Temporal population variability in local forest communities has mixed effects on tree species richness across a latitudinal gradient

Among the local processes that determine species diversity in ecological communities, fluctuation‐dependent mechanisms that are mediated by temporal variability in the abundances of species populations have received significant attention. Higher temporal variability in the abundances of species populations can increase the strength of temporal niche partitioning but can also increase the risk of species extinctions, such that the net effect on species coexistence is not clear. We quantified this temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. A fitted mechanistic model showed that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal variability in the abundances of species populations has no clear negative or positive contribution to the latitudinal gradient in tree species richness

Functional composition drives ecosystem function through multiple mechanisms in a broadleaved subtropical forest

Understanding the role of biodiversity (B) in maintaining ecosystem function (EF) is a foundational scientific goal with applications for resource management and conservation. Two main hypotheses have emerged that address B-EF relationships: niche complementarity (NC) and the mass-ratio (MR) effect. We tested the relative importance of these hypotheses in a subtropical old-growth forest on the island nation of Taiwan for two EFs: aboveground biomass (ABG) and coarse woody productivity (CWP). Functional dispersion (FDis) of eight plant functional traits was used to evaluate complementarity of resource use. Under the NC hypothesis, EF will be positively correlated with FDis. Under the MR hypothesis, EF will be negatively correlated with FDis and will be significantly influenced by community-weighted mean (CWM) trait values. We used path analysis to assess how these two processes (NC and MR) directly influence EF and may contribute indirectly to EF via their influence on canopy packing (stem density). Our results indicate that decreasing functional diversity and a significant influence of CWM traits were linked to increasing AGB for all eight traits in this forest supporting the MR hypothesis. Interestingly, CWP was primarily influenced by NC and MR indirectly via their influence on canopy packing. Maximum height explained more of the variation in both AGB and CWP than any of the other plant functional traits. Together, our results suggest that multiple mechanisms operate simultaneously to influence EF, and understanding their relative importance will help to elucidate the role of biodiversity in maintaining ecosystem function

Global importance of large-diameter trees

Aim: To examine the contribution of large‐diameter trees to biomass, stand structure, and species richness across forest biomes. Location: Global. Time period: Early 21st century. Major taxa studied: Woody plants. Methods: We examined the contribution of large trees to forest density, richness and biomass using a global network of 48 large (from 2 to 60 ha) forest plots representing 5,601,473 stems across 9,298 species and 210 plant families. This contribution was assessed using three metrics: the largest 1% of trees ≥ 1 cm diameter at breast height (DBH), all trees ≥ 60 cm DBH, and those rank‐ordered largest trees that cumulatively comprise 50% of forest biomass. Results: Averaged across these 48 forest plots, the largest 1% of trees ≥ 1 cm DBH comprised 50% of aboveground live biomass, with hectare‐scale standard deviation of 26%. Trees ≥ 60 cm DBH comprised 41% of aboveground live tree biomass. The size of the largest trees correlated with total forest biomass (r2 = .62, p < .001). Large‐diameter trees in high biomass forests represented far fewer species relative to overall forest richness (r2 = .45, p < .001). Forests with more diverse large‐diameter tree communities were comprised of smaller trees (r2 = .33, p < .001). Lower large‐diameter richness was associated with large‐diameter trees being individuals of more common species (r2 = .17, p = .002). The concentration of biomass in the largest 1% of trees declined with increasing absolute latitude (r2 = .46, p < .001), as did forest density (r2 = .31, p < .001). Forest structural complexity increased with increasing absolute latitude (r2 = .26, p < .001). Main conclusions: Because large‐diameter trees constitute roughly half of the mature forest biomass worldwide, their dynamics and sensitivities to environmental change represent potentially large controls on global forest carbon cycling. We recommend managing forests for conservation of existing large‐diameter trees or those that can soon reach large diameters as a simple way to conserve and potentially enhance ecosystem services

An Investigation of the Temperature-Drift Effect on Strain Measurement of Concrete Beams

This study investigates the temperature-drift effect on strain measurement of concrete beams and proposes a method for determination of the mechanical strain of stressed concrete beams. In the study, wireless electrical resistance strain gauges were used to measure the strain of concrete beams. This study first examined how temperature changes affected the strain gauge attached to concrete beams. Subsequently, a concrete beam experiencing changes in temperature and load was monitored for six consecutive days. The test results showed that the apparent strain response of the concrete beam was significantly affected by temperature changes. After adjusting for the temperature effect, the mechanical strain generated by a load could be obtained. However, temperature-induced drift was still observed in the mechanical strain response. Based on the assumption that temperature changes are slow and gradual, and mechanical strain changes are momentary, an adjacent data subtraction method can be used to eliminate the temperature-induced drift present in the mechanical strain data. The subtraction results show that the mechanical strain generated by a load was accurately obtained. The proposed data-processing method could also be used to find the residual strain of the nonelastic response of a beam subjected to substantial short-term forces

Impact-Echo Response of Plates Containing Thin Layers and Voids

An ongoing research program initiated in 1983 by Carino and Sansalone has been aimed at developing the theoretical basis and practical applications for a new nondestructive technique for detecting flaws in reinforced concrete structures. The technique, known as impact-echo, is well documented [1–6] and only a brief overview of the principle of the method, signal processing techniques, and instrumentation is presented. This paper highlights the results of a recent investigation into the feasibility of using the impact-echo technique to detect voids in plates containing thin layers of materials having different acoustic impedances</p

Impact-echo method for the deterioration evaluation of near-surface mounted CFRP strengthening under outdoor exposure conditions

This paper discusses the feasibility of using the impact-echo method (IE method) to evaluate the deterioration of near-surface mounted carbon fiber reinforced polymer (NSM CFRP) strengthening under outdoor exposure. In NSM CFRP strengthening materials, CFRP bars are embedded in pre-cut grooves on the surface of reinforced concrete members and the grooves are filled with epoxy. Prolonged outdoor exposure reduces the bonding strength of epoxy and generates debonding flaws at epoxy–concrete interfaces, preventing CFRP bars from fully exerting their material strength and inhibiting the reinforcement effectiveness of NSM CFRP. The IE method based on the principle of stress wave propagation was adopted in this study as a nondestructive technique for detecting debonding flaws at epoxy–concrete interfaces caused by the prolonged outdoor exposure of the NSM CFRP strengthening members. Firstly, the IE tests were conducted on the NSM CFRP beams without outdoor exposure. The IE spectra showed high-amplitude peaks at the dominant frequencies associated with the cross sectional mode of vibration of the NSM CFRP beams. Subsequently, the strengthening surfaces of the NSM CFRP beams were exposed to direct sunlight under outdoor environments. The IE and loading tests were conducted when the beams were exposed to the outdoor environment for 4, 8, 12, and 48 months. The experimental results indicated that prolonged outdoor exposure generated the debonding flaws at the epoxy–concrete interfaces and the IE method can be used to detect the interfacial debonding by identifying the decrease in the dominant frequencies. For a NSM CFRP beam with debonding interfaces, the result obtained from loading test revealed a decrease in the load-carrying capacity due to damages at the epoxy–concrete interfaces resulting in the inability of the CFRP bars to fully exert their material strength. Therefore, the IE method is applicable for assessing the deterioration of the NSM CFRP strengthening members under prolonged outdoor exposure

Distinction between crack echoes and rebar echoes based on Morlet Wavelet Transform of impact echo signals

In an Impact-Echo (IE) test, the Fast Fourier transform (FFT) of the impacted surface responses reveals frequency peaks of major periodical events and can be used to determine depth information, such as the thickness of a target specimen and the depth of a delaminated crack. Since rebar interfaces also result in comparable spectral frequencies but with different depth characteristics, IE testers might have difficulty to determine whether a certain peak frequency is associated with a crack or a rebar. This paper aims to illustrate the use of the time-frequency analysis with Morlet Wavelet transform (MWT) to discriminate the crack response from the rebar response. In additional to the traditional FFT spectrum, the time-frequency technique provides more informative data to assist the analysis task. The proposed method for recognizing crack and rebar can be carried out first by analyzing the peak frequencies in the FFT spectrum. Next, the amplitude histogram at the selected frequency is extracted from the time-frequency spectrogram. Through evaluating the decay pattern of the amplitude curve, an index representing the “duration of amplitude-decay” is defined. Based on the comparison between results of the two studied interfaces, namely concrete-void and concrete-steel, it is preliminarily summarized that the reflected signals are generally less significant at the concrete-steel interface. A rebar leads to weaker reflective energy and thus shortens duration of amplitude-decay. Therefore, it is concluded that the proposed methodology can be applicable to distinguish the difference between the crack echoes and the rebar reflections based on the spectrogram obtained with the time-frequency analysis. Accordingly, this innovated technique provides analysts with an alternative scheme in the recognition/verification of cracks and rebars via the IE tests

Kenting_Abundance_Data

The absolute abundances (i.e., individuals) of 67 woody plant species (DBH 1-2 cm) for the 2008 census and 2013 census of the Kenting Karst Forest Dynamics Plot, Taiwan. See the main text and Appendix S5 of Chao et al. (2019) for analysis details