Rhizosphere activity in an old-growth forest reacts rapidly to changes in soil moisture and shapes whole-tree carbon allocation

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought-induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests’ resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied 13CO2 pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in above- and belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

Spatial variability in terrestrial and aquatic carbon stocks and fluxes in boreal forested catchments

<p>Canada’s boreal zone is a complex mosaic of forests, wetlands, streams and lakes.  The pool of carbon (C) stored in each of these ecosystem components is vast, and significant to the global C balance.  However, C pools and fluxes are heterogeneous in time and space, which contributes to uncertainty in predicting how a changing climate will affect the fate of C in these sensitive ecosystems. The objective of this study was to investigate factors controlling spatial variability in soil C stocks and stream C export and assess the sensitivity of these stocks and fluxes to climatic factors. We conducted a detailed examination of soil C stocks and stream dissolved organic C (DOC) export from a 320 ha boreal forested catchment located in northwestern Ontario, Canada. High-frequency stream chemistry and discharge samples were collected from three inflow streams during snowmelt and rain events from 2016-2017. An intensive soil C sampling campaign resulting in 47 surface (0 – 30 cm) samples were collected during the summer of 2019. Stream hysteresis analysis revealed marked differences in flowpaths among sub-catchments during snowmelt and rain events. In the wetland-dominated catchment, near-stream sources contributed most of the DOC export during both rainstorms and snowmelt events, but in upland-dominated catchments, the sources of DOC depended on antecedent moisture conditions. Rainstorms in these catchments following prolonged droughts resulted in DOC flushing from distal regions of the catchment. Soil C stocks were also highly spatially variable, with much of the variability being explained by local-scale factors (e.g. gravel content, soil depth, distance to the nearest ridge). Taken together, these two findings emphasize the need to consider sub-catchment scale variability when calculating C pools and fluxes in boreal catchments. This is also important when predicting how C dynamics will shift in the future as a result of shorter winters, longer droughts and more intense rainstorms.</p>

Above-and Belowground Carbon Allocation in Post-Fire Lodgepole Pine Forests: Effects of Tree Density and Stand Age

Validating the different components of the carbon (C) budget in forest ecosystems is essential for developing allocation rules that allow accurate predictions of global C pools and fluxes. In addition, a better understanding of the effects of natural disturbances on C cycling is critical - particularly in light of changes in disturbance regimes that may occur with alterations in global climate. This study investigates the indirect effects of fire on C cycling in lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) stands in Yellowstone National Park by examining above- and belowground C pools, fluxes and allocation patterns in post-fire stands that vary in tree density and stand age (four forest types: low (50,000 trees/ha) in 13-yr­old stands; and ~110-yr-old mature stands). Above­ and belowground biomass were estimated to investigate the effect of tree density on biomass partitioning in young stands. The effect of tree density on soil-surface C02 efflux (Fs) and microbial biomass C (Cmic) in young stands was also examined, and data from mature stands were used as a proxy of pre-fire conditions to estimate the extent of ecosystem recovery 13 years after a stand replacing fire. Finally, the effects of tree density and stand age on ecosystem C pools, fluxes and allocation patterns were investigated. Partitioning of biomass to belowground increased with tree density in young stands primarily as a result of inherent differences associated with tree size, not competition. Fs and Cmic increased with tree density in young stands and with stand age, and both Fs and Cmic were correlated exclusively with biotic variables. These findings support recent studies demonstrating the prevailing importance of plants in controlling overall rates of Fs, and suggest that increased decomposition of older, recalcitrant soil C pools is relatively unimportant following fire. Fire, through influences on tree density and stand age, has important and lasting effects on the magnitude of C pools and fluxes in lodgepole pine ecosystems. However, results presented here suggest that overall C allocation patterns following fire are independent of tree density and stand age.