The effect of microbial activity on soil water diffusivity

In this study, we explored the effects of microbial activity on the evaporation of water from cores of a sandy soil under laboratory conditions. We applied treatments to stimulate microbial activity by adding different amounts of synthetic analogue root exudates. For comparison, we used soil samples without synthetic root exudates as control and samples treated with mercuric chloride to suppress microbial activity. Our results suggest that increasing microbial activity reduces the rate of evaporation from soil. Estimated diffusivities in soil with the largest amounts of added root exudates were one third of those estimated in samples where microbial activity was suppressed by adding mercuric chloride. We discuss the effect of our results with respect to water uptake by roots.

Highlights

• We explored effects of microbial activity on the evaporation of water from cores of a sandy soil.
• We found the effect of microbial activity on water release characteristic was small.
• Increasing microbial activity reduced evaporation from soil, while microbial suppression increased it.
• Effect of microbial activity on root water uptake was estimated to be equivalent to a change in soil structure.

     

An evaluation of farmers’ experiences planting native trees in rural Panama: implications for reforestation with native species in agricultural landscapes

In the Republic of Panama, reforestation with native species is of great interest, but many landholders often do not participate in tree planting projects and little information exists about landholder interest in, or experiences with, native trees. This study evaluates the experiences of farmers participating in a native species reforestation initiative in rural Panama to identify lessons learned that can guide on-going or future tree planting efforts. Based on the results of a questionnaire administered to program participants and non-participants (n = 68), we found that trees are important to farmers for multiple reasons, primary a variety of environmental and economic benefits. No relationship between the size of landholdings or land tenure status and the desire to plant trees was found. All participants in the program considered their experience to be positive, few had problems with their plantations, and most were interested in planting more native trees. The program’s frequent and ongoing technical support was an important factor for farmers. These results indicate widespread interest in, and success with, planting native species and underscore the need to systematically examine farmers’ interests and perceptions when planning, implementing, and evaluating reforestation initiatives.     

Variation in canopy structure, light and soil nutrition across elevation of a Sri Lankan tropical rain forest

Mixed dipterocarp forests are perhaps the single most important rain forest type in the wet tropics. Only a few studies have purposefully examined differences in resource availability across mixed dipterocarp forest landscapes by simply measuring the abiotic variables of light, soil nutrition and soil water availability in relation to forest structure. We sought to directly measure the environment of canopy gaps across elevation and geology—from lowland mixed dipterocarp forest (100 m amsl) to lower montane dipterocarp forest (1200 m amsl) in southwest Sri Lanka. Middle elevation gap sites (300–900 m amsl) were subdivided into valley, mid-slope and ridge topographic positions. Eighteen natural disturbances all of which were canopy openings caused by tree fall, were randomly selected within primary rain forest that ranged across 100–1200 m elevation. Plots were placed in gap centers and in adjacent understories and measurements taken of forest structure (basal area, canopy height, canopy cover index, CCI), shade (light sensors—photosynthetically active radiation [PAR], canopy hemispherical photographs—global site factor [GSF]) and soil nutrition (pH, exchangeable Al, K, Mg and Ca; Total N; and plant available P). Soil moisture was measured at bi-weekly intervals for five years across middle elevation sites only (300–900 m amsl). Stand basal area, mean canopy height, and canopy cover index all declined with increase in elevation. Understory PAR and GSF decreased with increases in canopy height, basal area and CCI. Size of canopy opening decreased with increase in elevation, but PAR and GSF increased. Valley sites had significantly greater levels of mean percent soil water content as compared to mid-slope and ridge sites of middle elevation sites. However, at the onset of the southwest monsoons in May all sites were similar. Differences were most pronounced during the dry season (December–April). No differences in soil moisture content could be found between gap and understory microsites. K and Ca in gap centers and adjacent forest understories increased with increase in elevation and change in associated geology. pH increased and Al decreased with elevation and associated geology but only for forest understory conditions. Results demonstrate strong differentiation in soil and light resources with elevation that appears related to size of tree-fall disturbance, stature of the forest, topographic position and underlying geology and soil-weathering environment. This suggests that forest management and conservation practices need to develop and tailor techniques and treatments (silviculture) to the forest that emulate and/or account for change in elevation, geology and topographic position. Further studies are needed to identify which are the primary underlying mechanisms (e.g. temperature, wind, soil nutrients, soil moisture availability) defining change in forest structure across elevation.     

Spatial and semantic dimensions of landscape heterogeneity

This paper addresses the challenge of measuring spatial heterogeneity in categorical map data. Spatial heterogeneity is a complex notion that involves both spatial variability and attribute variability, and metrics to capture this are a product of their developers’ simplifying assumptions about both spatial and attribute dimensions. We argue that the predominantly binary treatment of categorical data is frequently an unnecessary oversimplification that can be replaced by ordered measures based on semantic similarity evaluations. We develop a typology of autocorrelation metrics for categorical data that identifies a critical gap: existing measures are limited in their ability to capture variability of both spatial and attribute dimensions simultaneously. We demonstrate an approach to formally characterize the semantic similarity between pairs of categorical data classes as a continuous numeric variable. A series of experiments on synthetic and actual land cover data contrasts the information content provided by metrics representative of all portions of the typology: the recently proposed semantic variogram, the indicator variogram, the contagion index, and the edge contrast index. Experimental results suggest that the typology captures essential qualities of metric information richness. Among our findings is that the commonly used contagion index is directly correlated with Moran’s I for 2-class maps but it fails to distinguish between negatively and positively autocorrelated patterns. We identify the semantic variogram as the only metric that can simultaneously detect both spatial and semantic attribute aspects of categorical autocorrelation. The semantic variogram is also relatively robust to attribute scale changes and therefore less sensitive to class aggregation than the other metrics.     

The importance of demographic niches to tree diversity

Most ecological hypotheses about species coexistence hinge on species differences, but quantifying trait differences across species in diverse communities is often unfeasible. We examined the variation of demographic traits using a global tropical forest data set covering 4500 species in 10 large-scale tree inventories. With a hierarchical Bayesian approach, we quantified the distribution of mortality and growth rates of all tree species at each site. This allowed us to test the prediction that demographic differences facilitate species richness, as suggested by the theory that a tradeoff between high growth and high survival allows species to coexist. Contrary to the prediction, the most diverse forests had the least demographic variation. Although demographic differences may foster coexistence, they do not explain any of the 16-fold variation in tree species richness observed across the tropics.