Belowground morphology and population dynamics of two forest understory herbs of contrasting growth form

Forest understory herbs exhibit a large range of variation in morphology and life history. Here we expand the reported range of variation by describing the belowground structures of two very different species, Xerophyllum tenax and Chimaphila umbellata. We excavated individuals in forests of the Cascade Mountains, Pacific Northwest, USA. Xerophyllum tenax has short rhizomes, but an extensive root system that is exceptionally large among forest understory species. The roots reach 4 m in length and may occupy an area 50 times that of the aboveground canopy. In contrast, Chimaphila umbellata has very small roots, but an extensive rhizome system. The largest plant we excavated had 57 m of connected rhizomes and still had a seedling source. Both species have long-lived individuals but differ in response to disturbance. Based on monitoring of 151 permanent 1 m2 plots in an old-growth forest, X. tenax increased only minimally in density over 40 years following tephra deposition from the 1980 eruption of Mount St. Helens, whereas density of C. umbellata increased substantially. The very different morphology of these two species highlights the large range of variation found among forest herbs, which needs to be considered when examining the forest understory.

Functional traits of broad-leaved monocot herbs in the understory and forest edges of a Costa Rican rainforest

Broad-leaved monocot herbs are widespread and dominant components of the shaded understories of wet neotropical forests. These understory habitats are characterized by light limitation and a constant threat of falling branches. Many shaded understory herb species have close relatives that occupy forest edges and gaps, where light availability is higher and defoliation threat is lower, creating an opportunity for comparative analysis of functional traits in order to better understand the evolutionary adaptations associated with this habitat transition. We documented ecological, morphological and ecophysiological traits of multiple herb species in six monocot families from each of these two habitats in the wet tropical rainforest at the La Selva Biological Station, Costa Rica. We found that a mixture of phylogenetic canalization and ecological selection for specific habitats helped explain patterns of functional traits. Understory herbs were significantly shorter and had smaller leaves than forest edge species. Although the mean number of leaves per plant and specific leaf area did not differ between the two groups, the larger leaf size of forest edge species gave them more than three times the mean plant leaf area. Measures of leaf water content and nitrogen content varied within both groups and mean values were not significantly different. Despite the high leaf nitrogen contents, the maximum photosynthetic rates of understory herbs were quite low. Measures of δ13C as an analog of water use efficiency found significantly lower (more negative) values in understory herbs compared to forest edge species. Clonality was strongly developed in several species but did not show strong phylogenetic patterns. This study highlights many functional traits that differ between broad-leaved monocot species characteristic of understory and forest edge habitats, as well as traits that vary primarily by phylogenetic relatedness. Overall, plant functional traits do not provide a simple explanation for the relative differences in abundance for individual understory and forest edge species with some occurring in great abundance while others are relatively rare.

Influence of nitrogen fertilization on abundance and diversity of plants and animals in temperate and boreal forests

Aerial and land-based applications of nitrogen-based fertilizers to enhance forest growth makes nutrients potentially available to all trees, plants, and wildlife in a given ecosystem and, therefore, may have direct and indirect effects on wildlife and biodiversity. A scientific review of these potential effects was conducted with 106 published studies covering vascular and nonvascular plants, amphibians, birds, mammals, terrestrial invertebrates, and soil animals associated with fertilization in temperate and boreal forests, primarily in North America and Scandinavia. In terms of direct effects, amphibians and domestic mammals appear to be the most sensitive to urea used in fertilization programs. The avoidance behaviour and (or) mortality of amphibians in laboratory studies was species dependent. Ruminant animals, including wild ungulates, rapidly convert urea to ammonia and are susceptible to toxicity following ingestion of large amounts of urea. Feeding on urea pellets by small mammals or gallinaceous birds appears to be minimal as granules are unpalatable. In terms of indirect effects, the majority of responses of understory herbs to nitrogen fertilization showed an increase in abundance. Some shrubs in repeatedly fertilized stands eventually increased in abundance in long-term studies, whereas dwarf shrubs and abundance of bryophytes (mosses and terrestrial lichens) declined. In general, species richness and diversity of understory herbs and shrubs declined, or were unaffected, in fertilized stands. Response in abundance and species richness-diversity of vascular plants to a single application of nitrogen showed either an increase or no change. Repeated applications (2–5 and >5) usually resulted in declines in these responses. Relative abundance of mule deer (Odocoileus Rafinesque spp.), moose (Alces alces L.), and hares (Lepus L. spp.), and forage quantity and quality were usually increased by fertilization. Small mammal species generally showed increases or no change in abundance; decreases may be related to fertilizer-induced changes in food sources. Forest fertilization may provide winter feeding habitat for coniferous foliage-gleaning insectivorous birds in some cases. Six species of forest grouse showed no response to fertilizer treatments. Responses of soil animals to nitrogen fertilization appeared to be species- and dose-specific and are ameliorated by surrounding micro- and macro-habitat characteristics.