Land-cover changes and new ecosystem trajectories in Interior Alaska have altered the structure and function of landscapes, with regional warming trends altering carbon and water cycling. Notably, these changes include the increased distribution of tall woody vegetation, trees and shrubs, in landscapes that historically only supported low shrub vegetation cover. In Denali National Park, Alaska, this phenomenon has altered primary succession pathways towards tundra ecosystems with the establishment and expansion of balsam poplar (Populus balsamifera) trees. In this study, we examine how snow, soil, and vegetation processes interact within this altered successional pathway towards further landscape change following glacial recession. In a sequence of outflow terraces, we found that variations in snow depth, functional soil depth, leaf area index, overstory height, and understory height were all significantly correlated with each other, with those effects largely explained by the presence of poplar. Poplar-dominated plots had deeper snowpacks, deeper functional soil depths, taller overstory and shrub heights, and greater LAI than in non-poplar plots of the same landscape age. These findings suggest a feedback cycle where the establishment of taller vegetation (here, poplar) alters ecosystem processes in the following notable ways: taller vegetation is able to trap more snow by reducing wind exposure and limiting sublimation; this snow provides water through additional snowmelt and insulation, keeping soils warmer and lessening permafrost development, leading to deeper functional soil depths. This feedback demonstrates poplar's ability to modify the environment as an ecosystem engineer, engineering a trajectory away from the otherwise expected permafrost-underlain tundra.

Haloxylon ammodendron is a typical tree species in arid region for windbreak and sand fixation. Understanding how stand structure (i.e., afforestation density, forest type, and stand age) of H. ammodendron impacts ecological benefits and tree growth status has great implications for desertification control in arid area. We obtained a dataset of 446 observations from 79 studies related to H. ammodendron plantations for meta-analysis. Stand age was the key factor affecting performances of windbreak, tree canopy, and soil properties while afforestation density was more important in determining tree survival rate. Wind speed in H. ammodendron plantations was reduced by 52% in general compared to that at the site without any plantations and decreased as the increasing density. With the increase of stand age, soil carbon, nitrogen, and phosphorus accumulated continuously, but soil moisture in 0-60 cm depth decrease when trees grew up to nearly 20 years. Within 4000 plants ha(-1), responses of survival rate, tree height, ground diameter, and canopy decreased with the increase of afforestation density. Compared with mono-species plantation, mixed afforestation was more beneficial to windbreak and survival, while it damaged tree growth. Combining the ecological functions and tree growth of H. ammodendron plantations, the afforestation density and forest age should be controlled to 450-900 plants ha(-1) and 20 years, respectively, as the upper limit. Therefore, besides reasonable afforestation density, management measures for regulations of forest age and existing stand density were important to maintain higher ecological benefits and better tree growth of H. ammodendron plantations.