Leaf longevity in temperate evergreen species is related to phylogeny and leaf size

Leaf longevity (LL), the amount of time a photosynthetically active leaf remains on a plant, is an important trait of evergreen species, affecting physiological ecology and ecosystem processes. A long LL gives leaves more time to fix carbon but carries higher construction costs, while a short LL allows plants to respond more rapidly to changing environmental conditions. For many evergreen taxa, LL data are not readily available, and it is not known if LL is phylogenetically conserved. To address this gap, we measured LL for 169 temperate and boreal evergreen woody species at the Arnold Arboretum, a botanical garden in Boston, Massachusetts, along with metrics of leaf size and number known to be related to LL. We hypothesized that LL is phylogenetically conserved, and that longer LL is associated with a greater numbers of leaves, smaller leaves, and a colder hardiness zone of the species' native range. We found that average LL ranged from 1.4 years in Rhododendron tomentosum to 10.5 years in Abies cilicia. LL was phylogenetically conserved, with some genera, such as Abies and Picea, exhibiting long LL (> 3 years) and others, such as Ilex and Rhododendron, exhibiting short LL (< 3 years). Leaf length was negatively correlated with LL in conifers, due to differences between Pinus and other genera; however, there was no correlation between LL and number of leaves. This study highlights the considerable variation and phylogenetic pattern in LL among temperate evergreen species, which has implications for carbon budgets and ecosystem models.

Leaf functional trait variation in a humid temperate forest, and relationships with juvenile tree light requirements

The species-rich arborescent assemblages of humid tropical forests encompass much of the known range of the leaf economics spectrum, often including >20-fold variation in leaf lifespan. This suite of traits underpins a life-history continuum from fast-growing pioneers to slow-growing shade-tolerant species. Less is known about the range of leaf traits in humid temperate forests, and there are conflicting reports about relationships of these traits with the light requirements of temperate evergreen angiosperms. Here I quantify the range of leaf functional traits in a New Zealand temperate evergreen forest, and relationships of these traits with light requirements of juvenile trees and shrubs. Foliage turnover of saplings of 19 evergreen angiosperms growing beneath gaps (12-29% canopy openness) and in understories (1.2-2.9%) was measured over 12 months. Dry mass per area (LMA), dry matter content, thickness, density and nitrogen content (N) of leaves were also measured. Species minimum light requirements were indexed as the 10th percentile of the distribution of saplings in relation to canopy openness. Interspecific variation of leaf lifespan was ∼6-fold in gaps (0.6 to 3.8 yrs), and ∼11-fold in the understorey (0.7 to 7.7 yrs). Six small tree and shrub species are effectively leaf-exchangers, with leaf lifespans of c.1 year in gaps-albeit usually longer in the shade. Interspecific variation in other leaf traits was 2.5 to 4-fold. Lifespans and LMA of both sun and shade leaves were negatively correlated with species light requirements i.e., positively correlated with shade tolerance. However, light environment (gap vs shade) explained about the same amount of variation in LMA as species' identity did. Species light requirements were not significantly correlated with leaf N, dry matter content, density or thickness-except for a marginally significant correlation with dry matter content of shade leaves. Species light requirements were thus less consistently related to leaf structural traits than appears to be the case in humid tropical forests. Whereas the wide interspecific variation in leaf economic traits of tropical rainforest species outweighs plastic response to light availability, temperate evergreen woody angiosperms appear to occupy a narrower range of the leaf economic spectrum. Standardization of the light environments in which LMA is measured is vital in comparative studies of humid temperate forest evergreens, because of countergradient responses of this trait to light, and because of the relative magnitudes of plastic and interspecific variation in LMA in these forests.

Long-term simulated nitrogen deposition alters the plant cover dynamics of a Mediterranean rosemary shrubland in Central Spain through defoliation

Nitrogen (N) deposition due to anthropogenic pollution is a major driver of the global biodiversity loss. We studied the effect of experimental N and phosphorus (P) fertilization (0, 10, 20, and 50 kg N ha^-1 year^-1 and 14 kg P ha^-1 year^-1 over the background deposition levels) on plant cover dynamics of a rosemary (Rosmarinus officinalis L.) shrubland after 8 years of nutrient addition in a semiarid Mediterranean ecosystem from Central Spain. We specifically aimed at testing whether N deposition has the potential to influence the observed expanding trend of woody vegetation into areas dominated by grassland, biological soil crusts, and bare soil. Our results show that N addition loads above 10 kg N ha^-1 year^-1 reverted the cover dynamics of shrubs. Under N addition conditions, N was no longer a limiting nutrient and other elements, especially P and calcium, determined the seasonal growth of young twigs. Interestingly, N fertilization did not inhibit the growth of young shoots; our estimates point to a reduced rosemary leaf lifespan that is driving individuals to death. This may be triggered by long-term accumulation of N compounds in leaves, suggesting the need to consider the old organs and tissues in long-lived perennial plants, where N toxicity effects could be more mediated by accumulation processes. Shrublands are a widely distributed ecosystem type in biodiverse Mediterranean landscapes, where shrubs play a key role as nurse plants. Therefore, the disappearance of shrublands may accelerate the biodiversity loss associated with other global change drivers, hamper the recruitment of seedlings of woody species, and, as a consequence, accelerate desertification.

Phylogenetic structural equation modelling reveals no need for an 'origin' of the leaf economics spectrum

The leaf economics spectrum (LES) is a prominent ecophysiological paradigm that describes global variation in leaf physiology across plant ecological strategies using a handful of key traits. Nearly a decade ago, Shipley et al. (2006) used structural equation modelling to explore the causal functional relationships among LES traits that give rise to their strong global covariation. They concluded that an unmeasured trait drives LES covariation, sparking efforts to identify the latent physiological trait underlying the 'origin' of the LES. Here, we use newly developed phylogenetic structural equation modelling approaches to reassess these conclusions using both global LES data as well as data collected across scales in the genus Helianthus. For global LES data, accounting for phylogenetic non-independence indicates that no additional unmeasured traits are required to explain LES covariation. Across datasets in Helianthus, trait relationships are highly variable, indicating that global-scale models may poorly describe LES covariation at non-global scales.

Photosynthetic induction times in shade-tolerant species with long and short-lived leaves

In the understory of a tropical rainforest, light flecks can contribute 10-80% of the total light flux. We investigated the capacity of eight shade-tolerant species to use light flecks by examining the time required for full induction of photosynthesis during an artificial light fleck. CO₂ fixation rates were measured with a portable LiCor gas-exchange system for plants growing in the field on Barro Colorado Island, Panama. In all species induction to 50% of maximum CO₂ fixation occurred quickly, from 1 to 3 min. In species with short leaf lifetimes (1 year), induction to 90% of maximum also occurred quickly, in 3-6 min. In contrast, the species with longer lived leaves (>4 years) required 11-36 min for induction to 90% of maximum. Induction times for leaves from gap and understory plants of the same species were indistinguishable. Elevated CO₂ did not eliminate the slow induction phase of long-lived leaves. This suggests that slow induction did not result from stomatal limitation. O₂ evolution, measured on excised leaf disks, induced in 3-4 min in species with short-lived leaves, and 4-8 min in species with long-lived leaves. The rapid induction of O₂ evolution indicates that the slower induction of CO₂ fixation in long-lived leaves was not caused by a delay in the induction of electron transport. Activation of rubisco may be the major factor limiting response times in species with long-lived leaves. Species from Panama with short-lived leaves had remarkably rapid induction times that are comparable to those of algae or higher plant chloroplasts. We also found that understory forest plants induced two to seven times more quickly than did potted plants.