Photosynthetic capacity of male and female Hippophae rhamnoides plants along an elevation gradient in eastern Qinghai-Tibetan Plateau, China

Leaf trait networks of subtropical woody plants weaken along an elevation gradient

The leaf economic spectrum (LES) captures key leaf functional trait relationships, defining a conservative-acquisitive axis of plant resource utilization strategies. Examining the leaf trait network (LTN) is useful for understanding resource utilization strategies but also more broadly, the ecological strategies of plants. However, the relationship between the LES conservation-acquisition axis and LTN correlations across environmental gradients is unclear. To address this knowledge gap, we measured physiological, chemical, and structural traits in 52 broad-leaved tree species spanning an elevation gradient (1400 m, 1600 m, 1800 m) in Wuyi Mountain, China. A total of 12 leaf traits were selected, including: photosynthetic rate (A25), respiration rate (R25), optimum photosynthetic temperature (Topt), rate of photosynthesis at optimum temperature (Aopt), mean temperature at which 90 % of Aopt is reached (T90), temperature sensitivity of respiration (Q10), N and P content, N/P, leaf mass per area (LMA), photosynthetic nitrogen use efficiency (PNUE) and photosynthetic phosphorus use efficiency (PPUE). We found that leaf physiological traits exhibited signs of thermal acclimation along the elevation gradient. We also observed significant changes in leaf N and P content, N/P, photosynthetic phosphorus utilization efficiency (PPUE) and LMA with elevation. The resource utilization strategies of plants changed from conservative to acquisitive as elevation increased. The LTN analysis showed that as elevation increased, the links among traits weakened and modularity (modularity is used to describe the degree of separation between networks) increased. Collectively, our results indicate that elevation changes can trigger moderate shifts in the resource utilization and ecological strategies of plants via leaf functional traits.

Variation in photosynthetic capacity of Salvia przewalskii along elevational gradients on the eastern Qinghai-Tibetan Plateau, China

Elevational variation in plant growing environment drives diversification of photosynthetic capacity, however, the mechanism behind this reaction is poorly understood. We measured leaf gas exchange, chlorophyll fluorescence, anatomical characteristics, and biochemical traits of Salvia przewalskii at elevations ranging from 2400 m to 3400 m above sea level (a.s.l) on the eastern Qinghai-Tibetan Plateau, China. We found that photosynthetic capacity showed an initial increase and then a decrease with rising elevation, and the best state observed at 2800 m a.s.l. Environmental factors indirectly regulated photosynthetic capacity by affecting stomatal conductance (gs), mesophyll conductance (gm), maximum velocity of carboxylation (Vc max), and maximum capacity for photosynthetic electron transport (Jmax). The average temperature (T) and total precipitation (P) during the growing season had the highest contribution to the variation of photosynthetic capacity of S. przewalskii in subalpine areas, which were 25% and 24%, respectively. Photosynthetic capacity was mainly affected by diffusional limitations (71%-89%), and mesophyll limitation (lm) played a leading role. The variation of gm was attributed to the effects of environmental factors on the volume fraction of intercellular air space (fias), the thickness of cell wall (Tcw), the surface of mesophyll cells and chloroplasts exposed to intercellular airspace (Sm, Sc), and plasma membrane intrinsic protein (PIPs, PIP1, PIP2), independent of carbonic anhydrase (CA). Optimization of leaf tissue structure and adaptive physiological responses enabled plants to efficiently cope with variable climate conditions of high-elevation areas, and the while maintaining high levels of carbon assimilation.

Eco-physiological trait variation in widely occurring species of Western Himalaya along elevational gradients reveals their high adaptive potential in stressful conditions

Species distributed across a wide elevation range have broad environmental tolerance and adopt specific adaptation strategies to cope with varying climatic conditions. The aim of this study is to understand the patterns of variation in leaf eco-physiological traits that are related to the adaptation of species with a wide distribution in different climatic conditions. We studied the variability in eco-physiological traits of two co-occurring species of Western Himalaya (Rumex nepalensis and Taraxacum officinale), along elevational gradients. We conducted our study in elevations ranging from 1000 to 4000 m a.s.l. in three transects separated in an eco-region spanning 2.5° latitudes and 2.3° longitudes in the Western Himalaya. We hypothesized substantial variation in eco-physiological traits, especially increased net rate of photosynthesis (PN), Rubisco specific activity (RSA), and biochemicals at higher elevations, enabling species to adapt to varying environmental conditions. Therefore, the photosynthetic measurements along with leaf sampling were carried out during the months of June-August and the variations in photosynthetic performance and other leaf traits were assessed. Data was analyzed using a linear mixed effect model with 'species,' 'elevation' as fixed and 'transect' as random factor. Elevation had a significant effect on majority of traits. It was found that PN and maximum carboxylation rate of Rubisco (Vcmax) have unimodal or declining trend along increasing elevations. High RSA was observed at higher elevations in all the three transects. Trends for biochemical traits such as total soluble sugars, total soluble proteins, proline, and total phenolics content suggested an increase in these traits for the survival of plants in harsh environments of higher elevations. Our study reveals that although there is considerable variation in the eco-physiological traits of the two species across elevational gradients of different transects, there are certain similarities in the patterns that depict their high adaptive potential in varying climatic conditions.

Stomatal, mesophyll and biochemical limitations to photosynthesis and their relationship with leaf structure over an elevation gradient in two conifers

Photosynthetic responses across complex elevational gradients provides insight into fundamental processes driving responses of plant growth and net primary production to environmental change. Gas exchange of needles and twig water potential were measured in two widespread coniferous tree species, Pinus contorta and Picea engelmannii, over an 800-m elevation gradient in southeastern Wyoming, USA. We hypothesized that limitations to photosynthesis imposed by mesophyll conductance (gm) would be greatest at the highest elevation sites due to higher leaf mass per area (LMA) and that estimations of maximum rate of carboxylation (Vcmax) without including gm would obscure elevational patterns of photosynthetic capacity. We found that gm decreased with elevation for P. contorta and remained constant for P. engelmannii, but in general, limitation to photosynthesis by gm was small. Indeed, estimations of Vcmax when including gm were equivalent to those estimated without including gm and no correlation was found between gm and LMA nor between gm and leaf N. Stomatal conductance (gs) and biochemical demand for CO2 were by far the most limiting processes to photosynthesis at all sites along the elevation gradient. Photosynthetic capacity (A) and gs were influenced strongly by differences in soil water availability across the elevation transect, while gm was less responsive to water availability. Based on our analysis, variation in gm plays only a minor role in driving patterns of photosynthesis in P. contorta and P. engelmannii across complex elevational gradients in dry, continental environments of the Rocky Mountains and accurate modeling of photosynthesis, growth and net primary production in these forests may not require detailed estimation of this trait value.

Intraspecific and sex-dependent variation of leaf traits along altitude gradient in the endangered dioecious tree Taxus fuana Nan Li & R.R. Mill

Plant intraspecific trait variation (ITV) including sex-dependent differences are matters of many ecological consequences, from individual to ecosystem, especially in endangered and rare species. Taxus fuana is an endangered dioecious species with small and isolated populations endemic to the Himalayas region. Little is known about its trait variation between sexes, and among populations. In this study, 18 leaf traits from 179 reproductive trees (males and females) along the altitude (2600-3200m a.s.l.) of the T. fuana populations distributed in Gyirong County, Tibet, China, were measured. ITV and sources of variation in leaf traits were assessed. The relationship between leaf traits of males and females and altitude was analyzed separately. Variations in leaf traits of T. fuana ranged from 3.1% to 24.2%, with the smallest in leaf carbon content and the largest in leaf thickness to area ratio. On average 78.13% of the variation in leaf traits was from within populations and 21.87% among populations. The trends in leaf width, leaf nitrogen to phosphorus ratio, leaf carbon to nitrogen ratio, leaf carbon isotope ratio, and leaf nitrogen isotope ratio in relation to altitude were the same for males and females. Leaf length to width ratio varied significantly with altitude only in males, while leaf phosphorus content, leaf nitrogen content, and leaf carbon to phosphorus ratio varied significantly with altitude only in females. The correlation coefficients of most leaf traits of females with altitude were larger than that of males. In the relationship between leaf traits, there was a high similarity among males and females, but the altitude accounted for more explanation in females than in males. Our results suggested that the variation in leaf traits of T. fuana was small and did not dominate the interspecific competition in the local communities. Adaptation to the altitude gradient of T. fuana might be through altering nutrient storage processes and water use efficiency. Adaptation of male and female T. fuana to environmental changes showed differences, where the males were more tolerant and the females responded greatly to altitude. The differences in adaptation strategies between male and female T. fuana may be detrimental to the maintenance of their populations.

Functional Traits of Male and Female Leaves of Hippophae tibetana on the Eastern Edge of the Tibetan Plateau and Their Altitudinal Variability

To date, there have been few studies of the functional traits of the dioecious Hippophae tibetana Schlecht leaves, either male or female, in response to ecological factors such as altitude. Elucidating these relationships will establish an important scientific basis for vegetation restoration and reconstruction of the Tibetan Plateau ecosystem. The natural populations of H. tibetana, distributed across three field sites, at 2868 m, 3012 m and 3244 m, in Tianzhu, Gansu, were studied by field survey sampling and laboratory analysis. In particular, the adaptions of leaf functional traits to elevation in these dioecious plants were analyzed. The results show that: (1) there is no “midday depression” of photosynthetic activity in either male or female plants. Over a one-day period, the net photosynthetic rate (Pn) and transpiration rate (Tr) of H. tibetana female plants were higher than those of male plants (p < 0.05). This correlated to the period of vigorous fruit growth in the female plant. The measured Pn and Tr were maximal at the intermediate altitude (3012 m). The light compensation point (LCP) of the leaves of male and female plants were 57.6 and 43.2 μmol·m−2·s−1, respectively, and the light saturation points (LSP) of the leaves were 1857.6 and 1596.8 μmol·m−2·s−1. (2) Altitude had a significant effect on plant and leaf functional traits of male and female H. tibetana (p < 0.05), and no significant difference was noted between plants at the same altitude. The values for leaf area (LA), specific leaf weight (LMA), leaf phosphorus content per unit mass (Pmass) and leaf phosphorus content per unit area (Parea) were also maximal at the intermediate altitude. Leaf nitrogen content per unit area (Narea) and leaf nitrogen content per unit mass (Nmass) increased with altitude. This indicated that the functional traits of male and female plants and leaves of H. tibetana showed a strong “trade-off relationship” with altitude. (3) Pearson correlation analysis showed that there were significant correlations among functional traits of H. tibetana leaves. Redundancy analysis (RDA) showed that soil water content (SWC), altitude (Alt) and soil organic carbon (SOC) had significant effects on the functional traits of H. tibetana leaves (p < 0.05).

Lack of coordination between stomatal and vein traits provides functional benefits to the dioecious tropical tree Myrsine coriacea

Climate change will affect the distribution of many tropical plant species. However, the understanding of how dioecious tropical species cope with different environmental conditions is still limited. To address this issue, we investigated how secondary trait attributes in populations of the dioecious tropical tree Myrsine coriacea change along an altitudinal gradient. Eighty individual plants (40 male and 40 female) were selected among seven natural populations. Leaf variation in morphological and stomatal traits, and carbon and nitrogen isotopic compositions were analyzed. Female plants had greater isotopic leaf carbon composition (δ¹³ C) and nitrogen content than male plants, increasing their carboxylation capacity. Plants of both sexes had smaller stomata, greater water-use efficiency (greater δ¹³ C), and greater nitrogen isotopic composition (δ¹⁵ N) at higher altitudes. They also showed lower δ¹⁵ N and had greater carbon: nitrogen ratios at lower altitudes. There was a lack of coordination between stomatal and vein traits, which was compensated for by variation in specific leaf areas. This mechanism was essential for increasing plant performance under the limiting conditions found by the species at higher altitudes.