Cristina Castanha

The whole-soil carbon flux in response to warming

Warming the top meter increased soil CO2 production by 37%, with 40% of the response coming from soils more than 15 centimeters deep. Digging deeper into soils Soils contain about twice as much carbon as Earths atmosphere, so their response to warming is crucial to understanding carbon fluxes in a changing climate. Past studies have heated soil to a depth of 5 to 20 cm to examine such fluxes. Hicks Pries et al. heated the ground to a depth of 100 cm. Extending measurements to that depth revealed that 4°C of warming increased annual soil respiration by 34 to 37%—a considerable amount more than previously observed. Science, this issue p. 1420 Soil organic carbon harbors three times as much carbon as Earth’s atmosphere, and its decomposition is a potentially large climate change feedback and major source of uncertainty in climate projections. The response of whole-soil profiles to warming has not been tested in situ. In a deep warming experiment in mineral soil, we found that CO2 production from all soil depths increased with 4°C warming; annual soil respiration increased by 34 to 37%. All depths responded to warming with similar temperature sensitivities, driven by decomposition of decadal-aged carbon. Whole-soil warming reveals a larger soil respiration response than many in situ experiments (most of which only warm the surface soil) and models.

Conifer seedling recruitment across a gradient from forest to alpine tundra: effects of species, provenance, and site

Background: Seedling germination and survival is a critical control on forest ecosystem boundaries, such as at the alpine–treeline ecotone. In addition, while it is known that species respond individualistically to the same suite of environmental drivers, the potential additional effect of local adaptation on seedling success has not been evaluated. Aims: To determine whether local adaptation may influence the position and movement of forest ecosystem boundaries, we quantified conifer seedling recruitment in common gardens across a subalpine forest to alpine tundra gradient at Niwot Ridge, Colorado, USA. Methods: We studied Pinus flexilis and Picea engelmannii grown from seed collected locally at High (3400 m a.s.l.) and Low (3060 m a.s.l.) elevations. We monitored emergence and survival of seeds sown directly into plots and survival of seedlings germinated indoors and transplanted after snowmelt. Results: Emergence and survival through the first growing season was greater for P. flexilis than P. engelmannii and for Low compared with High provenances. Yet survival through the second growing season was similar for both species and provenances. Seedling emergence and survival tended to be greatest in the subalpine forest and lowest in the alpine tundra. Survival was greater for transplants than for field-germinated seedlings. Conclusions: These results suggest that survival through the first few weeks is critical to the establishment of natural germinants. In addition, even small distances between seed sources can have a significant effect on early demographic performance – a factor that has rarely been considered in previous studies of tree recruitment and species range shifts.

Warming and provenance limit tree recruitment across and beyond the elevation range of subalpine forest

Climate niche models project that subalpine forest ranges will extend upslope with climate warming. These projections assume that the climate suitable for adult trees will be adequate for forest regeneration, ignoring climate requirements for seedling recruitment, a potential demographic bottleneck. Moreover, local genetic adaptation is expected to facilitate range expansion, with tree populations at the upper forest edge providing the seed best adapted to the alpine. Here, we test these expectations using a novel combination of common gardens, seeded with two widely distributed subalpine conifers, and climate manipulations replicated at three elevations. Infrared heaters raised temperatures in heated plots, but raised temperatures more in the forest than at or above treeline because strong winds at high elevation reduced heating efficiency. Watering increased season-average soil moisture similarly across sites. Contrary to expectations, warming reduced Engelmann spruce recruitment at and above treeline, as well as in the forest. Warming reduced limber pine first-year recruitment in the forest, but had no net effect on fourth-year recruitment at any site. Watering during the snow-free season alleviated some negative effects of warming, indicating that warming exacerbated water limitations. Contrary to expectations of local adaptation, low-elevation seeds of both species initially recruited more strongly than high-elevation seeds across the elevation gradient, although the low-provenance advantage diminished by the fourth year for Engelmann spruce, likely due to small sample sizes. High- and low-elevation provenances responded similarly to warming across sites for Engelmann spruce, but differently for limber pine. In the context of increasing tree mortality, lower recruitment at all elevations with warming, combined with lower quality, high-provenance seed being most available for colonizing the alpine, portends range contraction for Engelmann spruce. The lower sensitivity of limber pine to warming indicates a potential for this species to become more important in subalpine forest communities in the coming centuries.

Declines in low‐elevation subalpine tree populations outpace growth in high‐elevation populations with warming

Author(s): Conlisk, Erin; Cristina Castanha; Matthew J. Germino; Thomas T. Veblen; Jeremy M. Smith; Lara M. Kueppers | Abstract: Species distribution shifts in response to climate change require that recruitment increase beyond current range boundaries. For trees with long life spans, the importance of climateâsensitive seedling establishment to the pace of range shifts has not been demonstrated quantitatively. Using spatially explicit, stochastic population models combined with data from longâterm forest surveys, we explored whether the climateâsensitivity of recruitment observed in climate manipulation experiments was sufficient to alter populations and elevation ranges of two widely distributed, highâelevation North American conifers. Empirically observed, warmingâdriven declines in recruitment led to rapid modelled population declines at the lowâelevation, âwarm edgeâ of subalpine forest and slow emergence of populations beyond the highâelevation, âcool edgeâ. Because population declines in the forest occurred much faster than population emergence in the alpine, we observed range contraction for both species. For Engelmann spruce, this contraction was permanent over the modelled time horizon, even in the presence of increased moisture. For limber pine, lower sensitivity to warming may facilitate persistence at low elevations â especially in the presence of increased moisture â and rapid establishment above tree line, and, ultimately, expansion into the alpine. Synthesis. Assuming 21st century warming and no additional moisture, population dynamics in highâelevation forests led to transient range contractions for limber pine and potentially permanent range contractions for Engelmann spruce. Thus, limitations to seedling recruitment with warming can constrain the pace of subalpine tree range shifts.

Annual grassland resource pools and fluxes: sensitivity to precipitation and dry periods on two contrasting soils

In ecosystems throughout the world climate models project increased variability in precipitation patterns that may strongly affect the above- and below-ground processes that control carbon, water, and nutrient cycles. Uncertainty about how plant and soil processes respond to wet and dry periods at different times in the growing season is a barrier to understanding how changing rainfall patterns will affect ecosystem function in annual grasslands. We used mesocosm systems to test the sensitivity to mid- and late-season dry periods of twenty response variables related to nitrogen, carbon, and water cycling in Avena barbata monocultures. We compared the responses of individual variables and of grassland systems under low and high cumulative rain treatments and between two contrasting soil types. Analysis of individual response variables demonstrated strong seasonal patterns: most soil and plant resource pools and fluxes changed between mid- and late-season, and many had larger responses to the late-season dry period. Under increasingly variable precipitation regimes, specific resource pools and fluxes such as soil nitrate and ecosystem CO2 flux may be more strongly affected when dry periods occur later in the growing season. Individual responses variables were also used as state variables in a principal components analysis of changes in grassland functional states between treatments and over time. There were dramatic functional state shifts between the mid- and late-season for both soil types, driven by changes in canopy height, leaf soluble protein, soil nitrate, gross N mineralization, and leaf N. However, we did not find evidence that the functional state of grassland systems was affected by rainfall patterns, indicating that interactions among below- and above-ground processes resulted in system-level resistance to changes in soil moisture. A strong association between plant canopy size and ecosystem functional state suggests that responses of plant phenology and growth to climate change may predict changes in ecosystem function.

Response to Comment on “The whole-soil carbon flux in response to warming”

Temperature records and model predictions demonstrate that deep soils warm at the same rate as surface soils, contrary to Xiao et al.’s assertions. In response to Xiao et al.’s critique of our Q10 analysis, we present the results with all data points included, which show Q10 values of >2 throughout the soil profile, indicating that all soil depths responded to warming.