ALBERT

Description: Despite the important role of tropical forest ecosystems in the uptake and storage of atmospheric carbon dioxide (CO2), the carbon (C) dynamics of tropical tree species remains poorly understood, especially regarding belowground roots. This study assessed the allocation of newly assimilated C in the fast-growing pioneer tropical tree species Ceiba pentandra (L.), with a special focus on different root categories. During a 5-day pulse-labelling experiment, 9-month-old (~3.5-m-tall) saplings were labelled with 13CO2 in a large-scale aeroponic facility, which allowed tracing the label in bulk biomass and in non-structural carbohydrates (sugars and starch) as well as respiratory CO2 from the canopy to the root system, including both woody and non-woody roots. A combined logistic and exponential model was used to evaluate 13C mean transfer time and mean residence time (MRT) to the root systems. We found 13C in the root phloem as early as 2 h after the labelling, indicating a mean C transfer velocity of 2.4 ± 0.1 m h−1. Five days after pulse labelling, 27% of the tracers taken up by the trees were found in the leaves and 13% were recovered in the woody tissue of the trunk, 6% in the bark and 2% in the root systems, while 52% were lost, most likely by respiration and exudation. Larger amounts of 13C were found in root sugars than in starch, the former also demonstrating shorter MRT than starch. Of all investigated root categories, non-woody white roots (NRW) showed the largest 13C enrichment and peaked in the deepest NRW (2–3.5 m) as early as 24 ± 2 h after labelling. In contrast to coarse woody brown roots, the sink strength of NRW increased with root depth. The findings of this study improve the understanding of C allocation in young tropical trees and provide unique insights into the changing contributions of woody and non-woody roots to C sink strengths with depth.

Description: Aims Climate warming and agricultural non-point source pollution both resulting from anthropogenic activities have been projected to affect plant reproduction and growth in wetlands worldwide. In order to predict and mitigate impacts of these anthropogenic activities, it is important to investigate how marsh plants respond to such environmental changes. Methods In this study, Bolboschoenus planiculmis, a tuberous sedge with a wide distribution range in Eurasia, was selected to examine the effect of air temperature changes (15, 20 and 25 °C over 24 h; 20/10 and 30/15 °C, day/night) and K supply (0, 1, 3, 9 and 18 mM) on its reproductive and growth traits in climate chambers. Important Findings We found that high constant temperatures (20 and 25 °C) were more beneficial for tuberization of B. planiculmis than high alternating temperatures (30/15 °C), whereas aboveground biomass and shoot height were generally largest at high temperatures (30/15 and 25 °C). Both reproductive and growth traits of B. planiculmis showed hump-shaped relationships with K supply, with an optimum K concentration of around 1–3 mM. The combination of high constant temperatures and optimal K concentrations promoted reproductive traits the most, whereas the combination of higher temperatures (30/15 and 25 °C) and K concentrations up to 9 mM increased growth traits only. We therefore conclude that population abundance of B. planiculmis might benefit from global warming and the additional K supply.

Description: Abies alba (Mill.) has a high potential for mitigating climate change in European mountain forests; yet, its natural regeneration is severely limited by ungulate browsing. Here, we simulated browsing in a common garden experiment to study growth and physiological traits, measured from bulk needles, using a randomized block design with two levels of browsing severity and seedlings originating from 19 populations across Switzerland. Genetic factors explained most variation in growth (on average, 51.5%) and physiological traits (10.2%) under control conditions, while heavy browsing considerably reduced the genetic effects on growth (to 30%), but doubled those on physiological traits related to carbon storage. While browsing reduced seedling height, it also lowered seedling water-use efficiency (decreased $delta ^{13}$C) and increased their $delta ^{15}$N. Different populations reacted differently to browsing stress, and for seedling height, starch concentration and $delta ^{15}$N, population differences appeared to be the result of natural selection. First, we found that populations originating from the warmest regions recovered the fastest from browsing stress, and they did so by mobilizing starch from their needles, which suggests a genetic underpinning for a growth-storage trade-off across populations. Second, we found that seedlings originating from mountain populations growing on steep slopes had a higher $delta ^{15}$N in the common garden than those originating from flat areas, indicating that they have been selected to grow on N-poor, potentially drained, soils. This finding was corroborated by the fact that nitrogen concentration in adult needles was lower on steep slopes than on flat ground, strongly indicating that steep slopes are the most N-poor environments. These results suggest that adaptation to climate and soil nitrogen availability, as well as ungulate browsing pressure, co-determine the regeneration and range limit of silver fir.