Secret lives of evergreen needles

Photosynthesis capacity forest needles Research Experience Undergraduates

Benjamin Russell pointed out different years of growth on a white spruce tree, using bud scars found on the back of the branch to segment off different growing seasons’ needles. “This is the new growth, the growth from this season. And see how abnormally large this is compared to the rest of the tree? That is a reflection of the unusually warm temperatures we’ve had this growing season in Alaska,” he told Frontier Scientists.

Benjamin Russell, a student in the National Science Foundation’s program Research Experience for Undergraduates, studied forest growth under the direction of Bjartmar Sveinbjornsson, professor of biological sciences and director of the Environment and Natural Resources Institute at the University of Alaska Anchorage. They examined white spruce trees in Alaska’s Chugach mountains, which host patches of forest representing both coastal rainforest and the boreal forest.

Russell was using equipment to measure tree needles’ photosynthesis and respiration. He clamped part of a machine which looked much like a large box-shaped clam around one segment of living white spruce branch. Inside such equipment, conditions are highly controlled. Russell can set the machine to maintain a constant relative humidity, and to control flow rates of air. He can control temperature via a fan that blows on the chamber, and control what area inside is being measured. Many factors came into play as he tested the branch segment.

“You want to set up the machine so that the concentration of carbon dioxide inside the chamber reflects the ambient concentration of CO2 in the air,” Russell said. “Twenty years ago they were setting these concentrations at 360-380 ppm [parts per million] and as we’ve come across the century we’ve had to start making these concentrations at 400. So you kind of get the idea of the change in ambient CO2 concentrations through time. Which is really interesting in and of itself.”

“Whenever we close this and there is no light we’re measuring respiration,” Russell said. Respiration is a chemical reaction that’s basically the reverse of photosynthesis. Plants respire all the time. During respiration plant cells put glucose (sugar) and oxygen together in a reaction which produces water, carbon dioxide, and usable energy. [ C6H12O6 + 6O2 → 6H2O + 6CO2 + energy ]. So respiration uses oxygen and produces carbon dioxide.

But add sunlight, and suddenly the plant is also able to photosynthesize. For this reaction carbon dioxide is taken in and oxygen is given out. During photosynthesis sunlight, water, and carbon dioxide combine to produce glucose (sugar) and oxygen gas. [ Sunlight + 6H2O + 6CO2 → C6H12O6 + 6O2 ]. “We can control the amount of light that shines into this chamber,” Russell explained, activating the light in the equipment. “With the light on,” he said, “We are actually getting a good estimate of the photosynthetic capacity of those needles in the chamber.” Russell was using the equipment’s fine controls to measure the photosynthetic rate of different years’ needles under optimal conditions.

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These specifics are helping the scientists look into how trees in the treeline zone grow. “It’s a zone above the forest where you have scattered trees,” Sveinbjornsson said. “And these trees might tell us something about why it is difficult to grow in this area. Why the forest doesn’t reach higher up in the mountains or further north on flatter areas.”

“What is limiting the spread of trees?” Sveinbjornsson asks in his research. “In order to answer these questions we need to know how trees work.” Usually there is less growth at or above the treeline than in the forest; you’ll see stunted trees growing at high elevations, often with branches stripped of most of their needles.

One might imagine that fewer needles mean less photosynthesis. “Things are never that simple,” Sveinbjornsson cautioned. “If you have few needles you are losing less water, so you might have better water balance. So they might compensate for that. And we want to be able to more exactly attribute the reduction in available carbohydrate, whether it’s increased respiration and decreased photosynthesis or the other way around.” Add to that, “The number of needles is very important as well as the rate at which they take up CO2 or lose it.”

Part of the reason for stunted, damaged treeline trees is harsh Alaskan winters. Branches are exposed to more harsh winds and blowing snow; ice crystals damage and shear away needles. Sveinbjornsson noted “Sometimes the snow is deep and sometimes it is low. And so the blasting zone is different in different years. But what we can see is: there is a lot of tissue loss on those trees, which means they have to grow a lot to compensate.”

“But there are years where there is as much and even more growth at the treeline,” Sveinbjornsson stated. “And this last summer was one of them,” he said, “We had a sharp onset of summer. Really warm temperatures, just all of a sudden.” Also, “The winters have also had very little snow and so presumably very little blasting of the needles off the trees,” Sveinbjornsson said. Overwintering conditions can have a big impact.

Sveinbjornsson explained that Russell’s work was designed to investigate “The importance of retaining those needles in terms of photosynthesis and in terms of how many nutrients we would lose– how much carbon, how much nitrogen.” Russell looked at the capacity of different age classes to photosynthesize. And he found a decline in the capacity– as tree needles age they become less able to photosynthesize. Sveinbjornsson said “Benjamin discovered that there is a gradual decline in photosyntheic capacity from young current season needles to older needles. And he discovered there was a co-occuring decline in the concentrations of nitrogen.” Older age classes were hardly doing any photosynthesis at all. Instead, they stored resources to allow for the growth of new needles. Over time old needles contribute the resources they have stored inside themselves to help their tree grow brand new needles. Evergreen needles (unlike deciduous tree leaves) serve as storage containers. Sveinbjornsson: “When they start growing in the spring they don’t have new photosynthetic needles. They have to rely on the photosynthesis of the needles that survived and the storage in these needles.”


“What happens to needles during the winter time is extremely important to needles in the present time,” Sveinbjornsson said. “Older tree needles are contributing carbon and nitrogen to young growth. And that this is similar to other conifers that are important in the boreal zone, mainly the black spruce.” Black spruce, abundant in interior Alaska, can keep their needles for as long as 25 years. Sveinbjornsson: “And you wonder why a tree is doing this… It costs a lot to retain those needles. It seems, primarily, these are storage organs for the new photosynthetic parts.” Although they cannot photosynthesize as well as young needles, old needles remain important because they store carbohydrates, glucose, sugar.


While white spruce trees growing in sheltered forests have needles with an average lifespan of 8 years and have been shown to sometimes keep their needles for a whopping 12 years, white spruce growing at higher elevations above the treeline have needles with an average lifespan of 4 years. The short lifespan, probably due to rough winter conditions, is a problem for treeline trees because damaged or missing needles mean damaged or missing resources, which impairs early season growth.

Wintertime and summertime conditions define future growth. “In good summers proceeded by good winters when we don’t have a needle loss we have good growth,” Sveinbjornsson stated. “We have now had two very good summers, with very little needle loss, and under those conditions we can see those trees can grow just as well up on the mountain as down here. ” Forest trees and treeline trees are genetically the same, and have the potential to grow as robustly. “The individuals that are up there are not small because they are genetically unable to grow. They are just as capable as forest trees, it seems like,” and “If conditions are amenable up there, they will grow more.” When warm years and mild winters allow treeline trees and forest trees to sport equal growth, needles produced at either location have similar photosynthetic capacity, similar number of stomates, and similar storage capacity with which to fund new growth.

“It’s a balance between this growth and the loss of tissue that determines the size of the organism,” the trees at the treeline. Sveinbjornsson: “And we feel that by studying these we can say something about the possible mechanisms behind forest expansion or forest retreat.” Forests can expand. “It’s possible for the trees to grow fast and well up there, if conditions are right for it.”

To understand the future of the forests “We need a mechanistic understanding of what is controlling the activities in these ecosystems,” Sveinbjornsson said, and this research helps build our understanding.

Laura Nielsen 2016

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