ENSO and NAO affect long‐term leaf litter dynamics and stoichiometry of Scots pine and European beech mixedwoods

Abstract

Litterfall dynamics (production, seasonality and nutrient composition) are key factors influencing nutrient cycling. Leaf litter characteristics are modified by species composition, site conditions and water availability. However, significant evidence on how large‐scale, global circulation patterns affect ecophysiological processes at tree and ecosystem level remains scarce due to the difficulty in separating the combined influence of different factors on local climate and tree phenology. To fill this gap, we studied links between leaf litter dynamics with climate and other forest processes, such as tree‐ring width (TRW) and intrinsic water‐use efficiency (iWUE) in two mixtures of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) in the south‐western Pyrenees. Temporal series (18 years) of litterfall production and elemental chemical composition were decomposed following the ensemble empirical mode decomposition method and relationships with local climate, large‐scale climatic indices, TRW and Scots pine's iWUE were assessed. Temporal trends in N:P ratios indicated increasing P limitation of soil microbes, thus affecting nutrient availability, as the ecological succession from a pine‐dominated to a beech‐dominated forest took place. A significant influence of large‐scale patterns on tree‐level ecophysiology was explained through the impact of the North Atlantic Oscillation (NAO) and El Niño–Southern Oscillation (ENSO) on water availability. Positive NAO and negative ENSO were related to dry conditions and, consequently, to early needle shedding and increased N:P ratio of both species. Autumn storm activity appears to be related to premature leaf abscission of European beech. Significant cascading effects from large‐scale patterns on local weather influenced pine TRW and iWUE. These variables also responded to leaf stoichiometry fallen 3 years prior to tree‐ring formation. Our results provide evidence of the cascading effect that variability in global climate circulation patterns can have on ecophysiological processes and stand dynamics in mixed forests.

Interactions between Climate and Nutrient Cycles on Forest Response to Global Change: The Role of Mixed Forests

“…Growth responses to environmental shifts are difficult to infer from tree responses to single factors [117]. However, observational or experimental multifactor studies in forests are still scarce (refer to [35,95,108,109,118]).…”

“…Litterfall constitutes a major proportion of nutrient cycling between plant and soils in forest ecosystems [168]. Climatic conditions are closely linked to variations in litterfall annual production [169][170][171][172][173], seasonal patterns [118,171,[174][175][176], and nutrient composition [118,[177][178][179][180]. Hence, projected increasing temperatures and alterations of precipitation regimes will have striking consequences on litterfall dynamics.…”

“…Globally, moist and warm conditions reduce and enhance N and P resorption efficiencies, respectively [178,193]. Furthermore, moisture conditions largely modulate the effect of rising Ca and N deposition on the stoichiometry of nutrient recycling [118,192].…”

“…Forests 2019, 10, x FOR PEER REVIEW 9 of 25 moisture conditions largely modulate the effect of rising Ca and N deposition on the stoichiometry of nutrient recycling [118,192]. The consumer-driven nutrient recycling theory predicts that the balance between litter stoichiometry and microbial element use efficiency directly determines nutrient recycling ratios [159].…”

Coarse Woody Debris’ Invertebrate Community is Affected Directly by Canopy Type and Indirectly by Thinning in Mixed Scots Pine—European Beech Forests

“…Controlling stand density by thinning could reduce CWD water content under a coniferous canopy as CWD are exposed to radiation and wind. However, under mixed conifer-broadleaf canopy in which a species such as European beech can expand its crown in detriment of Scots pine [56], thinning could actually have the opposite effect. In addition, these effects can change over time, as a moist CWD class 3 could be quickly colonized, generating a CWD class 4 with higher relative surface due to boring and drilling, which in turn could cause faster desiccation in a Mediterranean environment with greatly variable precipitation in late spring-early summer.…”