Twelve years of low nutrient input stimulates growth of trees and dwarf shrubs in the treeline ecotone

Abstract

Almost all natural terrestrial ecosystems are nutrient limited in terms of growth, and we expect treeline vegetation to be no exception. However, direct constraints of low temperature on tissue formation may superimpose effects of low nutrient availability. We examined growth responses of two tree (Larix decidua and Pinus uncinata) and two dwarf shrub species (Vaccinium myrtillus and Vaccinium gaultherioides) to 12 years of moderate fertilizer addition (NPK applied at a rate of 15 and 30 kg nitrogen ha−1 a−1) along an elevation gradient within the treeline ecotone in the Swiss Alps (2,083 to 2,225 m a.s.l.). We measured annual top‐ and side‐shoot increments as well as stem ring width in trees and shoot increments in dwarf shrubs. Fertilizer addition increased soil nutrient availability, indicated by enhanced soil extractable N, higher concentrations of N, P and K in leaves and higher foliar δ15N. Fertilizer addition stimulated annual growth of all four species: by 11%–20% for L. decidua and 15%–36% for P. uncinata (depending on trait) and by 8%–16% for the two dwarf shrub species. Growth stimulation by the higher fertilizer dose was not significantly stronger than by the lower dose (except for V. gaultherioides), suggesting an overall low nutrient demand for growth and saturation at a rather low nutrient input. Synthesis. Even slightly enhanced nutrient availability can stimulate growth of trees and dwarf shrubs in an alpine treeline ecosystem. Ongoing atmospheric nutrient deposition, in conjunction with global warming, may accelerate plant growth at the treeline.

Climate implications on forest above- and belowground carbon allocation patterns along a tropical elevation gradient on Mt. Kilimanjaro (Tanzania)

“…We assume that the temperature effect on N cycling and N supply and resulting reductions in leaf area (and possibly photosynthetic capacity) represent a major pathway through which elevation is acting on total C gain. In addition, tree growth was found to be stimulated by moderate nitrogen fertilization at the tree line ecotone in the Swiss Alps (Möhl et al 2018 ), supporting our assumption of nutrient limitation of aboveground tree productivity at high elevations. The replacement of macrophyllous, broadleaved trees by southern hemispheric conifers at the upper montane zone and ericoid trees with more sclerophyllous leaves and their specialized ericoid mycorrhiza at the subalpine zone can also be interpreted as an indication of increasing nutrient limitation at higher elevations.…”

Leaf and Soil δ15N Patterns Along Elevational Gradients at Both Treelines and Shrublines in Three Different Climate Zones

“…Although temperature has been suggested as a primary driver underlying the formation of such boundaries [20], low soil nutrient availability may also be responsible for reduced growth of trees and shrubs at their upper limits [21]. A recent study found that the growth of trees and shrubs in an alpine ecosystem increased with slightly increased nutrient availability [22], which in turn suggesting the potential increasing nutrient dynamics with the expansion of trees or shrubs. Thus, understanding changes of N cycling at alpine ecosystems is particularly important as both alpine treelines and shrublines may shift in the face of global climate change [23][24][25].…”

Contrasting plant responses to multivariate environmental variations among species with divergent elevation shifts

“…2013). These highland nutrient limitations are further supported by field experiments in which fertilization stimulated highland forest growth (Möhl 2019). Therefore, highland nutrient limitation could determine upper limits to plant distributions (Rehm and Feeley 2015).…”