TREES Functions, structure, physiology
Trees in Agroecosystems - 1 Microclimate effects lower soil temperature alter soil moisture reduce temperature fluctuations Maintain or increase soil fertility biological nitrogen fixation (legumes) reduce nutrient loss nutrient pumping from sub-soil litterfall, root turnover, and root exudates Potential for insect & weed control
Trees in agroecosystems -2 Maintain or increase soil organic matter leaf litter, root turnover, and root exudates improved soil physical qualities and nutrient availability increases microbial activity Reduce soil erosion Harvest potential food, fodder, or fuel wood
Tree Structure & Morphology Shoot - trunk and branches (stems), leaves, and fruits Roots - tap roots and feeder roots, +/- mychorrhizae Stems & branches Woody plant stems have the unique ability to undergo process of secondary growth or thickening Can do this by creation of a continuous vascular meristem or cambium as stem ages
Tissue types Cambium - meristematic cells that divide and produce other cell types Phloem - specialized elongated cells that transport carbohydrates, amino acids and other metabolites Xylem - specialized elongated cells that lignify (become woody). Contents die and they become conduits for water and nutrient transport from root to shoots
Cambium P X Typical annual stem Xylem Phloem
Vascular cambium 3 year old stem 5 year old stem Xylem rays phloem pith vessel
Types of buds Vegetative buds - grow new stem and leaves flower or fruit buds - produce flowers that develop into fruit. Compound or mixed buds - have both a vegetative and flower bud together.
Tree root systems Vary in spread, depth and distribution of root types among species Majority of feeder roots in top 1-2m, and in many cases within top 50cm or less depending on whether soil surface is bare, mulched or in sod.
Light Temperature Humidity Humans Wind Plants Crop Rainfall pests Fire Nutrients Topography Soil The Environmental Complex for a crop plant
Managing the resources trees require:water Soil-plant-air continuum Lower water potential in the root Higher water potential in soil
Water transport in trees Physically impossible? How can water move to tall tree canopies? Under perfect vacuum (i.e. 0 bars of pressure) water can be raised 10m - many trees taller than this! Xylem - must be under negative pressure (tension). Achieved via strong cohesive forces of water in the narrow channels and spaces between fibers in the xylem
Managing the resources trees require:nutrients C, H, O N Ca P Mg S Cl K Plant macro-nutrients Basic constituents of organic material Proteins, chlorophyll, enzymes etc Cell walls, cellular signals Energy transfer - ATP etc Chlorophyll, enzymes, protein synthesis Proteins, Light reaction, ionic balance, stomatal opening Ionic balance, osmosis, enzyme activator Micro-nutrients to watch for fruit trees: Boron, Iron, Zinc
Light & tree forms In all plants dry matter production directly related to total light interception Annual dry wt Total annual light interception Light environment within canopy critical
Canopy light environment Direct light - hits leaves at the edge of the canopy and in the interior as sunflecks Diffuse light - has passed through leaves or reflected off leaves Diffuse light has different quality than direct light - more green, less red & blue (absorbed by leaf pigments). Less effective at driving photosynthesis
Managing light environment For high fruit production want to reduce amount of self shading within the canopy Found that nitrogen allocation into the tree canopy correlates closely with light environment Flower buds will not develop without adequate sunlight
Apical dominance & plant hormones Four stages: I Lateral bud formation cytokinin promoted II Inhibition lateral bud growth suppression by auxin III Release of dominance cytokinin promoted (first few hours) IV Lateral shoot development promoted by auxin and gibberelins Note: within a tree shoots have different strengths of apical dominance. So apical dominance used to describe process in a single shoot, whereas apical control used to describe whole tree pattern of dominance which affects canopy shape.
Pruning - managing apical control and dominance A major goal of pruning is to create a canopy that results in a high light environment throughout. Fruit buds need high light to form Vegetative bud numbers also lower in reduced light environments in tree interior When a stem is cut lateral buds are released from apical dominance.
Resource allocation - trees Carbon not only major resource plants partition for different functions Nutrients (especially N) and water also critical How do plants respond when any of these are limiting? Development of resource allocation theory
Predictions Plants acquire resources at minimal cost and use at maximum benefit plants adjust allocation only until return is equal to costs of investment that is they increase expenditures for scarce resources Optimal allocation among competing processes would occur when each resource limits to same degree
CARBON ALLOCATION IN TREES Partitioning of carbon between different plant parts and processes. Carbon moves from sources to sinks Source: Sink: Storage: leaves e.g. roots, stem,growing leaves, flowers, fruit, seeds etc all tissues, but especially stems note many trees produce specialized storage proteins as well as starch paths: via phloem primarily in the form of sugars
Allocation controlled by signals and feedback systems between roots and shoots Root to shoot - e.g. low nutrient availability reduces cytokinin production by roots, less transported to shoot leading to reduced shoot growth an abscissic acid type of molecule produced by roots under water stress, reduces shoot growth, so plant has more C to increase root growth. shoot - root e.g. when leaf carbohydrates low, shift to produce more photosynthetic capacity, thus less sugar released into phloem and translocated to roots N status of leaves controls N uptake rate by roots
Temporal considerations Need to consider allocation patterns within and across seasons: low dry matter production due to poor environmental conditions in one year dramatically affects tree performance in subsequent year(s) see handout for patterns of C allocation throughout the year in fruit trees
Other key environmental interactions Temperature. growth increases with temperature within moderate range Low temperature - frost survival is important. Hardened trees are more cold tolerant than nonhardened. Need to understand chilling requirement for breaking bud dormancy. Varies with species and variety
Wind can lead to diminished growth via temp/moisture effects but also aids in frost prevention