Carbon isotopic signatures of soil organic matter correlate with leaf area index across woody biomes

Abstract

Summary1. Leaf area index (LAI), a measure of canopy density, is a key variable for modelling and understanding primary productivity, and also water use and energy exchange in forest ecosystems. However, LAI varies considerably with phenology and disturbance patterns, so alternative approaches to quantifying stand-level processes should be considered. The carbon isotope composition of soil organic matter (d 13 C SOM ) provides a time-integrated, productivity-weighted measure of physiological and stand-level processes, reflecting biomass deposition from seasonal to decadal time scales. 2. Our primary aim was to explore how well LAI correlates with d 13 C SOM across biomes.3. Using a global data set spanning large environmental gradients in tropical, temperate and boreal forest and woodland, we assess the strength of the correlation between LAI and d 13 C SOM ; we also assess climatic variables derived from the WorldClim database. 4. We found that LAI was strongly correlated with d 13 C SOM , but was also correlated with Mean

Multi‐century stasis in C3 and C4 grass distributions across the contiguous United States since the industrial revolution

“…they are grassland plots) (Asner, Scurlock, & Hicke, ). As such, comparing local‐ and ecosystem‐level variation in δ 13 C proxies might also be valuable for studies that examine δ 13 C across broader LAI gradients (similar to Ladd et al., ) or for combination with phytolith data for improving palaeo‐LAI proxies (Dunn, Stromberg, Madden, Kohn, & Carlini, ). The SEM path from MAP to soil δ 13 C was positive, and harder to explain than the other paths because rainfall is expected to increase carbon isotope fractionation in woody C 3 vegetation (resulting in more negative δ 13 C), although this has not been investigated in mixed C 3 and C 4 ecosystems (Diefendorf et al., ; Kohn, ).…”

“…Ladd et al. () suggest that leaf area index (LAI) correlates well with soil δ 13 C across ecosystems because it reflects water use, but LAI showed very little variation among all grid cells and was therefore not included. All additional environmental/vegetation data were resampled onto the same grid as the isotope data as a simple mean.…”

“…δ 13 C [VPDB]) from soils and herbivore tissues are widely used as proxies of ecological properties and processes such as the relative abundance of C 3 and C 4 plants, water use efficiency in C 3 plants, productivity, trophic position, aridity, and tree cover (e.g. Cerling et al., ; Dawson, Mambelli, Plamboeck, Templer, & Tu, ; Diefendorf, Mueller, Wing, Koch, & Freeman, ; Kohn, ; Ladd et al., ; Still et al., ). Yet, δ 13 C values from such proxies have only rarely been compared directly to abundances of C 3 and C 4 source vegetation at the spatial resolution and extent of many biogeographical processes (e.g.…”

“…For example, trees are almost exclusively C 3 (Sage & Sultmanis, ) but their δ 13 C values can vary widely with plant physiology/morphology, biome, along environmental gradients [i.e. with mean annual precipitation (MAP) (Diefendorf et al., ; Kaplan, Prentice, & Buchmann, ; Kohn, ; Ladd et al., ), and in lock step with long‐term changes to the δ 13 C value of the atmosphere. In general, the present‐day δ 13 C value for C 4 grasses centres around −12.5 (±1.1‰) while C 3 grasses have a mean of −26.7 (±2.3‰) (Cerling et al., ), although the data come from arid environments, which would bias the results toward more positive values (Kohn, ).…”

“…For example, Ladd et al. () show that leaf area index (LAI) measured in situ can be represented well by soil δ 13 C across many ecosystems, but that remotely sensed LAI at 1 km is poorly correlated with soil δ 13 C.…”