Temperate rain forest species partition fine-scale gradients in light availability based on their leaf mass per area (LMA)

Abstract

Background and Aims Ecologists are increasingly using plant functional traits to predict community assembly, but few studies have linked functional traits to species' responses to fine-scale resource gradients. In this study, it was tested whether saplings of woody species partition fine-scale gradients in light availability based on their leaf mass per area (LMA) in three temperate rain forests and one Mediterranean forest in southern Chile.Methods LMA was measured under field conditions of all woody species contained in approx. 60 plots of 2 m 2 in each site, and light availability, computed as the gap light index (GLI), was determined. For each site, species' pairwise differences in mean LMA (D LMA) and abundance-weighted mean GLI (D light response) of 2 m 2 plots were calculated and it was tested whether they were positively related using Mantel tests, i.e. if species with different LMA values differed in their response to light availability. Additionally linear models were fitted to the relationship between plot-level mean LMA and GLI across plots for each site.Key Results A positive and significant relationship was found between species' pairwise differences in mean LMA and differences in light response across species for all temperate rain forests, but not for the Mediterranean forest. The results also indicated a significant positive interspecific link between LMA and light availability for all forests. This is in contrast to what is traditionally reported and to expectations from the leaf economics spectrum.Conclusions In environments subjected to light limitation, interspecific differences in a leaf trait (LMA) can explain the fine-scale partitioning of light availability gradients by woody plant species. This niche partitioning potentially facilitates species coexistence at the within-community level. The high frequency of evergreen shadeintolerant species in these forests may explain the positive correlation between light availability and LMA.

Intraspecific trait variation and the leaf economics spectrum across resource gradients and levels of organization

“…This pattern, which is contrary to what is commonly found at the interspecific level in tropical and northern hemisphere temperate forests (Walters and Reich , Poorter et al. ), was also found across species in the temperate rainforest of southern Chile (Fajardo and Siefert ).…”

The interplay among intraspecific leaf trait variation, niche breadth and species abundance along light and soil nutrient gradients

“…Specifically, species that occurred across a wide range of light environments had high ITV in leaf N concentration, and species broadly distributed along the soil P availability gradient had high ITV in leaf N and P. Importantly, we found these positive relationships between niche breadth and ITV by comparing species’ ITV measured across a common set of environments. Research conducted in these southern hemisphere cool temperate rainforests thus far (Lusk , Fajardo and Siefert , Peltzer and Wardle ) has commonly focused on light availability as the limiting factor, especially, for regeneration, but here we show that soil N and P availability had a strong positive relationship with leaf trait variation, indicating that multiple environmental filters drive regeneration dynamics (see also Lusk and Laughlin , Fajardo and Siefert ). Although some studies have found that N is the most limiting nutrient in soils of temperate rainforests in southern Chile (Huygens et al ), the volcanic origin of these soils makes them very poor in available P for plants (large amount of allophane and sesquioxide in the soil, which have a high phosphate‐fixing capacity, making P not readily available for plants) (Fajardo and Piper ).…”

“…In temperate rainforests of southern Chile, where as many as 25 woody species per hectare can co‐occur within forest communities, species can be found in a variety of local environmental conditions, from deep shade to tree‐fall canopy gaps (Veblen 1985, Lusk , Fajardo and Siefert , ), and from spots with poor to rich nutrient availability. Given these apparent strong environmental gradients, we expected that species achieving high local abundance would be those that could tolerate a wide range of environments by adjusting their trait values to local conditions (Darwin 1859, Sides et al ).…”

“…We selected four second‐growth, temperate rainforests for our study (Fig. 1) (Fajardo and Siefert , ). The first site was Exploradores (EXP, 46°29′S, 73°12′W, 75 m above sea level), where the forest is on moraine deposits of glacial erosion originating from the Exploradores Glacier (Aniya et al ).…”

“…To test these hypotheses, we examined species’ niche breadth along three environmental gradients that are important drivers of community assembly in temperate rainforests: light, soil N and soil P availability. The partitioning of light and soil nutrient availability gradients encountered in the early phases of a plant's life cycle is known to play a central role in coexistence of woody species (Grubb , Poorter , Fajardo and Siefert ). To relate patterns of ITV to species’ niche breadth along these gradients, we focused on four leaf traits – leaf area (cm 2 ), leaf mass per area (LMA, g m −2 ), leaf nitrogen (N) and phosphorus (P, mg g −1 ) concentrations.…”