Determinants of conifer distributions across peatland to forest gradients in the coastal temperate rainforest of southeast Alaska

Abstract

Wetland determination relies on assumptions that site hydrologic and edaphic conditions limit plant species to certain environments. For example, using species' wetland indicator status for wetland determination assumes that tolerance of wetland conditions best explains distributional patterns. However, abiotic and biotic factors often interact to create complex plant responses across different environments. To evaluate these interactions, we used a hydrologic gradient in the coastal temperate rainforest of southeast Alaska to (1) quantify the primary determinants of conifer distributions, (2) identify thresholds in environmental factors limiting species' success and (3) assess current wetland indicator status of local conifers (Pinus contorta, Picea sitchensis, and Tsuga heterophylla). Data were collected using a hierarchical sampling scheme and analyzed within a Bayesian framework. Topography and hydrologic regime were the primary determinants of distributional patterns, but species were limited by specific microsite factors. Competitively dominant P. sitchensis occurred where hydrology, pH, and nitrogen were most favorable for establishment, while stress-tolerant P. contorta was competitively excluded from these sites. Tsuga heterophylla occurred across the gradient but took advantage of drier conditions, which promoted biomass accumulation. Tree distributions were limited by the interaction between abiotic and biotic factors rather than by abiotic tolerance alone. This knowledge of local and regional drivers of species' distributions and the relative importance of interacting abiotic and biotic drivers provide critical information for land management and regulation. Wetland delineation procedures can be improved through application of the regional empirical limits identified for plant species, as implemented and addressed in this study. Figure 2. Representative National Wetland Inventory (NWI) ecosystem types in the coastal temperate rainforest of southeast Alaska, including palustrine emergent (a), palustrine scrub-shrub (b), palustrine forested (c), and upland (d) sites.

From canopy to seed: Loss of snow drives directional changes in forest composition

“…Local hydrologic regime and associated ecosystem productivity are key factors in both species’ distributions (Bisbing et al, ) and in the extent of C. nootkatensis decline (D'Amore & Hennon, ). Mortality across this plot network was most extensive on wetter, lower‐productivity peatlands; however, proportional mortality was greatest in productive upland forests.…”

“…Sphagnum spp. are common in areas of poor drainage and low forest productivity, decreasing in abundance with increasing slope and increasing depth to groundwater (Bisbing, Cooper, D'Amore, & Marshall, ; Neiland, ).…”

“…Callitropsis nootkatensis is locally distributed across the NPCTR's hydrologic gradient from emergent wetlands to upland forests, which corresponds to a gradient of low to high forest productivity (Hennon et al, , ). This gradient drives the distribution, abundance, and biomass of the region's dominant tree species (Bisbing et al, ), with low‐lying saturated peatlands limiting the success of most species but providing low‐competition environments for stress‐tolerant species, such as P. contorta (Bisbing et al, ) and C. nootkatensis (Caouette et al, ; Hennon, Hansen, et al, ). Background mortality rates in healthy C. nootkatensis forests average <25% (Hennon, Hansen, et al, ).…”

“…Obtaining information on life stage response to climate-induced mortality, postdecline community composition, and stand dynamics in the NPCTR and beyond will be essential to scientifically based forest management and vital to supporting conservation efforts in the face of climate change. are common in areas of poor drainage and low forest productivity, decreasing in abundance with increasing slope and increasing depth to groundwater (Bisbing, Cooper, D'Amore, & Marshall, 2016;Neiland, 1971).…”

Spatial aspects of structural complexity in Sitka spruce – western hemlock forests, including evaluation of a new canopy gap delineation method

“…Extreme structural variants of edaphic gaps also occur that prevent the development of 384 a closed canopy. For example, low-productivity palustrine forested wetlands-sites that support 385 development of spruce-hemlock forests but where a shallow depth to the water table limits 386 productivity (Bisbing et al 2015), sometimes preventing development of a closed canopy.…”