Spatial patterns of seedlings dominated by proximity to deadwood and adult trees for Pinus flexilis and Pinus longaeva
The spatial patterns of trees are a defining feature of forests, yet because of the long lifespan of trees, the full origins of these spatial patterns remain unclear. We used 15.6 ha of a mapped forest plot to assess the local habitat characteristics and spatial structure of 1,550 naturally established seedlings and 1,955 adults of Pinus flexilis (limber pine) and Pinus longaeva (Great Basin bristlecone pine) in southern Utah, USA. For both species, proximity to deadwood (large end diameter ≥ 10 cm) and mature trees were key parameters of habitat, likely indicating abiotic or biotic facilitation. Both seedlings and mature trees were highly aggregated 0.0–2.5 m around large pieces of deadwood, which can persist in this ecosystem for centuries or millennia. Seedlings were most often located around deadwood 15–22 cm in diameter, though deadwood size had little influence on the distance at which seedlings established. Seedlings of both P. flexilis and P. longaeva were abundant near heterospecific adult Pinus. The elevational distribution of seedlings and adults of both species were not different, indicating a regeneration niche similar to the niche of mature trees and also that changing climate at this site has not yet affected species distributions. Low amounts of manganese and higher levels of pH and potassium in the mineral soil were associated with P. longaeva seedlings but were less important for P. flexilis. Edaphic patterns combined with the presence of deadwood define the overlap between seedling and mature species habitat and identify the key drivers influencing the spatial structure in this high-elevation, old-growth forest. Likely, persistence of these two Pinus species as well as regeneration or site colonization is more favorable in the presence of large deadwood.