The tolerance of plants to herbivory reflects the degree to which a plant can regrow and reproduce after damage from herbivores. Autoecological factors, as well as the influence of competitors and mutualists, affect the level of plant tolerance. Recent work indicates that there is a heritable basis for tolerance and that it can evolve in natural plant populations. Although tolerance is probably not a strict alternative to plant resistance, there could be inter- and intraspecific tradeoffs between these defensive strategies.

http://www.eeb.cornell.edu/agrawal/pdfs/1999/Tree-1999.pdf

The ecology and evolution of plant tolerance to herbivory.

The theoretical impacts of anthropogenic habitat degradation on genetic resources have been well articulated. Here we use a simulation approach to assess the magnitude of expected genetic change, and review 31 studies of 23 neotropical tree species to assess whether empirical case studies conform to theory. Major differences in the sensitivity of measures to detect the genetic health of degraded populations were obvious. Most studies employing genetic diversity (nine out of 13) found no significant consequences, yet most that assessed progeny inbreeding (six out of eight), reproductive output (seven out of 10) and fitness (all six) highlighted significant impacts. These observations are in line with theory, where inbreeding is observed immediately following impact, but genetic diversity is lost slowly over subsequent generations, which for trees may take decades. Studies also highlight the ecological, not just genetic, consequences of habitat degradation that can cause reduced seed set and progeny fitness. Unexpectedly, two studies examining pollen flow using paternity analysis highlight an extensive network of gene flow at smaller spatial scales (less than 10 km). Gene flow can thus mitigate against loss of genetic diversity and assist in long-term population viability, even in degraded landscapes. Unfortunately, the surveyed studies were too few and heterogeneous to examine concepts of population size thresholds and genetic resilience in relation to life history. Future suggested research priorities include undertaking integrated studies on a range of species in the same landscapes; better documentation of the extent and duration of impact; and most importantly, combining neutral marker, pollination dynamics, ecological consequences, and progeny fitness assessment within single studies.

/pdf/genetic-resource-impacts-of-habitat-loss-and-degradation-42s59nypfr.pdf

Genetic resource impacts of habitat loss and degradation; reconciling empirical evidence and predicted theory for neotropical trees

Summary 1 We examined how seed size and seedling growth rate influence recruitment success of neotropical pioneer species. We investigated the soil seed bank, and followed seedlings from emergence and establishment to the sapling stage in artificially created gaps in secondary forest on the Barro Colorado Nature Monument, Panama. 2 To simulate microsite variation within gaps, litter addition/removal and soil disturbance treatments were applied to seedling plots in a factorial design. 3 Seedling emergence was almost three-fold higher in litter-free plots than in control plots. 4 We found a negative relationship between seed mass and seed abundance in the soil, seed mass was positively correlated with seedling emergence success from the seed bank and with seedling survival through to the end of the first dry season. 5 In the 18 months following the first dry season we observed an uncoupling of seed mass from seedling performance. Seedling mortality was now a function of maximum relative growth rate, with higher mortality of fast-growing species attributable to herbivory, notably by shoot-borers. 6 We propose that a seed size-dependent trade-off between dispersal success (selecting for large seed number), and emergence‐establishment success (selecting for large seed size) can explain the maintenance of wide variation in seed size among pioneer species. Secondarily, a trade-off between growth rate and susceptibility to herbivores acting at the post-establishment phase may contribute to observed differences in light requirements among pioneer species.

Seed size, growth rate and gap microsite conditions as determinants of recruitment success for pioneer species

Habitat destruction and the resultant fragmentation of the remaining forest are a common phenomenon in the tropics. Most investigations emphasize the potential dangers of fragmentation in isolating patches of forest and exposing populations to loss of species diversity through founder effects, genetic drift, inbreeding, and restricted gene flow. However, a limited number of studies have shown that gene flow may be extensive in tropical trees, suggesting that it may occur between forest fragments and also “isolated” remnant trees. There is an urgent need to quantify pollen flow within and between forest fragments to test the veracity of such views and determine the genetic value of such fragments for in situ conservation. Microsatellite markers are used to genotype individuals of Swietenia humilis from a highly fragmented forest mosaic to directly quantify pollen-mediated gene flow. Distances of pollen flow more than 10 times greater than previously reported were detected. Our results show that some tropical angiosperm tree species may be much more adaptable and resilient to habitat destruction and fragmentation than previously considered. The description of many remnant trees as isolated or “living dead” may be more a conditioning of human perception than a true reflection of their potential conservation value.

/pdf/increased-pollen-flow-counteracts-fragmentation-in-a-opnai3pe0z.pdf

Increased pollen flow counteracts fragmentation in a tropical dry forest: An example from Swietenia humilis Zuccarini

Conservation Biology, Pages 7–20 Volume 15, No. 1, February 2001 This paper is a much abridged version of World Bank Environment Department Paper 75, “Biodiversity Conservation in the Context of Tropical Forest Management,” by the same authors. It is the first paper in the bank’s Impact Studies Series, which addresses the broader questions of the positive and negative effects of human activities on biodiversity. A more complete set of data supporting the arguments herein are in the appendices and text of the World Bank paper, which can be accessed from http://www.worldbank.org/biodiversity.

Tropical forest management and conservation of biodiversity: an overview

The mahogany shoot borer: prospects for control

Summary

1 Attack by shoot borers (Hypsipyla grandella Zeller) is the main factor limiting the cultivation of Cedrela odorata and Swietenia macrophylla, two economically important members of the mahogany family. No viable methods of pest control are currently available. To assess for genetic variation in susceptibility to pest attack, a combined progeny/provenance test of C. odorata and a provenance test of S. macrophylla were established separately at CATIE, Turrialba, Costa Rica, and assessed intensively over an 84-week period.



2 Variation in height growth between provenances was highly significant in both species, provenance means varying by a factor of 2.7 and 1.2 in C. odorata and S. macrophylla, respectively. Cedrela odorata also displayed provenance variation in foliar phenology, as 94.7% of the trees from a ‘wet zone’ provenance remained foliated, whereas > 35% of trees from three ‘dry zone’ provenances abscised their leaves during the dry season.



3 Both species displayed significant genetic variation in susceptibility to shoot borer attack. At the peak of attack during the second year of growth, the effect of provenance was highly significant in both species, the mean number of attacks per tree varying between 0.8–2.4 and 0.6–1.3 in different provenances of C. odorata and S. macrophylla, respectively. A provenance of C. odorata from San Carlos, Costa Rica, displayed consistently lower susceptibility to pest attack, being subjected to fewer attacks during the first year and demonstrating a higher mean height to first point of damage.



4 The nitrogen, total tannin and proanthocyanidin concentration of foliage varied significantly between C. odorata provenances: nitrogen concentration was significantly lower and tannin and proanthocyanidin contents were significantly higher in trees from the San Carlos provenance, Costa Rica. This study therefore suggests that proanthocyanidins may reduce susceptibility of C. odorata to H. grandella, at least during the early period of growth.



5 These results provide support for the development of plant resistance as a strategy for managing shoot borers. In particular, there may be scope for selecting for high foliar proanthocyanidin content and the ability to tolerate attack by vigorous apical growth.

Genetic variation in host susceptibility to attack by the mahogany shoot borer, Hypsipyla grandella (Zeller)

In order to evaluate the relationships between attacks by the mahogany shoot borer Hypsipyla grandella (Zeller) and host growth and phenology, field trials of Spanish cedar Cedrela odorata and American mahogany Swietenia macrophylla established in Costa Rica were assessed intensively over an 84 week period. Height growth of C. odorata was approximately twice that of S. macrophylla , with mean growth rates of 2.0 and 1.3 cm week −1 respectively. Cedrela odorata displayed pronounced temporal variation in leaf phenology, with a minimum of 51% of trees in leaf during a relatively dry period. In contrast, the minimum proportion of foliated S. macrophylla trees was 87%. No shoot borer attacks were recorded on S. macrophylla until week 50 after the onset of assessments, whereas initial attacks of C. odorata were recorded during week 6. By week 84, the proportion of trees attacked was 74% and 77%, respectively. In both species, attacks were concentrated in seasonal peaks, coinciding with the production of new shoots. Pronounced spatial variation in attack was observed in both species. At week 12, the number of attacks per block in C. odorata was negatively correlated with available calcium concentration (r=0.85, P C. odorata was positively correlated with available phosphate concentration (r=0.84; P C. odorata and 0.4% S. macrophylla remained undamaged to a height of 3 m. It is suggested that control methods for the mahogany shoot borer should take account of temporal dynamics in attack, and relationship to host phenology. Manipulation of the soil nutrient status may also form part of an integrated approach to pest management.

Variation in attack by the mahogany shoot borer, Hypsipyla grandella (Lepidoptera: Pyralidae), in relation to host growth and phenology

An assessment was made of genetic variation in apical dominance of Cedrela odorata L. using a decapitation test. Nineteen weeks after sowing, pot-grown seedlings belonging to 30 open-pollinated progenies from 5 provenances were decapitated by removal of the shoot apex, leaving shoots of a uniform height of 20 cm. Foliage was reduced to a single leaf with 2 pairs of leaflets on each plant. The length and number of lateral shoots subsequently formed were measured at 2 weeks, 4 weeks and 8 weeks after decapitation to determine the percentage bud activity and dominance ratio of the shoots. Percentage bud activity increased within the first 2 weeks to 4 weeks and declined thereafter as dominance was reimposed. Significant differences between provenances and progenies were recorded both in the timing and extent of peak bud activity (P < 0.001, ANOVA). Maximum bud activity ranged from 17.7% to 24.2% in the 5 provenances whereas mean values for the progenies differed by more than a factor of 2, ranging from 13.6% to 32.3%. Pronounced genetic variation in dominance ratio was also recorded; maximum values occurred 4 weeks after decapitation in all progenies. Dominance ratio was negatively correlated with percentage bud activity (r = 0.65, P < 0.001). These results indicate that significant potential exists for selection of Cedrela odorata genotypes with relatively high apical dominance, which may exhibit superior form and pest tolerance.

J.F. Mesén

Papers

The mahogany shoot borer: prospects for control

Genetic variation in host susceptibility to attack by the mahogany shoot borer, Hypsipyla grandella (Zeller)

Variation in attack by the mahogany shoot borer, Hypsipyla grandella (Lepidoptera: Pyralidae), in relation to host growth and phenology

Genetic variation of apical dominance of Cedrela odorata seedlings in response to decapitation

Mahogany as a genetic resource