Needle morphology in the boreal forest

Needle morphology stomata forest Research Experience Undergraduates

“We are just trying to get as much experience as we can with research, just delving into science.” Amanda Bonavia was a participant in the National Science Foundation’s program Research Experience for Undergraduates. She studied the boreal forest 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. The experience took her far from Rockford University, Chicago, Illinois, to Alaska’s Chugach mountains where she joined Sveinbjornsson to research white spruce trees. Bonavia described the science overall as tree line ecology; “We came up here to look at tree line trees and forest trees and contrast the differences.”

“My particular project is looking at needle morphology, more specifically looking at stomata.” Bonavi described stomata as pores or apertures in leaves and needles, tiny openings which control gas exchange. Carbon dioxide, oxygen, and water vapor enter and exit plants through stomata, so stomata help regulate photosynthesis. “I want to see: are there more stomata on the needles up at the treeline or are there less, compared with the forest trees? On top of that I’m looking at the stomatal size– are they going to be larger, are they going to be smaller?”

What was Bonavia expecting to see before beginning her REU project? She wasn’t sure… “The literature indicates a whole bunch of contradictions,” she said “Some studies find this, and some studies find that.” It’s a complicated field. Bonavia: “Species are different. Things change and that’s especially true for different types of trees, so one tree might behave a certain way, while another tree behaves another way. And I think it is a good idea to determine what species is behaving in what way. It also implies that some species might be able to adapt to certain things while others won’t be able to do that.”

The data gathered adds to science’s knowledge bank about the boreal forest, and helps forecast the future of white spruce trees as the years bring changes in factors like average temperatures and growing season length.

Some species have more phenotypic plasticity– ability to change in response to an altered environment– than others. “It’s good to be able to identify the dominating species up at the tree line and see what we can expect, what we can anticipate happening with them.”

Bonavia studied needles taken from white spruce trees at the tree line and needles taken from lower elevation white spruce trees growing in the forest. “I’m looking at a particular age class, that’s a cohort, a cohort of needles,” she said. “I look at last year’s needles and pluck a bunch off from the middle of the cohort,” avoiding particularly old or young needles from the age range. Bonavia would freeze the specimens until they could be imaged under a dissecting scope. She’d take pictures of the needles’ sides then use software called Image J to bring stomata into focus and tally them. The software can remove green pigment, causing stomata apertures to stand out like white dots, then count the dots. Bonavia noted it’s a time-consuming process. “It’s indicating there’s about 2500 stomata within that small little area.”


At higher elevations Bonavia stated “Needles are typically going to be shorter. And that’s because you have a lot of mechanical pressures going on up there. You have damages, abrasion from snow, wind, things like that. Temperatures are a lot colder.” Near the tree line vegetation tends to end up smaller– likely closer to the ground, wider. Needle growth tends to echo tree growth, Bonavia said. In general “You end up seeing that needles are wider and they have a tendency to be shorter,” when they belong to tree line trees and not more sheltered forest trees.

While that’s the trend, Bonavia pointed out tree line needles from a year with favorable conditions are sized similarly to lower elevation forest needles. “That implies that the growing season has been abnormal up there. So it looks like we’ve had more days of growth and more opportunity for growth,” compared to ‘normal’ years which produce stunted growth at the tree line.

Sveinbjornsson: “The reduced white spruce growth correlates with loss of needles in the winter. At the end of each summer, at the tip of each branch, there are three buds laid down. And these buds will then grow and extend out in a fan-shaped fashion the next year. The number of needles that are going to be forming next year has been determined by the bud.” Weather conditions during late summer and fall determine the condition of the bud, including the bud’s initiators which define how many needles will grow from the bud. “But the density of these needles and the size of these needles are affected by the climate when they are growing,” Sveinbjornsson said (underlines added by editor).

Sveinbjornsson: “If you have a summer when there is a gradual warming you may have 2 or 3 weeks earlier growth in the forest than you have at tree line. If on the other hand you have a sudden warming, very sudden and it’s quite warm, like we had the last two summers here, then they all start at about the same time and the difference is very small.”

With the unusually warm summers and the study area in mind, Bonavia reported what her research showed. “Right now my absolute counts indicate that there are more stomata on the tree line than there are down here. But the density overall, it seems to be relatively the same. So it doesn’t really matter which location you are at or which elevation you are at.” A snapshot of white spruce tree tree line tree stomata versus forest tree stomata in years with more typical temperature trends and growing season lengths might have shown her different numbers.

Then again, what’s ‘typical’ is in flux.

If she had more time with the project Bonavia would be eager pursue more boreal science: exploring evolutionary tradeoffs and DNA analysis. “Unfortunately I am not able to take a look at that stuff, I just don’t have enough time,” Bonavia said. In the Research Experience for Undergraduates program “They’ve given us a good amount of time but not enough to do all that we want.”


Laura Nielsen 2016

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

(Needle morphology stomata forest Research Experience Undergraduates)