Stem CO2 Efflux as an Indicator of Forests’ Productivity in Relict Juniper Woodlands (Juniperus thurifera L.) of Southern Spain

There are considerable uncertainties about the C cycle in semi-arid ecosystems. Hence, studies that have focused on Juniperus in Mediterranean woodlands are non-existent. This study provides a survey of the effect of the juniper woodland type (young and mature woodlands; joint effect of maturity and forest productivity) on stem respiration. We checked the seasonal variation of stem respiration, evaluating the effects of stem temperature on stem CO2 efflux. For this, we measured the stem CO2 efflux (µmol CO2 m−2 s−1) over the four seasons on 16 junipers using LI-6400 equipment. The results showed that in the more productive site (young woodland), the stem CO2 efflux was higher. This variable followed a clear seasonal trend, being higher during the spring and progressively decreasing in cold periods. In both juniper woodlands, and especially in the older forests, the Q10 coefficients were low (<2), typical of cold forests and slow-growing species. The exponential model also confirmed that the Q10 was significantly higher in young juniper trees. Thus, stem CO2 efflux was an indicator of the growth in this juniper woodland that is well adapted to a semi-arid climate.

Low-cost chamber design for simultaneous CO2 and O2 flux measurements between tree stems and the atmosphere

Tree stem CO2 efflux is an important component of ecosystem carbon fluxes and has been the focus of many studies. While CO2 efflux can easily be measured, a growing number of studies have shown that it is not identical with actual in situ respiration. Complementing measurements of CO2 flux with simultaneous measurements of O2 flux provides an additional proxy for respiration, and the combination of both fluxes can potentially help getting closer to actual measures of respiratory fluxes. To date, however, the technical challenge to measure relatively small changes in O2 concentration against its high atmospheric background has prevented routine O2 measurements in field applications. Here we present a new and low-cost field-tested device for autonomous real-time and quasi-continuous long-term measurements of stem respiration by combining CO2 (NDIR based) and O2 (quenching based) sensors in a tree stem chamber. Our device operates as a cyclic closed system and measures changes in both CO2 and O2 concentration within the chamber over time. The device is battery-powered with a &gt; 1 week power independence and data acquisition is conveniently achieved by an internal logger. Results from both field and laboratory tests document that our sensors provide reproducible measurements of CO2 and O2 exchange fluxes under varying environmental conditions.

Manipulating phloem transport affects wood formation but not nonstructural carbon concentrations in an evergreen conifer

Wood formation is a crucial process for carbon sequestration, yet how variations in carbon supply affect wood formation and carbon dynamics in trees more generally remains poorly understood.To better understand the role of carbon supply in wood formation, we restricted phloem transport using girdling and compression around the stem of mature white pines and monitored the effects on local wood formation and stem CO2 efflux, as well as nonstructural carbon concentrations in needles, stems, and roots.Growth and stem CO2 efflux varied with location relative to treatment (i.e., above or below on the stem). We observed up to a two-fold difference in the number of tracheids formed above versus below the manipulations over the remaining growing season. In contrast, the treatments did not affect mean cell size noticeably and mean cell-wall area decreased only slightly below them. Surprisingly, nonstructural carbon pools and concentrations in the xylem, needles, and roots remained largely unchanged, although starch reserves declined and increased marginally below and above the girdle, respectively.Our results suggest that phloem transport strongly affects cell proliferation and respiration in the cambial zone of mature white pine, but has little impact on nonstructural carbon concentrations. These findings contribute to our understanding of how wood formation is controlled.HighlightRestrictions in phloem transport designed to affect carbon supply, lead to changes in wood formation and stem respiration of mature white pines without substantially changing local nonstructural carbon concentrations.

Limited vertical CO2 transport in stems of mature boreal Pinus sylvestris trees

Several studies have suggested that CO2 transport in the transpiration stream can considerably bias estimates of root and stem respiration in ring-porous and diffuse-porous tree species. Whether this also happens in species with tracheid xylem anatomy and lower sap flow rates, such as conifers, is currently unclear. We infused 13C-labelled solution into the xylem near the base of two 90-year-old Pinus sylvestris L. trees. A custom-built gas exchange system and an online isotopic analyser were used to sample the CO2 efflux and its isotopic composition continuously from four positions along the bole and one upper canopy shoot in each tree. Phloem and needle tissue 13C enrichment was also evaluated at these positions. Most of the 13C label was lost by diffusion within a few metres of the infusion point indicating rapid CO2 loss during vertical xylem transport. No 13C enrichment was detected in the upper bole needle tissues. Furthermore, mass balance calculations showed that c. 97% of the locally respired CO2 diffused radially to the atmosphere. Our results support the notion that xylem CO2 transport is of limited magnitude in conifers. This implies that the concerns that stem transport of CO2 derived from root respiration biases chamber-based estimates of forest carbon cycling may be unwarranted for mature conifer stands.

Seasonally varying relationship between stem respiration, increment and carbon allocation of Norway spruce trees

Stem respiration is an important component of an ecosystem’s carbon budget. Beside environmental factors, it depends highly on tree energy demands for stem growth. Determination of the relationship between stem growth and stem respiration would help to reveal the response of stem respiration to changing climate, which is expected to substantially affect tree growth. Common measurement of stem radial increment does not record all aspects of stem growth processes, especially those connected with cell wall thickening; therefore, the relationship between stem respiration and stem radial increment may vary depending on the wood cell growth differentiation phase. This study presents results from measurements of stem respiration and increment carried out for seven growing seasons in a young Norway spruce forest. Moreover, rates of carbon allocation to stems were modeled for these years. Stem respiration was divided into maintenance (Rm) and growth respiration (Rg) based upon the mature tissue method. There was a close relationship between Rg and daily stem radial increment (dSRI), and this relationship differed before and after dSRI seasonal maximum, which was around 19 June. Before this date, Rg increased exponentially with dSRI, while after this date logarithmically. This is a result of later maxima of Rg and its slower decrease when compared with dSRI, which is connected with energy demands for cell wall thickening. Rg reached a maxima at the end of June or in July. The maximum of carbon allocation to stem peaked in late summer, when Rg mostly tended to decrease. The overall contribution of Rg to stem CO2 efflux amounted to 46.9% for the growing period from May to September and 38.2% for the year as a whole. This study shows that further deeper analysis of in situ stem growth and stem respiration dynamics is greatly needed, especially with a focus on wood formation on a cell level.

Tracing recent carbon from photosynthesis to stem and soil respiration in an experimental tropical rainforest in response to drought

&lt;p&gt;Tropical rainforests play a major role in the terrestrial carbon (C) cycle. However, to date little is known about the mechanisms and processes controlling C fluxes in tropical forests. Within the C cycle of a forest, trees allocate a substantial amount of photoassimilates belowground, and fuel respiration by stems, roots and microorganisms. This link between assimilation and respiration represents a significant pathway by which assimilated C is quickly returned to the atmosphere. However, the nature of this coupling, including the speed of above- to below-ground C allocation and the proportion of rapidly metabolized assimilates is yet unknown for mature tropical rainforest systems. Furthermore, the role of tree species and size and the relative roles of canopy versus understory plants are still unresolved.&lt;/p&gt;&lt;p&gt;Drought spells can exert a major control on the C balance of tropical forest ecosystems by altering C uptake, the partitioning of C and the dynamics of C allocation and belowground utilization. As such responses are difficult to measure in tropical rainforest, the consequences of drought for the dynamics of recent C in stem and soil respiration in this biome remain unclear.&lt;/p&gt;&lt;p&gt;To assess and quantify these processes, we made use of the Tropical Rain Forest at the Biosphere 2 research complex in Arizona, US. This infrastructure provides unique opportunities to study drought effects on the C dynamics in a controlled environment. We simulated a drought spell for eight weeks and continuously measured stem and soil CO&lt;sub&gt;2&lt;/sub&gt; fluxes using isotope laser spectroscopy before and during the drought as well as during the subsequent rewetting period. Our study is part of a large-scale experiment that aims to disentangle C- and water-cycle processes underpinning ecosystem responses to drought from a molecular to an ecosystem-scale level, with particular focus on plant-soil and plant-atmosphere interfaces.&lt;/p&gt;&lt;p&gt;We performed two canopy-scale &lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; pulse labeling campaigns under ambient environmental conditions and towards the end of the experimental drought. We traced the allocation dynamics of recently assimilated C to soil respiration and to stem respiration of dominant tree species. First results show that the allocation of assimilates from the canopy to soil-respired CO&lt;sub&gt;2&lt;/sub&gt; took several days and was affected by tree size and species identity. Under drought, tracer efflux from stems and soils was &amp;#160;slowed down, with strong species-specific differences. Our results will allow novel insights into the combined effects of tree size, species identity and drought on the allocation dynamics and respiratory utilization of photoassimilates in tropical rainforest.&lt;/p&gt;