16
absorption features of plant chemical or structural characteristics at specific wavelengths relate to phenotypic information. However, the effects plant traits have on
spectra are only partially understood. Identifying the regions of the spectrum that
are influenced by specific traits is complicated by overlapping absorption features
and subtle differences in plant chemical, structural, morphological, and anatomical
characteristics that simultaneously influence the shape of the spectral response
(Ustin and Jacquemoud, Chap. 14).
2.4 Patterns in Plant Diversity
One of the most intensively studied patterns in biodiversity is the latitudinal gradient, in which low-latitude tropical regions harbor more species, genera, and families
of organisms than high-latitude regions. In particular, wet tropical areas tend to
reveal higher diversity of organisms than colder and drier climates (Fig. 2.2).
Humboldt (1817) documented these patterns quite clearly for plant diversity.
Naturalists since then have sought to explain these patterns.
Tropical biomes have existed longer than more recent biomes, such as deserts,
Mediterranean climates, and tundra, which expanded as the climate began to cool
some 35 million years ago. Tropical biomes also cover more land surface area than
other biomes. Tropical species thus have had more time and area (integrated over
the time since their first appearance) for species to evolve and maintain viable populations (Fine and Ree 2006). Lineages that originally evolved in the tropics may also
have been less able to disperse out of the tropics and to evolve new attributes adapted
to cold or dry climates—due to phylogenetic conservatism—restricting their ability
to diversify (Wiens and Donoghue 2004). However, not all lineages follow this latitudinal gradient. Ectomycorrhizal fungi, for example, show higher diversity at temperate latitudes, where they likely have higher tree host density (Tedersoo and Nara
2010). Moreover, other measures of diversity do not necessarily follow these patterns. Variation in functional attributes of species, for example, follow different patterns depending on the trait (Cavender-Bares et al. 2018; Echeverría-Londoño et al.
2018; Pinto-Ledezma et al. 2018b). Specific leaf area, one of the functional traits
that is highly aligned with the leaf economic spectrum (discussed below), shows
higher variation at high latitudes than low latitudes across the Americas. In contrast,
seed size shows higher variation at low latitudes (Fig. 2.2b).
At regional scales, variation in the environment, as discussed by Record et al.
(Chap. 10), sets the stage for variation in biodiversity because species have evolved
to inhabit and can adapt to different environments, which allows them to partition
resources and occupy different niches created by environmental variation. Thus,
habitat diversity begets biodiversity, and remotely sensed measures of environmental variation have long been known to predict biodiversity patterns (Kerr et al. 2001).
Land area is another long-observed predictor of species diversity, first described
for species within certain guilds on island archipelagoes (Diamond and Mayr 1976).
J. Cavender-Bares et al.
absorption features of plant chemical or structural characteristics at specific wavelengths relate to phenotypic information. However, the effects plant traits have on
spectra are only partially understood. Identifying the regions of the spectrum that
are influenced by specific traits is complicated by overlapping absorption features
and subtle differences in plant chemical, structural, morphological, and anatomical
characteristics that simultaneously influence the shape of the spectral response
(Ustin and Jacquemoud, Chap. 14).
2.4 Patterns in Plant Diversity
One of the most intensively studied patterns in biodiversity is the latitudinal gradient, in which low-latitude tropical regions harbor more species, genera, and families
of organisms than high-latitude regions. In particular, wet tropical areas tend to
reveal higher diversity of organisms than colder and drier climates (Fig. 2.2).
Humboldt (1817) documented these patterns quite clearly for plant diversity.
Naturalists since then have sought to explain these patterns.
Tropical biomes have existed longer than more recent biomes, such as deserts,
Mediterranean climates, and tundra, which expanded as the climate began to cool
some 35 million years ago. Tropical biomes also cover more land surface area than
other biomes. Tropical species thus have had more time and area (integrated over
the time since their first appearance) for species to evolve and maintain viable populations (Fine and Ree 2006). Lineages that originally evolved in the tropics may also
have been less able to disperse out of the tropics and to evolve new attributes adapted
to cold or dry climates—due to phylogenetic conservatism—restricting their ability
to diversify (Wiens and Donoghue 2004). However, not all lineages follow this latitudinal gradient. Ectomycorrhizal fungi, for example, show higher diversity at temperate latitudes, where they likely have higher tree host density (Tedersoo and Nara
2010). Moreover, other measures of diversity do not necessarily follow these patterns. Variation in functional attributes of species, for example, follow different patterns depending on the trait (Cavender-Bares et al. 2018; Echeverría-Londoño et al.
2018; Pinto-Ledezma et al. 2018b). Specific leaf area, one of the functional traits
that is highly aligned with the leaf economic spectrum (discussed below), shows
higher variation at high latitudes than low latitudes across the Americas. In contrast,
seed size shows higher variation at low latitudes (Fig. 2.2b).
At regional scales, variation in the environment, as discussed by Record et al.
(Chap. 10), sets the stage for variation in biodiversity because species have evolved
to inhabit and can adapt to different environments, which allows them to partition
resources and occupy different niches created by environmental variation. Thus,
habitat diversity begets biodiversity, and remotely sensed measures of environmental variation have long been known to predict biodiversity patterns (Kerr et al. 2001).
Land area is another long-observed predictor of species diversity, first described
for species within certain guilds on island archipelagoes (Diamond and Mayr 1976).
J. Cavender-Bares et al.