Stade Tolerance and Intolerance

• Shade tolerance

  The relative capacity of tree species to compete for survival under shaded (less-than-optimal) conditions
• Shade tolerance

  • It is a tree trait, a functional adaptation that varies among species
  • It has outsized influence on tree survival and stand growth
  • It is a pillar of silviculture
• Ranking of tree species by shade tolerance

  • Very shade intolerant
  • Very shade tolerant
• Shade tolerance rankings are a fundamental precept, a basis for a forester's management decisions about which trees to keep or cut and when
• Shade preference

  Green plants prefer light, they require it, there's no such thing as a shade-loving tree
• Shade tolerance

  Certain species can make do with less light, a bit of shade makes them more competitive than they would be under more light because they can outgrow less shade-tolerant species
• Shade tolerance levels

  • Shade-tolerant species (e.g. eastern hemlock, American beech) can survive on 1-3% of full light
  • Shade-intolerant species (e.g. trembling aspen, red pine) require up to 60% of full light
  • Intermediate shade-tolerant species (e.g. yellow birch, white pine) work in the 10-30% of full light range
• Intermediate shade-tolerant species

  1. Germinate and establish under the existing forest canopy

  2. Respond with accelerated growth to fill a gap in the canopy when a local disturbance provides new light
• Shade tolerance is not necessarily always so simple and constant
• Shade tolerance can vary with the age of trees, within a species across site types and regional climates, and has a genetic component
• Shade-tolerant trees

  They are better than shade-intolerants at balancing photosynthesis and respiration under low light levels
• Photosynthesis

  How plants make food
• Respiration

  How plants consume food
• Shade-tolerants can capture and use low levels of light to make food without burning it all in the process, they don't build solar cells they have no sun for, they're thrifty
• Shade-intolerants can photosynthesize and grow rapidly in full light, which enables them to outgrow the shade-tolerants in open conditions, but they have similarly lavish rates of respiration
• Under shade, shade-intolerant seedlings languish as they cannot capture enough light to pay the costs, whereas shade-tolerant seedlings linger and last