Reconciling niches and neutrality in a subalpine temperate forest

Abstract

The Unified Neutral Theory of Biodiversity has been put forth to explain species coexistence in forests worldwide, but its assumption of species equivalence has been met with much debate. Theoretical advancements have reconciled the opposing concepts of neutral and niche theories as two ends of a continuum, improving our understanding of global patterns in diversity and community assembly. However, the relative importance of niche and neutral processes remains understudied in temperate forests. To determine the balance of niche and neutral processes in climatically limited subalpine temperate forests, we established the Utah Forest Dynamics Plot, a 13.64‐ha plot comprising 27,845 stems ≥1 cm diameter at breast height (1.37 m) representing 17 species at 3100 m elevation on the Colorado Plateau. We examined the fit of niche‐ and neutral‐based models to the species abundance distribution (SAD), and tested three underlying assumptions of neutral theory. The neutral model was a poor fit to the SAD, but we did not find the alternative model to provide a better fit. Using spatial analyses, we tested the neutral assumptions of functional equivalence, ecological equivalence, and habitat generality. Half of species analyzed were characterized by non‐neutral recruitment processes, and the two most abundant species exhibited asymmetric competitive and facilitative interactions with each other. The assumption of habitat generality was strongly contradicted, with all common species having habitat preferences. We conclude niche‐based processes play the dominant role in structuring subalpine forest communities, and we suggest possible explanations for variation in the relative importance of niche vs. neutral processes along ecological gradients.

Global importance of large‐diameter trees

“…With large plot sizes and global distribution, ForestGEO is uniquely suited to capture structural variation (i.e., the heterogeneity in the abundance of trees of all diameter classes). The relatively large area required (6.5 ha, on average) to estimate biomass to within 5% of the entire plot value reinforces conclusions that the distribution of largediameter trees is not homogeneous within forests (e.g., Table 2; Furniss, Larson, & Lutz, 2017;Lutz et al, 2012Lutz et al, , 2013. We note that this…”

Genetic and Spatial Structuring of Populus tremuloides in a Mixed-Species Forest of Southwestern Utah, USA

“…The UFDP is a 13.64-ha forest dynamics plot affiliated with the Smithsonian ForestGEO network (Anderson-Teixeira et al 2015, Lutz 2015, Lutz et al 2018b) within which all 23,177 live woody stems ≥1 cm diameter at breast height (dbh; 1.37 along the main stem) have been identified, mapped, and tagged (Furniss et al 2017). The UFDP has 8 habitat types differentiated by soil parent material and topographic position (Furniss et al 2017), with all 8 habitat types showing positive or negative associations with the principal woody species (Table S3 in Furniss et al 2017). Approximately 35% of the UFDP has a relatively uniform density of larger-diameter (≥25 cm dbh) Populus tremuloides, but the remaining 65% of the plot is mixed-species forest interspersed with aspen ( Fig.…”

“…In the summer of 2016, we performed a stratified random sampling of 96 out of the 2742 total Populus tremuloides stems ≥1 cm dbh within the UFDP. Using the locations of the 2742 stems, we sampled from across the range of slopes, aspects, and plant communities identified in Furniss et al (2017) to achieve a representative sample of the distribution of the species in the UFDP. Sampled trees were selected to represent ten 5-cmdiameter classes (first diameter class 1 cm ≤ dbh < 5 cm).…”

“…Habitat types were defined by Furniss et al (2017) based on geology (derived from Rowley et al 2013) and aspect. Geology was used to delineate habitat types because there is a high degree of geologic diversity within the plot Fig.…”

Determinants of spatial patterns of canopy tree species in a tropical evergreen forest in Gabon

“…The inhomogeneous Cox processes were fitted using the two-step procedure described by Waagepetersen and Guan (2009) using the minimum contrast method based on the PCF to fit the Cox process. Goodness-of-fit of the observed point patterns to the models was tested using the Diggle-Cressie-Loosmore-Ford (DCLF) test based on Monte Carlo simulations of the model because differently structured tests of spatial patterns may identify significant patterns (Furniss, Larson, & Lutz, 2017;Harms et al, 2001).…”