Hugh A. L. Henry

Cold truths: how winter drives responses of terrestrial organisms to climate change

Winter is a key driver of individual performance, community composition, and ecological interactions in terrestrial habitats. Although climate change research tends to focus on performance in the growing season, climate change is also modifying winter conditions rapidly. Changes to winter temperatures, the variability of winter conditions, and winter snow cover can interact to induce cold injury, alter energy and water balance, advance or retard phenology, and modify community interactions. Species vary in their susceptibility to these winter drivers, hampering efforts to predict biological responses to climate change. Existing frameworks for predicting the impacts of climate change do not incorporate the complexity of organismal responses to winter. Here, we synthesise organismal responses to winter climate change, and use this synthesis to build a framework to predict exposure and sensitivity to negative impacts. This framework can be used to estimate the vulnerability of species to winter climate change. We describe the importance of relationships between winter conditions and performance during the growing season in determining fitness, and demonstrate how summer and winter processes are linked. Incorporating winter into current models will require concerted effort from theoreticians and empiricists, and the expansion of current growing‐season studies to incorporate winter.

Coordinated distributed experiments: an emerging tool for testing global hypotheses in ecology and environmental science

There is a growing realization among scientists and policy makers that an increased understanding of todays environmental issues requires international collaboration and data synthesis. Meta-analyses have served this role in ecology for more than a decade, but the different experimental methodologies researchers use can limit the strength of the meta-analytic approach. Considering the global nature of many environmental issues, a new collaborative approach, which we call coordinated distributed experiments (CDEs), is needed that will control for both spatial and temporal scale, and that encompasses large geographic ranges. Ecological CDEs, involving standardized, controlled protocols, have the potential to advance our understanding of general principles in ecology and environmental science.

Worldwide evidence of a unimodal relationship between productivity and plant species richness

Grassland diversity and ecosystem productivity The relationship between plant species diversity and ecosystem productivity is controversial. The debate concerns whether diversity peaks at intermediate levels of productivity—the so-called humped-back model—or whether there is no clear predictable relationship. Fraser et al. used a large, standardized, and geographically diverse sample of grasslands from six continents to confirm the validity and generality of the humped-back model. Their findings pave the way for a more mechanistic understanding of the factors controlling species diversity. Science, this issue p. 302 The humped-back model of plant species diversity is confirmed by a global grassland survey. The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.

Interactive effects of lateral shade and wind on stem allometry, biomass allocation, and mechanical stability in Abutilon theophrasti (Malvaceae)

The effects of lateral shade and wind on stem allometry, whole-plant biomass allocation, and mechanical stability were examined for Abutilon theophrasti in a fully factorial glasshouse experiment. Lateral shade from neighboring plants increased stem height by 33% relative to control plants grown individually, despite a decrease in plant dry mass. Intermittent wind decreased stem height by 18% in unshaded plants, but by only 3% in shaded plants. Surprisingly, both lateral shade and wind caused decreases in stem diameter, even with diameter controlled for height, resulting in low diameter : height ratios in wind-treated plants relative to untreated plants. Under shade, wind-treated plants had higher root allocation than untreated plants, which allowed wind-treated shade plants to compensate for a low diameter : height ratio. This did not occur in the absence of shade, where stem tissue density and root allocation of wind-treated plants did not exceed that of untreated plants. Nevertheless, wind-treated plants experienced low drag relative to untreated plants due to a lower leaf area. Consequently, stem deflections of wind-treated plants did not exceed those of untreated plants at any given windspeed. Our results document a complex interaction between shade and wind on plant morphology and suggest that the nature of this interaction is generally that lateral shade acts to reduce or eliminate thigmomorphogenic responses.

Additive Effects of Warming and Increased Nitrogen Deposition in a Temperate Old Field: Plant Productivity and the Importance of Winter

Both climate warming and atmospheric nitrogen (N) deposition are predicted to alter plant productivity and species composition over the next century. However, the extent to which their effects may interact is unclear. For example, over winter, the effects of warming on soil freezing dynamics may promote ecosystem N losses, thereby limiting increases in productivity in response to warming, yet these losses may be compensated for by increased N deposition. We measured plant production and species composition in response to warming (winter-only or year-round) and N addition in a temperate old field. We used shoot allometric relationships to estimate aboveground production non-destructively and sampled root biomass destructively throughout two growing seasons. We also used spectral data (normalized difference vegetation index—NDVI) to examine the treatment effects on the timing of plant green-up and senescence. In 2007, which featured an exceptionally dry summer, there were no treatment effects on plant growth. However, in 2008, warming (both winter-only and year-round) and N addition combined approximately doubled aboveground productivity, and these effects were additive. Warming increased root biomass, but no N effect was evident. Conversely, N addition increased NDVI, but NDVI was unresponsive to warming. Overall, our results do not support the hypothesis that warming-induced changes to soil freezing dynamics limit plant productivity in our system. On the contrary, they demonstrate that winter warming alone can increase primary productivity to the same extent as year-round warming, and that this effect may interact very strongly with interannual variation in precipitation.

Frost damage and winter nitrogen uptake by the grass Poa pratensis L.: consequences for vegetative versus reproductive growth

Frost damage can decrease nitrogen uptake by grasses over winter, and it can also decrease biomass production over the following growing season. However, it is not clear to what extent reduced nitrogen uptake over winter decreases grass production, or whether is it merely a symptom of root damage. We examined the growth response of the grass Poa pratensis L. (Kentucky bluegrass) to variation in the timing of freezing and nitrogen availability over winter in London, Ontario, Canada. All tillers were transplanted into untreated soil in early spring, and at peak seed maturation, root, shoot, and reproductive biomass were measured. There was an interaction between freezing and increased winter nitrogen availability, whereby nitrogen addition increased tiller biomass under ambient temperatures, but decreased tiller biomass in combination with a late winter freeze. The nitrogen response of ambient temperature tillers occurred primarily via increased seed production, whereas for frozen tillers seed production was generally absent. Our results support the hypothesis that nitrogen uptake over winter can increase growing season productivity in P. pratensis, but also demonstrate that increased nitrogen availability increases tiller vulnerability to frost. These results have important implications for grass responses to the alteration of soil freezing dynamics with climate change.

Plant responses to climatic extremes: within-species variation equals among-species variation.

Within-species and among-species differences in growth responses to a changing climate have been well documented, yet the relative magnitude of within-species vs. among-species variation has remained largely unexplored. This missing comparison impedes our ability to make general predictions of biodiversity change and to project future species distributions using models. We present a direct comparison of among- versus within-species variation in response to three of the main stresses anticipated with climate change: drought, warming, and frost. Two earlier experiments had experimentally induced (i) summer drought and (ii) spring frost for four common European grass species and their ecotypes from across Europe. To supplement existing data, a third experiment was carried out, to compare variation among species from different functional groups to within-species variation. Here, we simulated (iii) winter warming plus frost for four grasses, two nonleguminous, and two leguminous forbs, in addition to eleven European ecotypes of the widespread grass Arrhenatherum elatius. For each experiment, we measured: (i) C/N ratio and biomass, (ii) chlorophyll content and biomass, and (iii) plant greenness, root (15) N uptake, and live and dead tissue mass. Using coefficients of variation (CVs) for each experiment and response parameter, a total of 156 within- vs. among-species comparisons were conducted, comparing within-species variation in each of four species with among-species variation for each seed origin (five countries). Of the six significant differences, within-species CVs were higher than among-species CVs in four cases. Partitioning of variance within each treatment in two of the three experiments showed that within-species variability (ecotypes) could explain an additional 9% of response variation after accounting for the among-species variation. Our observation that within-species variation was generally as high as among-species variation emphasizes the importance of including both within- and among-species variability in ecological theory (e.g., the insurance hypothesis) and for practical applications (e.g., biodiversity conservation).

Soil freezing and N deposition: transient vs multi‐year effects on plant productivity and relative species abundance

Plant responses to increased atmospheric nitrogen (N) deposition must be considered in the context of a rapidly changing climate. Reductions in snow cover with climate warming can increase the exposure of herbaceous plants to freezing, but it is unclear how freezing damage may interact with increased N availability, and to what extent freezing effects may extend over multiple years. We explored potential interactions between freezing damage and N availability in the context of plant productivity and relative species abundance in a temperate old field using both snow removal and mesocosm experiments, and assessed the legacy effects of the freezing damage over 3 yr. As expected, N addition increased productivity and freezing damage decreased productivity, but these factors were nonadditive; N addition increased productivity disproportionately in the snow removal plots, whereas extreme freezing diminished N addition responses in the mesocosm experiment. Freezing altered relative species abundances, although only the most severe freezing treatments exhibited legacy effects on total productivity over multiple growing seasons. Our results emphasize that while both increased N deposition and freezing damage can have multi-year effects on herbaceous communities, the interactions between these global change factors are contingent on the intensities of the treatments.

Multi-year and short-term responses of soil ammonia-oxidizing prokaryotes to zinc bacitracin, monensin, and ivermectin, singly or in combination

A field experiment was initiated whereby a series of replicated plots received annual applications of ivermectin, monensin, and zinc bacitracin, either singly or in a mixture. Pharmaceuticals were added at concentrations of 0.1 mg/kg soil or 10 mg/kg soil. The authors collected soil samples in 2013, before and after the fourth annual application of pharmaceuticals. In addition, a 30-d laboratory experiment was undertaken with the same soil and same pharmaceuticals, but at concentrations of 100 mg/kg soil. The impact of the pharmaceuticals on nitrification rates, on the abundance of ammonia-oxidizing bacteria (AOB), and on the abundance of ammonia-oxidizing archaea (AOA) was assessed. None of the pharmaceuticals at 0.1 mg/kg had any effect on nitrification. Referenced to control soil, nitrification was accelerated in soil exposed to 100 mg/kg zinc bacitracin or 10 mg/kg of the pharmaceutical mixture, but none of the treatments inhibited nitrification. Neither AOB abundance nor AOA abundance was affected by the pharmaceuticals at 0.1 mg/kg. At 10 mg/kg, monensin, zinc bacitracin, and a mixture of all 3 pharmaceuticals suppressed the abundance of AOB, and zinc bacitracin and the mixture increased AOA abundance. The decrease in AOB abundance and increase in AOA abundance when exposed to 10 mg/kg soil suggests that AOB are more sensitive to these chemicals and that AOA populations can expand to occupy the partially vacated niche.

Soil Freezing Dynamics in a Changing Climate: Implications for Agriculture

Soil freezing can affect winter cereals and perennial crops directly over winter, and it can affect all crops indirectly by modifying soil physical structure, soil moisture, microbial communities, and weed growth over the following growing season. Although climate warming is expected to increase mean air temperatures over winter, changes in snow cover and extreme temperature events could complicate the responses of soil freezing dynamics to climate change. I examined projections of soil freezing responses to climate change obtained using a variety of modeling, observational, and hindcasting approaches. Overall, despite a general pattern of a decreased numbers of days of frozen soil and decreased numbers of days with snow on the ground, projected responses of soil freezing dynamics to climate change have been regional in nature. Specifically, although southern temperate regions that currently experience mild winters will likely cease to experience soil freezing in some years, many northern temperate regions may experience an increased number of soil freeze-thaw cycles (FTC) as a result of reduced snow cover. Nevertheless, from the standpoint of agricultural impacts of soil freezing, an increased number of soil FTC may be of little consequence if it is not also accompanied by changes in the timing, severity or depth of soil freezing. The latter factors may be influenced strongly by changes in the timing of precipitation, which affects snow cover. The increased occurrence of extreme warming events may also affect the timing and strength of plant cold acclimation and deacclimation responses. The management of snow and plant litter cover may play an important role in modulating interactions between climate change and soil freezing responses in some systems.