Rainfall drives leaf traits and leaf nutrient resorption in a tropical dry forest in Mexico.

Core leaf traits, such as leaf mass per area (LMA) and leaf nitrogen (N) and phosphorus (P) concentrations in green and senesced leaves, are currently used to discern patterns of variation, constraints, and relations to nutrient availability and climate among plant species up to the biomes and global scale. These traits and nutrient resorption were measured in 21 woody species in three phenological groups of the Chamela tropical dry forest (TDF) during a 3-year study period under conditions of contrasting rainfall. The LMA was similar among years and showed negative relationships with green-leaf N and P concentrations, in agreement with the global assessments. Community-level mean N concentrations in green and senesced leaves did not change among years, but P concentrations decreased with lower water availability in dry years. Both mean and minimum foliar N:P mass ratios increased with diminishing rainfall, suggesting that leaf stoichiometry reflects a water control on soil P limitation in this forest. In the wet year, green-leaf N and P concentrations were not correlated, and there were high coefficients of variation among species in leaf P and N:P ratios, indicating that species variability and physiological constraints are expressed in leaf stoichiometry upon the removal of water limitation. The resorption efficiency and proficiency of P, but not N, increased in years with low rainfall, suggesting that the costs of nutrient resorption relative to acquisition from soil differ between N and P and that P conservation increases when rainfall decreases in the Chamela TDF. Our results support the proposal that water availability controls P dynamics in this ecosystem.

Precipitation extremes in recent decades impact cattle populations at the global and national scales.

Cattle populations are one of the most important global ecological drivers. The global cattle population tripled during the past century, leading to large impacts on nutrient cycling, greenhouse gas emissions and biodiversity loss. Nonetheless, their populations have not increased uniformly through the last seven decades (1961-2018), with large unexplained variation between years. We hypothesized a main driver for such fluctuation was climate variability and thus examined global and national level relationships between cattle population growth and precipitation anomalies for the period 1961-2017. We showed that the variation in the global cattle population growth rate was related to precipitation anomalies following a distinctive parabolic relationship, where extreme wetness or dryness decreased population growth. When the analysis was downscaled to the national level, we found the strength of such relationship to be determined by the background climate and management intensity. Countries in drier climates and with less intensive cattle management showed the largest susceptibility to extreme annual precipitation. We propose a general model to explain the relationship between precipitation extremes and cattle populations at multiple scales, based on ecological processes applicable to grazing systems.

The decreasing range between dry- and wet- season precipitation over land and its effect on vegetation primary productivity.

One consequence of climate change is the alteration of global water fluxes, both in amount and seasonality. As a result, the seasonal difference between dry- (p < 100 mm/month) and wet-season (p > 100 mm/month) precipitation (p) has increased over land during recent decades (1980-2005). However, our analysis expanding to a 60-year period (1950-2009) showed the opposite trend. This is, dry-season precipitation increased steadily, while wet-season precipitation remained constant, leading to reduced seasonality at a global scale. The decrease in seasonality was not due to a change in dry-season length, but in precipitation rate; thus, the dry season is on average becoming wetter without changes in length. Regionally, wet- and dry-season precipitations are of opposite sign, causing a decrease in the seasonal variation of the precipitation over 62% of the terrestrial ecosystems. Furthermore, we found a high correlation (r = 0.62) between the change in dry-season precipitation and the trend in modelled net primary productivity (NPP), which is explained based on different ecological mechanisms. This trend is not found with wet-season precipitation (r = 0.04), These results build on the argument that seasonal water availability has changed over the course of the last six decades and that the dry-season precipitation is a key driver of vegetation productivity at the global scale.

Short-term consequences of slash-and-burn practices on the arbuscular mycorrhizal fungi of a tropical dry forest.

Rates of land conversion from forest to cultivated land by slash-and-burn practices are higher in tropical dry forest (TDF) than any other Neotropical forest type. This study examined the short-term consequences of the slash-and-burn process on arbuscular mycorrhizal fungi (AMF). We expected that slash-and-burn would reduce mycorrhizal colonization and propagules and change species richness and composition. Soil and root samples were taken from TDF control and pasture plots originated after slash-and-burn at four dates during the year of conversion to examine species composition, spore abundance, and infective propagules. Additionally, spore abundance and viability and viable intraradical colonization were measured twice during the second year after conversion. Forest and pasture plots maintained similar species richness and an overall 84% similarity during the first year after conversion. Infective propagules were reduced in pasture plots during the first year after slash-and-burn, whereas spore abundance and intraradical colonization remained similar in TDF and pasture plots both years of the study. Our results suggest, contrary to the expected, that forest conversion by means of slash-and-burn followed by cultivation resulted in few immediate changes in the AMF communities, likely because of the low heat conductivity of the soil and rapid combustion of plant residues.

Rainfall and labile carbon availability control litter nitrogen dynamics in a tropical dry forest.

N cycling in tropical dry forests is driven by rainfall seasonality but the mechanisms involved are not well understood. We studied the seasonal variation in N dynamics and microbial biomass in the surface litter of a tropical dry forest ecosystem in Mexico over a 2-year period. Litter was collected at 4 different times of the year to determine changes in total, soluble, and microbial C and N concentrations. Additionally, litter from each sampling date was incubated under laboratory conditions to determine potential C mineralization rate, net N mineralization, net C and N microbial immobilization, and net nitrification. Litter C concentrations were highest in the early-dry season and lowest in the rainy season, while the seasonal changes in N concentrations varied between years. Litter P was higher in the rainy than in the early-dry season. Water-soluble organic C (WSOC) and water-soluble N concentrations were highest during the early- and late-dry seasons and represented up to 4.1 and 5.9% of the total C and N, respectively. NH (4) (+) and NO (3) (-) showed different seasonal and annual variations. They represented an average 23% of soluble N. Microbial C was generally higher in the dry than in the wet seasons, while microbial N was lowest in the late-dry and highest in the early-rainy seasons. Incubations showed that lowest potential C mineralization rates and C and N microbial immobilization occurred in rainy season litter, and were positively correlated to WSOC. Net nitrification was highest in rainy season litter. Our results showed that the seasonal pattern in N dynamics was influenced by rainfall seasonality and labile C availability, and not by microbial biomass. We propose a conceptual model to hypothesize how N dynamics in the litter layer of the Chamela tropical dry forest respond to the seasonal variation in rainfall.