Natural root grafts (anastomoses) are morphological unions formed between roots of different trees. Common root systems allow translocation of water, nutrients and photosynthesis products between grafted trees, affecting their growth and their physiology. As carbohydrates are redistributed among grafted trees, the formation of a common root system could reduce the negative effect of intraspecific competition for light or soil resources within stands. The aim of this study was to investigate the role of root grafting on intraspecific competition and growth of balsam fir Abies balsamea. We studied inter-tree relationships in three natural balsam fir stands of the boreal forest of Quebec (Canada) that contained an average 36% of grafted trees. At each stand, ring width and basal area of trees were measured using dendrochronology techniques. We used mixed linear models to test the effect of root grafting and intraspecific competition on annual basal area increment of trees. Trees before grafting had higher growth rates than trees once grafted. Thus, root grafting did not improve tree growth. Growth of grafted trees was more negatively affected by intraspecific competition than growth of non-grafted trees. Thus, grafted trees cannot be considered as better competitors than non-grafted trees. Under high intraspecific competition, growth of larger grafted trees was less affected than that of smaller trees suggesting that they were able to divert resources at their advantage within a union. Our study demonstrated that grafted trees acted on each other's growth and provides support for the idea that grafted trees respond to competition for resources more as a community rather than as individual trees.

Natural root grafts (anastomoses) result from the fusion of the vascular systems of two roots and allow trees to share water, nutrients and photosynthesis products, affecting tree growth and physiology. The aim of this study was to investigate the frequency of root grafting in balsam fir (Abies balsamea (L.) Mill) of the boreal forest of Quebec (Canada), and to identify ecological drivers such as tree proximity or size of stems and roots. One 50 m2 area was hydraulically excavated in each of three natural balsam fir stands of various ages, tree diameters and densities. For each area, we measured the number of roots and grafts per tree, and the diameter and age of all stems, roots and grafts using dendrochronology techniques. Percentages of grafted trees and number of grafts per tree were similar between stands, corresponding to 36% (± 2.86 SE) and 1.30 (± 0.03 SE), respectively. Root grafting occurred at a wide range of tree ages from 12 to 106 years old. Mean distance between grafted trees was 47.91 cm (± 5.10 SE) and we did not observe any graft beyond a 2 m distance between trees. The number of grafts per tree increased with number of roots per tree and decreased with distance between trees. Root grafting also occurred at a wide range of root ages, from 5 to 64 years old. However, roots were relatively small at graft initiation, with an average root diameter of 3.94 cm (± 0.33 SE). These results demonstrate that balsam fir stands are highly connected through root grafting, occurring early in stand development and continuing throughout the life of the stands. The number of roots per tree and distance between trees were the best predictors for root grafting.

1.A broad and diversified group of compounds, secondary metabolites, are known to govern species interactions in ecosystems. Recent studies have shown that secondary metabolites can also play a major role in ecosystem processes, such as plant succession or in the process of litter decomposition, by governing the interplay between plant matter and soil organisms. 2.We reviewed the ecological role of the three main classes of secondary metabolites and the methodological challenges and novel avenues for their study. We highlight emerging general patterns of the impacts of secondary metabolites on decomposer communities and litter decomposition and argue for the consideration of secondary compounds as key drivers of soil functioning and ecosystem functioning. 3.Synthesis. Gaining a greater understanding of plant-soil organisms relationships and underlying mechanisms, including the role of secondary metabolites, could improve our ability to understand ecosystem processes. We outline some promising directions for future research that would stimulate studies aiming to understand the interactions of secondary metabolites across a range of spatio-temporal scales. Detailed mechanistic knowledge could help us to develop models for the process of litter decomposition and nutrient cycling in ecosystems and help us to predict future impacts of global changes on ecosystem functioning.

Background and Aims

Abandoned lands are increasingly used to establish fast-growing tree plantations, and are often rapidly colonized by a high density of herbaceous undergrowth. These weeds are generally removed since they compete with trees for resources, in particular soil nutrients. However, mixing herbaceous litter with the litter of planted trees could also stimulate the activity of decomposers and associated nutrient release due to an increase of litter quality (lower C:N ratio), plant diversity (more diverse litter traits) and water holding capacity. The objective was to determine the impact of herbaceous litter on the litter decomposition process of white spruce and hybrid poplar litters alone or in mixtures.

Methods

Litter mass loss rate, nutrient release and decomposer communities were monitored on single and mixed-species litters using litterbags during two years in three plantations types (hybrid poplar, white spruce and mixed plantations). Litters within mixtures were separated by species to identify species-specific responses of leaf mass loss.

Results

N release of all litter types increased with the presence of herbaceous litter. This finding could be linked to the greater abundance of decomposers and fungal biomass brought about by the herbaceous litter. Addition of herbaceous litter had no effect on spruce litter mass loss but had positive effects on poplar and mixed spruce/poplar litter mass loss. Abundance of fungi and mites was more affected by litter quality, whereas the abundance of collembola was more affected by the diversity of resources than by litter quality.

Conclusions

In these 10-year plantations with poplar, increased litter mass loss for poplar and mixed litters and N release associated to the presence of herbaceous litter showed that weeds may change soil C sequestration and N cycling.

Tree species influence the litter decomposition process by influencing litter quality and soil microclimate. Furthermore, over the long term, trees could promote soil communities that are particularly capable of degrading the litter they encounter most often. Thus, plant litter could decompose faster when placed in the habitat from which it was derived than in a foreign habitat, which has been termed home field advantage (HFA) of litter decomposition. In mixed-plant species environments however, it is not known whether a specific decomposer community under one tree species is affected by the presence of another tree species in the vicinity. To address this question, we tested if spruce and poplar litters showed HFA in mono-specific and in mixed species plantations under each tree species by reciprocally transplanting litter in the two plantation types. Decomposition rates, as well as the composition and ability of decomposer communities to degrade the different types of litter, were monitored during two years. Only spruce litter exhibited a faster decomposition rate at home. This HFA could be explained by higher abundance of decomposers. Furthermore, cellulose and poplar litter decomposed less or similarly in spruce plantations, suggesting that soil communities of that environment were capable of specifically degrading spruce litter. In mixed plantations, HFA was in the same direction as in mono-specific plantations, but was not as strong, indicating that HFA is sensitive to the surrounding plant community. Furthermore, this “mixed environment” had synergistic effects on decomposition rates under poplar trees. These ‘tree environment-specific’ results highlighted the possible importance of spatial distribution of each litter on decomposition rates in mixed stands. Thus, the influence of litter dispersal should be taken into account in future studies.

The use of trees under intensive management is particularly important for rapid fiber production in boreal regions. Mixed-species plantations using species that have complementary ecological niches, such as hybrid poplar and white spruce, potentially can maximize the use of resources and, consequently, increase productivity. In the context of climate change, vegetation and soil carbon sequestration is of a particular interest as part of a possible means of compensating for CO2 emissions. Since higher productivity leads to higher CO2 sequestration, the use of mixed-species plantations could improve the ecological service of carbon storage compared to mono-specific plantations. We compared above-ground and soil C storage of nine-year-old mono-specific plantations of white spruce and hybrid poplar with mixed plantations of these two species. Soil carbon was evaluated by separately sampling four soil horizons, while aboveground carbon was assessed from tree biomass estimates using allometric relationships. Mixing white spruce and hybrid poplar exerted a substantial synergistic effect on above-ground and soil carbon storage. This positive effect was due to greater productivity of poplar (47% of biomass increase) and great accumulation of litter in soil surface horizons (52% L-horizon carbon increase) of mixed-species compared to mono-specific plantations. These results imply that in addition to wood production gains by poplar trees, mixed-species plantations of hybrid poplar and white spruce promotes greater carbon sequestration than mono-specific plantations of either hybrid poplar or white spruce, an important aspect of forest ecosystem services.