Medium-term impact of an increase of soil temperature and nitrogen enrichment on the growth of black spruce

Context

The circumpolar boreal region is home to 32% of the world's forests. In Canada, the boreal forest accounts for 60% of all revenues generated from the logging sector, when considering direct jobs only (wood cutting, paper pulp production, lumber). Despite the vital importance of this ecosystem, its sensitivity to climate change is still poorly understood. A better understanding of the factors that influence the productivity and structure of the boreal forest is essential for exploiting this ecosystem in a sustainable way and for developing reliable tools for estimating its evolution in the near future.

Objective(s)

To better understand the impacts of climate change and, in particular, the effects of changes in temperature, precipitation and atmospheric nitrogen deposition on nutrient uptake, photosynthesis and the use of sugars as well as on the radial growth of balsam fir and black spruce in a mature boreal forest.

Methodology

Real-time monitoring of the intra-annual growth of mature trees using electronic dendrometers and conducting anatomical analyses of tree-ring growth (xylogenesis);
Insertion of heating cables in root systems to produce early snowmelt; installation of sprinklers above the canopy to subject selected trees to an artificial precipitation regime enriched in labelled nitrogen (15N);
Monitoring of the cell development of growth rings by sampling different parts of trees and ecosystem compartments to determine the nutrient balance.

Results

Original experimental methods were established in a mature fir stand and in two mature black spruce stands. Canopy irrigation systems using sprinklers were used to simulate the future contribution of additional nitrogen from rainfall. Heating cables placed in the ground around the root systems of treated trees raised the soil temperature by 4 degrees, simulating global warming. These methods allowed for various configurations (heated trees only; heated and watered; watered only and controls). Sprinklers simulating rain enriched with labelled nitrogen (15N) made it possible to determine in which part of the tree this additional nitrogen was located and whether it could contribute to the formation of wood.

Innovative methods were used to monitor the effects of these treatments on tree growth as well. The first was to measure the cell development of the growth ring. Microsamples of the growth ring taken once a week during the growing season were used to determine the developmental phenology and compare the cell production of the treated and untreated trees. A second method based on electronic dendrometers was used to measure the radial variations of the trunks at a frequency of 5 minutes.

Overall, after eight years of watering and soil heating, the spruce trees showed no significant difference in terms of cell number or the other cellular parameters measured, such as cell wall thickness or cell size. Contrary to what was anticipated, the growth of black spruce did not increase significantly compared to controls. After three years of measurement, the growth of the spruce trees located at the colder site started earlier and a trend towards an increase in the number of cells produced seemed to emerge. However, this trend did not continue over the next five years. Furthermore, although budburst occurred earlier at the colder site, this effect faded over time. Black spruce, which is a very conservative species, shows that mature trees are highly resistant to reacting to different treatments. Heated soils affected the proportion of ectomycorrhizae and the number of root apices only at the coldest site; no difference was observed with the other setup. However, it would be useful to study the addition of nitrogen according to its dispersion in the ecosystem (addition via the canopy instead of the soil). The analysis of wood microcores will allow us to determine whether this nitrogen can contribute significantly to the creation of wood cells and possibly increase growth.

Phases phénologiques projet épinette noire

Figure 1 : Phenological phases of xylem differentiation, duration and total number of mature xylogenesis cells in C (control), H (heated), N (nitrogen), and HN (heated and nitrogen) trees in the BER and SIM zones during 2008-2013.

Benefits for adaptation

This work shows that the most widespread and harvested species in the boreal forest of eastern Canada, due to its exceptional qualities, shows surprising resistance to treatments simulating medium-term global warming (8 years). Even if this method does not perfectly reproduce global warming, it allows us to conclude that black spruce will not react spontaneously and drastically to regional warming and that growth should not increase rapidly in the coming decades.