Nonlinear response of stream ecosystem structure to low‐level phosphorus enrichment

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Impacts of multiple extreme winter warming events on sub‐Arctic heathland: phenology,reproduction, growth,and CO2 flux responses

Extreme weather events can have strong negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme, short‐lived, winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (for instance, 2–10 °C for 2–14 days) but upon return to normal winter climate exposes the ecosystem to much colder temperatures due to the loss of insulating snow. Single events have been shown to reduce plant reproduction and increase shoot mortality, but impacts of multiple events are little understood as are the broader impacts on community structure, growth, carbon balance, and nutrient cycling. To address these issues, we simulated week‐long extreme winter warming events – using infrared heating lamps and soil warming cables – for 3 consecutive years in a sub‐Arctic heathland dominated by the dwarf shrubs Empetrum hermaphroditum, Vaccinium vitis‐idaea (both evergreen) and Vaccinium myrtillus (deciduous). During the growing seasons after the second and third winter event, spring bud burst was delayed by up to a week for E. hermaphroditum and V. myrtillus, and berry production reduced by 11–75% and 52–95% for E. hermaphroditum and V. myrtillus, respectively. Greater shoot mortality occurred in E. hermaphroditum (up to 52%), V. vitis‐idaea (51%), and V. myrtillus (80%). Root growth was reduced by more than 25% but soil nutrient availability remained unaffected. Gross primary productivity was reduced by more than 50% in the summer following the third simulation. Overall, the extent of damage was considerable, and critically plant responses were opposite in direction to the increased growth seen in long‐term summer warming simulations and the ‘greening’ seen for some arctic regions. Given the Arctic is warming more in winter than summer, and extreme events are predicted to become more frequent, this generates large uncertainty in our current understanding of arctic ecosystem responses to climate change.     

Interactive responses of old‐field plant growth and composition to warming and precipitation

As Earth's atmosphere accumulates carbon dioxide (CO₂) and other greenhouse gases, Earth's climate is expected to warm and precipitation patterns will likely change. The manner in which terrestrial ecosystems respond to climatic changes will in turn affect the rate of climate change. Here we describe responses of an old‐field herbaceous community to a factorial combination of four levels of warming (up to 4 °C) and three precipitation regimes (drought, ambient and rain addition) over 2 years. Warming suppressed total production, shoot production, and species richness, but only in the drought treatment. Root production did not respond to warming, but drought stimulated the growth of deeper (> 10 cm) roots by 121% in 1 year. Warming and precipitation treatments both affected functional group composition, with C₄ grasses and other annual and biennial species entering the C₃ perennial‐dominated community in ambient rainfall and rain addition treatments as well as in warmed treatments. Our results suggest that, in this mesic system, expected changes in temperature or large changes in precipitation alone can alter functional composition, but they have little effect on total herbaceous plant growth. However, drought limits the capacity of the entire system to withstand warming. The relative insensitivity of our study system to climate suggests that the herbaceous component of old‐field communities will not dramatically increase production in response to warming or precipitation change, and so it is unlikely to provide either substantial increases in forage production or a meaningful negative feedback to climate change later this century.     

Responses of phenology and biomass production of boreal fens to climate warming under different water‐table level regimes

Climate change affects peatlands directly through increased air temperatures and indirectly through changes in water‐table level (WL). The interactions of these two still remain poorly known. We determined experimentally the separate and interactive effects of temperature and WL regime on factors of relevance for the inputs to the carbon cycle: plant community composition, phenology, biomass production, and shoot:root allocation in two wet boreal sedge‐dominated fens, “southern” at 62°N and “northern” at 68°Ν. Warming (1.5°C higher average daily air temperature) was induced with open‐top chambers and WL drawdown (WLD; 3–7 cm on average) by shallow ditches. Total biomass production varied from 250 to 520 g/m², with belowground production comprising 25%–63%. Warming was associated with minor effects on phenology and negligible effects on community composition, biomass production, and allocation. WLD clearly affected the contribution of different plant functional types (PFTs) in the community and the biomass they produced: shrubs benefited while forbs and mosses suffered. These responses did not depend on the warming treatment. Following WLD, aboveground biomass production decreased mainly due to reduced growth of mosses in the southern fen. Aboveground vascular plant biomass production remained unchanged but the contribution of different PFTs changed. The observed changes were also reflected in plant phenology, with different PFTs showing different responses. Belowground production increased following WLD in the northern fen only, but an increase in the contributions of shrubs and forbs was observed in both sites, while sedge contribution decreased. Moderate warming alone seems not able to drive significant changes in plant productivity or community composition in these wet ecosystems. However, if warming is accompanied by even modest WL drawdown, changes should be expected in the relative contribution of PFTs, which could lead to profound changes in the function of fens. Consequently, hydrological scenarios are of utmost importance when estimating their future function.