Global dependence of field‐observed leaf area index in woody species on climate: a systematic review

Abstract

Aim Leaf area index (LAI) is one of the key variables related to carbon, water and nutrient cycles in terrestrial ecosystems, but its global distribution patterns remain poorly understood. We evaluated the dependence of LAI on mean annual temperature (MAT) and wetness index (WI; a ratio of annual precipitation to potential evapotranspiration) for three plant functional types (PFTs: deciduous broadleaf, DB; evergreen conifer, EC; evergreen broadleaf, EB) at the global scale.Location Global. MethodsWe developed a new global database of unprecedented size (2606 published values) of field-observed LAI (site-specific maximum) values for vegetation of woody species. To maximize the generic applicability of our analysis, we standardized the definition of LAI, and corrected or excluded potentially erroneous data obtained from indirect optical methods. ResultsThe global dependence of LAI on MAT showed a reverse S-shaped pattern, in which LAI peaked at around 8.9 and 25.0°C and was lowest at around −10.0 and 18.8°C. The dependence on WI followed a saturation curve levelling off at around log WI = 0.30. LAI for evergreen forests increased linearly with increasing WI, but that for DB showed a curvilinear pattern saturating at log WI = 0.03. EC forests had higher LAI values than those of DB forests under cool conditions (MAT ≤ 8.9°C), but similar values under temperate conditions (MAT = 8.9-18.8°C).Main conclusions This analysis of global LAI−climate relationships supports the general belief that temperature limits LAI under cool conditions whereas water availability plays a predominant role under other conditions. We also found that these patterns differed significantly between PFTs, suggesting that the LAI of different PFTs may respond differently to climate change. Our study provides a broad empirical basis for predicting the global distribution of LAI and for analysing the effects of global climate change on vegetation structure and function.

Tropical forest canopies and their relationships with climate and disturbance: results from a global dataset of consistent field-based measurements

“…We emphasize that we have used a consistent method to derive our canopy structure estimates for each plot. This is an important difference to previous analyses (Iio et al 2014), because optical instruments can produce large discrepancies between canopy structure estimates in particular for lowheight canopies and canopies with senescent vegetation and high spatial heterogeneity (Garrigues et al 2008). …”

“…Previous studies focussing on temperate natural forests and forest plantations (Iio et al 2014) and tropical forests in East Africa (Pfeifer et al 2014) have found a decline in canopy LAI in regions with very high levels of rainfall and water availability. This is in line with studies showing a decline in NPP of humid tropical forests under high rainfall regimes, with NPP peaking at around 2500 mm mean annual rainfall, with subsequent declines linked to decreased radiation inputs (high cloud cover), increased nutrient leaching, or reduced soil oxygen Fig.…”

“…The latter is echoed by canopy structure data on natural forests, which are largely from temperate regions (Iio et al 2014), but also by data from tropical East Africa (Pfeifer et al 2014). This suggests that canopy LAI is limited by water availability at the lower end of the rainfall spectrum and by cloud cover variability at the upper end, the latter regulating incident solar radiation on forest canopies and, hence, constraining vegetation productivity (Nemani et al 2003).…”

“…In order to project future global change in tropical canopy structure variables, a benchmark database for contemporary climates is needed. Although there are large and increasing databases for Northern hemisphere temperate and boreal ecosystems (Iio et al 2014), measurements of key forest canopy variables for tropical regions are underrepresented (Pfeifer et al 2014).…”

Leaf Area Prediction Using Three Alternative Sampling Methods for Seven Sierra Nevada Conifer Species

“…Leaf area index (LAI) is an important variable for understanding architecture and biological processes in trees [1], designing silvicultural treatments [2], for landscape-level ecosystem process models [3], or global forest functions [4]. LAI has been defined as the sum of projected (one-sided), or all-sided leaf areas, for all trees in a given area divided by the land surface area the trees cover [5].…”