Early life history transitions and recruitment of Picea mariana in thawed boreal permafrost peatlands

Abstract

Black spruce (Picea mariana) is the most abundant tree species in the boreal biome, but little is known about how climate warming may change recruitment in peatlands, especially those affected by permafrost thaw. We used results from a seven-year study in northern Manitoba, Canada, to address the following questions: (1) What is the relative importance of early life history transitions on P. mariana recruitment? (2) How are these transitions mediated by biological and environmental factors, including competition, facilitation, disease, herbivory, water table depth, and soil nutrients? (3) Do interactions among these factors create additional recruitment limitations beyond those imposed by environmental factors changing with climate warming, such as hydrology? Seed rain was measured over six years on forested permafrost plateaus and in neighboring collapse scar bogs. Seed germination and seedling survival and growth were measured over 4-5 years in collapse scars and assessed across a three-level water table treatment. Survival and growth experiments examined additional combinations of above- and belowground vascular plant competition and fertilizer addition. Results showed that failure of germination and survival on growing moss surfaces and reduced survival of seedlings in wetter microsites were primary constraints. Seed influx was significantly lower in collapse scars but likely did not limit recruitment. Biological and environmental factors mediating these life history transitions also differed in relative importance, and interactions among them tended to amplify recruitment limitation. Seedling survival was most strongly controlled by fast-growing mosses in wet microsites but also was influenced by apparent drowning in wet plots, herbivory, and loss of foliage caused by a fungal pathogen. Seedling growth was strongly controlled by water table depth, nutrient and competition levels, and fungal pathogens. Multiple, interacting factors will affect P. mariana establishment in boreal peatlands during climate warming. Generalizations about recruitment relying on few environmental gradients sensitive to climate change, such as water table, may therefore not fully capture the complexities of establishment.

Positive shrub–tree interactions facilitate woody encroachment in boreal peatlands

“…In addition to amelioration of abiotic stress directly and reduced moss competition indirectly, shrubs may facilitate tree seedlings by reducing herbivore damage (Camill et al . ).…”

“…However, recent field and greenhouse experiments have demonstrated a complex interplay of positive and negative interactions between vascular plants and Sphagnum mosses (Camill et al . ; Pouliot et al . ).…”

Permafrost thaw and wildfire: Equally important drivers of boreal tree cover changes in the Taiga Plains, Canada

“…Poorly drained landscapes are generally characterized by ice‐rich permafrost leading to larger surface subsidence upon thaw compared to less ice‐rich, well‐drained landscapes [ Jorgenson et al ., ; Hinzman et al ., ]. Surface subsidence and subsequent saturation of soils have been identified as an important cause of mortality and reduced recruitment of black spruce in the northern boreal forest [ Osterkamp et al ., ; Camill et al ., ; Baltzer et al ., ]. Therefore, we argue that permafrost thaw and disappearance mainly cause these observed reductions in PTC.…”

“…Within the same permafrost zones, declining PTC was only observed for poorly drained landscapes, while well‐drained landscapes were characterized by increasing PTC. Enhanced drought intensities and frequencies could induce PTC declines across all drainage classes [ Beck et al ., ; Walker and Johnstone , ] or improve growth conditions in poorly drained landscapes by ameliorating anoxic stress [e.g., Camill et al ., ; Baltzer et al ., ]. Small ΔPTC in the zone with no permafrost further indicate that permafrost thaw is mainly causing PTC declines at its southern limit.…”

Do plant traits explain tree seedling survival in bogs?

“…In contrast, factors limiting shoot growth, such as foliage loss by fungal pathogens or herbivory (Camill et al . ), may increase the likelihood of seedlings being overgrown by moss. Once seedlings have reached a safe height above the moss surface, root competition with shallow‐rooting herbs or shrubs may likely further take its toll (Sarkkola et al .…”