Effects of Climate and the Urban Heat Island Effect on Urban Tree Growth in Houston

The growing conditions of urban trees differ substantially from forest sites and are mainly characterized by small planting pits with less water, nutrient and aeration availability, high temperatures and radiation inputs as well as pollution and soil compaction. Especially, global warming can amplify the negative effects of urban microclimates on tree growth, health and well-being of citizens. To quantify the growth of urban trees influenced by the urban climate, ten urban tree species in four climate zones were assessed in an overarching worldwide dendrochronological study. The focus of this analysis was the species water oak (Quercus nigra L.) in Houston, Texas, USA. Similar to the overall growth trend, we found in urban trees, water oaks displayed an accelerated growth during the last decades. Moreover, water oaks in the city center grew better than the water oaks growing in the rural surroundings of Houston, though this trend was reversed with high age. Growth habitat (urban, suburban, rural and forest) significantly affected tree growth (p < 0.001) with urban trees growing faster than rural growing trees and forest trees, though a younger age of urban trees might influence the found growth patterns. Growing site in terms of cardinal direction did not markedly influence tree growth, which was more influenced by the prevalent climatic conditions of Houston and the urban climate. Higher temperatures, an extended growing season and eutrophication can cause an accelerated growth of trees in urban regions across, across all climatic zones. However, an accelerated growth rate can have negative consequences like quicker ageing and tree death resulting in higher costs for new plantings and tree management as well as the decrease in ecosystem services due to a lack of old trees providing greatest benefits for mitigating the negative effects of the urban climate.

Sperm traits differ between winged and wingless males of the ant Cardiocondyla obscurior

Size and shape of sperm cells vary tremendously throughout the animal kingdom.The adaptive significance of this variation is not fully understood.In addition to sperm-female interactions and the environmental condi­tions,the risk of sperm competition might affect number,morphology and other“quality”traits of sperm.In the male-diphenic ant Cardiocondyla obscurior,winged sneaker males have limited sperm number,because their testes degenerate shortly after adult emergence,as is typical for males of social Hymenoptera.In contrast,wing­less fighter males continuously replenish their sperm supply due to their exceptional lifelong spermatogene­sis.While winged males usually have to compete with several other winged males for virgin queens,wingless males are able to monopolize queens by killing all other rivals.Hence,this presents a unique system to inves­tigate how alternative reproductive tactics and associated physiology affect sperm morphology and viability.We found that sperm-limited males invest into sperm number instead of sperm size.Variance in sperm length is smaller in winged males,probably reflecting that they have to compete with several other males.Finally,sperm viability is equally high in both male phenotypes.