Abstract
AbstractThe 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.
Highlights
One of the most notable contemporary ecological theories is the Unified Neutral Theory of Biodiversity, which posits ecological equivalence between trophically similar species and assumes neutrality as a first approximation for investigating ecological processes (Hubbell 2001)
We propose three possible explanations for the lack of support of neutral theory in our study: (1) The spatial scale of this study was not sufficient to capture the true neutral nature of subalpine forest systems; (2) past disturbance has shifted this forest out of a neutral state, but it would eventually conform to neutral models given enough time; and (3) neutral theory is not a practical model for subalpine forest ecosystems
The first two points consider spatial and temporal scale as important factors determining the relative importance of niche and neutral processes
Summary
One of the most notable contemporary ecological theories is the Unified Neutral Theory of Biodiversity, which posits ecological equivalence between trophically similar species and assumes neutrality as a first approximation for investigating ecological processes (Hubbell 2001). While studies in temperate grasslands (Fargione et al 2003, Adler 2004, Harpole and Tilman 2006) and temperate forest understories (Gilbert and Lechowicz 2004, Kern et al 2012) have found support for the dominance of niche processes, studies that consider temperate forests have found contradictory results (Shibata et al 2010, Wang et al 2011, Myers et al 2013, Masaki et al 2015, Qiao et al 2015) This lack of clarity may be partially explained by the absence of studies that directly address the niche–neutrality gradient in climatically extreme temperate environments where the contrast between temperate and tropical forests is most evident. We assess the relative importance of niche and neutral processes in temperate forests by testing a prediction and three fundamental assumptions of neutral theory in a high-elevation subalpine ecosystem near the altitudinal limit of forest cover at an extreme end of the temperate–tropical ecological gradient
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