Search

• Different species have the diverse responses to climate change. • Explored the growth-climate relationships of subtropical six tree species. • Most tree species are suffering from severe drought stress. • Broadleaved trees are generally better adapted to drought events than conifers. The young secondary forests in subtropical regions of China play an important role in the global carbon cycle and China's ecological and economic security. These values are closely related to tree radial growth (RW), primarily affected by climate changes, especially climate warming. Here, we measured the RW of six common tree species in subtropical China (Cunninghamia lanceolata , Pinus massoniana , Metasequoia glyptostroboides , Liquidambar formosana , Choerospondias axillaris , and Lithocarpus glaber) and quantified how each responded to climate warming. Temperature and moisture were the main climate factors limiting the growth of tree species in the study area. The growth of tree species other than L. formosana and P. massoniana was negatively correlated with temperature and positively correlated with relative humidity. Precipitation in the growing season was negatively correlated with the growth of L. formosana and M. glyptostroboides and positively affected the growth of the other tree species. The radial growth trends of trees growing in similar terrain and climate conditions were similar, but their degree of variation was different. Compared to broad-leaved tree species (L. formosana , C. axillaris , and L. glaber), coniferous tree species (C. lanceolata , P. massoniana , and M. glyptostroboides) were more climate-sensitive, particularly C. lanceolata. Climate warming can potentially extend the growing period and promote the radial growth of the study tree species, but more often leads to drought stress that inhibits radial growth. The long-term drought during 2004–2015 significantly reduced the radial growth of most species, especially conifers which are more vulnerable to drought than broad-leaved trees. Our results highlight that most tree species in the humid subtropical forest in South China are suffering from severe drought stress, which significantly affects forest carbon sequestration. It is probably a good warning for current plantation policy and species selection to better achieving the "Carbon Peaking & Carbon Neutrality" goals of the Chinese government. [ABSTRACT FROM AUTHOR]

Subtropical forests across south China have been extensively affected by anthropogenic disturbances with potential profound effects on plant biodiversity and ecosystem function (BEF). However, whether and to what degree biodiversity influences biomass of subtropical forests remains uncertain. In this study, we analyzed the effects of tree biodiversity (species diversity, functional diversity and phylogenetic diversity) on stand biomass at three stages (early, mid and late) of succession in subtropical forests. As forest succession progressed, we found that the changes in species diversity were different compared with those of functional and phylogenetic diversity. Most of the indices of species and functional and phylogenetic diversity were not significantly correlated with stand biomass at two spatial grains (plot sizes of 10 m × 10 m and 20 m × 30 m). However, when other abiotic and biotic factors (i.e., altitude, canopy light, stem density and forest phase) were included with diversity indices in mixed models to explain biomass, the effect of functional diversity on biomass was significant at both resolution. For species diversity indices, the effect was significant at the 20 m × 30 m resolution, and phylogenetic diversity had no significant effect. Our results indicate that biodiversity affects forest biomass during the succession of a subtropical forest and suggest that functional diversity may provide a better metric than species or phylogenetic diversity for determining the biodiversity effect on biomass. Moreover, our results indicate that the diverse conclusions reached in other BEF analyses might be explained by differences in the plot size examined and the diversity metrics selected. Because the bivariate relationship between biodiversity and biomass can be strongly confounded by covarying abiotic and biotic factors, we suggest that the effects of abiotic factors, succession stage, and management practices on BEF should be carefully considered in future research. [ABSTRACT FROM AUTHOR]

Abstract: Belowground interactions in diverse plant communities may be decisive for the performance of individual species and community stability. Here we assessed the effect of tree species richness on belowground fine-root morphology and belowground competition between four different species in a 6-year-old field biodiversity experiment to test the hypotheses: (i) overall fine-root exploitation (total fine-root length and surface area) increases with tree species richness; (ii) belowground interspecific competition is size-symmetric. Overall fine-root length and surface area in the centre of neighbourhoods of four saplings were initially low (1.03kmm−2 and 2.00m2 m−2), but reached 3.13kmm−2 and 6.50 m2 m−2, respectively, across all species combinations after two growing seasons in the ingrowth cores. However, no significant differences were found among the different tree species richness levels. The saplings of different tree species grew in proportion to their initial sizes with respect to aboveground basal area increments. For belowground fine-root growth in mixed neighbourhoods, however, Pseudotsuga menziesii and Picea abies had higher fine-root growth rates in ingrowth cores than in monocultures, whereas the reverse was true for Fagus sylvatica and Quercus petraea. After two years of root ingrowth, the competitive ability indexes (P. abies =0.07, P. menziesii =0.08, F. sylvatica =−0.19, Q. petraea =−0.18) revealed that belowground competition in this sapling stand was size-asymmetric and that conifers showed a higher competitive ability, when fine-root growth was related to aboveground standing basal area. Nutrient enrichment in ingrowth cores did not affect proliferation rates and morphology of fine roots significantly. Fine-root morphologies of different species were remarkably different, but within each species the morphology was not significantly influenced by tree species richness of neighbourhoods. Our results show that belowground competition may occur earlier than aboveground in mixed forest stands and fine-root growth of dominant species benefitted more from mixing with other species than that of inferior species. [Copyright &y& Elsevier]

• Available phosphorus (P) content in soil increased from plantation until deciduous broadleaved forest. • The labile (easily available for plants) P content decreased during forest succession. • Available P was gradually transformed to stable pools during forest succession from plantation to evergreen broadleaved forest. • Phosphorus utilization in forest ecosystems shifts during succession from acquiring to recycling process. Although phosphorus (P) is a limiting nutrient for plant growth in subtropical forests, the effects of forest succession on soil P dynamics, which in turn influences P availability, are unclear. The objective was to access the impacts of forest succession on P fractions of different availability (Hedley sequential fractionation) in highly weathered subtropical soils. We compared the P dynamics and availability under the chronosequence of forest succession from four stages: i) Cunninghamia lanceolata plantation, ii) through mixed broadleaf-conifer, iii) deciduous broadleaved, and finally iv) evergreen broadleaved forest. The soil P was dominated by stable P fractions (69–76%) in all successional stages. Forest succession increased total P content from plantation (199 mg kg−1) to evergreen broadleaved forest (253 mg kg−1), whereas P reached the peak in deciduous broadleaved forest and then remains stable due to balance between input with litter and litter decomposition and tree uptake. Stable P (non-available P) increased for 31–39% with forest succession because soil acidification led to more Fe and Al (oxyhydr)oxides strongly bounding P. Moderately labile P (moderately available P) contents under deciduous and evergreen broadleaved forests were higher than under plantation and mixed forest due to organic matter accumulation. However, labile (easily available) P content was reduced 35–50% by succession because of P removal by plant uptake. Available P content reached the peak under deciduous broadleaved forest (64–81 mg kg−1) and decreased again, indicating that forest ecosystem transit from P acquiring to P recycling system (litter input and plant uptake of mineralized P). Fine root biomass was the primary driver that controlled total, moderately labile and stable P contents during forest succession. Available P increased with soil organic carbon (SOC), suggesting that organic matter is crucial to maintain P availability. The C/P ratio of litter was the primary driver decreasing available P because litter decomposition released P is the main source determining P availability in soil. The increase of moderately labile P following forest succession played a crucial role for accumulation of available P. These results suggest that forest succession increases soil P availability until deciduous broadleaved forest. Therefore, strong measures to facilitate succession to the deciduous broadleaved forest stage should be a key approach to increase long-term soil P availability in subtropics. [ABSTRACT FROM AUTHOR]

• Change of soil N contents as stands age was pronounced in the topsoil. • Ratios of NO 3 −: NH 4 + increased while MBN: (NH 4 + + NO 3 −) decreased as stands aged. • Soil N availability increases and N cycling becomes more open in older stands. • Soil total N is co-regulated by trees' N demand and litter biomass return. • Gross N mineralization and nitrification control mineral N availability. Soil nitrogen (N) is frequently limiting forest productivity, especially in plantations. To investigate the soil N dynamics and the consequences of N accumulation as well as the losses in the ecosystem, we studied a chronosequence of Chinese-fir (Cunninghamia lanceolata [Lamb.] Hook) plantations with stands aged 3, 16, 25, 32, and >80 years. Total N (TN), dissolved organic N (DON), microbial biomass N (MBN) and mineral N (i.e., ammonium (NH 4 +) and nitrate (NO 3 −)) in the top 0–5 cm and 5–15 cm soils were compared with the annual N accumulation by trees, annual litter N return and microbial N transformation rates. Stronger changes with stand age were observed in all N forms in the top 5 cm compared to the 5–15 cm layer. Topsoil TN and DON increased steadily in stands from ages 3- to >80-years-old by 1.9-times and 2.1-times, respectively. MBN increased from 206 mg kg−1 to 327 mg kg−1 in the first 25 years and stabilized thereafter. NH 4 + content increased sharply from 3 to 16 years and then stabilized, while NO 3 − increased linearly with stand age. Both N demand of Chinese-fir and litter N input were clearly age dependent and defined the soil N status: rapid tree biomass accumulation in younger stands greatly depleted soil N, while the older stands with larger litter return and slower growth enabled N re-accumulation in the topsoil. The close positive correlation between total mineral N and gross mineralization rate, as well as between NO 3 − and gross nitrification rate indicated that the mineralization of soil organic matter and the nitrification of released NH 4 + were the two primary microbial processes controlling the available N supply. The topsoil NO 3 −: NH 4 + ratio in older stands (32- and >80 years) was greater than 1.0, and the MBN: (NH 4 + + NO 3 −) ratio was greatly reduced. This indicates that N availability increase and N cycling is accelerated with stands age. [ABSTRACT FROM AUTHOR]

• How nutrient limitation and nutrient-use strategy change with stand age are unknown. • Driver effects on resorption are differentiated by variation partitioning analyses. • Soil nutrient shift from N limitation to P limitation with plantation ageing. • Young plantations adopt more conservative nutrient-use strategy than older stands. • Tree growth (demand) and soil nutrient (supply) co-determine nutrient resorption. Nutrient resorption is a key nutrient-use strategy exhibited by perennial plants to overcome nutrient limitations and meet their nutritional demands. However, the factors that drive nutrient resorption with stand development remain largely unknown. In this study, we examined how a nutrient source (soil) and sink (trees) affected the resorption efficiency (RE) of nitrogen (N) and phosphorus (P). Fresh leaves, leaf litterfall, and soil samples were collected from a chronosequence of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook., Taxodiaceae) plantations at the ages of 3, 8–11, 16, 20, 25, 29, and 32 years. The N and P concentrations in stems, branches, leaves, coarse roots, and fine roots, stocks of N and P in soils, and the relative growth rate (RGR) of trees were measured. Fresh leaf N and P concentrations and RE decreased with stand age, while the N:P ratio in fresh leaves as well as the N and P concentrations in leaf litterfall increased. The RGR of trees declined with stand age. Soil P stocks increased but soil N stocks slightly decreased with stand age. A variation partitioning analysis revealed that the combined effect of RGR and soil P stocks on the RE of P (PRE) was stronger than the individual effect of each factor, while the RE of N (NRE) was greatly affected by the individual effect of RGR. With increasing age, Chinese fir plantations shifted from N-limiting to P-limiting conditions, and the nutrient-use strategy changed from "conservative consumption" to "resource spending". These patterns were co-regulated by the tree growth rate and soil nutrient status. Thus, supply-demand relationships reflected the nutrient cycling status and could provide valuable information for sustainable nutrient management practices in tree plantations. [ABSTRACT FROM AUTHOR]