3
Tree-Leaf Chemicals in Seasonal Environments
3.1
General Patterns
Most trees live long but may change in their physiological conditions seasonally or
between dry and wet periods. The mixed forest in temperate zones consists of both
deciduous and evergreen trees. In winter, deciduous trees fall leaves throughout
autumn to the next spring, and herbivores use only the leaves of evergreen trees (Fig.
1). The newly expanded leaves from winter buds are available in spring and after that
herbivores can use both deciduous and evergreen tree leaves. Seasonally fluctuating
temperatures and humidity may alter leaf chemical components. If so, availability of
profitable leaves for arboreal herbivores change greatly with seasons.
Although the data on the seasonal changes in tree-leaf chemicals are still limited,
we can see some general patterns. First, young leaves appeared in spring include
more chemicals than mature leaves, and once matured, their contents are kept nearly
constant or gradually decreased.
Sugars are included more in young leaves than matures, and after maturation their
contents are nearly constant in Quercus robur [90, 91], Quercus agrifolia [92],
Quercus suber [93], Quercus acutissima [39, 73], and Betula pubescens [94].
However, all kinds of sugars do not follow this pattern. In Quercus robur leaves,
maltose and oligosaccharide contents considerably fluctuate during early summer,
but sucrose content shows a marked increase in early summer and keeps increasing
until autumn [90]. Inositol (a sugar alcohol) is present in low concentrations
throughout the summer, but almost disappeared from senescing leaves [94].
The protein content of Quercus robur and Betula pubescens leaves also decreases
markedly according to the leaf expansion and maturation processes from spring to
early summer [90, 94]. Protein levels of leaves (Alnus incana, Tilia americana,
Prunus serotine, Ulmus americana, Acer rubrum, Betula papyrifera, Populus
deltoides, Quercus velutina, Quercus macrocarpa, Quercus alba, Carya glabra,
and Juglans nigra) decrease by 22% in mature leaves compared with immature
leaves [95]. N content of leaves decreases to less than half of the initial level in
summer in Acer palmatum, Acer saccharum, Quercus crispula, and Betula
alleghaniensis [96, 97] and in many other species of trees [98]. N and P contents
in Quercus agrifolia leaves are higher in new leaves compared with matures [92].
Also, N, P, K, and ash contents of Quercus suber leaves are highest in young leaves
in spring, reaching the minimum during the hot and dry summer, and increase
slightly during the rainy period of autumn-winter, although Na, Mg, and Ca contents
are lowest in spring-early summer and increase during summer and autumn-winter
[93]. N and P fractions in leaves of Larix laricina, Betula papyrifera, and Alnus
crispa are highest in young leaves and declined in concentration through the season,
probably because the concentration is diluted by increasing leaf biomass in the
former case and because organic N and P fractions are hydrolyzed and inorganic P
and amino acid N are translocated out of leaves in the latter [99].
The water content of leaves decreases from spring to summer because water is
included at higher percentage in buds and young leaves than mature leaves [94]. This
15 Tree-Leaf Chemicals and Feeding Behavior of Arboreal Mammals in Seasonal. . .
363
Tree-Leaf Chemicals in Seasonal Environments
3.1
General Patterns
Most trees live long but may change in their physiological conditions seasonally or
between dry and wet periods. The mixed forest in temperate zones consists of both
deciduous and evergreen trees. In winter, deciduous trees fall leaves throughout
autumn to the next spring, and herbivores use only the leaves of evergreen trees (Fig.
1). The newly expanded leaves from winter buds are available in spring and after that
herbivores can use both deciduous and evergreen tree leaves. Seasonally fluctuating
temperatures and humidity may alter leaf chemical components. If so, availability of
profitable leaves for arboreal herbivores change greatly with seasons.
Although the data on the seasonal changes in tree-leaf chemicals are still limited,
we can see some general patterns. First, young leaves appeared in spring include
more chemicals than mature leaves, and once matured, their contents are kept nearly
constant or gradually decreased.
Sugars are included more in young leaves than matures, and after maturation their
contents are nearly constant in Quercus robur [90, 91], Quercus agrifolia [92],
Quercus suber [93], Quercus acutissima [39, 73], and Betula pubescens [94].
However, all kinds of sugars do not follow this pattern. In Quercus robur leaves,
maltose and oligosaccharide contents considerably fluctuate during early summer,
but sucrose content shows a marked increase in early summer and keeps increasing
until autumn [90]. Inositol (a sugar alcohol) is present in low concentrations
throughout the summer, but almost disappeared from senescing leaves [94].
The protein content of Quercus robur and Betula pubescens leaves also decreases
markedly according to the leaf expansion and maturation processes from spring to
early summer [90, 94]. Protein levels of leaves (Alnus incana, Tilia americana,
Prunus serotine, Ulmus americana, Acer rubrum, Betula papyrifera, Populus
deltoides, Quercus velutina, Quercus macrocarpa, Quercus alba, Carya glabra,
and Juglans nigra) decrease by 22% in mature leaves compared with immature
leaves [95]. N content of leaves decreases to less than half of the initial level in
summer in Acer palmatum, Acer saccharum, Quercus crispula, and Betula
alleghaniensis [96, 97] and in many other species of trees [98]. N and P contents
in Quercus agrifolia leaves are higher in new leaves compared with matures [92].
Also, N, P, K, and ash contents of Quercus suber leaves are highest in young leaves
in spring, reaching the minimum during the hot and dry summer, and increase
slightly during the rainy period of autumn-winter, although Na, Mg, and Ca contents
are lowest in spring-early summer and increase during summer and autumn-winter
[93]. N and P fractions in leaves of Larix laricina, Betula papyrifera, and Alnus
crispa are highest in young leaves and declined in concentration through the season,
probably because the concentration is diluted by increasing leaf biomass in the
former case and because organic N and P fractions are hydrolyzed and inorganic P
and amino acid N are translocated out of leaves in the latter [99].
The water content of leaves decreases from spring to summer because water is
included at higher percentage in buds and young leaves than mature leaves [94]. This
15 Tree-Leaf Chemicals and Feeding Behavior of Arboreal Mammals in Seasonal. . .
363