Convergence and decoupling of ecosystem properties at treeline ecotones of the world
Elevation gradients, as natural experiments, have long offered researchers the ability to understand the response of ecosystem properties to elevation associated processes at macroecological timescales. Elevation associated temperature gradients impact both above and below ground ecosystem properties and offer insight into future climate change factors. The formation of treelines – the threshold where trees can no longer grow large due to cold conditions – is perhaps the most conspicuous incarnation of temperature controls over ecosystem properties. Recently, it has been proposed that nutrient limitations resulting from temperature inhibition of microbial activity may also contribute to treeline formation and nutrient cycling processes. Determining if such processes are widespread, rather than idiosyncratic, has been difficult because most studies originate from single-site transects or use syntheses of data derived from divergent methodologies or temperature ranges. We overcame these limitations by using replicate transects and consistent methodologies across seven regions in both Northern and Southern hemispheres. We asked how above and belowground ecosystem properties vary along 300 m transects spanning thermally consistent treeline transition zones – a range representing an approximately 2ºC decline in temperature.
Tree heights and relative basal areas declined yet stem densities increased in response to declining temperatures towards treeline. Community weighted plant N and P concentrations declined with elevation but were coupled only to soil N availability in most regions. Plant and root N/P ratios at lower forested elevations suggest variable nutrient limitation to plant productivity across regions, yet global similarities at high elevation indicate a stoichiometric convergence under colder and more stressful N-limited alpine environments. Decomposed components of community weighted plant nutrient variation indicated turnover of plant species along the elevation gradients contributed more than intraspecific variability or the presence of tree cover. Below ground properties, such as soil fertility, microbial community composition, and microbial activity, similarly varied with elevation but were more responsive to the influence of tree cover on soil properties than to gradual changes in temperature. As such, the coupling of ecosystem responses suggests that the effects of global climate change will in part depend on plant-soil feedbacks largely dependent on slowly expanding treelines. Such gradual processes will likely lag behind and delay the upward migration of many plant species adapted to high nutrient availabilities and less stressful growing conditions.