Controls on fallen leaf chemistry and forest floor element masses in native and novel forests across a tropical island

Abstract

Abstract. Litter chemistry varies across landscapes according to factors rarely examined simultaneously.We analyzed 11 elements in forest floor (fallen) leaves and additional litter components from 143 forest inventory plots systematically located across Puerto Rico, a tropical island recovering from large-scale forest clearing. We assessed whether three existing, independently-derived, landscape classifications (Holdridge life zone, remotely sensed forest type (leaf longevity combined with geology generalized to karst vs. non-karst), and plot-based measures of forest assemblage) would separate observed gradients. With principal component and regression analyses, we also tested whether climate-, landscape-(geology, elevation, aspect, percent slope, slope position, distance from coast), and stand-scale (tree species composition, basal area, density, stand age) variables explained variation in fallen leaf chemistry and stoichiometry. For fallen leaves, C, Ca, Mg, Na, and Mn concentrations differed by Holdridge life zone and C, P, Ca, Mn, Al, and Fe concentrations differed by forest type, where leaf longevity distinguished C and Ca concentrations and geology distinguished C, P, Ca, Mn, Al, and Fe concentrations. Fallen leaf C, P, Ca, and Mn concentrations also differed, and N concentrations only differed, by forest assemblage. Across several scales, fallen leaf N concentration was positively related to the basal area of putatively N 2 -fixing tree legumes, which were concentrated in lower topographic positions, providing for the first time a biological explanation for the high N concentrations of fallen leaves in these locations. Phosphorus concentrations in fallen leaves by forest assemblages also correlated with the basal area of N 2 -fixing legumes, and P and N concentrations decreased with mean age of assemblage. Fallen leaves from younger (,50 yr, 86% of the plots) and often novel forests had higher P, Fe, and Al and lower C concentrations and lower C/P and N/P ratios than fallen leaves from older forests, the latter due to a decrease in P rather than changes in N. These findings suggest that both N and P availability may currently be greater on the island than pre-deforestation, and substantiate the unique roles that state factors play in contributing to the spatial heterogeneity of fallen leaf chemistry.

Novelty and Its Ecological Implications to Dry Forest Functioning and Conservation

“…Although these communities are commonly found in the Dry Forest Life Zone (sensu Holdridge), each community contains plots located in wetter life zones. We note that there are novel forest communities with greater mean annual precipitation than these five that are considered in the Erickson et al [13] paper but not here, where the emphasis is on drier novel and native communities. Modeled mean annual precipitation (cf.…”

“…Modeled mean annual precipitation (cf. [13]) for individual plots in this study ranges from 787 mm to 2322 mm. Although the species assemblages used in this analysis are identified by the dominant tree species, the leaf litter samples that were chemically analyzed represent the litter of all the species in the stand, not necessarily a monospecific leaf litter.…”

“…For the analysis of element chemistry and stoichiometry of novel and native dry forests, we build on the study of Erickson et al [13] who analyzed fallen leaf litter mass and chemistry (11 elements) in 140 plots located across Puerto Rico within 14 of the species assemblages described by Brandeis et al [14]. Using the same data set as in Erickson et al [13], we selected the five driest forest communities for a total of 41 plots.…”

“…Using the same data set as in Erickson et al [13], we selected the five driest forest communities for a total of 41 plots. Although these communities are commonly found in the Dry Forest Life Zone (sensu Holdridge), each community contains plots located in wetter life zones.…”

“…Our objective is to assess the ecological implications of novelty in Puerto Rico's dry forests through a synthesis of published information coupled with a new analysis of island-wide fallen leaf chemistry, first reported in Erickson et al [13]. To accomplish this goal, we need to place dry forests in environmental and historical contexts so that the adaptive role of novelty can be revealed.…”