Botanizing in Alaska: Black Spruce

Black spruce (Picea mariana) are a common forest tree up in central Alaska, ranging north until the tundra. The trees in the image at left are in Fairbanks, Alaska.

The trees grow slowly, eventually topping out at 20 meters in the southern parts of their range. The trees in Fairbanks are generally much shorter. The trees here are maybe 30 ft tall, growing less than a foot apart. They can get away with such crowding because those two trees are probably separate trunks growing from a unified root system. Large connecting roots grow horizontally just under the surface and graft together with their neighbors. The individual trunks share nutrients and carbohydrates and thus don't suffer from competitive shading as much as trees that don't cooperate in this manner.

This style of growth also potentially helps them stay upright in the swampy soils they're usually found in. The horizontal grafted root structure spans wider than the cluster of trunks, allowing the cluster to stay upright even if the ground beneath part of the cluster can't support their weight. This style of growth would help them grow horizontally out onto a bog, with some trees suspended over the lake hidden below.

References:

• Black spruce: https://www.oecd-ilibrary.org/docserver/9789264095434-10-en.pdf?expires=1571892005&id=id&accname=guest&checksum=D3380DF0E335EC3E021633EB1523689A
• Root grafts:
• https://www.nrcresearchpress.com/doi/pdf/10.1139/cjfr-2016-0121
• https://www.sciencedaily.com/releases/2011/06/110608141529.htm