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A Soil-Plant Analysis Development (SPAD) chlorophyll meter can be used as a simple tool for evaluating N concentration of the leaf and investigating the combined effects of nitrogen rate and leaf age on N distribution. We conducted experiments in a paddy field over two consecutive years (2008–2009) using rice plants treated with six different N application levels. N distribution pattern was determined by SPAD readings based on the temporal dynamics of N concentrations in individual leaves. At 62 days after transplantation (DAT) in 2008 and DAT 60 in 2009, leaf SPAD readings increased from the upper to lower in the rice canopy that received N levels of 150 to 375 kg ha−1The differences in SPAD readings between the upper and lower leaf were larger under higher N application rates. However, as plants grew, this atypical distribution of SPAD readings in canopy leaf quickly reversed to the general order. In addition, temporal dynamics of the leaf SPAD readings (N concentrations) were fitted to a piecewise function. In our model, changes in leaf SPAD readings were divided into three stages: growth, functioning, and senescence periods. The leaf growth period lasted approximately 6 days, and cumulative growing days were not affected by N application rates. The leaf functioning period was represented with a relatively stable SPAD reading related to N application rate, and cumulative growing days were extended with increasing N application rates. A quadratic equation was utilized to describe the relationship between SPAD readings and leaf age during the leaf senescence period. The rate of decrease in SPAD readings increased with the age of leaves, but the rate was slowed by N application. As leaves in the lower canopy were physiologically older than leaves in the upper canopy, the rate of decrease in SPAD readings was faster in the lower leaves.

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.

The selection of tree species is critical for soil organic carbon (SOC) stock and dynamics under afforestation. Exotic tree species were widely introduced and have been considered as a form of human-induced global change. However, how exotic tree species, as compared with native ones, affect SOC storage and soil respiration (R s ) has not been sufficiently investigated. Here we studied this issue in an even-aged (22 years) plantation composed by native Masson pine (Pinus massoniana Lamb.) (MP) and exotic slash pine (Pinus elliottii Engelm.) (SP) in subtropical China. A trenching method was used to partition R s into heterotrophic (R h ) and autotrophic respiration (R a ). R s and R h were measured approximately biweekly during August 2007 to December 2009. Our results showed that seasonal patterns of R s and R h in both pine forests were mainly affected by soil temperature and moisture (R 2 = 0.56-0.78, P b 0.001). There was no difference in SOC stock (forest floor + mineral soil of 0-40 cm), R h and turnover rate (R h /SOC stock) between MP and SP forests. However, annual R s , R a and R a /R s of MP were significantly 24%, 119% and 74% higher than those of SP, respectively. In contrast, even though MP and SP had similar fine and coarse root biomass, MP had significant higher ratios of R a /fine root biomass and R a /coarse root biomass than SP, suggesting a higher respiration rate in native pine species on a root biomass basis. We concluded that the exotic SP, as compared with indigenous MP, did not alter SOC stock, R h and turnover rate but depressed R a and R s . Therefore, both pine species were recommended in case of reforestation in terms of SOC sequestration.

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