{"id":44,"date":"2019-04-06T21:39:49","date_gmt":"2019-04-07T01:39:49","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/?post_type=chapter&p=44"},"modified":"2020-10-31T15:32:54","modified_gmt":"2020-10-31T19:32:54","slug":"topic-4-1-trees","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/chapter\/topic-4-1-trees\/","title":{"raw":"Topic 4.1: Trees","rendered":"Topic 4.1: Trees"},"content":{"raw":"Because of their large sizes and great longevities, trees have tremendous effects on the ecology of tropical forests.\u00a0 Tropical trees are also important at the planetary level; carbon stored in their trunks and roots is important in maintaining global carbon balance and in slowing global warming and sea level rise.\u00a0 At the level of large watersheds like the Amazon, it is important to recognize that a large portion of the rain falling on forests is derived from water transpired by trees.\u00a0 At a more local level, light rains may never reach the forest floor, the water instead being intercepted by and evaporated from tree leaves and branches.\u00a0 Trees also provide structural support for a variety of other growth forms, such as epiphytes and vines.\u00a0 Although knowledge about other types of plants is important when trying to manage forests sustainably, we obviously need to know a lot about how trees function, starting with below-ground processes.\r\n\r\nThe root systems of tropical trees vary a great deal by species and with environmental conditions (e.g., depth to mechanically impeding horizons, bedrock, or the water table). Tropical forests are famous for their diversities of aerial roots, buttressed roots, apogeotropic (=upward growing) roots, and pneumatophores (=so-called \u201cbreathing\u201d roots).\u00a0 But most species of tropical trees have roots that are similar to those of trees in temperate forests in both structure and distribution.\r\n\r\nAlthough most fine roots of tropical trees are generally concentrated near the soil surface where nutrients are more available, deep roots also develop where conditions allow.\u00a0 These deep roots may be particularly important for water uptake when surface soils have dried; the discovery of roots at depths of 10-14 m in Amazonia suggests that roots may be exploiting soil resources substantially deeper than typical samples taken by soil scientists.\r\n\r\nThe horizontal spread of tree roots has not been much studied in the tropics but, like rooting depth, most likely varies by species and site conditions.\u00a0 That roots extend well beyond the crown margins of many trees is abundantly clear.\u00a0 Even at the seedling stage, horizontal root growth can be substantial. Allowing the roots of tree seedlings to expand freely is often problematic in nurseries where \u201cpot-bound\u201d plants are all too common.\u00a0 Out-planted seedlings can also suffer the same \u201cpot-bound\u201d problem when set out in compacted soils.\r\n\r\nRoots proliferate in resource-rich volumes of soil.\u00a0 Given that the distributions of nutrients and water in soils are extremely heterogenous, fine root densities also can vary substantially over even very short distances.\u00a0 And these distributions change over time, sometimes very rapidly as when an animal dies or defecates, creating thereby a local, if temporary, concentration of nutrients.\r\n\r\nMaintenance of the appropriate balance between root surface area for absorption and leaf surface area for photosynthesis and transpiration is critical for tree survival and growth.\u00a0 As environmental conditions change seasonally or due to silvicultural practices, so must this so-called \u201croot-shoot ratio.\u201d\u00a0 For example, because evapo-transpirational demands in the understory are low, understory saplings tend to have relatively low root-shoot ratios.\u00a0 When the canopy is opened due to logging or thinning, these saplings must rapidly adjust their root-shoot ratios for the new conditions of higher light intensities, higher temperatures, and higher vapor pressure deficits.\u00a0 For the same reason, nursery-grown seedlings are often intentionally \u201chardened up\u201d by gradual deprivation of water and nutrients before out-planting so as to reduce \u201ctransplant stock.\u201d\u00a0 Pruning off a portion of leaf area of these seedlings can also help to reduce transplant shock by increasing root-shoot ratios.\u00a0 Pre-planting shoot pruning is made even more critical when seedlings are purposefully or inadvertently root-pruned to facilitate lifting and planting.\r\n\r\nLittle is known about the long-term impacts on tropical trees of root damage, but enough is known about temperate oaks (Quercus spp.) and other hardwoods to raise concern.\u00a0 Root damage is common where ground-based timber yarding operations are carried out, particularly if they continue during wet weather.\u00a0 But which species are most susceptible to root-invading pathogens?\u00a0 Do trees that suffer mechanical root damage subsequently develop butt and heart rots?\u00a0 Do trees with damaged roots suffer elevated mortality rates and decreased growth rates?\u00a0 Research is needed to answer these and other questions, but in the meantime, logging damage to roots needs to be minimized.\r\n\r\nRoots of virtually all species of tropical trees are symbiotically associated with fungi that assist in nutrient uptake.\u00a0 Very fine hyphae (=fungal threads) of the fungi attached to tree roots grow out into the soil, thereby greatly increasing the host trees\u2019 below-ground surface area and rates of uptake of nutrients, especially of phosphorous.\u00a0 The fungi benefit from the relationship primarily by receiving sugar from their host tree. \u00a0There are different sorts of these mycorrhizal associations (e.g., endo- and ectomycorrhizae) involving many different species of fungi.\u00a0 Generally the spores and other inocula of these fungi are abundantly available in tropical forest soils.\u00a0 Only where species are newly introduced outside of their natural geographical range, on extremely degraded soils, and in nurseries is lack of mycorrhizae sometimes an issue.\u00a0 Generally by incorporating forest soil into planting mixes, this problem can be avoided.\r\n\r\nDue to their commercial importance for timber and the fact that they grow above ground where we can see them, tree stems have received more attention from researchers than tree roots.\u00a0 Among tropical trees are species with typically fluted, anastomosing, locally swollen, and other variations on the basic tapered cylinder.\u00a0 Tree stems are subjected to abiotic stresses (e.g., wind damage and fire) and biological attacks (e.g., entwining vines and heartrot fungi) that affect their marketability.\u00a0 Stem quality, from a timber production perspective, is generally recorded in permanent plots and during inventories using a classification system such as that presented in Figure 3-1 (stem form classes, crown classes).\u00a0 Note, for example, that if a primary objective of forest management is maintaining an increasing populations of animals that use tree cavities for nesting or denning, these \u201cquality\u201d classes might be arranged quite differently.\r\n

\"\"<\/p><\/div>\r\n

Figure 4.1.1. A common crown class system in which D = dominant, C = co-dominant, I = indirect light (or subdominant), and O = overtopped. Sometimes a fifth class is included for trees that receive substantial illumination from the side.<\/p>\r\nCharacteristics of tree crowns such as width and depth serve as good indicators of the local conditions under which a tree has been growing and may also be useful in predicting a tree\u2019s response to thinning.\u00a0 Broad and deep crowns indicate, for example, that the tree had plenty of growing space.\u00a0 Trees with small or incomplete crowns (Figure 3-1) typically do not respond well to release treatments due to having been suppressed for too long.\u00a0 When the canopy surrounding such a tree is radically opened, it may die or at least take a very long time to respond.\u00a0 This poor reaction is likely due to some combination of having a root-shoot ratio inappropriate to the new conditions, mechanical instability when neighbors are removed, and bark scald.\r\n\r\nA more detailed examination of a tree\u2019s branching patterns can reveal even more about its history and even its wood properties.\u00a0 The duration of branch retention and the process by which they are finally shed, for example, influence the knottiness of wood and whether the knots are loose or tight.\u00a0 The high value of knot-free or \u201cclear\u201d timber can sometimes justify branch pruning in intensively managed stands.\u00a0 Note that when pruning it is always preferable to remove branches while they are small.\u00a0 A rule of thumb is that branches >30% of the diameter of the main trunk themselves function as trunks, i.e., they have lost the characteristic branch collar at the base of smaller branches that helps keep decay from penetrating into the main stem from the branch stub.\r\n\r\nBark is the first line of defense of a tree stem against mechanical damage and attack by herbivores and pathogens.\u00a0 The often high concentration of extractives (e.g., resins and gums) as well as the abundance of long \u201cbast\u201d fibers in bark constitute part of this defense of the vascular cambium and xylem.\u00a0 Bark also protects the cambium from high temperatures during fires; predictions of tree survival through controlled burns, for example, are often based on bark thickness.\u00a0 Direct solar radiation on the stems of previously shaded can also cause damage if the bark is not thick or rough enough to protect the cambium from the heat load.\u00a0 Generally 60\uf0b0C is taken to be the lethal temperature in the cambium.\r\n\r\nVascular cambia vary seasonally in their activity cycles and hence in their susceptibility to damage.\u00a0 When the cambium is active, for example, bark is more easily peeled by herbivores and also inadvertently by timber yarding machinery.\u00a0 Some tree species produce successive cambia and hence develop included phloem (layers of phloem alternating with layers of xylem).\u00a0 Whether this form of developmental anatomy in trees confers resistance to or capacity to recuperate from damage has apparently not been determined in trees but seems important in some woody vines (see below).\u00a0 Due to the substantial potential long-term negative consequences of even minor mechanical damage, protecting trees during forest management operations is obviously critical.\r\n\r\nTrees respond to damage by depositing defensive compounds around the damaged tissues, thereby compartmentalizing the decay.\u00a0 Trees therefore do not \u201cheal\u201d in the same sense as animals, but they can restrict the proliferation of decay.\u00a0 Living wood cells (parenchyma) must be present near the site of damage for compartmentalization to occur.\u00a0 Due to scarcity of living parenchyma in the heartwoods of many tree species, the compartmentalization potential is limited and the likelihood of heartrots is increased.\u00a0 A few decades after destructive logging operations or ground fires, 50% or more of the large trees may be hollow or have heartrots.\r\n\r\nMany silvicultural operations are applied to optimize the leaf areas and crown exposures of potential crop tree. It is important to recognize that all leaves are not constructed the same and that during the life span of a single leaf (ranging from several months to many years), physiological characteristics change as well.\u00a0 Leaves that develop where light intensities are high, for example, are typically thick, small, covered with a thick cuticle, and held at a steep angle to horizontal.\u00a0 Shade leaves, in contrast, tend to be larger, thinner, and darker green due to a higher concentration of chlorophyll.\u00a0 Based on these differences between \u201csun\u201d and \u201cshade\u201d leaves, it is easy to understand why some trees must shed and replace their leaves before they can respond favorably to canopy-opening treatments.\u00a0 As leaves age and become encrusted with lichens, algae, and other epiphylls, their photosynthetic capacity declines.","rendered":"

Because of their large sizes and great longevities, trees have tremendous effects on the ecology of tropical forests.\u00a0 Tropical trees are also important at the planetary level; carbon stored in their trunks and roots is important in maintaining global carbon balance and in slowing global warming and sea level rise.\u00a0 At the level of large watersheds like the Amazon, it is important to recognize that a large portion of the rain falling on forests is derived from water transpired by trees.\u00a0 At a more local level, light rains may never reach the forest floor, the water instead being intercepted by and evaporated from tree leaves and branches.\u00a0 Trees also provide structural support for a variety of other growth forms, such as epiphytes and vines.\u00a0 Although knowledge about other types of plants is important when trying to manage forests sustainably, we obviously need to know a lot about how trees function, starting with below-ground processes.<\/p>\n

The root systems of tropical trees vary a great deal by species and with environmental conditions (e.g., depth to mechanically impeding horizons, bedrock, or the water table). Tropical forests are famous for their diversities of aerial roots, buttressed roots, apogeotropic (=upward growing) roots, and pneumatophores (=so-called \u201cbreathing\u201d roots).\u00a0 But most species of tropical trees have roots that are similar to those of trees in temperate forests in both structure and distribution.<\/p>\n

Although most fine roots of tropical trees are generally concentrated near the soil surface where nutrients are more available, deep roots also develop where conditions allow.\u00a0 These deep roots may be particularly important for water uptake when surface soils have dried; the discovery of roots at depths of 10-14 m in Amazonia suggests that roots may be exploiting soil resources substantially deeper than typical samples taken by soil scientists.<\/p>\n

The horizontal spread of tree roots has not been much studied in the tropics but, like rooting depth, most likely varies by species and site conditions.\u00a0 That roots extend well beyond the crown margins of many trees is abundantly clear.\u00a0 Even at the seedling stage, horizontal root growth can be substantial. Allowing the roots of tree seedlings to expand freely is often problematic in nurseries where \u201cpot-bound\u201d plants are all too common.\u00a0 Out-planted seedlings can also suffer the same \u201cpot-bound\u201d problem when set out in compacted soils.<\/p>\n

Roots proliferate in resource-rich volumes of soil.\u00a0 Given that the distributions of nutrients and water in soils are extremely heterogenous, fine root densities also can vary substantially over even very short distances.\u00a0 And these distributions change over time, sometimes very rapidly as when an animal dies or defecates, creating thereby a local, if temporary, concentration of nutrients.<\/p>\n

Maintenance of the appropriate balance between root surface area for absorption and leaf surface area for photosynthesis and transpiration is critical for tree survival and growth.\u00a0 As environmental conditions change seasonally or due to silvicultural practices, so must this so-called \u201croot-shoot ratio.\u201d\u00a0 For example, because evapo-transpirational demands in the understory are low, understory saplings tend to have relatively low root-shoot ratios.\u00a0 When the canopy is opened due to logging or thinning, these saplings must rapidly adjust their root-shoot ratios for the new conditions of higher light intensities, higher temperatures, and higher vapor pressure deficits.\u00a0 For the same reason, nursery-grown seedlings are often intentionally \u201chardened up\u201d by gradual deprivation of water and nutrients before out-planting so as to reduce \u201ctransplant stock.\u201d\u00a0 Pruning off a portion of leaf area of these seedlings can also help to reduce transplant shock by increasing root-shoot ratios.\u00a0 Pre-planting shoot pruning is made even more critical when seedlings are purposefully or inadvertently root-pruned to facilitate lifting and planting.<\/p>\n

Little is known about the long-term impacts on tropical trees of root damage, but enough is known about temperate oaks (Quercus spp.) and other hardwoods to raise concern.\u00a0 Root damage is common where ground-based timber yarding operations are carried out, particularly if they continue during wet weather.\u00a0 But which species are most susceptible to root-invading pathogens?\u00a0 Do trees that suffer mechanical root damage subsequently develop butt and heart rots?\u00a0 Do trees with damaged roots suffer elevated mortality rates and decreased growth rates?\u00a0 Research is needed to answer these and other questions, but in the meantime, logging damage to roots needs to be minimized.<\/p>\n

Roots of virtually all species of tropical trees are symbiotically associated with fungi that assist in nutrient uptake.\u00a0 Very fine hyphae (=fungal threads) of the fungi attached to tree roots grow out into the soil, thereby greatly increasing the host trees\u2019 below-ground surface area and rates of uptake of nutrients, especially of phosphorous.\u00a0 The fungi benefit from the relationship primarily by receiving sugar from their host tree. \u00a0There are different sorts of these mycorrhizal associations (e.g., endo- and ectomycorrhizae) involving many different species of fungi.\u00a0 Generally the spores and other inocula of these fungi are abundantly available in tropical forest soils.\u00a0 Only where species are newly introduced outside of their natural geographical range, on extremely degraded soils, and in nurseries is lack of mycorrhizae sometimes an issue.\u00a0 Generally by incorporating forest soil into planting mixes, this problem can be avoided.<\/p>\n

Due to their commercial importance for timber and the fact that they grow above ground where we can see them, tree stems have received more attention from researchers than tree roots.\u00a0 Among tropical trees are species with typically fluted, anastomosing, locally swollen, and other variations on the basic tapered cylinder.\u00a0 Tree stems are subjected to abiotic stresses (e.g., wind damage and fire) and biological attacks (e.g., entwining vines and heartrot fungi) that affect their marketability.\u00a0 Stem quality, from a timber production perspective, is generally recorded in permanent plots and during inventories using a classification system such as that presented in Figure 3-1 (stem form classes, crown classes).\u00a0 Note, for example, that if a primary objective of forest management is maintaining an increasing populations of animals that use tree cavities for nesting or denning, these \u201cquality\u201d classes might be arranged quite differently.<\/p>\n

\n

\"\"<\/p>\n<\/div>\n

Figure 4.1.1. A common crown class system in which D = dominant, C = co-dominant, I = indirect light (or subdominant), and O = overtopped. Sometimes a fifth class is included for trees that receive substantial illumination from the side.<\/p>\n

Characteristics of tree crowns such as width and depth serve as good indicators of the local conditions under which a tree has been growing and may also be useful in predicting a tree\u2019s response to thinning.\u00a0 Broad and deep crowns indicate, for example, that the tree had plenty of growing space.\u00a0 Trees with small or incomplete crowns (Figure 3-1) typically do not respond well to release treatments due to having been suppressed for too long.\u00a0 When the canopy surrounding such a tree is radically opened, it may die or at least take a very long time to respond.\u00a0 This poor reaction is likely due to some combination of having a root-shoot ratio inappropriate to the new conditions, mechanical instability when neighbors are removed, and bark scald.<\/p>\n

A more detailed examination of a tree\u2019s branching patterns can reveal even more about its history and even its wood properties.\u00a0 The duration of branch retention and the process by which they are finally shed, for example, influence the knottiness of wood and whether the knots are loose or tight.\u00a0 The high value of knot-free or \u201cclear\u201d timber can sometimes justify branch pruning in intensively managed stands.\u00a0 Note that when pruning it is always preferable to remove branches while they are small.\u00a0 A rule of thumb is that branches >30% of the diameter of the main trunk themselves function as trunks, i.e., they have lost the characteristic branch collar at the base of smaller branches that helps keep decay from penetrating into the main stem from the branch stub.<\/p>\n

Bark is the first line of defense of a tree stem against mechanical damage and attack by herbivores and pathogens.\u00a0 The often high concentration of extractives (e.g., resins and gums) as well as the abundance of long \u201cbast\u201d fibers in bark constitute part of this defense of the vascular cambium and xylem.\u00a0 Bark also protects the cambium from high temperatures during fires; predictions of tree survival through controlled burns, for example, are often based on bark thickness.\u00a0 Direct solar radiation on the stems of previously shaded can also cause damage if the bark is not thick or rough enough to protect the cambium from the heat load.\u00a0 Generally 60\uf0b0C is taken to be the lethal temperature in the cambium.<\/p>\n

Vascular cambia vary seasonally in their activity cycles and hence in their susceptibility to damage.\u00a0 When the cambium is active, for example, bark is more easily peeled by herbivores and also inadvertently by timber yarding machinery.\u00a0 Some tree species produce successive cambia and hence develop included phloem (layers of phloem alternating with layers of xylem).\u00a0 Whether this form of developmental anatomy in trees confers resistance to or capacity to recuperate from damage has apparently not been determined in trees but seems important in some woody vines (see below).\u00a0 Due to the substantial potential long-term negative consequences of even minor mechanical damage, protecting trees during forest management operations is obviously critical.<\/p>\n

Trees respond to damage by depositing defensive compounds around the damaged tissues, thereby compartmentalizing the decay.\u00a0 Trees therefore do not \u201cheal\u201d in the same sense as animals, but they can restrict the proliferation of decay.\u00a0 Living wood cells (parenchyma) must be present near the site of damage for compartmentalization to occur.\u00a0 Due to scarcity of living parenchyma in the heartwoods of many tree species, the compartmentalization potential is limited and the likelihood of heartrots is increased.\u00a0 A few decades after destructive logging operations or ground fires, 50% or more of the large trees may be hollow or have heartrots.<\/p>\n

Many silvicultural operations are applied to optimize the leaf areas and crown exposures of potential crop tree. It is important to recognize that all leaves are not constructed the same and that during the life span of a single leaf (ranging from several months to many years), physiological characteristics change as well.\u00a0 Leaves that develop where light intensities are high, for example, are typically thick, small, covered with a thick cuticle, and held at a steep angle to horizontal.\u00a0 Shade leaves, in contrast, tend to be larger, thinner, and darker green due to a higher concentration of chlorophyll.\u00a0 Based on these differences between \u201csun\u201d and \u201cshade\u201d leaves, it is easy to understand why some trees must shed and replace their leaves before they can respond favorably to canopy-opening treatments.\u00a0 As leaves age and become encrusted with lichens, algae, and other epiphylls, their photosynthetic capacity declines.<\/p>\n","protected":false},"author":656,"menu_order":1,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-44","chapter","type-chapter","status-publish","hentry"],"part":42,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/pressbooks\/v2\/chapters\/44","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/wp\/v2\/users\/656"}],"version-history":[{"count":7,"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/pressbooks\/v2\/chapters\/44\/revisions"}],"predecessor-version":[{"id":283,"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/pressbooks\/v2\/chapters\/44\/revisions\/283"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/pressbooks\/v2\/parts\/42"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/pressbooks\/v2\/chapters\/44\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/wp\/v2\/media?parent=44"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/pressbooks\/v2\/chapter-type?post=44"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/wp\/v2\/contributor?post=44"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/fode014notebook\/wp-json\/wp\/v2\/license?post=44"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}