a shorter growing seasons may reduce the time for seed
(A) (B) (C)
Figure 3. Variation in seed mass with altitude, plant height, and habitat. (A) Correlation between seed mass and altitude; (B) Correlation
between seed mass and plant height; (C) variation in seed mass in alpine shrub (AS), rocky slope (RS), and forest (F).
Table 2. The GLM results of the relationship between seed traits and altitude, plant height, habitat, subgenus, section, subsection, and species.
Variables
Source
Altitude Plant height Habitat Subgenus Section Subsection Species
Seed mass df 3 4 2 2 2 21 41
F 11.744 3.681 1.909 0.672 0.42 2.701 5.29
Sig. <0.01 <0.01 <0.05 0.515 0.659 <0.01 <0.01
R
2
0.06 0.032 0.242 0.005 0.003 0.133 0.184
Seed length df 3 4 2 1 2 14 36
F 2.102 1.335 6.839 0.989 5.223 1.945 3.372
Sig. <0.05 0.271 <0.01 0.326 <0.05 <0.05 <0.01
R
2
0.028 0.003 0.01 0.001 0.005 0.01 0.039
Seed width df 3 4 2 1 2 14 36
F 1.801 1.638 4.341 0.093 0.672 3.218 9.117
Sig. <0.05 0.18 <0.05 0.762 0.517 <0.05 <0.01
R
2
0.004 0.005 0.007 0 0.001 0.011 0.041
Seed thickness df 3 4 2 1 2 14 36
F 0.175 1.677 1.301 0.264 0.144 1.587 0.442
Sig. 0.548 1.171 0.281 0.61 0.866 0.158 0.977
R
2
0.001 0.003 0.017 0.002 0.002 0.188 0.515
Seed length to width df 3 4 2 1 2 14 36
F 0.116 0.994 0.45 1.952 4.337 1.469 1.157
Sig. 0.951 0.42 0.64 0.17 <0.05 0.2 0.394
R
2
0.0001 0.0001 0 0.001 0.003 0.007 0.011
Seed width to thickness df 3 4 2 2 2 17 36
F 3.798 3.295 4.908 6857 2.624 2.663 2.827
Sig. 0.16 <0.05 <0.01 <0.01 0.083 <0.05 <0.05
R
2
0.003 0.004 0.167 0.219 0.102 0.618 0.872
Seed surface area df 3 4 2 1 2 14 36
F 2.358 1.556 6.648 0.533 2.419 3.243 6.14
Sig. 0.083 0.202 <0.01 0.47 0.103 <0.01 <0.01
R
2
0.128 0.114 0.029 0.001 0.007 0.034 0.127
Seed wing length df 3 4 2 1 2 14 33
F 6.376 2.962 5.632 2.658 14.87 5.417 5.857
Sig. <0.01 <0.05 <0.01 0.111 <0.01 <0.01 <0.01
R
2
0.03 0.021 0.03 0.008 0.055 0.082 0.941
Note: Analysis of variance in bold type is statistically significant at P<0.05.
1918 ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Variation in Seed Traits of Rhododendron Species Y. Wang et al.
development and seed provisioning, thereby reducing
mature seed mass (Baker 1972). The smaller seeds at high
altitude also may evolve by natural selection if the growing
season is not long enough to produce large seeds (Venable
and Rees 2008).
In contrast, Pluess et al. (2005) reported a positive corre-
lation between seed mass and altitude across species and
within species. Pluess et al. (2005) argued that natural
selection should favor production of larger seeds in species
at higher altitudes because larger seeds exhibit superior sur-
vivorship in stressful environments, which accounted for
the pattern they observed. The ecological sorting of species
across elevations could also generate this pattern directly
on seed mass. Indeed, the fact that Pluess et al. (2005)
found no relationship between seed mass and altitude
within species argues against a strong role for in situ natu-
ral selection. Therefore, although seed mass has been found
to be associated with altitude, the patterns observed are not
highly consistent, and the underlying mechanisms have not
been identified.
Seed morphology variation among congeneric
species
In our study, seed morphology varied with environmental
factors. The ratio of seed length to width and seed width
(A) (B) (C)
Figure 4. Variation in seed length with altitude, plant height, and habitat. (A) Correlation between seed length and altitude; (B) Correlation
between seed length and plant height; and (C) Variation in seed length in alpine shrub (AS), rocky slope (RS), and forest (F).
(A) (B) (C)
Figure 5. Variation in seed width with altitude, plant height, and habitat. (A) Correlation between seed width and altitude; (B) Correlation
between seed width and plant height; and (C) variation in seed width in alpine shrub (AS), rocky slope (RS), and forest (F).
(A) (B) (C)
Figure 6. Variation in seed thickness with altitude, plant height, and habitat. (A) Correlation between seed thickness and altitude; (B) Correlation
between seed thickness and plant height; and (C) Variation in seed thickness in alpine shrub (AS), rocky slope (RS), and forest (F).
ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 1919
Y. Wang et al. Variation in Seed Traits of Rhododendron Species
for Rhododendron species were positively correlated with
altitude. Seed length, seed width, ratio of seed width to
thickness, seed surface area, and seed wing length had a
significant negative relationship with the increasing alti-
tude. Seed morphology was related to seed dispersal
(Howe and Smallwood 1982). Seeds of Rhododendron spe-
cies mainly are dispersed by wind, but wind speed is rela-
tively low in the low-altitude area. However, the flat,
circular shape maximizes surface area, and the large wings
are conductive to flight. With the increasing altitude, the
wind becomes stronger and thus more conducive for seed
dispersal. Seed and seed wing are narrow at high altitudes,
which reduces flight ability. The possible explanation of
this pattern is that the wind at high altitudes is strong
(A) (B) (C)
Figure 8. Variation in ratio of seed width to thickness with altitude, plant height, and habitat. (A) Correlation between ratio of seed width to
thickness and altitude; (B) Correlation between ratio of seed width to thickness and plant height; and (C) Variation in ratio of seed width to
thickness in alpine shrub (AS), rocky slope (RS), and forest (F).
(A) (B) (C)
Figure 9. Variation in seed surface area with altitude, plant height, and habitat. (A) Correlation between seed surface area and altitude; (B)
Correlation between seed surface area and plant height; and (C) Variation in seed surface area in alpine shrub (AS), rocky slope (RS), and forest
(F).
(A) (B) (C)
Figure 7. Variation in ratio of seed length to width with altitude, plant height, and habitat. (A) Correlation between ratio of seed length to
width and altitude; (B) Correlation between the ratio of seed length to width and plant height; and (C) Variation in ratio of seed length to width
in alpine shrub (AS), rocky slope (RS), and forest (F).
1920 ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Variation in Seed Traits of Rhododendron Species Y. Wang et al.
enough for seed dispersal, and seeds do not need to
develop significant structures for flight.
Correlation between seed trait and plant
height
Seed mass variation
We detected a significant positive relationship between seed
mass and plant height (Fig. 5). Some studies have reported
a positive relationship because the data sets included spe-
cies representing many growth forms (Moles and Westoby
2004; Grubb et al. 2005; Venable and Rees 2008; Queen-
borough et al. 2009). For example, herbs produce relatively
smaller seeds than woody plants (Mazer and Percival 1989;
Leishman et al. 1995). In our study, all species are woody;
thus, variation due to growth form is avoided. Seed mass
was significantly positively correlated with plant height
among populations across species, but not within species,
which is in accordance with the study by Moles and West-
oby (2004). This suggests that mechanisms are different at
different taxonomical levels. Positive correlations are more
often found among a taxonomically highly diverse group of
taxa, and the reason may be phylogenetic constraints.
Increase in seed mass with plant height has been pro-
posed to reflect adaptive responses to dispersal require-
ments and to architectural constraints or competitive
interactions among seedlings (Grubb et al. 2005; Moles
et al. 2007). Alternatively, differences in seed mass and
(A) (B) (C)
Figure 10. Variation in seed wing length with altitude, plant height, and habitat. (A) Correlation between seed wing length and altitude; (B)
Correlation between seed wing length and plant height; and (C) Variation in seed wing length in alpine shrub (AS), rocky slope (RS), and forest
(F).
(A) (B) (C)
Figure 11. Relationships between seed mass and altitude among populations of (A) R. thomsonii (B) R. aganniphum var. flavorufum (C) R.
cerasinum.
Table 3. Variation in seed mass (Mean SD, n=3, 1000 seeds for each replicate) of three Rhododendron species.
Species
Number of
populations
Altitudinal
range (m)
Thousand seed weight (g)
(mean SD)
CVs among
populations (%)
Rhododendron thomsonii 13 3220–4200 0.0792 0.0169 21.3
Rhododendron aganniphum
var. flavorufum
8 4000–4540 0.0932 0.0142 23.1
Rhododendron cerasinum 8 2900–3768 0.0704 0.0161 15.44
ª2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 1921
Y. Wang et al. Variation in Seed Traits of Rhododendron Species
plant height among populations or taxa may be due to
plastic responses to local environmental conditions (Baker
1972; Moles et al. 2005; Guo et al. 2010).
Variation in seed morphology
Species with high dispersal ability may be more widely dis-
tributed than those with low dispersal ability (Gutierrez
and Men
endez 1997). Dispersal ability is significantly cor-
related with the seed mass (Rees 1995). Smaller and lighter
seeds are readily transported by dispersal agents (Venable
and Brown 1988; Greene and Johnson 1993), and thus they
have an advantage in colonization and in becoming abun-
dant. Larger and heavier seeds are relatively less abundant,
but they can produce seedlings that are more competitive
than those produced by small seeds, which enable them to
establish and survive under various stress conditions such
as defoliation, shading, competition, herbivory, drought,
and disturbance (Armstrong & Westoby 1993). We found
that taller plants of Rhododendron had larger seeds and seed
wings compared to shorter plants. With decrease in plant
height, the seed wing became smaller and even disappeared,
presumably because there is not enough energy to be allo-
cated to production of wings. There is a trade-off between
plant growth and production of wings (Ginwal et al. 2005).
Seed trait responses to habitat
Seed mass, seed length, seed width, ratio of seed width to
thickness, seed surface area, and seed wing length varied
significantly among habitats for populations of the same
species. These traits had their highest values in rocky
slope habitat, and the reason may be that, compared with
forest and alpine shrub, the plants on rocky slopes are
exposed to high solar irradiance. Thus, the plants had
more energy for reproductive growth and production of
large seeds, which are more favorable for germination and
seedling growth.
Seed trait responses to phylogeny
Mass and morphology of the Rhododendron seeds were
correlated with taxonomic membership mainly at the spe-
cies and subsection levels. This phylogenetic pattern of
seed size previously has been shown for different kinds of
genera (Kelly et al. 1996; Westoby et al. 1996; Hodkinson
et al. 2002). However, two corresponding questions
remain unsolved: How to interpret this phylogenetic cor-
relation, and how to consider both phylogenetic and eco-
logical correlations.
Seed mass and morphology might be the result of both
selective pressure over long-term ecological time and the
constraints over long evolutionary history of the taxo-
nomic group. Thus, seed mass will be similar in more
closely related species regardless of ecological factors.
Therefore, we maintain that correlates of ecology and
phylogeny should be taken into account in comparative
studies on seed mass and morphology among species.
Conclusions
In summary, our results indicate elevation, habitat, plant
height, and phylogeny were all correlated with seed mass
and morphology among species of Rhododendron.We
found a selection pressure for species with lighter and
smaller seeds, and shorter seed wings at higher altitude.
Seed mass was in positive correlation with plant height,
seed traits varied with habitats, and phylogeny constrains
the seed traits variation.
Acknowledgments
This work was funded by the Ministry of Science and
Technology of China [2012GB24910654].
Conflict of Interest
None declared.
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Y. Wang et al. Variation in Seed Traits of Rhododendron Species
