Contrasting root systems of two coexisting tree species in northern hardwood forest ecosystems

Background/Question/Methods

The species composition of forest ecosystems influences forest productivity and other important functions such as the retention of nutrients and maintenance of water quality. Niche separation and resource partitioning are key ways in which ecologically similar species coexist in a community and influence vital ecosystem functions. Understanding resource partitioning by different tree species in complex ecosystems like forest can help interpret species interactions and species responsiveness as anthropogenic change continues to alter resource availability. The current research aims to understand belowground resource partitioning by examining depth distributions of root growth by coexisting species, American beech (Fagus grandifolia Ehrh.) and sugar maple (Acer saccharum Marsh.) in three mature (>100 years) northern hardwood forest stands in central New Hampshire. We focused on survivorship of seedlings in the forest understory. This important stage in forest succession can indicate future regeneration potential, especially for shade tolerant species like beech and sugar maple, which require the presence of young individuals in the understory to take advantage of gaps that arise in the canopy over time. We examined differences in species-specific traits by testing the hypothesis- that beech and sugar maple differ in root depth distributions.

Results/Conclusions

Beech and sugar maple showed distinct species-specific root depth distribution in all the three forest stands (p= 0.00), however the total root length did not differ between species (p= 0.1). Sugar maple roots were concentrated at 2-8 cm depth, whereas beech roots were distributed primarily from 5-15 cm depth.  First-year survivorship of sugar maple seedlings (45%) was marginally (p= 0.06) lower than that of beech (67%) and root to shoot ratios were similar (beech =0.53 and sugar maple = 0.52).  A positive relationship between sugar maple root: shoot and soil moisture (p= 0.01) suggests the importance of water availability to belowground growth in this relatively more shallowly rooted species.  Our results indicate spatial partitioning of belowground resources for these two ecologically similar coexisting species, and justify the need to further study species-specific traits that contribute to recruitment in northern hardwood forests.

Key words: root architecture, seedlings, American beech, sugar maple, northern hardwood forest.