COS 124-2 - Spatial associations of tree seedlings and other forest plants form complex interaction networks along climate-edaphic gradients of the northeastern United States.

Abstract: Tree species are expected to migrate along climate gradients to higher latitudes and elevations as climate warms. Early evidence supports these trends for some species but not for others, suggesting that our understanding of the mechanisms behind tree migrations is still rudimentary. Since tree seedling establishment, survival, and growth is required for range expansion, experimental studies have often investigated how climate affects survival, growth, and interspecific competitive interactions of tree seedlings. While in many forests tree seedlings are more likely to experience competitive or facilitative interactions with understory plants that are not tree seedlings, studies of such interactions along climatic gradients are rare. To address this gap, we investigated how tree seedling abundance (by species) and their associations with other plant species vary with forest plant compositional changes along sharp climate-edaphic gradients.

We surveyed forest vegetation along elevational gradients on ten mountains from low-elevation temperate deciduous forests to high-elevation conifer forests (500 to 1,000m above sea level) in the Northeastern U.S. Understory plants and tree seedlings were surveyed on 1,710 plots in total (each 1x1m large). We surveyed forest canopy structure and soil depth (to cobble) in the field and characterized climate using PRISM data. Compositional changes with elevation were examined with non-metric ordination (NMS), PERMANOVA, and Indicator species analysis (ISA). Pairwise species associations were examined with scatter plots matrices and correlation coefficients across the full gradient and separately in low- and high-elevation forests. Each species was characterized by the number of significant associations.

Tree seedling abundance and associations with other plant species varied primarily with elevation, temperature, and precipitation, and secondarily with canopy openness. Importantly, tree seedling associations with other plant species were more numerous (overall and by species) in more species rich low-elevation forests and less common (and more positive) in high-elevation conifer forests. Within the same broad climatic zone (e.g., in low- or high- elevations), different tree seedling species established in different microhabitats and interacted with different sets of plant species. Thus, tree seedling establishment and early survival occurs within a complex web of relationships with other plants and the entire forest plant community needs to be considered when predicting or managing climate-induced tree migrations. While we identified specific positive and negative associations of tree seedlings with other plants, experiments are needed to test how these associations relate to plant interactions such as competition or facilitation.