Peaks in frequency, but not relative abundance, occur in the center of tree species distributions on climate gradients

The Conditioned Environmental Center-Periphery Hypothesis of Biogeography: Statistical Evidence From Tree Species

It has been discussed for decades whether species occur most frequently at their geographic center, and more recently at their environmental niches' center. The aim here is to analyze for each environmental gradient separately the ecological niche of 12 Mexican tree species and 16 abiotic environmental gradients, in the form of statistical probability density functions. Is a symmetrically positioned center always possible by searching for additional data? For each species-variable combination, the occurrences along an environmental gradient were grouped in histograms. Logistic regression was used to fit a polynomial equation, whose degree depended on the number of significantly different bins. A highest-probability interval on the gradient was determined, where 25% of the individuals were found with the highest probability. The relative distance from the center (midpoint) of the variable's range was calculated, and the feasibility of expanding the encountered interval on the environmental gradient for symmetry was analyzed. For 183 species-variable combinations, in only 22 cases (12.0%) did the highest-probability intervals include the midpoint of the environmental gradient. Furthermore, for 55% of the species-variable combinations, the truncation of the environmental gradients for species makes it impossible to expand the measured range with additional data for the shorter tail. For example, precipitation cannot be negative. This truncation frequently causes asymmetry around the highest-probability intervals. In those cases, the classical environmental center-periphery hypothesis turns out to be wrong, whereas in the remaining cases it could apply. This has implications for biogeographical assumptions, such as where to identify the best areas for conservation or how to predict the effects of climate change. We propose a new conditioned environmental center-periphery hypothesis: "On an environmental gradient, a given species is able to cover a certain range. For environmental gradients, where natural truncation of the environmental gradient is not limiting, the highest probability of occurrence is found away from the range's endpoints, tending towards its midpoint."

Vapour pressure deficit modulates hydraulic function and structure of tropical rainforests under nonlimiting soil water supply

Atmospheric conditions are expected to become warmer and drier in the future, but little is known about how evaporative demand influences forest structure and function independently from soil moisture availability, and how fast-response variables (such as canopy water potential and stomatal conductance) may mediate longer-term changes in forest structure and function in response to climate change. We used two tropical rainforest sites with different temperatures and vapour pressure deficits (VPD), but nonlimiting soil water supply, to assess the impact of evaporative demand on ecophysiological function and forest structure. Common species between sites allowed us to test the extent to which species composition, relative abundance and intraspecific variability contributed to site-level differences. The highest VPD site had lower midday canopy water potentials, canopy conductance (gc ), annual transpiration, forest stature, and biomass, while the transpiration rate was less sensitive to changes in VPD; it also had different height-diameter allometry (accounting for 51% of the difference in biomass between sites) and higher plot-level wood density. Our findings suggest that increases in VPD, even in the absence of soil water limitation, influence fast-response variables, such as canopy water potentials and gc , potentially leading to longer-term changes in forest stature resulting in reductions in biomass.

Poor acclimation to experimental field drought in subalpine forest tree seedlings

The ability of tree species to acclimate and tolerate projected increases in drought frequency and intensity has fundamental implications for future forest dynamics with climate change. Inquiries to date on the drought tolerance capacities of tree species, however, have focused almost exclusively on mature trees with scant in situ work on seedlings, despite the central role that regeneration dynamics play in forest responses to changing conditions. We subjected naturally established seedlings of co-dominant subalpine conifer species (Abies lasiocarpa and Picea engelmannii) in the southern Rocky Mountains to 2 years of in situ summer precipitation exclusion, simulating summer drought conditions similar to a failure of the North American monsoon. We compared the morphological and physiological responses of seedlings growing in drought vs. ambient conditions to assess the relative changes in drought tolerance traits as a function of seedling size. Drought treatments had a marked impact on soil moisture: volumetric water content averaged ≈5-8 % in drought treatments and ≈8-12 % in ambient controls. We detected no significant shifts in morphology (e.g. root biomass, leaf:stem area ratio) in response to drought for either species, but net photosynthesis in drought treatments was 78 % lower for spruce and 37 % lower for fir. Greater stomatal control associated with increasing stem diameter conferred greater water use efficiencies in larger seedlings in both species but was not significantly different between drought and ambient conditions, suggesting an overall lack of responsivity to water stress and a prioritization of carbon gain over investment in drought mitigation traits. These results indicate a canonization of traits that, while useful for early seedling establishment, may portend substantial vulnerability of subalpine seedling populations to prolonged or recurrent droughts, especially for spruce.