Leaf economics and plant hydraulics drive leaf : wood area ratios.

Biomass and area ratios between leaves, stems and roots regulate many physiological and ecological processes. The Huber value Hv (sapwood area/leaf area ratio) is central to plant water balance and drought responses. However, its coordination with key plant functional traits is poorly understood, and prevents developing trait-based prediction models. Based on theoretical arguments, we hypothesise that global patterns in Hv of terminal woody branches can be predicted from variables related to plant trait spectra, that is plant hydraulics and size and leaf economics. Using a global compilation of 1135 species-averaged Hv , we show that Hv varies over three orders of magnitude. Higher Hv are seen in short small-leaved low-specific leaf area (SLA) shrubs with low Ks in arid relative to tall large-leaved high-SLA trees with high Ks in moist environments. All traits depend on climate but climatic correlations are stronger for explanatory traits than Hv . Negative isometry is found between Hv and Ks , suggesting a compensation to maintain hydraulic supply to leaves across species. This work identifies the major global drivers of branch sapwood/leaf area ratios. Our approach based on widely available traits facilitates the development of accurate models of above-ground biomass allocation and helps predict vegetation responses to drought.

The hemiparasitic angiosperm Bartsia alpina has the potential to accelerate decomposition in sub-arctic communities.

We investigated the hypothesis that hemiparasites accelerate nutrient cycling in nutrient-poor communities. Hemiparasites concentrate nutrients in their leaves, thus potentially producing high quality litter that releases nutrients that would otherwise remain in host tissues or in slowly decomposing plant litter. This hypothesis was tested using species from a European sub-arctic community where root hemiparasites are abundant. The N content of green leaves, and the N, P and C content of leaf litter were measured in seven species of root hemiparasitic Scrophulariaceae, and nine species of commonly co-occurring dwarf shrubs, graminoids and herbs. Fresh leaves of the hemiparasites had greater N concentrations than leaves of dwarf shrubs, graminoids or herbs. This difference was even more marked in litter, with hemiparasite litter containing 1.8-4.1% N, between 1.8 and 8.5 times as much N as in the litter of commonly co-occurring species. Litter of the hemiparasitic plant Bartsia alpina and of three commonly co-occurring dominant species of dwarf shrub was decomposed alone and in two species mixtures, in a laboratory microcosm experiment. Bartsia litter decomposed faster and lost between 5.4 and 10.8 times more N than that of the dwarf shrubs over the 240 days of the experiment. Mixtures of dwarf shrub and hemiparasite litter showed significantly more mass loss and CO2 release than expected, while nutrient release was the same as or less than expected. It is concluded that hemiparasites have the potential to enhance decomposition and nutrient cycling in nutrient-poor environments.