Editorial: Natural tree seedling establishment and forest regeneration under climate change

Forests provide valuable ecosystem services including carbon storage, water capture and supply, and local climate regulation (Miura et al., 2015) but are increasingly threatened by climatic changes and associated disturbances such as forest fires, drought, and storms. Largescale mortality events have occurred in all forest biomes and are associated with hotter droughts (Hammond et al., 2022). The hotter, drier edge of species' ranges are becoming unsuitable in some landscapes, initiating conversion of forests to non-forested states (Davis et al., 2019).Natural regeneration is required for long-term forest survival and relies on successful seedling establishment. Many variables influence forest regeneration, including soil temperature, soil fertility, and the existence of favorable microsites. Newly emerged seedlings are vulnerable before their root system develops, whereas established seedlings and mature trees have carbon and water stores and are more resilient to episodic climatic events. A better understanding of the processes influencing seedling survival is needed for predicting changes in forest extent. This Research Topic of six articles explores the factors influencing seedling establishment and survival, from seedling to sapling to tree. The collection outlines critical knowledge gaps and advances our understanding of how forest regeneration may change in the future. These studies span boreal forests in Canada and Sweden, conifer forests in the western USA, and temperate and subtropical forests in Asia.In their review, Brodersen et al. outline a gap in forestry research -understanding the mechanisms by which climate, and other factors, inhibit seedling establishment at mountain forest boundaries. The upward elevational movement of timberlines depends on recruitment in favorable microsites facilitated by microtopography, existing vegetation, or inanimate objects (e.g., rocks, fallen logs). To enable cross-study comparisons, the authors proposed terminology to differentiate 'emergent seedlings' -the initial 1-2 years of growth when cotyledons are present -from 'established seedlings' that have developed their first set of leaves. They advocated for further research aimed at long-term monitoring of first-year seedling abundance and mortality at both upper and lower forest boundaries, identification of the nature and frequency of favorable microsites, and a better understanding of seedling ecophysiology.To help predict future forest distribution in fire-prone landscapes across the western USA, Rank et al. assessed soil surface temperature as a predictor of conifer seedling survival and forest regeneration potential. First, the authors extracted survival data from past laboratory experiments that exposed seedlings to elevated soil surface temperatures for varying durations. Next, they developed logistic models to predict survival of individual seedlings for different species, including species with very few observations. They found that the concept of a lethal temperature strongly depends on the duration of exposure, particularly from 45-60 °C. Their findings suggest soil surface temperature is a promising climate metric for characterizing the environmental boundaries suitable for seedling survival.index can be used for real-time monitoring of gross primary productivity (GPP) in subtropical and temperate monsoon regions of southwest China. They used satellite observations from 2000-2015 to assess the potential for large-scale monitoring of drought and GPP. At a monthly scale, SIF was more strongly correlated with GPP than traditional vegetation indices such as the normalized difference vegetation index (NDVI). In forests with complex canopy structure, monthly NDVI was relatively stable and failed to fully capture the effects of a winter drought event. In hot, wet regions that lack seasonal water limitations, SIF was a useful real-time monitoring tool for predicting GPP and monitoring drought effects over large spatial scales.Using an experimental approach, Marty et al. investigated the effects of soil warming and increased nitrogen deposition on soil organic N mineralization and tree growth in two eastern Canada boreal forests. In response to 9 years of soil warming (+2-4 °C) and canopy N addition (+0.30-0.35 kg N ha -1 yr -1 ), the soil was remarkably stable. Net soil N mineralization, soil chemistry and fertility, and soil organic matter quality were barely impacted, despite evidence that some of the added N reached the forest floor. Minor changes in seedling growth and foliar chemistry may have been driven by mycorrhizal fungi. The main effect of soil warming was to accelerate bud development and budburst, suggesting boreal seedlings may be more vulnerable to late spring frosts in the future.In boreal forests in Sweden, Jessen et al. investigated the effects of warming, both directly and indirectly through changes in understory vegetation, on growth and survival of seedlings. They planted 6400 seedlings and removed competing understory plants (feather moss and/or ericaceous shrubs), then installed passive open-top chambers to increase air temperature (+0.4 °C) at sites with varying successional status. Seedling growth and survival was affected by understory vegetation and successional status, but not warming. Seedling survival increased with feather moss removal, with the greatest benefit in old, late-successional forests with thick moss layers. For birch seedlings, this effect was canceled by shrub removal, possibly because shrubs protected young seedlings from herbivores. For pine species, growth of seedlings was promoted by both shrub and moss removal, suggesting facilitation among understory functional groups is highly species-specific. Ugawa et al. investigated the effects of elevated CO2 and/or O3 on the growth and composition of organic constituents of stems in planted two-year-old seedlings of Japanese oak. Elevated CO2 (550 ppm) increased leaf and stem growth, plus structural stem components such as holocellulose, while decreasing the extractive content of stems by 21%. Extractive compounds such as tannins, flavonoids, and terpenoids are non-structural but have an antimicrobial function that inhibits biodegradation. Seedling growth and organic constituents were not sensitive to elevated O3 (twice ambient), but O3 reduced the influence of elevated CO2. Their findings suggest elevated CO2 and O3 may lead to faster decomposition of Japanese oak and altered C cycling of temperate forests in East Asia. This collection identified potential approaches for monitoring forest productivity and better understanding and predicting forest distribution. Other studies highlighted that many interacting environmental factors, such as warming, nitrogen deposition, and changing atmospheric gas concentrations, affect seedling survival. Filling the remaining research gaps will help to better understand forest regeneration and safeguard forest ecosystems.

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