The intriguing paradox of leaf lifespan responses to nitrogen availability

Changes in leaf lifespan, nitrogen resorption, and mean residence time of leaf nitrogen along a soil fertility gradient in an evergreen oak tree

The ability of plants to retain nitrogen (N) for a long period of time is critical to their N use efficiency, growth, and fitness, particularly in infertile environments. The mean residence time of leaf N (MRTL) and its two determinants, leaf lifespan and N resorption efficiency (rN, the fraction of the total leaf N pool that is resorbed during leaf senescence), have been hypothesized to increase plastically with decreasing soil N fertility but this remains to be fully tested. To avoid confusion by random changes in these characteristics in a relatively narrow N fertility range, MRTL, leaf lifespan, and N resorption efficiency were measured in Quercus glauca over a broad N fertility range. In the high to moderate N fertility range, leaf lifespan and rN increased with decreasing N addition rate, and thus the MRTL increased. However, in the moderate to low N fertility range, leaf lifespan increased but rN decreased significantly, so MRTL decreased. The decrease in rN occurred because the senesced leaf N concentration was almost constant at the lower limit while the green leaf N concentration decreased in this range. The hump-shaped quadratic responses of MRTL and rN along the N fertility gradient suggest that incorrect conclusions about the response of these traits to N fertility variation may be drawn from experiments that include only a few fertility levels, and N recycling within leaf canopy alone cannot achieve efficient N use in infertile environments.

Ethylene-regulated leaf lifespan explains divergent responses of plant productivity to warming among three hydrologically different growing seasons

Leaf senescence is known to be regulated by the plant hormone ethylene, but how leaf lifespan responds to global environmental change and links to ecosystem-level responses remains largely unexplored. Here we investigated the effects of climate warming and nitrogen addition on plant functional traits, plant hormone ethylene and net primary production in a 13-year field experiment in a desert steppe. Across the last 3 years of the experiment (2016-2018), plant productivity increased under warming only in 2016, when there was above normal precipitation, but consistently increased with nitrogen addition. Warming enhanced net photosynthesis, leaf nitrogen and ethylene production and reduced leaf lifespan in 2016 (a wet year), but not in 2017 (a drought year); the effect of warming in 2018 (a year with normal precipitation) was opposite to 2016, likely due to the below-normal precipitation in the mid-growing season in 2018. Nitrogen addition led to increases in leaf nitrogen, ethylene production and net photosynthesis, and declines in leaf lifespan in 2016 and 2018, but not in 2017. The ethylene-regulated lifespan was further evidenced by the addition of CoCl₂ (an ethylene biosynthesis inhibitor) that reduced ethylene production and prolonged lifespan. Structural equation modeling showed that leaf lifespan had a negative effect on plant productivity, both directly and indirectly via its negative effect on net photosynthesis, across all 3 years. Our results demonstrate the divergent responses of leaf lifespan and, in turn, plant productivity to warming under inter-annual and intra-annual precipitation variation, thus linking plant hormone production, functional traits and ecosystem functioning in the face of global environmental change.

Leaf Traits of Trees in Tropical Dry Evergreen Forests of Peninsular India

A plant functional trait study was conducted to know the existing relationship between important leaf traits namely, specific leaf area (SLA), leaf dry matter content (LDMC), and leaf life span (LL) in tropical dry evergreen forest (TDEFs) of Peninsular India. Widely accepted methodologies were employed to record functional traits. The relationships between SLA and LDMC, LDMC and LL, and SLA and LL were measured. Pearson’s coefficient of correlation showed a significant negative relationship between SLA and LDMC, and SLA and LL, whereas a significant positive relationship was prevailed between LDMC and LL. The mean trait values (SLA, LDMC, and LL) of evergreens varied significantly from deciduous species. SLA had a closer relationship with LDMC than LL. Similarly, LL had a closer relationship with SLA than LDMC. Species with evergreen leaf habits dominated forest sites under study. Evergreen species dominate the study area with a high evergreen-deciduous ratio of 5.34:1. The S strategy score of trees indicated a relatively higher biomass allocation to persistent tissues. TDEFs occur in low elevation, semiarid environment, but with the combination of oligotrophic habitat, high temperature and longer dry season these forests were flourishing as a unique evergreen ecosystem in the drier environment. The relationships found between leaf traits were in concurrence with earlier findings. Trees of TDEFs survive on the poor-nutrient habitat with a low SLA, high LDMC, and LL. This study adds baseline data on key leaf traits to plant functional trait database of India.

Leaf shedding increases the photosynthetic rate of the canopy in N2-fixing and non-N2-fixing woody species

It has long been hypothesized that timing of leaf shedding is critical for plant fitness but there is little experimental evidence to support the hypothesis. According to an optimality theory, shedding of old leaves increases canopy photosynthesis despite some nitrogen (N) being lost as litterfall, when the ratio of daily photosynthesis to leaf N (N-use efficiency, ε) in old leaves, expressed as a fraction of ε in new leaves, becomes lower than the fraction of leaf N that is resorbed before shedding (RN). This was shown to be true for N-poor plants but not for N-rich plants in a pot experiment; however, the use of planting pots imposes a variety of physical, chemical and biological constraints that could change the experimental results. Here we conducted a 3-year field survey in a cool temperate deciduous forest to examine whether Alnus sieboldiana Matsum. (N2-fixing) and Carpinus tschonoskii Maxim. (non-N2-fixing) shed their leaves to increase canopy photosynthesis in accord with the above criterion. These species often grow sympatrically and were chosen as representatives of N-rich and N-poor plants, respectively. Overall, daily photosynthesis decreased with leaf age, accompanied by small changes in leaf N, resulting in a decrease in ε. In both species, ε of leaves at shedding expressed as a fraction of ε in new leaves was nearly equal to RN in all years, implying that the old leaves were shed to increase canopy photosynthesis. Our results, together with those of previous field surveys, suggested that the timing of leaf shedding is explained by N use in maximizing canopy photosynthesis across broad groups of species.

Leaf economics of evergreen and deciduous tree species along an elevational gradient in a subtropical mountain

The ecophysiological mechanisms underlying the pattern of bimodal elevational distribution of evergreen tree species remain incompletely understood. Here we used leaf economics spectrum (LES) theory to explain such patterns. We measured leaf economic traits and constructed an LES for the co-existing 19 evergreen and 15 deciduous species growing in evergreen broad-leaved forest at low elevation, beech-mixed forest at middle elevation and hemlock-mixed forest at high elevation in Mao'er Mountain, Guangxi, Southern China (25°50'N, 110°49'E). Leaf economic traits presented low but significant phylogenetic signal, suggesting trait similarity between closely related species. After considering the effects of phylogenetic history, deciduous species in general showed a more acquisitive leaf strategy with a higher ratio of leaf water to dry mass, higher leaf nitrogen and phosphorous contents, higher photosynthetic and respiratory rates and greater photosynthetic nitrogen-use efficiency. In contrast, evergreen species exhibited a more conservative leaf strategy with higher leaf mass per area, greater construction costs and longer leaf life span. With the elevation-induced decreases of temperature and soil fertility, both evergreen and deciduous species showed greater resource conservation, suggesting the increasing importance of environmental filtering to community assembly with increasing elevation. We found close inter-specific correlations between leaf economic traits, suggesting that there are strong genetic constraints limiting the independent evolution of LES traits. Phylogenetic signal increased with decreasing evolutionary rate across leaf economic traits, suggesting that genetic constraints are important for the process of trait evolution. We found a significantly positive relationship between primary axis species score (PASS) distance and phylogenetic distance across species pairs and an increasing average PASS distance between evergreen and deciduous species with increasing elevation, implying that the frequency of distantly related evergreen and deciduous pairs with wide spreading of leaf economic values increases with increasing elevation. Our findings thus suggest that elevation acts as an environmental filter to both select the locally adapted evergreen and deciduous species with sufficient phylogenetic variation and regulate their distribution along the elevational gradient based on their coordinated spreading of phylogenetic divergence and leaf economic variation.