Pacific Northwest Research Station

Canopy Processes in Temperate Mesic Forests

2008 Science Accomplishments

As Douglas-fir get taller, growth is limited by changes in leaf function

Douglas-fir compromise growth to maintain photosynthetic abilities. Credit: John Laurence

Height-related changes in leaf function may affect tree growth and forest productivity because leaf stomata are responsible for maximizing photosynthetic carbon gains while simultaneously limiting transpiration to avoid damaging levels of dehydration. The xylem of Douglas-fir needles undergoes structural changes with increasing tree height that make the needles less vulnerable to hydraulic failure caused by entry of gas bubbles or emboli. This increased hydraulic safety is associated with a reduction in the hydraulic efficiency of the needles, causing them to act as hydraulic bottlenecks. Although this syndrome is likely to result in the maintenance of photosynthesis under conditions of greater foliar water stress, it is also likely to result in increased stomatal restriction of transpiration and carbon dioxide uptake in taller trees while gas exchange is occurring in this foliage, thus contributing to height-related reductions in tree growth. These findings are fundamental to understanding how trees cope physiologically with increases in water stress and timing of tree growth in different sites and environments. Basic physiological information informs models used to project the effects of climate change on tree growth and ability to adapt to stresses of a changing environment.

Partner: Oregon State University

To learn more, contact David Woodruff at dwoodruff@fs.fed.us.

Dry season has little effect on tree transpiration in tropical savanna

Vast areas in central Brazil are covered by this type of tropical savanna known as cerrado. Credit: Sandra Bucci

Tropical savannas in Brazil are characterized by large variations in tree density over short distances. As expected, stand-level water loss to the air increased with increasing abundance and coverage of trees. Surprisingly though, tree transpiration showed little seasonality despite a 5-month dry season. Stability of transpiration is attained via reduced leaf stomatal opening that balances the higher evaporative demand during the dry season, and a deep rooting habit that ensures a reliable supply of water year-round.

Scientists are using these findings to better understand the mechanisms that are important in modeling the effects of changing climate.

Partners: Universidad de Buenos Aires, Argentina; Universidade de Brasilia, Brazil;
University of Miami, Florida

To learn more, contact Rick Meinzer at fmeinzer@fs.fed.us.

Sapwood water storage helps protect trees against catastrophic xylem failure

Water stored in living and dead sapwood tissue can be released into the transpiration stream where it buffers changes that can provoke formation of air emboli and consequent loss of water transport capacity in the tree. Studies conducted on tropical trees in Panama showed that species with higher sapwood water storage capacity experienced smaller daily fluctuations in xylem tension than species with lower sapwood water storage capacity and did not allocate as much of their carbon resources to producing embolism-resistant xylem. These relationships are being explored in Pacific Northwest conifers to better understand how they transport sapwood water along gradients of increasing aridity and summer drought.

Understanding the fundamental physiology of tree species is critical to predicting growth and vigor under variable conditions. The findings are being used by tree physiologists and groups modeling tree species distribution and performance under different climate scenarios.

To learn more, contact Rick Meinzer at fmeinzer@fs.fed.us

Partners: Universidad de Buenos Aires, Argentina; University of Miami, Florida

Photo: Sapwood water.jpg
Caption: Researchers use the Smithsonian Tropical Research Institute canopy crane in the Parque Natural Metropolitano near Panama City to measure the photosynthetic capacity of the canopy foliage.